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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Instrumentation For additional information on this subject, contact

File Reference: PCI20402 E.W. Reah on 875-0426

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Specifying Control Valves

For Severe Service Applications

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Engineering Encyclopedia Instrumentation


for Severe Service Applications

Saudi Aramco DeskTop Standards

Contents Pages

SELECTING CONTROL VALVE MATERIALS FOR CORROSIVE

FLUID APPLICATIONS.....................................................................................1

Corrosion And Its Consequences..............................................................1

Basic Corrosion Mechanisms........................................................1

Common Forms Of Corrosion.......................................................2

Quantifying Corrosion Intensity..................................................36

Consequences Of Corrosion........................................................36Corrosive Service Flags And Typical Corrosive Applications ...............37

Flags For Corrosive Fluid Applications ......................................37

Common Corrosive Fluid Applications.......................................38

Critical Control Valve Specification Considerations..............................41

Selection Of Appropriate Valve Types .......................................41

Material Considerations ..............................................................42

Importance Of Specifying Specific Material Grades ..................48Significance Of An Accurate Fluid Description..........................48

Significance Of Providing Accurate Service Conditions ............53

Resources For Control Valve Selection..................................................53

SAES-L-008................................................................................53

Vendor’s Corrosion Guidelines...................................................53

SELECTING CONTROL VALVES FOR EROSIVE FLUID

APPLICATIONS................................................................................................55

Erosion And Its Consequences ...............................................................55

Common Forms Of Erosion ........................................................55

Quantifying Erosion Intensity .....................................................55

Consequences Of Erosion ...........................................................57

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Erosive Service Flags And Typical Erosive Fluid Applications.............58

Flags For Erosive Fluid Applications..........................................58Common Erosive Fluid Applications ..........................................61

Critical Control Valve Specification Considerations..............................62

Control Valve Styles ...................................................................62

Materials Selection Considerations .............................................80

Sizing Issues................................................................................71

Information Sources................................................................................97

SELECTING CONTROL VALVE OPTIONS FOR HIGH-

TEMPERATURE FLUID APPLICATIONS .....................................................98

High Temperature Applications And Their Consequences.....................98

Categories Of High Temperature Applications ...........................98

Common Applications.................................................................98

Consequences Of High Temperature Fluids On

Incompatible Components.........................................................100

Consequences Of Thermal Cycling Applications .......................79

High Temperature Service Flags ..........................................................107

Saudi Aramco Definition Of High Temperature .......................107

Thermal Cycling Flags..............................................................107

Critical Control Valve Specification Considerations............................108

Valve Design Considerations ....................................................108

Material Temperature Ratings...................................................109

Extended Bonnets For Packing Protection................................120

Achieving Tight Shutoff At Elevated Temperatures.................121

SELECTING AND SIZING CONTROL VALVES FOR CAVITATING

FLUID APPLICATIONS.................................................................................123

Cavitation And Its Consequences .........................................................123

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The Cavitation Phenomenon .....................................................123

Cavitation Versus Other Flowstream Phenomenon...................124

Common Forms Of Cavitation ..................................................125

Consequences Of Cavitation .....................................................127

Predicting The Potential For Cavitation................................................130

Saudi Aramco And Manufacturer’s System Cavitation

Indices .......................................................................................130

Subjective Factors For Analyzing The Potential For

Cavitation Damage....................................................................133

Cavitation Service Flags And Typical Cavitating Applications ...........143

Flags For Cavitating Fluid Applications ...................................143

Specific Applications ................................................................143

Anti-Cavitation Valve Technology.......................................................144

General Anti-Cavitation Valve And Trim Design

Strategies...................................................................................144

Specific Anti-Cavitation Valve And Trim Designs...................147

Custom Valves ..........................................................................156

Control Valve Selection Considerations...............................................159

Performance Objective: Cavitation Damage Control

Versus Cavitation Prevention....................................................159

Manufacturers Control Valve Selection Procedures .................159

Valve Performance Contingency Requirements .......................161

Sensitivity To Accurate Data ....................................................161

Importance Of Defining Worst Case Cavitating

Conditions .................................................................................164Cavitation In Combination With Other Severe Conditions.......164

Anti-Cavitation Trim And Flashing Applications .....................164

Non-Valve Methods Of Reducing The Potential For

Cavitation ..................................................................................165

ISA System Indices From ISA-dRP75.23.................................167

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SELECTING AND SIZING CONTROL VALVES FOR FLASHING

FLUID APPLICATIONS.................................................................................172

Flashing And Its Consequences............................................................172

Review Of Flashing Phenomenon.............................................172

Common Forms Of Flashing.....................................................174

Quantifying Flashing.................................................................178

Consequences Of Flashing........................................................179

Flashing Service Flags And Typical Flashing Applications.................179

Flags For Flashing Fluid Applications ......................................179

Typical Flashing Fluid Applications .........................................179

Critical Control Valve Selection Considerations ..................................180

Basic Control Valve Selection Criteria .....................................180

Erosion Resistant Control Valve Types.....................................180

Materials Of Construction .........................................................183

System Design Considerations..................................................183

Valve Sizing Procedures ...........................................................184

Flashing In Combination With Particle Erosion Or

Corrosion...................................................................................185

Importance Of Accurate Data....................................................185

SELECTING AND SIZING CONTROL VALVES TO ATTENUATE

AERODYNAMIC CONTROL VALVE NOISE..............................................186

Sources Of Control Valve Noise...........................................................186

Types Of Control Valve Noise..................................................186

Mechanics Of Aerodynamic Noise Generation AndTransmission .............................................................................189

Quantifying Noise Intensity..................................................................190

Measurement Parameters ..........................................................190

Measurement Units And Scales ................................................190

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Measurement Techniques..........................................................193

Consequences Of Control Valve Noise.....................................198

Flags For Excessive Noise And Common Noise Applications.............201

SPL> 90 dBA For A Standard Valve ........................................201

Outlet Velocity Greater Than 0.3 Mach....................................202

P1/P2 > 5 For Dry Gas And Superheated Steam Services........202

SPL > Limits That Are Established By Saudi Aramco

Engineering Standards ..............................................................202

Specific Applications ................................................................202

Predicting Control Valve Noise............................................................202

Introduction...............................................................................202

Influences On Noise Generation And Transmission .................203

Noise Prediction Equations .......................................................204

Control Valve Options For Attenuating Control Valve Noise..............209

Source Treatments Vs. Path Treatments ...................................209

Valve Style Versus Noise Attenuation......................................210

Body Options For Globe And Angle Valves.............................210

Noise Abatement Trim Design Strategies .................................210

Commonly Available Noise Abatement Valve Options............215

Characterizing Noise Abatement Trim......................................219

Common Selection Problems And Specification Errors.......................220

Absence Of Industry Standards For Noise Prediction

Equations...................................................................................220

Specifier's Failure To Identify Worst Case ServiceConditions .................................................................................221

WORK AID 1: FLUID COMPATIBILITY INFORMATION THAT IS

USED TO SELECT CONTROL VALVES FOR CORROSIVE FLUID

APPLICATIONS..............................................................................................222

Work Aid 1A: NACE Compliant Materials Of Construction ...............222

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Work Aid 1B: Recommended Materials Of Construction For

Seawater And Brine Services ...............................................................237

Work Aid 1C: Valve And Material Selection Guidelines For

Amine (DGA) Letdown Applications...................................................242

WORK AID 2: HIERARCHICAL LISTINGS OF EROSION

RESISTANT VALVE STYLES AND CONSTRUCTION

MATERIALS ...................................................................................................243

Work Aid 2A: Hierarchy Of Erosion Resistant Valve Styles That

Is Used To Select Control Valves For Erosive Fluid Applications.......243

Work Aid 2B: Hierarchies Of Erosion Resistant Body And TrimMaterials That Are Used To Select Control Valves For Erosive

Fluid Applications ................................................................................257

Hierarchy Of Erosion Resistant Body Materials .......................257

Hierarchy Of Erosion Resistant Trim Materials........................260

WORK AID 3: PROCEDURES THAT ARE USED TO SELECT

CONTROL VALVE OPTIONS FOR HIGH TEMPERATURE FLUID

APPLICATIONS..............................................................................................269

Body And Bonnet Material Selection .......................................269Trim Material Selection.............................................................269

Gasket Material Selection .........................................................269

Packing Material Selection........................................................269

Bonnet Type Selection ..............................................................269

Thermal Cycling Considerations...............................................269

WORK AID 4: PROCEDURES THAT ARE USED TO SELECT AND

SIZE CONTROL VALVES FOR CAVITATING FLUID

APPLICATIONS..............................................................................................245

Work Aid 4A: Procedures That Are Used To Perform Basic

Selection And Sizing With The Use Of Control Component’s

Inc. Sizing Software..............................................................................245

Preliminary Entries....................................................................245

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Entering Fluid Properties And Service Conditions ...................245

Design Information ...................................................................245

Change Menu ............................................................................245

Calculation Results....................................................................271

Work Aid 4B: Procedures That Are Used To Perform Basic

Selection And Sizing With The Use Of Valtek’s Sizing Software .......271


Project Identification .................................................................247

Valve Selection .........................................................................247

Valve Sizing..............................................................................247

Work Aid 4C: Procedures That Are Used To Perform Basic

Selection And Sizing With The Use Of Fisher Control’s Sizing

Program ................................................................................................273


Setting Options..........................................................................273

Data Entry And Sizing Calculations..........................................273

WORK AID 5: GUIDELINES FOR VALVE STYLE AND

MATERIAL SELECTION AND PROCEDURES THAT ARE USED

TO SIZE CONTROL VALVES FOR FLASHING FLUID

APPLICATIONS..............................................................................................251

Work Aid 5A: Procedures That Are Used To Size Control

Valves For Flashing Fluid Applications ...............................................251

Work Aid 5B: Guidelines For Valve Style And Material

Selection That Are Used To Select Control Valves For Flashing

Fluid Applications ................................................................................251

Valve Style Selection Guidelines ..............................................251

Body and Trim Material Selection Guidelines..........................251

WORK AID 6: PROCEDURES THAT ARE USED TO SELECT AND

SIZE CONTROL VALVES TO ATTENUATE AERODYNAMIC

CONTROL VALVE NOISE ............................................................................253

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Work Aid 6A: Procedures That Are Used To Select And Size

Noise Attenuating Control Valves With The Fisher Sizing

Program ................................................................................................253


Setting Options..........................................................................253

Data Entry And Sizing Calculations..........................................253

Work Aid 6B: Procedures That Are Used To Select And Size

Noise Attenuating Control Valves With Control Components

Sizing Software.....................................................................................255


Entering Fluid Properties And Service Conditions ...................255

Design Information ...................................................................255

Change Menu ............................................................................283

Calculation Results....................................................................283

GLOSSARY.....................................................................................................284

ADDENDUM...................................................................................................289

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Saudi Aramco DeskTop Standards 1

Selecting Control Valve Materials For Corrosive Fluid Applications

Corrosion And Its Consequences

Basic Corrosion Mechanisms

Electrochemical Action - Most forms of corrosion can be viewed as an electrochemical

reaction. The basic mechanics of the electrochemical reaction are illustrated in Figure 1.

Use Word 6.0c or later to

view Macintosh picture.

Figure 1

Basic Corrosion Process

When zinc is placed in dilute hydrochloric acid as shown in the above Figure, avigorous reaction occurs; hydrogen gas is evolved and the zinc dissolves,

forming a solution of zinc chloride; i.e.:

Zn H Zn H+ → ++ +2 22

Deterioration (corrosion) of the zinc occurs at the area where the electrons

leave the metal. This area is referred to as theanode and it is where damageis observed. The area to which the electrons migrate is thecathode. Thecathode is a protected area and it is not subject to corrosion damage.

In some corrosion reactions, the oxidation reaction occurs uniformly on thesurface of the affected metal. In other cases, the corrosive reactions occur only

at specific areas. The differences in the location of the electrochemical reaction

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provide a basis for categorizing the various forms of corrosion that will be

discussed throughout this section.

Passivation - When sufficient oxygen is available, some metals develop a

protective oxide film or passive layer. The passive layer often addsconsiderably to the corrosion-resistance of the metal. Passivation requires the

presence of oxidizing agents. The effectiveness of the passive layer dependsupon the oxidizing power of the solution as shown in Figure 2. When the initialsolution oxidizing power is low, the rate of corrosion increases as a direct

function of the solution oxidizing power. At some point, the metal undergoes atransition from the active to the passive region. At this point, a dramaticreduction in the corrosion rate is observed. Further increases in the solution

oxidizing power ultimately cause the material to lose its corrosion-resistance.

Many of the alloys that are used in control valve assemblies achieve their corrosion resistance from the phenomenon of passivation.



Figure 2

Passivation As A Function Of Solution Oxidizing Power

Common Forms Of Corrosion

Uniform attack , also known as general corrosion, occurs when the metal is

uniformly dissolved by the environment. Refer to Figure 3. As a result of

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uniform attack, the metal becomes thinner and thinner and eventually fails.

During this type of corrosion, the corrosion product may form a protective layer

on the metal surface; e.g., rust on iron or the passive layer that forms on somestainless steels. The protective layer may slow corrosion, or, the corrosion

product may also be attacked (dissolved) by the corrosive media. Uniformattack can be prevented through the selection of corrosion resistant materials,through the use of protective coatings, or by adding corrosion inhibitors to the

process fluid. Uniform attack, or general corrosion, is not of great concern froma technical viewpoint because fluid/material compatibility issues can be

established by immersing a particular metal specimen in a particular fluid andmeasuring the material loss. The results of such tests are well documentedand can be used to develop material/environment compatibility guidelines.

Other, more localized forms of corrosion that will be discussed in this section

present a greater challenge to the valve specifier.



Figure 3

General Corrosion

Pitting Corrosion - Pitting is an extremely localized attack that results in holes inthe metal as shown in Figure 4. Pitting is often difficult to detect because of thesmall size of the pits, and because the pits are often covered with corrosion

products. Pitting is generally associated with the presence of chlorides in theflow stream. Carbon steels are not generally subject to pitting; however, theconditions that lead to pitting do generally result in unacceptable levels of

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general corrosion of carbon and alloy steels. Ironically, many stainless steels

that are selected for their general corrosion resistance are particularly

susceptible to pitting. To improve stainless steel’s resistance to pitting,increasing amounts of chromium, nickel, and/or molybdenum are included in

the stainless steel; e.g., the 300 series stainless steels. In these alloys, thechromium and/or molybdenum combine with oxygen at the material surface toform a tough, adherent oxide layer that is resistant to attack in many

environments. The passive layer may be damaged or removed by extremelyhigh velocity flows or by direct chemical attack. A damaged protective oxide

layer may reform (repassivate) if sufficient oxygen is available. If the film doesnot immediately reform (repassivate), pitting may occur. The initial attack isfollowed by penetration of the corrosive fluid into the metal.



Figure 4

Pitting Corrosion

Crevice Corrosion is similar to pitting corrosion but it is observed in areas whereaccess to oxygen is restricted; e.g., in crevices, at gasket surfaces, and under

deposits or biological organisms. Refer to Figure 5. When access to oxygen isrestricted, the passive layer is either nonexistent or weak. Because reducedflow rates limit the available oxygen, low flow rates can also promote crevicecorrosion. Reduced flow rates also increase the potential for scaling and

fouling which results in oxygen-restricted areas where repassivation cannotoccur. Crevice corrosion is likely to occur in any application where pitting is

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anticipated; however, crevice corrosion is likely to begin earlier and to produce

more dramatic damage than pitting corrosion.



Figure 5

Crevice Corrosion

Erosion-Corrosion - The passive layer can be damaged by high velocity flows or by the impingement of erosive particles on material surfaces. If the passive

layer becomes eroded, the base material is exposed directly to theenvironment and the rate of corrosion may increase. Refer to Figure 6. Toproduce a tougher, more adherent passive layer that can resist erosion,

materials that include increased amounts of chromium and molybdenum arespecified. It is generally acknowledged that molybdenum is more influential

than chromium in increasing erosion-corrosion resistance. Another solution toerosion-corrosion is to select materials that do not depend upon the passivelayer for corrosion protection; for example, the Monels (N04400 and N05500)

and Hastelloy B2 (N10665) do not include chromium and do not depend uponthe passive oxide layer for corrosion resistance.

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Figure 6

Erosion Corrosion

Intergranular Corrosion is a form of corrosion in which the material along thegrain boundaries of the metal is removed. Refer to Figure 7. Intergranular corrosion begins with a phenomena known as sensitization. Sensitization is a

process in which exposure to high temperature causes corrosion resistantalloys to precipitate out of the material matrix, leaving a zone at the grainboundary that is not protected from corrosion attack. For example, in some

stainless steels (primarily the 300 series) that have been subjected to highheat from welding, chromium carbides may precipitate at the grain boundaries

thus depleting the chromium from the immediately adjacent material. In acorrosive environment, the area of the grain boundary that has been depletedof chromium is susceptible to attack by the corrosive atmosphere. The

corrosion that results is known as intergranular corrosion.

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Figure 7

Intergranular Corrosion

Galvanic Corrosion - When two dissimilar metals are immersed in a conductivesolution, an electron flow may be established between the two. The standardcarbon-zinc battery that is shown in Figure 8 is a familiar example of electron

flow between two dissimilar metals. If electron flow is established, galvanic

corrosion of the less corrosion-resistant material (the anode) will occur.

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Figure 8

Carbon Zinc Battery As An Example Of Electron Flow Between Two Metals

As a general rule of thumb, the potential for galvanic corrosion increases asthe separation between the two metals on agalvanic series increases. Agalvanic series is a list of metals that is ordered according to the relativemagnitude of the electrical potential that each produces when paired with a

reference electrode. Refer to Figure 9. The metals near the top of the list areconsidered “noble”, or cathodic (less likely to give up electrons and thereforeless susceptible to corrosion). The metals at the bottom of the list are

considered to be active or anodic (they are more likely to give up electrons andtherefore more susceptible to corrosion). Electron flow can be establishedbetween two metals either through direct contact or through an electrolyte (a

conductive medium). The process fluid can serve as an electrolyte. When a

circuit is completed between two metals that are close together in the galvanicseries, the potential for corrosion is small or non-existent. As the separation of

two paired metals on the chart increases, the potential for electron flow andcorrosion increases dramatically.

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Platinum

Gold

Graphite

Titanium

Silver

Chlorimet 3

Hastelloy C

316 stainlesssteel(passive)

304 stainlesssteel(passive)

Inconel

Nickel

Monel

Bronzes

Copper

Brasses

Hastelloy B

Inconel (active)

Nickel (active)

Tin

Lead

316 stainlesssteel(active)

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304 stainlesssteel

(active)

Cadmium

Aluminum

Zinc

Magnesium

Figure 9

Galvanic Series Of Common Materials In Seawater



Figure 10

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Zinc Washers As Sacrificial Elements To Protect Stainless Steel Valve

Stems

SCC, or stress corrosion cracking, occurs when certain alloys are exposed to

specific environments and the affected component is subjected to tensilestress. Tensile stress is present in virtually all components. Tensile stress maybe the result of process pressure that is exerted on a valve component,

misalignment of piping, thermal expansion, and from the residual stress of coldwork, welding, or heat treatments. Examples of alloy-environment pairs thatare susceptible to SCC are listed in Figure 11. The concentration of the

environment, the operating temperature, and the operating pressures impactthe extent of SCC. Insulfide stress cracking (SSC), the corrosive action ismost intense at ambient temperatures because at low temperatures the

diffusion process is slowed, and, at elevated temperatures the diffusion rate isso fast that a critical concentration is never reached. Withchloride stresscracking, which is commonly encountered in deep, sour wells and in seawater

and brine applications, SCC occurs at temperatures above 130 degrees F. Thesteps that are taken to prevent sulfide stress cracking are embodied in aguideline titled NACE MR0175 that will be discussed later.



Envir Allo

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Chlorid 300

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Hydrog Hard

Hydrog Hard

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Sodiu Steel

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Ammo Cop

Figure 11

Mechanics Of Stress Corrosion Cracking

The susceptibility of a material to SCC is related to its hardness level.Hardness is a physical property that relates the resistance of a material to

penetration or indentation. In metals, hardness is usually measured in thelaboratory by loading an indenter into a material and measuring either thedepth or the surface area of the indentation. Several test procedures and

scales of hardness have been established. A popular scale is the Rockwell Cscale, which is abbreviated as HRC (Hardness Rockwell C). The range for the

Rockwell C scale is from HRC 20 to HRC 60. For reference, hardness levels of some common materials are listed in Figure 12.

M

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3

1

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(

A

(

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4

(

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Figure 12

Hardness Levels Of Some Common Materials

Figure 13 illustrates the relative time to failure (in hours) of bolting materialswith varying hardness levels. Because of the relationship of hardness levels

and SSC, the hardness of valve construction materials must be less thanallowable hardness levels that have been determined by test and evaluation.



Figure 13

Effects Of Material Hardness On Failure Caused By SCC

NACE MR0175 - The National Association of Corrosion Engineers (NACE) hasissued Standard MR0175 that specifies the proper materials, the heat-treating

conditions, and the strength levels that are required to provide good service lifein sour gas and oil environments.

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Figure 14 lists some of the NACE approved materials, hardness information,

and pertinent remarks. Note that the maximum hardness that is allowed under

the NACE guidelines depends on the material type.

Re

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Re

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Re

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Po

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Ca

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Ma

Ex

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Mo

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Hi

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Hi

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NA

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Mu

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NA

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NA

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NA

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Co

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Bel

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Figure 14

Common NACE Approved Materials

NACE MR0175 does not address elastomer and polymer materials. However, the

importance of these materials for critical sealing must be considered. User experience has

shown that nitrile, neoprene, and PTFE can be applied within their normal respective

temperature ranges.

Because hardness is a required property for a spring, the NACE MR0175 specifications for

maximum material hardness make it difficult to manufacture NACE compliant springs. Most

manufacturers offer a limited number of material options when NACE compliant springs are

required. To solve the problem of spring selection of control valve packing arrangements,

jam-style packings that do not require a spring are typically specified.

According to NACE MR0175, NACE compliant external bolting must be specified

whenever the bolting will be deprived of contact with the atmosphere. External boltingincludes the bonnet-to-body bolting, packing flange bolting, and line flange bolting.

Conditions that deprive the bolting of access to the environment include the use of

insulation, flange protectors, or burial of the valve.

Quantifying Corrosion Intensity

Because there is no standard "corrosion coefficient" upon which to base valve and material

selection decisions, corrosion intensity is generally discussed in subjective terms. For

example, corrosion intensity is often expressed as mild , moderate, or extreme. In advanced

corrosion engineering studies, other parameters such as the millimeters of material lost per

some unit of time provide a more precise index of a material’s corrosion resistance to

specific fluids.

Consequences Of Corrosion

Body Damage - Figure 15 illustrates that corrosion damage to control valvebodies can range from thinning of the body wall to loss of gasket surfaceintegrity to total failure of the body.

Trim Damage - Figure 15 also illustrates several different forms of trim damage. Any loss of material at seating surfaces will degrade the ability of the valve to

shutoff. Material removal may also enlarge plug-to-cage and stem-to-bushingclearances thereby allowing vibration of the valve plug and progressive

damage to the plug and seat. Crevice corrosion may be observed on gasketsurfaces, on the portion of the valve stem that is in the packing bore, and atother stagnant areas within the valve. SCC and intergranular corrosion aregenerally seen as small leaks before they result in catastrophic failures.

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Figure 15

Common Forms Of Corrosion Damage

Corrosive Service Flags And Typical Corrosive Applications

Flags For Corrosive Fluid Applications

Flags, or clues that a particular process is corrosive can come from a variety of sources.

Several flags are discussed below.

Specific Applications And Fluids - Seasoned valve specifiers learn fromexperience that certain applications involve corrosive fluids. For example, all

sour hydrocarbons, sour water, seawater, amine stripping processes, andboiler feedwater control are universally recognized as applications that canpresent significant corrosion challenges.

Construction Materials Of Related Equipment - If the piping, pumps, block valves,instruments, and other equipment in the control loop are made of corrosionresistant materials, the specifier has received a clear signal that the processfluid is corrosive.

Corrosion Guides And Compatibility Charts - Specifiers may consult variouscorrosion guides and material compatibility charts to determine if a specific

fluid will present the potential for corrosion with standard valve materials.

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Common Corrosive Fluid Applications

Sour Services (NACE) - Many crude oils and natural gasses contain hydrogensulfide; therefore, sulfide stress cracking is very common in most gas and oil

producing operations. In gas and oil production, SSC and SCC may beencountered in any application until all the sulfides and the chlorides have

been removed. Refer to Figure 16.



Figure 16

Sour And Sweet Processes In Gas And Oil Production

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As discussed previously, the susceptibility of a material to any form of SCC is

related to its hardness level. Some valve manufacturers have established

standard policies and practices that ensure compliance with NACE guidelineswhenever a valve is specified for sour service. For example, the following

summarizes the procedures that are followed by Fisher Controls.

• Carbon steel bodies and bonnets are heat treated to 22 HRC maximum,and they are post-weld heat-treated.

• Hardened martensitic stainless steels are not used.

• Control valve packing sets are jam style only (springless or externally live-

loaded).

• Valve stems are made from Nitronic 50 when higher strength is required.

• Primary trim materials are S31600 and Alloy 6.

• No machining operations that cause work hardening of the materials areperformed in the manufacturing process.

• Platings and coatings are appliedover NACE approved base metals, and

the coatings are not intended to provide corrosion protection.

• Bolting in Class III material is standard when the bolting is not exposed to

the sour atmosphere. Bolting in Class I and Class II material is availablewhen bolting is buried, insulated, or otherwise exposed to H2S.

Most valve manufacturers offer specific valve constructions and/or trim optionsthat comply with the NACE guidelines. Refer to Table 7 in Bulletin 51.1:ES

(Fisher Catalog 71) and note the standard trim options that are NACEapproved.

Seawater - Seawater is commonly injected for recovery purposes. Regardlessof the application, seawater can present difficult and complex problems for thematerials specifier. The concerns for corrosion include the following:

• General corrosion will be observed in many carbon and low alloy steels.

• Pitting and crevice corrosion is common in many stainless steels.

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• Above 160 degrees F, chloride stress corrosion cracking may develop.Chloride SCC disallows the use of any 300 series or 400 series stainless

steel.

• Because the seawater that is used in secondary recovery oilfield operationsis often high in NaCl, CO2, and H2S, the NACE guidelines must be

observed.

Brine - Brines are commonly used as refrigerants or low temperature heat

exchange media. In these recirculating applications, brines are often treatedwith corrosion inhibitors. At temperatures below 160 degrees F, corrosioninhibitors may prevent corrosion of carbon steel bodies and bonnets and 300

series stainless steel trim components. At temperatures above 160 degrees F,

chloride SCC will occur and materials should be selected accordingly.

High Pressure DGA - A popular process for removing acid gasses from naturalgas involves stripping the gas with an amine such as diglycolamine (DGA) or

diethanolamine (DEA). DGA stripping is common in Saudi Aramco operations. As shown in Figure 17, lean liquid amine enters the top of the tower and itflows downward across the trays. As the acid-rich gas flows upward, the amine

absorbs H2S and CO2 from the natural gas. Clean gas exits the top of thetower and acid-rich amine leaves the bottom of the absorption tower. The richamine passes through a letdown valve into a flash tank where a portion of the

of the absorbed gasses flash out of the liquid. Following the flash tank, the

amine passes through a regeneration process. The regenerated amine isreused in the process.

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Figure 17

Typical Amine (DGA) Adsorption Process

During amine letdown, large amounts of entrained gas will come out of

solution. The process, which is referred to as “outgassing”, results in two-phase flow. One phase is the liquid amine and the other phase is the gaseousCO2 and/or H2S that has flashed out of the amine solution. The amine itself

does not pose a corrosion problem; however, wet CO2 can result in theformation of carbonic acid which is highly corrosive to carbon steel (not to

stainless steel). Also, the presence of H2S results in the potential for stresscorrosion cracking. Materials selection for sour service is not changed by thepresence of CO2 and the NACE guidelines should be followed.

Critical Control Valve Specification Considerations

Selection Of Appropriate Valve Types

When selecting valves for corrosive fluid applications, specifiers may select either standard

alloy valves or they may choose to evaluate lined valves.

Lined Valves - Lined valves are made from an inexpensive base metal such ascarbon steel to which a non-metallic coating, cladding, or lining is applied in

order to achieve corrosion resistance. Control valve body liners and trimcoatings of polyvinyl chloride, rubber, PTFE, and other elastomers are

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common. All liners have limitations in terms of temperature ratings,

permeability, vibration sensitivity, susceptibility to mechanical damage, thermal

degradation, and adherence of the lining or coating to the base metal.Generally speaking, lined valves are selected only for highly corrosive fluids

under the conditions of low pressure, low pressure drop, and low to moderatetemperatures.

Alloy Valves - Because of the flow, pressure, and temperature conditions thatare characteristic of most Saudi Aramco operations, standard control valve

constructions with corrosion-resistant material options are generally preferredover lined valves. The common limiting factors are the availability of thedesired material options for the selected valve, and the high cost of

increasingly corrosion resistant alloys. For example, an alloy valve that is

resistant to sour seawater may cost 3 to 4 times as much as the same valvewith standard material options. (WCB body and bonnet and 316 stainless steel

trim).

Material Considerations

To provide good performance and long life, control valve components (bodies, bonnets,

trim, packing, and gaskets) must be selected of materials that are resistant to the prevailing

environment. The available materials are numerous and the subject of proper material

selection can easily become a career study. Fortunately, Saudi Aramco standards and vendor

supplied suggestions are available to assist the specifier. It is useful to acquire a fundamental

understanding of the corrosion resistance of some of the popular material options. For the

discussion that follows, refer to the item in the Addendum that is titled “Composition,

Characteristics, And Typical Uses For Common Control Valve Materials”.

Stainless Steels - Stainless steels are the most commonly selected materials for corrosion service applications. The broad range of materials that are

commonly referred to as stainless steels (SST’s) can be segmented accordingto their basic structure and according to the alloying elements that are includedin the composition of the material. The popular stainless steels for valve

components are the martensitics, austenitics, precipitation hardened stainlesssteels, and duplex stainless steels.

• Martensitic stainless steels (the 400 series stainless steels) were the firststainless steels to be developed. Engineers and metallurgists soon notedthat the addition of 12 percent chromium imparted greatly improvedcorrosion and oxidation resistance to steel. The improved corrosion

resistance results from the chromium that produces a protective passivelayer. Compared to other types of stainless steel, martensitics have arelatively high carbon content. The addition of carbon to a stainless steel

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results in increased hardness and an increased susceptibility to

sensitization and SCC. Martensitic stainless steels can achieve great

strength and hardness through various heat-treatments.

• Austenitic stainless steels (the 300 series stainless steels) typically provide

increased corrosion resistance as compared to the martensitics. Type 304stainless steel is sometimes referred to as an “18-8” because itscomposition includes 18 percent chromium and 8 percent nickel. The

increase in chromium content results in a better chromium oxide passivelater than a 400 series stainless steel. If the chromium oxide passive layer becomes damaged and pitting attack occurs, the nickel content provides

increased resistance to further penetration. Because chromium increasesresistance to oxidizing environments and nickel increases resistance to

reducing environments, the austenitic stainless steels are resistant to abroader range of environments than the martensitic stainless steels. The316 stainless steels also include Molybdenum which makes the passive

layer tougher and more adherent, thereby increasing the material’sresistance to pitting in reducing environments. Austenitic stainless steelsare hardened by cold work but they cannot be hardened by heat

treatments.

• Super-Austenitic stainless steels are those that include increased alloycontent. One of the most popular super austenitic stainless steels is Avesta

254 SMO. It should be pointed out that Avesta is the name of a Swedish

steel company that manufactures many alloys and “Avesta” is not a specificalloy designation. For clarity when describing this alloy, specific material

designations are most appropriate. Super austenitics are sometimesreferred to as the “6 Mo’s”, referring to the nominal 6 percent molybdenumcontent of the material. The increased chromium content provides a

tougher, more adherent passive layer, the increase in nickel providesincreased pitting resistance in reducing environments, and the increasedmolybdenum and addition of nitrogen increase the resistance to pitting by

chlorides.

• Precipitation-Hardened stainless steels can be heat treated to achieve

great strength and high hardness levels. 17-4PH is the material that is mostcommonly used for control valve components. This material is commonlyheat treated to a variety of conditions. The H900 condition is the hardestand the strongest and has been popular for many valve trim components;

however, because of the exceptional hardness, material in the H900condition is susceptible to stress corrosion cracking. To minimize SCC

problems, the H1075 condition has become popular.

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Procedure

Heat to 900 degrees F for 1 hour, air cool

Heat to 1,025 degrees F for 4 hours, air cool


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Heat to 1,150 degrees F for 4 hours, air cool to ambient, reheat to 1,150degrees F for 4 hours, air cool

Heat to 1,400 degrees F for 4 hours, air cool to ambient, reheat to 1,150degrees F for 4 hours, air cool

Figure 18

Common Heat Treatment Procedures

• Duplex stainless steels are becoming increasingly popular because of their

superior resistance to general corrosion and SCC in both sour and chlorideenvironments. In addition, the yield strength of the duplex stainless steels isapproximately double that of the annealed austenitic stainless steels. A

major consideration is that the common duplex stainless steels are NACE

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approved in the wrought form only; only one duplex (Z6CNDU20.08M) is

currently NACE approved in the cast form.

Nickel Base Alloys - A broad range of nickel base alloys are available for

specific corrosive environments. In general, these alloys are known as theMonels, the Inconels, and the Hastelloys; however, it is imprecise and

potentially dangerous to specify a nickel alloy without giving the full description.For example, instead of describing an alloy as “Hastelloy”, the specifier shouldgive the complete description; e.g., N06022 (Hastelloy C22 in the wrought

form).

• Nickel-Copper Alloys - The first nickel-copper alloys were known as the“Monels”. Nickel-copper alloys are the industry standards for dry chlorine

and hydrogen chloride gasses, hydrofluoric acid, and oxygen. They arealso selected for brine and sea water applications where chloride stresscorrosion cracking of S31600 is a problem. N04400 (alloy 400) is the most

common grade and it is often used as a soft gasket material. N05500 (alloyK500) is a high strength, age hardenable grade and is routinely used as ahigh-strength shaft and stem material.

• Nickel-Chromium Alloys- The nickel-chromium alloys were originallymarketed under the Inco tradename Inconel 600 (N06600 or alloy 600).Because alloy 600 does not include Molybdenum, its corrosion resistance

is poor in comparison to many other nickel-based alloys.

• Nickel-Iron-Chromium Alloys- The most common alloy in this group isknown as Carpenter 20®, Alloy 20, or alloy 20Cb-3. Alloy 20 is the industry

standard for sulfuric acid below 160 degrees F and it is often selected for chloride environments such as brine and sea water.

• Nickel-Molybdenum Alloys- The most popular alloy in this family is known

as Hastelloy B2 (N10665). N10665 has excellent resistance in allconcentrations and temperatures of hydrochloric acid; however, if ferric or cupric ions are present, severe attack will occur. N10665 is also compatible

with hydrogen chloride, sulfuric acid, acetic acid, and phosphoric aid.

Cobalt Base Alloys - The most common cobalt base alloy is often referred to as Alloy 6 or Stellite. The correct designations are R30006 for castings, CoCr-Afor rod and powder (the materials that are used to apply hardfacings), and alloy

6B for wrought forms. Alloy 6 has good corrosion resistance in a variety of environments but it is inferior to most corrosion resistant nickel base alloys.

Alloy 6 performs well whenever S31600 is acceptable and it is compatible with

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steam, sea water, and brine applications. The various forms of Alloy 6 may be

used when, in addition to corrosion resistance, erosion resistance is required.

Elastomers - Elastomers that are compatible with corrosive service applications

are generally not difficult to identify. The most common elastomer componentsare packing rings and valve plug seal rings. Standard PTFE and PTFE-based

materials are compatible with most corrosive fluids.

Coatings and Platings - Coatings and platings may be applied to base metals to

improve either the corrosion resistance or the erosion resistance of the basemetal. Corrosion resistant coatings are generally applied to carbon and lowalloy steels or to aluminum. Wear resistant coating are generally applied to

prevent wear, or to prevent galling when a single corrosion resistant base

metal is selected for components that are in a sliding wear application; e.g., astainless steel plug and a stainless steel cage.

• Hard chromium platingsare deposited by an electroplating process thatrequires an aqueous solution, electrodes, and an applied current.Chromium platings can exhibit very high hardness (equivalent to 70 HRC).

However, all hard chromium platings include small cracks that allowcorrosive fluids to contact the base metal. Accordingly, the plating offerslittle or no corrosion resistance and the base metal must be compatible with

the environment. Chromium is compatible with steam, boiler feedwater, anddry gasses. Hard chrome plating is often applied to actuator diaphragm

rods, cages, and other components that require wear resistant surfaces attemperatures up to 600 degrees F.

• Electrolyzing is a proprietary method of applying a hard, chrome coating.

The chromium deposit that results from the Electrolyzing process isreferred to as a “coating” rather than a “plating”. While the traditionalchrome plating process and the Electrolyzing process are similar, the

coating displays improved performance. The coating retains good wear resistance and hardness at temperatures up to 1100 degrees F while theupper temperature limit for traditional hard chrome plating is 600 degrees F.

Chrome coatings also provide superior galling resistance compared to

traditional hard chromium plating.

• Electroless Nickel Coating (ENC)is applied in much the same way as other

platings except that electrodes and an applied current are not used. Thecoating is very homogenous with no crystalline structure; it is actually ametallic glass. ENC deposits are more uniform than conventional platings

and they will uniformly cover small holes. Traditional platings cannot

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uniformly cover small holes in the base metal’s surface. ENC is applied to

plugs and cages in order to improve wear resistance and to prevent galling.

Its corrosion resistance is similar to that of 316 stainless steel.

• Nitriding is a process in which the base metal is heat treated (in a furnace)

in the presence of a specific chemical atmosphere. During the process,nitrogen ties up the chromium in the alloy to form chromium nitrides andother compounds at or near the surface. This layer improves the surface

hardness and overall wear properties of the treated material. However,because chromium is the primary element that produces the excellentcorrosion resistance of stainless steels; all forms of nitriding severely

degrade the corrosion resistance of the stainless steels. Because of theexcellent wear resistance and the poor corrosion resistance that results

from nitriding, nitriding is typically specified for components that will beexposed to fluids such as steam, boiler feedwater, organic solvents, anddry gasses such as nitrogen, argon, and methane.

Importance Of Specifying Specific Material Grades

Failure to specify a specific material grade is a common error in valve specification. For

example, a specification for “stainless steel” trim would be satisfied by any of a large

number of 300 and 400 series stainless steels that have widely differing characteristics.

Another example of an incomplete material specification is when a material is specified

simply as “Hastelloy”. There are more than 25 different grades of Hastelloy and each has

unique properties. A more complete specification would include the specific grade; e.g.,

Hastelloy B2, Hastelloy C276, Hastelloy G30, and so forth.

Significance Of An Accurate Fluid Description

Problems in valve specification are often traced to an incomplete or imprecise description of

the process fluid. Figure 19 lists some common fluid descriptions and notes concerning the

nature of the problems that can result.

Comment

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Rich DGAincludesH2SandCO2,ishighlycorrosive,

and issubject toNACEguidelines.

Lean DGA doesnotrequir eNACEcompliantmater ials.

DGA, DEA, andMDAare

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usedin

hydrocarbonprocessingoperations.Thesefluidsareoftenassociated

withSCCanderosionproblems.

Alloy6(Stellite) isoftenselect

edbecause of itscorrosionanderosionresistance.

Amines such as

hydrazineinboiler feedwater applicationsattack

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andcorro

de Alloy6.440Cor S44004stainlesssteeltrim isoftenselected for

thisapplication.

Temperature

andconcentrationhaveapronouncedaffectoncorrosionintens

ityandmater ialselection.

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Figure 19

Common Problems With Inaccurate Descriptions Of Fluids

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Significance Of Providing Accurate Service Conditions

The corrosion resistance of a particular alloy to a particular fluid is often a strong function of

the concentration and temperature of the fluid. For example, the alloys that are commonlyselected to resist corrosion by hydrofluoric acid are show in Figure 20. Note that at high

concentrations and low to moderate temperatures, a WCB cast steel body is acceptable and

that at reduced concentrations, more exotic alloys are required to provide the needed

corrosion resistance. Failure to provide accurate service data for this fluid could easily lead

to the selection of totally incompatible materials.



Figure 20

Material Compatibility Chart For Hydrofluoric Acid

Resources For Control Valve Selection

SAES-L-008

The Materials Appendix that is included in SAES-L-008 provides a great deal of material

compatibility information to the valve specifier. Compared to other compatibility tables and

chars, the Materials Index in SAES-L-008 is unique because it includes (1) a listing of compounds that are particularly germane to Saudi Aramco operations and, (2) some

temperature and concentration information.

Vendor’s Corrosion Guidelines

Most valve manufacturers also provide fluid compatibility information in the form of charts,

tables, applications guides, and so forth. The limitations of most general compatibility charts

is that they do not include information that relates to temperature, concentration, or wear-

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couple compatibility. In addition, it must be emphasized that valve manufacturers do not

recommend materials, but rather suggest material specifications. Most manufacturers believe

that the user is more knowledgeable of the process than a vendor.

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Selecting Control Valves For Erosive Fluid Applications

Erosion And Its ConsequencesWhile corrosion is an electrochemical action, erosion - in its simplest form - is mechanical

damage that results in the gradual destruction of a material.

Common Forms Of Erosion

Erosion damage results from the impingement of solid particles, liquids, or liquid droplets

on the target material. The various forms of erosion are shown in Figure 21 and they are

discussed below.



Figure 21

Common Forms Of Erosion

Particle Erosion results when solid particles such as fines, soot, sand, dirt, or

scale impinge on material surfaces.

Flashing Erosion results when a liquid falls below its vapor pressure and some

portion of the liquid vaporizes. The velocity of the vapor phase can increasesignificantly. The liquid particles, driven by the high-velocity vapor, can impact

valve components and result in significant erosion damage.

Erosion/Corrosion - While it is convenient to discuss erosion as an independent

phenomena, erosion generally occurs simultaneously with corrosion. Eachindependent phenomenon accelerates the other.

Quantifying Erosion Intensity

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There is no erosion coefficient, no industry standard, and no scientific means for predicting

the occurrence or intensity of erosion damage. However, a better understanding of the

potential for erosion can be gained by evaluating the parameters that increase the potentialfor erosion in a given application. These parameters are shown in Figure 22. The fluid

parameters that influence the potential for erosion damage include the size of the particles,

the sharpness of the particles, the volume ratio of particles in the fluid stream, the angle of

particle impingement on a material surface, and the fluid velocity. The relative susceptibility

of a specific material to erosion damage is a strong function of the material’s mechanical

properties (hardness, toughness, etc.) and, in many instances, the corrosion resistance of the

affected material.



Figure 22

Influences On Erosion Intensity

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Consequences Of Erosion

The possible consequences of erosion in a control valve are shown in Figure 23 and they are

discussed below.



Figure 23

Typical Erosion Damage In Control Valves And Piping

Valve Body Damage - Erosive flows commonly result in a thinning of the valvebody casting in the area immediately below the valve port. The loss of

pressure-retaining capability and the total failure of the valve body are potentialresults of this type of erosion.

Trim Damage - The erosion of seat rings, valve plugs, cages, guide bushings,and stems are often the first steps in a progressive failure. For example, the

loss of material at seating surfaces generally results seat leakage and high

velocity clearance flows. High velocity flows may cause a type of erosionknown as wiredraw; a highly localized form of erosion occurs when small, high-

velocity jets cut fine slices or slots into the affected components. Any highvelocity clearance flows at the seat can cause accelerated erosion - evendisintegration - of the seat and plug. Abrasion and wear of the guiding

mechanism (either the cage or the guide bushing) can result in lateral pluginstability and vibration of the valve plug and stem. Lateral stem movement can

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cause packing to wear and fail. High frequency vibration of the plug is also a

cause of valve stem breakage.

Valve Plugging And Sticking - If grit, fines, or other forms of solid particles

become lodged between sliding contact surfaces such as cages and plugs or shafts and bearings, the valve may seize altogether.

Piping Erosion Damage - When fluids leave the control valve at high velocity,erosion of downstream piping may also occur.

Erosive Service Flags And Typical Erosive Fluid Applications

Flags For Erosive Fluid Applications

Because erosion is a strong function of fluid velocity, many of the flags that indicate the potential for erosion are based on valve outlet velocity. The fluid velocities at which the

potential for erosion damage should be given additional engineering attention are listed in

Figure 24 and they are discussed below.

Velocity

F

la

g

F

o

r

E

r

o

si

o

n

V > 0.3(6

0

-D)m

ete

r

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s/

seco

nd,

or

V > (60-

D)f e

et/s

eco

n

d

Experience

Low

er

osio

npo

t

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en

tial

V > 0.15M

ach

,o

r P1

/P2

>3

Figure 24

Fluid Velocities That Indicate Significant Potential For Erosion Damage

Velocity Limits For Clean Fluids - Liquids, even clean liquids, can be seen aspresenting the potential for erosion damage when the fluid velocity is greater than 0.3 (60-D) meters/second or (60-D) feet second, where D is the nominal

pipeline diameter. The velocity limits for clean liquids are based on thefollowing concerns:

• Liquids, even those that are described as clean, are rarely truly “clean"; i.e.,

they include some dirt, scale, or other particulate.

• High velocity liquids accelerate the removal of protective passive layersthereby hastening the erosion/corrosion process.

• High velocity liquids are prone to flashing. The liquid droplets that formduring flashing can impinge on critical valve surfaces

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The high-velocity flow of clean gasses and vapors are generally not thought of

as presenting a significant potential for erosion damage.

Velocity Limits For Fluids With Entrained Particles - If a fluid includes fines, scale,

sand, or other particles, erosion damage can occur at relatively low velocities.For gasses and vapors with entrained particles, the potential for erosion should

be considered whenever the fluid velocity is greater than 0.15 Mach, or whenever the ratio of P1/P2 >3.

For liquid flows, there is no specific flag in terms of velocity or pressureconditions; experience is the guide.

Common Erosive Fluid Applications

Any Application Near The Wellhead - Any valve application near the wellheadshould be evaluated for erosion because of the sand, dirt, and other particlesthat are commonly present in crude oils and natural gasses. High pressure gas

wells, because of the high velocity flows that are encountered, should alwaysreceive additional attention in order to assess the potential for erosion.

Separators - The dump valves on separators are nearly always subjected toerosive flows because of the sand, gravel, dirt, and other solids that arepresent in the crude oil that is being processed.

Fluidized Cat Cracking - Catalytic cracking processes typically

include several control valves that must be compatible

with erosive fluids. The feed valve may be subjected to

impurities that passed through the initial separation

process. Other valves in the process are required to resist

erosion from coke (a fine, gritty form of carbon) and

catalyst fines (which are generally very hard and very

sharp).

Wet Steam - In applications that involve saturated steam, liquid droplets that aretransported at high velocity can impinge upon critical valve surfaces and cause

significant erosion damage.

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When selecting control valves for erosive service applications, specifiers typically consider:

• Valve styles that have been uniquely designed to direct the erosive flowstream away

from critical valve surfaces.

• Materials of construction with the mechanical properties that are needed to resist erosion

damage.

• Sizing issues.

Control Valve Styles

Guiding Methods - If the fluid contains solid particles, cage-guided valves may be apoor choice because of the abrasion to guiding surfaces and the plug binding that can

occur if particulates becomes wedged between the plug and the cage. To reduce

friction and prevent plug binding, post-guided valves are typically preferred. In somevalve designs, the guide bushing is located in an area that is separated from the main

flow stream in order to prevent particulates from damaging the guide bushing. Refer to Figure 25.



Figure 25

Post-Guiding Versus Cage Guiding For Erosive Fluid Applications

Flow Geometry - As previously illustrated, the standard flow-down globe bodyconstruction is particularly susceptible to erosion damage because of the

tortuous path and the numerous opportunities for particle impingement oncritical surfaces. In contrast, valve designs that provide a straight-through or

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line-of-sight flow path generally provide increased protection against erosion

damage. For example, an angle body construction provides a flow path with a

single turn and minimizes the potential for erosion damage. Line-of-sight rotaryvalves also minimize the potential for particle impingement. Figure 26 shows

the differences in the flow path of standard globe style bodies and anglebodies. Erosion resistant trim materials such as tungsten carbide and ceramicsare commonly available for valves of this type.



Figure 26

Globe Versus Angle Bodies For Erosive Fluid Applications

ANSI Class Shutoff - When a control valve that is selected for an erosive fluid

application must shut off, ANSI Class V shutoff should be selected. The tightshutoff specification will help to minimize high-velocity leakage across the seatand the increased potential for erosion. In some applications, it may be

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advisable to achieve system shutoff with the use of a separate block valve. If

the control valve is not used to attain shutoff, then the damage that can result

when solids are trapped between the plug and seat can also be avoided.

Thick Seals Vs. Thin Seals - Because erosion manifests itself as the wearingaway of material, the life of vulnerable components can be extended if they are

robust and massive rather than thin and fragile. For example, the thin seals of a standard ball segment valve or a high-performance butterfly valve will nottolerate erosive fluids nearly as well as the massive seal of the eccentric rotary

plug valve that is shown in Figure 27. In addition, the plug and seat ring of theeccentric plug valve are typically available in a variety of erosion resistantmaterials including tungsten carbide and ceramics.



Figure 27

Thick Seals Of An Eccentric Rotary Plug Style Valve

Special Valve Constructions - For particularly erosive fluids and for other difficultapplications, special valve constructions may be considered. The valve that isshown in Figure 28 is a sweep flow, venturi outlet style valve that is available

from many manufacturers. The valve performs well in highly erosiveapplications and in coking applications. To protect critical surfaces from coke

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buildup, the valve stem and the plug guiding surfaces are protected from the

fluid stream. The flow path around the plug - referred to as sweep flow - also

helps to prevent the accumulation of coke deposits. The enlarged outletreduces the outlet velocity and aids in minimizing flashing and erosion

damage. The valve includes provisions for injecting warm oil to ensureadequate lubrication and to prevent the build up of coke. Steam may also beinjected to help prevent highly viscous fluids such as furnace bottoms from

clogging the valve. For highly erosive flows, the plug tip and seat ring areavailable in tungsten carbide and ceramics.



Figure 28

Sweep Flow Valve Design

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Hierarchy Of Erosion Resistant Valve Styles - To provide a summary of thepreceding discussion, Figure 29 lists a hierarchy of erosion resistant valvetypes and options.

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ValveS

t

y

l

e

Comment

Cage-gu

id

ed

v

alv

es

Concernf o

r

the

plu

g

bi

ndin

g

i

n

t

he

cag

e.

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Su

itab

le

wh

en

the

vo

lum

e

r

ati

o

o

f p

ar t

icu

la

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te

is

ver

y

lo

w

or

wh

en

the

er

osi

ve

me

dia

i

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s

a

f l

ash

in

g

liqu

id

(wi

th

no

sol

id

s).

Cage-

gui

Reduces

bod

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de

d

an

gle

v

al

ves

y

dama

ge.

Post-g

uid

ed

val

ves

Reducesp

lug

bi

ndi

ng.

De

s

ig

ns

wit

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h

pr ot

ect

ed

b

ushi

ngs

of

f er

in

cr e

as

ed

pr o

te

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ct

ion.

Post-

Gui

de

d

A

ngl

e

V

alv

es

Angle

bod

y

min

imi

zes

bo

dy

dam

ag

e

.

Post-G

uid

e

Liner r

edu

c

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d

Angl

e

V

al

ve

s

wi

th

har

den

ed,

r ep

la

cea

ble

o

es

er o

sio

n

to

val

ve

out

let

and

se

r v

es

as

a

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ut

letl

ine

r

sa

cr if

ici

al

me

mber

.

EccentricRo

tar

y

P

lug

V

a

lv

es

Straightth

r ou

gh

f lo

w

p

at

h

m

ini

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mi

zes

imp

in

ge

ment

on

cr i

tic

alp

ar t

s.

Rat

ing

s

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to

ANS

ICl

as

s

600.

Sweep

Flo

w

(

Ven

tur

iS

t

yl

e)

Ang

Very

r ug

ged

co

nst

r u

c

ti

on.

Swe

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le

Val

ves

ep

f lo

w

d

es

ig

n

di

r ec

ts

f lo

w

a

way

f

r om

cr

it

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ic

alsu

r f a

ce

s.

Rati

ngs

to

AN

SIC

las

s

900

.

Figure 29

Hierarchy Of Erosion Resistant Valve Styles

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Materials Selection Considerations

For erosive fluid applications, the general guideline for material selection is to specify

materials with sufficient hardness or toughness to provide extended life in erosive fluid

applications.

Material Selection For Valve Bodies - When erosion of a standard, cast carbonsteel body such as WCC or WCB is anticipated, specifiers may consider theselection of a variety of increasingly erosion resistant alloys. Figure 30 lists a

number of alloys and their relative resistance to erosion damage. The bodymaterial specification can also be affected by the flow geometry of the selectedvalve; i.e., if the flow is directed away from the body (as in an angle valve or an

eccentric rotary plug valve), a standard WCB body may provide long life. Incontrast, the direct impingement of the same erosive fluid on the body of astandard globe may require the selection of a WC9 (chromium-molybdenum)

body in order to provide satisfactory valve life.

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Remarks

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As

tan

dar

d

m

a

te

r ia

l.M

ay

be

se

lec

ted

f o

r

m

ild

ly

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Muchg

r ea

ter

er

o

si

on

r es

ist

an

ce

tha

n

c

ar b

on

ste

el

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Superior e

r os

ion

an

d

c

or r

osi

on

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sis

tan

ce.

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Figure 30

Hierarchy Of Erosion Resistant Valve Body Materials

Material Selection For Valve Trim - Because trim components are “wetted”components, they are exposed to the high velocity fluid stream. Consequently

trim for erosive service applications is always selected in erosion resistantalloys. Figure 31 lists a hierarchy of erosion resistant construction materials.

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Remarks

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o

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s

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n

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Figure 31

Hierarchy Of Erosion Resistant Valve Trim Materials

Problems With Focusing On Material Hardness Only - A common misconception isthat erosion resistance is a function of materialhardness only. Hardness is

defined as a material’s resistance to penetration or indention. In metals,hardness is measured by loading an indenter into the metal and measuringeither the depth or the of penetration or the surface area of the indentation.

Hardness may provide an approximation of the relative erosion resistance of one material compared to another in the same alloy family. However, differentfamilies of metals may achieve their erosion-resistant properties in different

ways. Mechanical properties such astoughness can have significant impact onthe erosion resistance of a particular alloy. Although the property of “toughness” is difficult to precisely define, it can be viewed as the opposite of

brittleness. For example, a tough metal that is subject to a heavy impact willdeform before it breaks whereas a less tough material will break beforedeforming.

The cobalt based Alloy 6 (Stellite) is well known for its erosion resistant

qualities even though its hardness is well below the hardness of most erosion-resistant stainless steels. Alloy 6 derives its erosion resistance from itstoughness. The unusual process by which Alloy 6 achieves its toughness is

shown in Figure 32 and it is summarized below.

1. In its as-manufactured state, Alloy 6 has a specific crystalline structure.

2. Following impact, the crystalline structure of the alloy 6 material actually

changes from one basic form to another. The new crystalline structuredisplays a much higher resistance to strain fracture than the material’snative structure. As a result, the material’s resistance to erosion actuallyincreases during impact.

3. Under static conditions (following an impact), the alloy reverts to its

initial, as-manufactured crystalline structure.

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Figure 32

Structural Changes In Alloy 6 That Improve Erosion Resistance

Erosion Resistance and NACE MR0175 - When the potential for stress corrosion

cracking and erosion are both present, the specifier is confronted withselecting materials according to two seemingly contradictory guidelines: (1)selecting materials with sufficient hardness or toughness to withstand the

erosive elements and (2) selecting materials with hardness levels thatsufficiently low to satisfy the NACE MR0175 guidelines. Fortunately, manymaterial options are available that meet both requirements. Type S41000

stainless steel is reasonably hard and it is NACE compliant if its hardness islimited to HRC 22. 316 stainless steel with Alloy 6 hardfacing provides a very

popular solution that is superior to S41000 when both erosion resistance andNACE compliance must be achieved.

Erosion In Combination With Other Severe-Conditions - Erosion in combinationwith other severe conditions such as corrosion, cavitation, and hightemperatures can further increase the complexity of the control valve and

material selection process. To clearly identify all requirements, the fluidproperties and service conditions must be closely evaluated.

Sizing Issues

Valve sizing can have a significant impact on the life of a control valve in an erosive fluid

application. The sizing issues typically relate to velocity control.

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Valve Body Size Selection - Because erosion intensity is a strong function of fluid

velocity, valve body sizes should be selected that will not significantly increase

the fluid velocity. For example, valve body sizes that are smaller than thepipeline size should be avoided.

Valve Trim - Oversized valves can present many problems and they are

particularly troublesome in erosive fluid applications. If the trim is oversized,then the valve will throttle near the seat resulting in high velocity erosive flowsacross flow-controlling surfaces. To prevent these flows, extra efforts should

be made to ensure that the valve trim is not oversized.

Information Sources

When selecting materials for erosive fluid applications, specifiers may apply information

that is included in manufacturer’s specification bulletins and in other resources such as the

charts previously shown in Figures 30 and 31. In addition, specifiers may draw upon the

expertise of vendors and peers who have experience in equivalent or similar applications.

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SELECTING CONTROL VALVE OPTIONS FOR high-temperature FLUID

Applications

High Temperature Applications And Their Consequences

Categories Of High Temperature Applications

High temperature Applications are those in which the normal operatingtemperature is sustained above a specific temperature limit. Refer to Figure 33.

Thermal Cycling Applications are those in which the operating temperaturerepeatedly rises and falls over a wide range of temperatures. Refer to Figure

33.



Figure 33

High Temperature Application Vs. A Thermal Cycling Application

Common Applications

High Temperature Applications - Control valves that are used to

control flu gasses in furnace applications, control valves

that are used to control feedstocks in various refinery

operations, and control valves in high-pressure steam

generation systems are all subjected to sustained high

temperatures.

Thermal Cycling Application - A common example of a control valve that issubjected to thermal cycling is the valve that is used to perform the soot-

blowing process in a fossil-fuel boiler. The boiler tubes are delicate and cannot

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tolerate hot spots that would develop if soot were allowed to build up on the

tubes. In many boiler systems, the tubes are cleaned by periodically directing

high pressure steam at the tubes. The steam is controlled by a control valvethat is operated at predetermined intervals, often several times a day.

Another example of a thermal cycling application is the desiccant dehydration

process that is used to remove moisture from many natural gasses. Refer toFigure 34. Desiccant dehydration towers use trays filled with a solid desiccant(a substance that attracts moisture). In operation, inlet gas enters into the top

of the tower, the desiccant removes moisture from the gas as the gas passesdownward through the tower, and dry gas exits the bottom of the tower. Over time, the desiccant becomes saturated and will not hold any more moisture. To

regenerate the desiccant, the tower is heated as shown in the middle tower of

Figure 34. During the heating cycle, valves A and C are closed and valves Band D control the flow of hot (400 to 500 degree F) gas upward through the

tower. When the desiccant is dry, cool gas (120 degrees F) is introduced intothe bottom of the tower as shown in the third tower in Figure 34. To enablecontinuous operation, many desiccant dehydration units include three towers.

The adsorption, heating, and cooling cycle may be repeated several times aday in each tower. Refer to Figure 34 and note that the temperature of the gasthat passes through valves B and D cycles between 120 degrees F and 400 to

500 degrees F. As a result, valves B and D are in a thermal cycling application.

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Figure 34

Thermal Cycling Application: Dry Desiccant Adsorption Process

Consequences Of High Temperature Fluids On Incompatible Components

General Effects Of Elevated Temperature On Materials - Most metal alloys aremetastable, meaning that during the manufacture and subsequent working of the alloy component, a unique but unnatural and unstable structure is

purposely developed in the alloy. The unique structure of each alloy impartsthe alloy’s mechanical properties such as strength, ductility, toughness, etc.

When an alloy is subjected to elevated temperatures, it tends to transform toits stable or natural structure. Examples of the effect of elevated temperatureson some materials are as follows:

• S17400 and similar precipitation hardenable materials become brittle.

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• Cold worked 300 series stainless steels lose the effects of cold work.

• Duplex steels become brittle.

Graphitization Of Carbon Steel - Carbon steels possess a two-phasemicrostructure that includes ferrite (pure iron) and iron carbides. At

temperatures above 800 degrees F, the carbides decompose into iron andgraphite flakes during a process that is known asgraphitization.



Figure 35

Graphitization In Carbon Steels

Sensitization Of Stainless Steels - Because carbon improves a materials strengthat elevated temperatures, it is often desirable to select materials with a highcarbon content. However, the addition of carbon increases the potential for

sensitization. Refer to Figure 36. Recall that sensitization is a process in whichexposure to high temperature causes corrosion resistant alloys to precipitateout of the material matrix, leaving a zone at the grain boundary that is not

protected from corrosion attack. In a corrosive environment, the area of the

grain boundary that has been depleted of chromium is susceptible to attack bythe corrosive atmosphere. The corrosion that results is known as intergranular

corrosion.

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Figure 36

Sensitization And Intergranular Corrosion

That Result From Exposure To High Temperatures

Creep - When exposed to stress such as an increase in fluid pressure or an

increase in a loading force, most alloy components strain (deform) inproportion to the amount of stress. When the stress is relieved, the component

reverts to its initial shape. The ability of a material to return to its initial formafter being exposed to stress is known aselasticity . In high-temperatureenvironments, the elasticity of a material can be affected by the phenomenon

of creep. The effects of creep are illustrated in Figure 37. At temperatures thatare sufficiently high, the amount of strain (deformation) may slowly increaseover time and the strain may become permanent. The main effects of creep in

control valves are the long-term loss of bolting forces, loss of gasket forces,and the deformation of trim parts.

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Figure 37

Effect Of Creep On The Elasticity Of A Material

Effects Of Mismatched Expansion Coefficients - When metallic materials areheated, they expand in a predictable and repeatable manner. Each alloy has

its own characteristic thermal expansion vs. temperature curve. In general,materials with similar chemical compositions have similar thermal expansionproperties. The carbon steels, alloy steels, and 400 series stainless steels

have fairly low thermal expansion coefficients whereas the 300 series stainlesssteels have very high expansion rates. At elevated temperatures, differential

thermal expansion coefficients of the trim components and the body andbonnet can cause different types of problems. Figure 38 illustrates twoscenarios. If the body and bonnet expands more than the cage and seat ringas shown in view B, gasket unloading will occur and the fluid will leak across

the gasket surfaces. If the cage and seat ring expand more than the body andbonnet as shown in view C, the cage and/or seat ring may be damaged bycrushing and the gaskets may be damaged by overloading.

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Figure 38

Effects Of Mismatched Thermal Expansion Coefficients

Gasket Failures - Gaskets that are exposed to temperatures that are greater

than the gasket material rating may become brittle and lose their ability todeform, thereby preventing them from sealing against their mating surfaces.

Any such failure can result in fluid leaks erosion damage.

Packing Failures - When standard PTFE packing materials are exposed to

temperatures that are above the packing material’s temperature rating asshown in Figure 39, the PTFE pacing rings may deform, they may sublimate,and/or they may begin to flow and extrude out of the packing bore as the valve

stem strokes. PTFE-based packing arrangements may display all of thesebehaviors at temperatures above 450 degrees F.

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Figure 39

Effects Of Elevated Temperature On PTFE Packing

Consequences Of Thermal Cycling Applications

Thermal Fatigue - As a hot fluid is introduced into a control valve, the component

surfaces that are in contact with the fluid are the first to respond to the increasein temperature. While the outermost surfaces of the components areattempting to expand, the material that is just behind the outermost surfaces

remains cool and resists expansion. During each heating and cooling cycle, astress gradient occurs in the components. The gradient can cause a form of thermal fatigue that, in extreme cases, results in cracking. Failures that result

from thermal fatigue are rare; however, if an application frequently cyclesacross an extreme range of temperatures, specifiers should be alert to thepotential for this form of damage.

Gasket Unloading - During thermal cycling, the bonnet-to-body bolting may

repeatedly load and unload the gaskets in the control valve assembly asshown in Figure 40. Continuous loading and unloading of the gaskets cancause the gaskets to take a set (lose their elasticity). If the gaskets lose their

elasticity and fail to seal, leaks can result in high-velocity erosive flows. Suchflows are generally the starting point for a progressive failure of the controlvalve.

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Figure 40

Thermal Cycling And Gasket Failure

Loosening Of Threaded Components - It has long been known that temperature cycling

has the tendency to loosen threaded components. Refer To Figure 41. In controlvalves, thermal cycling applications have been known to loosen threaded seat rings,threaded bonnet assemblies, and bonnet-to-body bolting.

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Figure 41

Effects Of High Temperature On Threaded Joints

High Temperature Service Flags

Saudi Aramco Definition Of High Temperature

Within Saudi Aramco, the definition of a high temperature application is based on the upper

operating temperature limit of PTFE. According to Section 4.1.5 of SAES -J-700, the upper

temperature limit of PTFE is 400 degrees F. Therefore, within Saudi Aramco, the definition

of a high temperature application is any application with an operating temperature that is

greater than 400 degrees F. Refer to Figure 42.

Thermal Cycling Flags

Thermal cycling flags are not defined by Saudi Aramco but are instead defined by valve

manufacturers. Refer to Figure 42. The temperature at which thermal cycling is considered

to be a problem can vary with each different valve construction; however, whenever anapplication repeatedly cycles over a range of 300 degrees F, problems from thermal cycling

can be anticipated.

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Figure 42

High Temperature Vs. Thermal Cycling Flags


Valve Design Considerations

Seat Ring Retention - Screwed-in seat rings are popular for many general

service applications because they do not require loading from a cagecomponent to ensure a tight fit in the valve body. However, screwed-in seat

rings are generally not selected for thermal cycling applications because of thetendency of the seat ring to loosen. Screwed-in seat rings may be usedsuccessfully when the seat ring is tack welded into the valve body (see Figure43) or if the seat ring is held firmly in place by an indexing lug on a cage or

cage-like component.

Bonnet-To-Body Attachment - Many small, high-pressure valves are designed

with threaded bonnet-to-body connections. Such constructions should beavoided for high temperature and thermal cycling applications unless optionsare available to tack weld the bonnet to the body as shown in Figure 43.

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Figure 43

Tack Welding Of Bonnets And Seat Rings

For High-Temperature And Thermal Cycling Applications

Material Temperature Ratings

Refer to Figure 44 for the discussion that follows.

Body And Bonnet Materials - Refer to Figure 44 for the discussion that follows.Carbon steel bodies such as WCC and WCB are commonly compatible with

temperatures up to a maximum of 800 degrees F. Above this limit, thephenomenon of graphitization can occur.

Between 800 degrees F and approximately 1050 degrees F, alloy steels thatinclude additional amounts of chromium and/or molybdenum may be selected.

The addition of chromium and/or molybdenum enhances the alloy’s resistanceto tempering and graphitization at elevated temperatures. Grades C5 and WC9are common. The WC9 material provides better castings and it is easier to

weld.

For increased high-temperature compatibility and/or for increased pressureretaining capability, alloys with still more chromium and molybdenum arespecified. CF8M stainless steel (the cast version of S31600) is commonly usedfor temperatures up to 1500 degrees F. The pressure and temperature limits

for body and bonnet materials are listed in the ANSI/ASMEpressure/temperature tables and may also be listed in control valvespecification bulletins.

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M Upper

Te

m

p

e

r

a

t

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i

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i

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A Approximat

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C Approximat

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1500

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Figure 44

Upper Temperature Limits Of Common Valve Body And Bonnet Materials

Trim Materials - The trim material options that are available for high-temperatureapplications vary according to the valve manufacturer. In addition, the specific

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temperature rating for a complete trim package depends on several factors,

including:

• the valve type

• the valve body and bonnet material

• the valve size

• the pressure drop

• other materials in the trim package

Because of the number of variables that must be considered, manufacturersestablish charts, tables, and other selection methods for selecting a pre-

engineered trim package for specific temperature and pressure conditions. Asan example, Figure 45 illustrates the temperature and pressure drop limits of various trim options in a Cr-Mo steel body. Each trim option number refers tospecific materials of construction for the plug, cage, and seat ring. Note also

the cautions and selection guidance that is listed in the section below the chart.

-200 0 400 800 1200

-20 1100

1400

1200

1000

800

600

400

200

0

FLUID TEMPERATURE, DEGREES F

PRESSURE

DROP,PSI 1, 3

37H

3H

WITH CLASS 600 1 WC9OR C5 CHROME MOLY STEEL BODY

1 DO NOT EXCEED THE MAXIMUM PRESSURE AND TEMPERATURE FOR THE CLASS RATINGOF THE BODY MATERIAL USED, EVEN THOUGH THE TRIMS SHOWN MAY HAVE HIGHERCAPABILITIES

1

12 BE ESPECIALLY CAREFUL TO SPECIFY SERVICE TEMPERATURE IF TRIM 3 OR 37 IS SELECTED

AS DIFFERENT THERMAL EXPANSION RATES REQUIRE SPECIAL PLUG CLEARANCES. SPECIFYTRIM 37H FOR TEMPERATURES ABOVE 4510 DEGREES F. SPECIFY TRIM 3H FOR TEMPERATURESABOVE 800 DEGREES F.

13 TRIM 29 MAY BE USED UP TO 1440 PSI WITH CLEAN, DRY GAS.

14 USE TRIM 27 INSTEAD OF TRIM 29 FOR NONLUBRICATING FLUIDS SUCH

AS SUPERHEATED STEAM OR DRY GASSES BETWEEN 300 AND 600 DEGREES F.

2

2

FIG74

Figure 45

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Pressure/Temperature Ratings For Various Trim Material Options

Gasket Materials - Most control valves include two different types of gaskets;

spiral wound gaskets and flat sheet gaskets. The characteristics andtemperature ratings of several common gasket materials are listed in Figure46.

• Spiral Wound Gasket Options - A spiral wound gasket is made of a metal

alloy that is formed into a V-shape and then wound into a spiral form.During the manufacture of the gasket, a filler is inserted between each coilof the V-shaped material. Of the options that are listed in Figure 46 below,

Inconel is the strongest alloy material, it has the highest temperature rating,

and it will maintain its spring properties longer than the other options. As aresult, the Inconel/graphite gasket is typically recommended for thermal

cycling applications.

• Flat Sheet Gasket Options - A standard material for flat sheet gaskets is a

composition material. Options such as PTFE coated Monel providecorrosion resistance, but at reduced temperature ratings, as shown in thetable below.

The selection of a suitable gasket material is based on the following:

• The temperature rating of the gasket material.

• Whether or not thermal cycling will occur.

• The corrosion resistance of the gasket material.

Standard

M

a

t

e

r i

a

l

Optional

M

a

t

e

r i

a

l

s

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Figure 46

Common Gasket Materials

Bolting Materials - As mentioned previously, each different alloy has a differentthermal expansion vs. temperature curve. However, different alloys in the

same family generally have similar thermal expansion and contractioncharacteristics. Accordingly, the general guidelines for bolting materialselection are:

• If possible, select steel bolting (for example, B7 or B16) for alloys steelbodies and bonnets.

• If possible, select stainless steel bolting (for example, 316 or 304 stainless

steel) for stainless steel bodies.

• Whenever non-standard bolting is considered or the above guidelinescannot be followed, investigate the need for pressure and/or temperature

derating to compensate for the differential in thermal expansion coefficients,differences in bolting strength, and other influences.

Packing Materials - Packing material selection is based upon the temperature atthe packing bore. The temperature at the packing bore is often considerably

less than the temperature of the process fluid, especially if the valve isinsulated below the packing bore or if an extended-height (extension) bonnet isspecified. For temperatures below 400 degrees F, PTFE base packing

arrangements are compatible with most fluids. Above 400 degrees, packingarrangements that are based on graphite materials are the industry standard.Graphite materials are compatible with a wide range of fluids; however,

graphite base packing arrangements should not be selected for hot oxidizingacids (nitric acid and sulfuric acid) or for oxygen services that operate above700 degrees F.

Extended Bonnets For Packing Protection

An extended bonnet locates the packing at an increased distance from the process fluid,thereby reducing the influence of the process fluid on the packing temperature. Refer to

Figure 47. Section 4.1.5 of SAES-J-700 requires the selection of extended bonnets or the

selection of special packing materials for applications in which the fluid temperature is

greater than 450 degrees F.

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Figure 47

Extended Bonnets That Are Used At Temperatures Above 450 Degrees F

Achieving Tight Shutoff At Elevated Temperatures

Metal Seats - ANSI Class VI shutoff is typically achieved with the use of soft-seated valve constructions. However, Saudi Aramco standards define anupper temperature limit of 400 degrees F for PTFE and many other materials

that are included in soft-seating arrangements. Therefore, at temperatures

above 400 degrees F, ANSI Class V shutoff or better is generally achieved byspecifying an unbalanced valve construction with metal-to-metal seats that

have been precision lapped to achieve the shutoff specification.

High Temperature Seal Rings For Balanced Valves - To achieve ANSI Class V or

better shutoff with a balanced valve construction in a high-temperatureenvironment, many manufacturers offer special high-temperature PTFE seal

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ring designs. Refer to Figure 48 and note the following features of a soft seal

arrangement that is rated for temperatures up to 600 degrees F.

• The PTFE “omni seal” is pressure loaded to improve seal performance.

• The PTFE seal includes a spring which helps to maintain a seal between theplug and cage at elevated temperatures where the PTFE material loses itselasticity.

• The PTFE material includes a high percentage of carbon and graphite to

improve its high-temperature performance.

• An anti-extrusion ring prevents any of the hot and potentially flowing PTFE

material from extruding out of the seal area.



Figure 48

High-Temperature Balanced Plug Seal Configuration

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SELECTing AND Sizing CONTROL VALVEs FOR Cavitating FLUID

Applications

Cavitation And Its Consequences

The Cavitation Phenomenon

Vapor Cavity Formation and Collapse - When, in a liquid flow, the fluid pressurefalls below the fluid’s vapor pressure, the fluid begins to vaporize; i.e., vapor

bubbles form in the flow stream. In a control valve, the onset of vaporizationoften occurs near the vena contracta, as shown in Figure 49. If thedownstream pressure P2 increases to a value that is greater than the fluid’s

vapor pressure, the bubbles collapse and the fluid is cavitating.



Figure 49

Cavitation

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Cavitation Versus Other Flowstream Phenomenon

Cavitation Vs. Flashing - Up to the point where the decrease in the localpressure causes bubbles to form in the fluid stream, flashing and cavitation are

similar phenomenon. In a flashing fluid, however, the downstream pressure P2is below the vapor pressure of the liquid and the bubbles that form near the

vena contracta remain in the fluid stream as shown in Figure 50. Flashing willbe discussed later in this Module.



Figure 50

Cavitation Vs. Flashing

Cavitation Vs. Outgassing - When a fluid includes dissolved gasses and the fluidis subject to pressure reduction or to agitation (both of which occur as the fluid

flows through a control valve), the dissolved gas may come out of solution in a

process that is known as outgassing. Refer to Figure 51. Outgassing differsfrom cavitation and flashing in that it is not a thermodynamic event and it

occurs independently of the values of the fluid’s vapor pressure and thepressure at the vena contracta. In addition, the bubbles that form as a result of outgassing may remain in the downstream flow regardless of the value of P2.

An increase in pressure andtime may both be required to force the gasbubbles back into solution.

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Figure 51

Cavitation Vs. Outgassing

Common Forms Of Cavitation

Hard Cavitation Vs. Soft Cavitation

The term “hard” cavitation is used to describe the worst-case scenario in termsof the potential for cavitation damage. Hard cavitation implies that there are no

circumstances or conditions present in the application that will have amitigating effect on the intensity of the cavitation or the potential for cavitation

damage. Cold water is the classic example of a fluid that will exhibit hardcavitation.

The phrase “soft cavitation” is used to describe any application in which either

the fluid properties or the service conditions serve to lessen the potential for cavitation damage. For example, the cavitation that occurs in a multi-species

fluid such as a hydrocarbon mixture may be less likely to cause significantcavitation damage because the mixture includes components with severaldifferent vapor pressures. As the local fluid pressure is reduced, not all of the

components will vaporize, and the components that remain in the liquid formmay cushion the collapse of the vapor cavities. In addition, fluids that are

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viscous and outgassing fluids may provide a cushioning effect on vapor cavity

implosions. Refer to Figure 52.



Figure 52Hard Vs. Soft Cavitation

Specifiers typically view the cavitation that occurs in crude oil as “soft

cavitation”. In a crude oil flow, the cavitation damage that occurs as a result of vapor cavity implosion may not present as great a concern as the noise andvibration that occurs. As hydrocarbon liquids become more refined (less

viscous and closer to a single species fluid), the damage from vapor cavityimplosions becomes a major concern.

Incipient Vs. Full Blown Cavitation

Specifiers will often encounter the term “incipient” cavitation. The term“incipient” cavitation defines the point at which the first vapor cavities form inthe fluid stream. On a plot of flow (Q) versus the square root of the pressure

drop that is shown in Figure 53, this point is observed as the first deviation of the actual flow plot from the plot of predicted flow. Incipient cavitation occurs

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when the local fluid pressure first dips below the fluid’s vapor pressure.

Damage may or may not occur at this point.

At increased pressure drops, more and more bubbles form and collapse in the

fluid stream. At the condition of fully choked flow, the cavitation that occurs isoften described as “fully blown cavitation” or as “choked flow cavitation”. These

terms indicate there is a substantial potential for cavitation damage; however,they are highly subjective and they provide little real guidance to the valvespecifier.



Figure 53

Incipient Vs. Choked Flow Cavitation

Consequences Of Cavitation

Valve And Piping Damage - If the vapor bubbles that are formed during thecavitation cycle implode on or near fluid boundaries such as valve componentsand pipe walls, high-velocity microjets and sonic waves can result in rapid and

catastrophic damage to the components as shown in Figure 54.

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Figure 54

Cavitation Damage That Results From Imploding Vapor Cavities

Vibration - In many liquid flows, vibration of the valve and piping is as great a

concern as the potential for damage from the implosion of vapor cavities.Figure 55 shows a representative plot of valve and pipeline vibration versus

the value of sigma (σ = P1-Pv/P1-P2). Following the occurrence of incipientcavitation, the intensity of the vibrations increases rapidlyas the value of sigma

decreases. Cavitation has been known to cause vibrations of sufficientintensity to break welded joints and damage pipeline supports.

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Figure 55

Valve And Pipeline Vibration Versus The Value Of Sigma

Hydrodynamic Noise - Cavitation may also be accompanied by moderate to highlevels of hydrodynamic noise. However, the intensity of cavitation that is

required to generate objectionable levels of hydrodynamic noise is generallysufficient to cause rapid and catastrophic damage to the valve and piping. As aresult, the concern for damage from vapor cavity implosion and from pipeline

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vibration is generally a far greater concern than the concern for high levels of

hydrodynamic noise.

Predicting The Potential For Cavitation

Saudi Aramco And Manufacturer’s System Cavitation Indices

Saudi Aramco Index Ksa - Saudi Aramco makes use of the cavitation index Ksa.Refer to Figure 56. Ksa is defined as Ksa = P1-P2/P1-Pv. As the value of P1-P2

approaches the value of P1-Pv, the pressure dip that occurs at the venacontracta is more likely to drop below the value Pv; hence, an increasing valueof Ksa indicates an increased potential for cavitation. Values of Ksa that are

greater than approximately 0.75 indicate a substantial potential for cavitation

and cavitation damage. A Ksa value of 0.99 signals the maximum potential for cavitation and cavitation damage. If the value of Ksa is 1.0 or greater, P2 is less

than Pv and the fluid is flashing.

Fisher Ar and Mokveld K cs - Fisher Controls and Mokveld each use a cavitation

index that is identical to the Saudi Aramco index Ksa. However, Fisher usesthe term Ar instead of Ksa and Mokveld uses the term Kcs instead of Ksa.



Figure 56

Cavitation Indices K sa, Ar , and Kcs

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Valtek Sigma - Valtek uses the cavitation indexsigma. Values of σ thatapproach 0 signal an increasing potential for cavitation. A sigma value of 0 or

less indicates flashing conditions. The significant relationship is the pressuredifferential between P2 and Pv. Refer to Figure 57.



Figure 57

Valtek Cavitation Index σσ

∆∆Pchoked (Masoneilan) - Some manufacturers (including Masoneilan and others)

evaluate the system potential for cavitation by calculating the value of ∆Pchoked

(∆Pcriticial in Masoneilan nomenclature). Note that a valve must be initiallyselected in order to obtain a value of FL. If the actual pressure drop is greater than the allowable pressure drop, the flow is choked and, if P2>Pv, the flow is

also assumed to be cavitating. The problem of associating cavitation with the

choked flow pressure drop is that the calculated value of ∆Pchoked predicts the

choked flow flow rate only; it does not predict the precise∆P at which choked

flow will occur nor does it provide any clear indication of cavitation intensity. As

shown in Figure 58, incipient cavitation is likely to occur at pressure drops thatare lower than the choked flow pressure drop.

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Figure 58

∆∆Pallow (∆∆Pchoked or ∆∆Pcritical) As An Indicator Of Cavitation

Ci (CCI) - Although cavitation is a function of pressure conditions (Pvc < Pv andP2 > Pv) some manufacturers, including Control Components Incorporated,

prefer to evaluate thevelocity conditions rather than pressure conditions thatwill cause cavitation to occur. Because of the pressure/velocity relationshipsthat are defined by Bernoulli's theorem, the relative tendency of a system to

cavitate can be expressed in terms of fluid velocity as well as in terms of fluidpressure. The cavitation index that is used by CCI is Ci .

Ci = 9724 (P-Pv)/ V2

where:

Ci

Cavitation index

9724 A constant

P The fluid pressure at any point in the valve, psia

Pv The fluid vapor pressure, psia

fluid density, lbm/ft3

V Fluid velocity at the point where P is measured, ft/sec

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If, as shown in Figure 59, the computed value of Ci is 1.0 or less, the system

will cavitate. In essence, this means that the fluid pressure P at the point that is

being examined will be less than the fluid’s vapor pressure and cavitation willoccur.



Figure 59

Control Components Cavitation Index C i

Because the values of V and P are not readily known, the index Ci is not easilyor quickly determined.

Subjective Factors For Analyzing The Potential For Cavitation Damage

In addition to the empirical methods that predict the occurrence of cavitation, several

subjective factors can be evaluated in order to assess the relative potential for cavitation

related problems. These factors are discussed below and they are listed in Figure 60.

Fluid Viscosity- As previously mentioned, highly viscous fluids such as heavycrude oils can lessen the effects of cavitation. Viscous fluids have two effects

on cavitation:

1. Viscous fluids impede the nucleation and growth of vapor cavities

2. Viscous fluids help to cushion the collapse of the vapor cavities.

Dissolved Gas Volume - If the liquid flow includes a large volume of entrained(dissolved) gas that comes out of solutions as flow passes through the valve,

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the presence of the vapor in the fluid stream may help to cushion the collapse

of the vapor bubbles.

Fluid Composition - If a liquid flow consists of a mixture of substances with

widely varying vapor pressures (which is often the case with hydrocarbonliquids), the classical “single fluid” model for cavitation does not apply. With

fluid mixtures, fluid vaporization may occur over a range of pressures asopposed to the single vaporization pressure of a single-species fluid. The netimpact is generally a reduction in the intensity of cavitation related problems.

Duty Cycle - If a valve will only be subjected to severe cavitating conditions for short periods of time, e.g., at startup, shutdown, or during rare transients, the

valve may be able to provide long life and good performance even though

cavitation does occasionally occur. In some applications where the occurrenceof cavitation is rare and occurs for short periods of time, the selection of

hardened trim materials may be sufficient to resist cavitation damage.

Pressure Scale Effects - The potential for cavitation is not absolutely defined by

indices such as Ksa, Ar , or σ. Laboratory tests indicate that the potential for

cavitation damage increases as the upstream pressure increases.

Size Scale Effects - Investigators have determined that the potential for cavitation and cavitation damage increases as the valve size increases. Thesize scale effect is also independent of the popular cavitation indices such as

Ksa, Ar , or σ.

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Figure 60

Subjective Factors For Analyzing The Potential For Cavitation Damage

Cavitation Service Flags And Typical Cavitating Applications

Flags For Cavitating Fluid Applications

Each unique application must be studied carefully in order to determine the potential for

cavitation. However, a general rule of thumb is that any application with a value of K sa

that

is greater than or equal to 0.8 should be closely examined to determine the potential for

cavitation and cavitation related problems.

Specific Applications

Many applications, because of the nature of the fluid properties and/or the service

conditions, are universally recognized as cavitating applications. Several such applications

are discussed below.

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Boiler Feedwater - A boiler feedwater control valve is susceptible to cavitation

because the fluid is a single-species, the feedwater control system operates at

elevated temperatures (with the effect of reducing the vapor pressure), and thevalve operates at very high pressure drops.

Tanker Loadout −− A tanker loadout application is susceptible to cavitation

because, for economy, the control valves are typically high efficiency types. Atthe beginning of the loading cycle, there is large pressure drop across thevalve because P2 approaches atmospheric pressure until the level in the

tanker increases. This application presents the potential for mild and periodiccavitation as opposed to the constant and severe cavitation that is inherent inother services.

Pump Bypass Or Recirculation Valve - The recirculating valve or bypass valve ona pump typically controls a high pressure drop, low flow stream. The high

pressure drops create a significant potential for cavitation, especially for single-species fluids such as water.

Water Injection - For secondary recovery operations, high pressure water thatoften includes brine, sour liquids, and sand is pumped, at high pressure, into

the reservoir. Because the pressure drops across the valve are often verylarge, cavitation is a natural result. In addition, salt can cause chloride stresscracking, the sour liquids can cause sulfide stress cracking, and any particles

such as sand can cause rapid erosion. The combination of cavitation,

corrosion, and erosion can dramatically shorten valve life unless the specifier selects appropriate anti-cavitation valve designs that are made of corrosion

and erosion resistant materials.

Anti-Cavitation Valve Technology

General Anti-Cavitation Valve And Trim Design Strategies

Low Recovery Trim Designs - The most common design strategy that is used toprevent the occurrence of cavitation is the selection of low recovery valves and

trim. The objective is to maintain the fluid pressure at the vena contracta at a

pressure that is greater than the fluid’s vapor pressure. As shown in Figure 61,the pressure dip at the valve vena contracta is not nearly as large as it is in a

high recovery trim. As the recovery coefficient (FL or Km) approaches a valueof 1.0, the pressure dip becomes smaller and smaller. If FL or Km = 1.0 there isno pressure recovery, Pvc will remain above Pv, and, if P2 > Pv, the fluid will

not vaporize.

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Figure 61

The Potential For Cavitation As A Function Of Pressure Recovery

Pressure Drop Staging - In order to maintain Pvc above Pv, most anti-cavitationvalve trims employ a pressure drop staging strategy. Pressure drop staginginvolves directing the fluid through a series of several small restrictions, or

stages, as opposed to directing the flow through a single large restriction. Eachsuccessive restriction dissipates a certain amount of the available energy andpresents a lower inlet pressure to the next stage. As shown in Figure 62, the

pressure dip that occurs at each stage is much smaller than the pressure dipthat would result from a single large restriction.

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Figure 62

Pressure Drop Staging

Damage Resistant Materials Of Construction- Although the recovery

characteristics of a valve or trim may determine the pressure conditions under

which cavitation occurs, the recovery characteristic of a device does notnecessarily predict the occurrence of cavitationdamage. Cavitation damage is

influenced to a large degree by the ability of the selected trim materials toresist cavitation damage. The material properties that provide the greatest

resistance to cavitation damage are hardness and toughness. As a generalguideline, materials that provide resistance to cavitation damage include - inorder of increasing resistance to damage - 316 stainless steel, 440C stainless

steel, 17-4 stainless steel, tungsten carbide, and Stellite (Alloy 6).

Figure 63 illustrates an application in which mild cavitation will be expected

because there is some fluid vaporization. However, if the valve materials are

sufficiently cavitation resistant, cavitation damage may not occur.

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Figure 63

Preventing Cavitation Damage With Damage Resistant Materials Of

Construction

Specific Anti-Cavitation Valve And Trim Designs

Straight Through Holes, Radial Flow Designs - The cage that is shown in Figure

64 includes multiple, straight-through holes. The holes serve several functions.

• In a flow-down configuration, the holes direct the collapsing vapor cavitiesto the center of the cage. The flowstream loses some energy as the

individual flow streams impinge upon one another. In addition, vapor cavitycollapse is likely to occur in the center of the cage rather than near critical

boundary surfaces.

• The holes separate the large free jet into many small flow streams and thetotal flow stream energy is divided into many small energy sources. By

breaking the single, large flow stream into many small streams, thefrequency of the noise that is generated is shifted upward. At higher

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frequencies, valve and pipeline vibration are less likely to produce

significant problems.

This trim style is useful for treating low levels of cavitation. For large valves (>

12 to 16 inches) and for large pressure drops (>300-400 psid), somemanufacturers have successfully minimized low level, low frequency vibration

problems by installing this trim in a flow-up configuration.



Figure 64

Single Stage Cavitation And Noise Control

Multi-Stage, Parallel Hole, Radial Flow Design - This method of pressure staging is

incorporated in many manufacturers trim designs. Figure 65 shows anexample of Fisher Controls’ Cavitrol III trim. The geometry of the holes isspecially designed to provide effective pressure staging while maintaining

maximum flow capacity. Trims are available to provide one, two, three, or four stages of pressure reduction.

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Figure 65

Fisher Controls Cavitrol III Trim

Multi Stage, Offset Hole, Radial Flow Design - This design is incorporated inValtek’s ChannelStream trim. The trim is essentially a cartridge that is made of

several concentric cylinders. As shown in Figure 66, the flow travels first

through the holes in the outer cylinder and it then enters a channel that ismachined into the second cylinder. This flow path is repeated in successive

stages to provide up to six stages of pressure reduction.

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Figure 66

Flow Concept Of The Valtek ChannelStream Trim

Stacked Plate, Tortuous Path, Radial Flow Designs - The “stacked plate” or

tortuous path approach to pressure reduction is employed in the Valtek Tiger-

Tooth trim and in the CCI Drag trim.

In Valtek’s Tiger-Tooth trim (see Figure 67), concentric grooves (or teeth) aremachined on both sides of a series of circular stacked disks. Flow passes from

the center of the disc in a radial, wave-like manner. The numerous turns in theflow path provide the staged pressure reduction that is desired. Trim isavailable with up to seven stages of pressure reduction.

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Figure 67

Flow Path Through The Valtek Tiger Tooth Trim

CCI’s DRAG trim also includes a number of plates. Each plate includes

multiple flow passages and each passage includes a number of right-angleturns as shown in Figure 68.

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Figure 68

Control Components Incorporated Drag Trim

Axial Flow, MultiStep Design - This trim design is the basis of Masoneilan’s VRT(Variable Resistance Trim) product. As shown in Figure 69, the flow is directedupward and parallel to the axis of the valve plug and stem. The trim is made up

of a number of plates that are drilled or machined to create a flow path thatincludes many turns or stages. When the valve plug is throttling near the seat,the flow is forced through a maximum number of stages. As the valve plug

approaches the open position, the flow is directed through fewer and fewer stages. As a result, this trim is most suitable for applications where thepressure drop decreases with increasing flow; i.e., the potential for cavitation

and cavitation related problems decreases at the normal and maximum flowrate. In constant pressure drop applications where cavitation could occur atany or every point in valve travel, this trim may not provide the required

cavitation protection.

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Figure 69

Masoneilan VRT Trim Concept

Notched Plug, Axial Flow Design - This trim design is the basis for Masoneilan’s

“Lincoln Log” trim that is shown in Figure 70. In this axial flow design, the plugand cage are machined to form several throttling surfaces, or stages, along thelength of the plug. As the valve is stroked, each stage throttles in unison and

the pressure drop is divided among each of the stages. Because the flowpassages in the Lincoln Log trim are larger than the flow passages in mostother anti-cavitation trim designs, the Lincoln Log trim is especially tolerant of

dirty fluids.

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Figure 70

Flow Pattern Of The Masoneilan Lincoln Log Trim

Combination Axial And Radial Flow Design - Fisher Controls Cav IV trim is anaxial flow design that includes a drilled-hole, radial flow cage element for each

axial stage. As shown in Figure 71, the flow is directed downward through thevalve. After the flow passes each axial stage, the flow is directed through a

drilled hole cage. The advantage of this design is that the large number of stages can eliminate cavitation in highly demanding applications.

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Flow

Drilled Hole,Radial FlowCages

Axial Flow PathThrough The Valve

Cav4

Figure 71

Combination Axial Flow With Radial Flow Cage Elements

Brute Strength Approach To Cavitation Damage Control - Some applicationspresent challenges that cannot be met by sophisticated anti-cavitation valve

technology. For example, when a fluid is extremely erosive, is alternately

flashing and cavitating, and includes large particles that are not compatiblewith the small flow passages of most anti-cavitation trim designs, specifiers

may select a “brute strength” approach to damage control. Figure 72 shows asweep flow, angle body valve with tungsten carbide trim and a hardened outletliner. This particular valve design has provided long valve life in difficult

applications where other, more sophisticated approaches have failed.

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Figure 72

A “Brute Strength” Approach To Cavitation Damage Prevention

Custom Valves

Characterized Anti-Cavitation Trim - Most standard anti-cavitation trims produce

an approximately linear inherent flow characteristic. In applications wheremultiple stages of pressure reduction are required at low flow conditions only, astandard multi-stage trim can unnecessarily reduce the maximum capacity of

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the trim and require the selection of a very large valve size. When the pressure

drop and the potential for cavitation related problems decrease at the

maximum flow condition, most anti-cavitation trims can be characterized asshown in Figure 73. In a characterized anti-cavitation trim, the trim includes the

number of stages that are required to prevent cavitation when the valve isthrottling near the seat and the pressure drop is at maximum. At mid travelpositions where the pressure drop decreases, the number of stages is

reduced. When the valve is fully open (or nearly so) and the pressure drop is atits minimum value, the number of stages may be further reduced or, if there is

no potential for cavitation, the trim may include straight-through flow passages. Although Figure 73 illustrates the means by which a drilled-hole cage ischaracterized, nearly all trim designs can be custom characterized.



Figure 73

Decreasing Pressure Drop Application And Characterized Anti-Cavitation

Cage

Super Severe Service Custom Valves - Many manufacturers have the capability to

design and manufacture super-special valves for difficult, super-severe serviceapplications. Super-special valves are unique, one-of-a-kind designs that are

designed specially designed for unique and especially demanding applications.For example, Figure 74 shows a custom valve that was designed for use as a

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liquid level control valve on a high pressure separator. The valve was designed

to provide cavitation protection, corrosion resistance, and erosion resistance.

The initial flow direction is flow down through a drilled hole cage. The upper cage provides one stage of cavitation protection and forces the flashing and

outgassing to occur in the void between the upper and lower plugs. The lower cage provide the benefits of a flow-up orientation; i.e., the flow is broken intoseveral smaller jets to prevent valve plug instability that can result from

flashing and outgassing. Although custom valves have a high first cost, theymay be the most economical solution when “standard” valves do not provide

satisfactory performance or valve life.



Figure 74

Super-Severe Service Valve For A Cavitating, Erosive, Corrosive, And

Outgassing Fluid

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Control Valve Selection Considerations

Performance Objective: Cavitation Damage Control Versus CavitationPrevention

When selecting valves for cavitating applications, one must determine whether the objective

is to select trim that will totally eliminate the potential for cavitation (by preventing bubble

formation) or to eliminate the potential for cavitation damage.

Cavitation Prevention - For a majority of applications, trims are available that will

totally eliminate cavitation by preventing the formation of bubbles in the flowstream. In high pressure and high pressure drop applications, the prevention of cavitation may require a large number of stages which in turns leads to larger

and larger body sizes and more costly valves. For critical applications that are

constantly operated at severe conditions, the selection of valves and valve trimthat will totally eliminate cavitation may be the most cost-effective solution over

time.

Prevention Of Cavitation Damage - In applications where the potential for

cavitation damage only occurs at system startup, system shutdown, or duringoperating transients, it may be more economical to:

1. Select a trim design that will prevent fluid vaporization during normaloperating conditions.

2. Select materials of construction that will resist cavitationdamage duringstartup, shutdown, and other periods of operating transients.

For example, it may be more economical to select a smaller two-stage trim that

is made of 316 stainless steel with Alloy 6 hardfacing than a larger valve with afour-stage trim that is made of a less damage resistant material such as astandard 410 or 416 stainless steel.

Manufacturers Control Valve Selection Procedures

Various techniques have been developed by valve manufacturers to evaluate system

conditions and select anti-cavitation control valves. Although each manufacturer’s methods

and techniques are different, most methods involve two major steps.

Step 1. Assessment of the system potential for cavitation with the use of a system

cavitation index.

Step 2. Selection a valve with a valve cavitation index that is appropriate for the

value of the system cavitation index.

Several manufacturer’s methods for valve selection are discussed below.

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Fisher Controls - Fisher Controls’ method for anti-cavitation valve and trim

selection is a two-step process.

1. Calculate the system cavitation index. The system cavitation index is

the application ratio, or Ar, where:

Ar = P1-P2/P1-Pv

2. Select a valve with a Kc rating that is greater than the value of Ar .

The Kc values that are published by Fisher Controls are designed to guide one

to the selection of a specific valve and trim that will prevent cavitation relatedproblems; i.e., damage from vapor cavity implosion, excessive noise, or

excessive valve and piping vibration. The Kc values are based on the recoverycoefficient of the valve as well as experiential factors that also take intoaccount the materials of construction, the valve size, and the pressure drop.

Valtek - To select a Valtek anti-cavitation valve and trim, one also performs atwo-step procedure.

1. Calculate the system cavitation index,σoperating, where:

σoperating = P2-Pv/P1-P2

2. Select a valve with aσmin rating that is less than the value of σoperating.

The σmin values that are published by Valtek are designed to guide one to an

estimated valve size only. Valtek’s literature indicates that final sizing must beperformed by factory personnel who will account for pressure scaling effects,size scaling effects, trim exit velocity, and other factors.

Masoneilan - To select an appropriate Masoneilan anti-cavitation valve and trim,

an initially selected valve is evaluated in terms of its pressure recovery

coefficient and the calculated value of ∆Pcritical (∆Pallow or ∆Pchoked) versus the

value of the actual pressure drop (∆Pactual).

1. Calculate the value of ∆Pcrit, where:

∆Pcrit = Cf 2(∆Ps)

Cf = FL

∆Ps = P1 - Ff Pv

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Note: The value of P crit is the same as the choked flow pressure drop that is

calculated with the use of the standard ISA liquid flow sizing equations.

2. Compare the value of ∆Pcrit with the actual∆P. If the value of ∆Pactual is

less than the value of ∆Pcrit, the selected valve is satisfactory. If the

value of ∆Pcrit is less than the value of ∆Pactual, then a valve with a

higher Cf (FL) should be selected.

CCI - The basic criteria for selection of a particular CCI Drag trim is the

selection of a trim that will limit fluid velocity at the trim exit to a value that isless than 100 feet per second. Because the indices and calculations that are

used to calculate fluid velocities throughout the valve are somewhat complex,most specifiers make use of CCI’s valve selection and sizing software in order

to select an appropriate valve and trim.

Valve Performance Contingency Requirements

Changes In Service Conditions - Specifiers should always allow for the possibilitythat the valve will be operated at pressure drops that are higher than those thatare specified on the ISS or on the process and piping drawings. In addition, the

system may be operated at elevated fluid temperatures which will cause anincrease in the value of Pv and an increase in the potential for cavitation. Inorder to minimize the potential for cavitation related problems when service

conditions change, specifiers should always specify a valve with an extra

margin of cavitation resistance. For example, if the value of Ksa for a givenapplication is 0.85, then a valve with a Kc of approximately 0.9 should be

considered.

Size Scale Effects - Control valve manufacturers often interpolate the cavitationindices for large valves on the basis of research that has been performed onsmaller valves. Because of size scale effects (larger valves often cavitate more

readily and more intensely than smaller valves of the same design), the valvecavitation index for a large valve may be somewhat overrated. Within Saudi

Aramco, it has been observed that manufacturers often ignore or miscalculate

the effects of valve size on cavitation damage resistance. Therefore, when

large anti-cavitation valves and trim are being selected, the specifier shouldallow for an additional margin of cavitation protection; i.e., the specifier should

select a valve with a higher Kc, lower σmin, etc. than is indicated by the normal

calculations. Generally speaking, size scale effects should be considered for all valves that are larger than 6 inches unless size scale affects have been fullyconsidered by the manufacturer.

Sensitivity To Accurate Data

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Importance Of Accurate Fluid Properties - When specifying anti-cavitation valves,

the specifier must make a concerted effort to secure accurate fluid properties.

For example, the calculations that are used to predict cavitation are highlydependent upon the value that is given for the fluid’s vapor pressure. Figure 75

shows that if an incorrect value is given for the vapor pressure, one maydetermine that an application is flashing when it is actually cavitating, or viceversa.



Figure 75

Fluid Behavior Versus The Value Of P v

Importance of Accurate Service Conditions - The need to secure accurate service

conditions is illustrated in Figure 76 which shows a plot of vibration as afunction of P1-P2/P2-Pv. Note the rapid increase in vibration that follows theonset of incipient cavitation. If the pressure drop is understated or overstated in

this range, then cavitation intensity cannot be accurately predicted.

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Figure 76

Vibration Intensity As A Function Of P1-P

2 /P

2-P

v

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Importance Of Defining Worst Case Cavitating Conditions

When specifying valves for cavitating applications, the specifier should make a concerted

effort to identify the worst-case conditions for cavitation. Some of the conditions that help to

define the “worst-case” scenario are described below.

Startup And Shutdown - During system startup and shutdown, the system is

often operated at low flow conditions for some period of time. In manyapplications, low flow operation is accompanied by high pressure drops thatincrease the potential for cavitation. To protect against cavitation damage

during system startup and shutdown, the service conditions should be clearlyidentified and considered during the selection process.

Changes In Operating Conditions - Specifiers should remain alert to the

possibility of changes in operating conditions. For example, if the system islikely to be operated at an elevated temperature, with a higher inlet pressure,

or with a reduced outlet pressure, an allowance for additional cavitationdamage prevention should be made during valve selection.

Reduced Throughput - If it can be anticipated that the system will be operated atreduced capacity (extreme turndown), the reduced capacity service conditions

should be evaluated during the valve selection process and a valve should beselected that will prevent cavitation related problems.

Cavitation In Combination With Other Severe Conditions

Cavitation in combination with other severe service conditions such as corrosion and/or erosion can quickly compound the rate and intensity of cavitation damage. When fluids are

erosive or corrosive, specifiers must give special attention to the materials of construction

that are selected.

Anti-Cavitation Trim And Flashing Applications

In some instances, a fluid may be cavitating at the normal flow condition and the maximum

flow condition while it is flashing at a low flow condition. Two general guidelines help to

guide the specifier under this circumstance.

1. Of the two phenomenon, cavitation is by far more damaging than flashing; therefore,

cavitation must be treated with an appropriate anti-cavitation trim.

2. In multi-stage, anti-cavitation trims, the flashing damage is most likely to occur between the trim stages (inter-stage flashing). Therefore, the following guidelines

apply.

a. If possible, select single stage anti-cavitation trim.

b. If multi-stage anti-cavitation trim must be selected to prevent cavitationrelated problems, select materials of construction that are highly

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resistant to damage from vapor cavity implosion and from flashing

erosion; i.e., 316 stainless steel with Alloy 6 hardfacing, solid Alloy 6,

tungsten carbide, etc.

Non-Valve Methods Of Reducing The Potential For Cavitation

During the valve selection process, the specifier should remain alert to means of minimizing

cavitation related problems other than valve selection. Two such possibilities are discussed

below.

System Design - In some applications, a change in valve placement can help tominimize cavitation related problems. For example, moving a feed valve from amid-line position to a tank mounted position can reduce the potential for

cavitation damage to the valve and piping. By mounting the valve on or near

the tank as shown in Figure 77, the vapor cavities will implode inside thevessel where they will not cause damage to valve parts or pipe walls.

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Figure 77

Minimizing Cavitation Damage Through Valve Placement

Valve Elevation - A change in the elevation of a valve can also have a significant

impact on the potential for cavitation and cavitation related problems. For

example, Figure 78 shows the difference in the values of P1 and P2 of adistillation column feed valve when the valve is located near the top of thecolumn (Installation A) and when the valve is located near the bottom of thecolumn (Installation B). When the valve is located near the bottom of the

vessel, P1 is increased because there is less friction loss and less head loss.P2 is also increased because of the additional head at the valve outlet. Bothpressure conditions serve to decrease the potential for cavitation.

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Figure 78

Minimizing Cavitation Damage By Changing The Elevation Of The Valve

ISA System Indices From ISA-dRP75.23

Standardization - Given the broad range of methods for predicting cavitation andfor rating the cavitation resistance of a particular control valve, there is

considerable confusion and controversy concerning the preferred methods for system assessment and for assigning valve indices. In an effort to standardizesystem assessment and valve selection procedures, ISA subcommittees haveprepared a draft recommended practiceISA-dRP75.23 Considerations For

Evaluating Control Valve Cavitation . This standard provides a recommendedmethodology for evaluating the potential for cavitation and for rating thecavitation resistance of control valves.

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System Cavitation Parameter σσsystem - In order to evaluate the potential for

cavitation in a given system, the ISA recommends the parameter σsystem where

σsystem = (P1-Pv)/(P1-P2). Using this analysis, the potential for cavitationincreases as σ approaches 1.0. Refer to Figure 79. As the value of σsystem

increases from 1.0 to approximately 17, the potential for cavitation decreases.

σsystem is related to the Saudi Aramco index Ksa as follows:

σsystem = 1/Ksa.

The parameter σ as used by the ISA is not the same as theσ parameter that isused by Valtek.



Figure 79

ISA System Cavitation Index σσsystem

σσ: Universal Control Valve Cavitation Index - The parameter σsystem only

quantifies the service conditions. By itself, the value of σsystem does not

convey any information about the performance of a particular valve in aparticular application. In order to gain utility from the parameter σsystem, the

ISA recommended practice describes a methodology in which the behavior of

a specific valve can be predicted as a result of the value of σsystem. The ISArecommended practice suggests that manufacturers test their valves under standard test conditions and assign several valve performance indices.

Several indices are illustrated in Figure 80 and they are discussed below.

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Figure 80

ISA-dRP75.23 Control Valve Cavitation Indices

• σI - The coefficient of incipient cavitation is the value of (P1-Pv)/(P1-P2) at

which cavitation can first be detected. This coefficient can be determinedwith noise or vibration measurements as shown in Figure 80.

• σc - The coefficient of constant cavitation is the value of (P1-Pv)/(P1-P2) at

which mild, steady cavitation occurs. Damage is not usually associated withthis level of cavitation. This coefficient can be determined with noise or vibration measurements as shown in Figure 80.

• σid - The coefficient of incipient cavitation damage is the value of (P1-Pv)/(P1-P2) at which the onset of cavitation damage occurs. This value

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cannot be determined from noise or vibration measurements; it must be

evaluated in the laboratory.

• σch - The coefficient for choking cavitation is the value of (P1-Pv)/(P1-P2)that causes choking in the valve. The maximum levels of noise, vibration,

and material damage have been observed to occur at or just prior to this

condition. The value of σch may be estimated with the following:

σchv

L F v

P P

F P F P=

−

−1

21( )

• σmv - The coefficient of maximum vibration can be determined by identifying

the value of (P1-Pv)/(P1-P2) at which the maximum vibration or noise

occurs on a plot such as the one shown in Figure 80.

• σmr - The manufacturer’s recommended minimum limit (σmr ) is an

operational limit that is supplied by the valve manufacturer. Thedetermination of this value may be based on laboratory analysis,experience with specific applications, or an understanding of specific valve

features.

σσ Parameters That Are Used During Valve Selection - The specifier may select a

specific valve on the basis of any of the aboveσ parameters. For example, if

the value of σsystem is 2.5 and the specifier’s objective is to limit cavitation to

the level of constant cavitation, the specifier would select a valve with aσc of 2.5 or less. The decision of which parameter to use during the selection of aparticular valve is largely subjective and may depend upon many factors suchas valve style, percentage of valve travel, duty cycle, location, desired life, and

past experience. According to the draft recommended practice, “the valvemanufacturer should be consulted in this matter.”

Scale Effects - ISA-dRP75.23 includes provisions for calculating size scaleeffects (SSE) and pressure scale effects (PSE) for a particular control valve.The calculations are explained in the draft.

Piping Factors - Upstream pipe reducers and downstream expansions cause avariation in cavitation levels and in sizing coefficients. To account for theseeffects, ISA-dRP75.23 defines a mathematical procedure for evaluating theeffects of reducers and expanders (swages) on the performance of a particular

valve.

Future Application Of ISA -dRP75.23 - When manufacturers fully endorse and

comply with the recommended practiceISA-dRP75.23, and when

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manufacturers test and publish the various values of σ that describe valveperformance in cavitating applications, the specifier will have a massive

amount of unbiased data on which to base his valve selection decisions.However, considerable time will be required for manufacturers to test their products and to publish the results.

Immediate Application Of ISA-dRP75.23 - Until valve manufacturers completecomprehensive testing of their products and until the results of the tests are

published, the only data that is likely to be available is a listing of σmr values.

The σmr values that will be initially published will likely to be translated from

existing cavitation indices.

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SELECTING AND SIZING CONTROL VALVES FOR FLASHING FLUID

Applications

Flashing And Its Consequences

Review Of Flashing Phenomenon

Flashing Compared To Cavitation - When, in a liquid flow, the fluid pressure fallsbelow the fluid’s vapor pressure, the fluid begins to vaporize; i.e., vapor

bubbles form in the flow stream. In a control valve, the onset of vaporizationoften occurs near the vena contracta, as shown in Figure 81. If thedownstream pressure (P2) increases to a value that is greater than the fluid’s

vapor pressure (Pv), the bubbles collapse and the fluid is cavitating. If the

downstream pressure P2 is less than the fluid’s vapor pressure, the bubbles, or vapor cavities, remain in the fluid stream and the fluid is flashing. It is important

to note that flashing occurs only as a function of the values of Pv and P2; i.e.,flashing is independent of the inlet pressure P1 and the vena contractapressure Pvc.



Figure 81

Flashing Phenomena That Occurs When P 2<Pv

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Flashing And Choked Flow - The pressure drop vs. flow plot that is shown in

Figure 82 shows that flashing normally occurs at higher pressure drops than

are required to cause cavitation and choked flow. Under certain conditions,however, flashingcan occur prior to choked flow and cavitation. For example,

a valve with a very high pressure recovery coefficient (Km or FL approaching1.0) will have a very high allowable pressure drop and the fluid may begin to

flash at a pressure drop that isless than the ∆Pallow.



Figure 82

Relative Values of ∆∆P That Cause Cavitation, Choked Flow, And Flashing

Changes In Fluid Density and In Fluid Velocity - When a liquid flashes (vaporizes),

the fluid density decreases (specific volume increases). To pass the requiredflow, the fluid velocity must increase. These conditions present two concernsfor the valve specifier.

1. The expansion of the fluid may require a larger valve in order to pass the

required flow.

2. The high-velocity flow can present the potential for erosion damage to the

valve and to the downstream piping.

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Figure 83

Major Concerns With Flashing Fluids

Common Forms Of Flashing

Unwanted Flashing - In some applications, flashing occurs as an unwanted

phenomenon that results from rigorous service conditions, improper systemdesign, or operating transients. For example, the water dump valve on the high

pressure separator that is shown in Figure 84 has a high inlet pressure and alow outlet pressure (P2 = atmospheric pressure). In this application, flashingcannot be avoided.

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Figure 84

Flashing As A Result Of Severe Conditions

Flashing By Design - In some applications, the process isdesigned to flash (P2

is intentionally set below Pv) in order to achieve a specific process objective.For example, Figure 85 shows that an individual fluid component can be“knocked out” of a liquid hydrocarbon mixture by setting the control valve outletpressure to a pressure that is less than the vapor pressure of the fluid

component that is to be removed. In this manner, the component that is to beextracted is converted to its gas or vapor phase and piped away from the liquidfor downstream processing.

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Figure 85

Flashing As A Means Of Achieving A Process Objective

Flashing Versus Outgassing - Outgassing is a phenomenon that is similar toflashing. Outgassing occurs when a gas that has been dissolved in a liquidcomes out of solution as a result of pressure reduction or agitation. Refer to

Figure 86. There are several important differences between flashing andoutgassing.

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Figure 86

Flashing Versus Outgassing

Outgassing is a physical reaction that can occur instantaneously upon a

reduction in fluid pressure or upon significant agitation of the fluid. Most valvespresent sufficient obstructions at the valve inlet to cause the onset of outgassing; therefore, outgassing can occur within the valve. Depending on thepressure drops and the amount of gas that comes out of solution, outgassing

can result in areas of high and low pressure within the valve. Areas of high andlow pressure around the valve plug can cause valve plug instability anddestructive vibrations, especially in larger valves. In addition, the standard

liquid sizing model and the equations for choked flow do not apply tooutgassing flows.

Flashing is a thermodynamic event that requires a change in the latent heat of

vaporization for a change of state to occur. It is commonly believed that thetime that is required for the change of state to occur is generally longer than

the transit time of the fluid moving through the region of the vena contracta. Asa result, the majority of the flashing phenomenon typically occurs downstream

of the primary flow restriction. The standard liquid sizing model and theequations for choked flow generally provide acceptable results.

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Quantifying Flashing

Flashing And Control Valve ∆∆P - There are no useful equations or indices thatallow the specifier to predict flashing intensity; however flashing intensity can

be viewed as a function of the pressure drop across the valve; i.e., the valve∆P is an indicator of the total energy that can be transformed into changes of state, vibration, and noise.

Flashing, Velocity, And Erosion - Erosion is a strong function of fluid velocity, andbecause any degree of flashing will cause the fluid velocity to increase,

specifiers should apply additional engineering attention to any flashingapplication.

Single-Species Liquids Vs. Liquid Mixtures - For liquid mixtures, the value that isgiven for the vapor pressure is often the “bubble point”; i.e., the pressure atwhich the lightest component will vaporize. The entire mixture will not flash at

the bubble point. Instead, only those components of the mixture whose vapor pressures are greater than P2 will vaporize. As shown in Figure 87, the amountof flashing that will occur in a mixture may be substantially less than what one

might predict for a single-species fluid. As a result, the choked flow sizingequations may calculate a conservative valve size.



Figure 87

Flashing Of One Component In A Liquid Mixture

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Consequences Of Flashing

Flashing Erosion - Because the fluid velocity is greatly accelerated during theliquid-to-vapor transformation, flashing is always accompanied by an increased

potential for valve and piping damage. Flashing erosion is the result of highvelocity vapor particles impinging upon critical valve surfaces such as valve

plugs, valve seats, valve body castings, and pipewalls. If the fluid includesparticles such as fines, sand, or other solids, the particles are swept along athigh velocity by the vapor phase of the fluid and the potential for erosion

damage is further increased. Most industry authorities agree that the potentialfor erosion damage increases exponentially as the fluid velocity increases; i.e.:

Potential For Erosion Damage∝ Velocityn

The value of the exponentn is estimated to be in the range of 5 to 8.

Choked Flow - Choked flow is usually present when flashing occurs. Chokedflow is treated as a sizing issue.

Flashing Service Flags And Typical Flashing Applications

Flags For Flashing Fluid Applications

Liquids With High Vapor Pressures - Any liquid with a high vapor pressure has an

increased potential for vaporization and flashing.

High Temperature Liquids - When the temperature of a liquid is elevated, its

vapor pressure also increases thereby increasing the potential for flashing.

Letdown Applications - Flashing is likely to occur in any application in which thefluid pressure is let down to a low pressure, to atmospheric pressure, or to avacuum.

Typical Flashing Fluid Applications

In many applications, the fluid flashes as a result of system objectives or as a result of the

normal service conditions.

Flash Drum Feed Control Valve - The valve that controls the fluid flow into a flashdrum reduces the fluid pressure for the purpose of separating gasses andvapors from the liquid phase. Although the fluid is generally outgassing, some

flashing may also occur.

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High Pressure Separator Condensate Drain - The condensate drain valve (or liquid

level control valve) on a high pressure separator often lets down to

atmospheric pressure or to a very low pressure.

Steam Generation Applications - Many of the control valves that are used tocontrol the flow of fluids in steam generators and boilers are subjected to

flashing conditions. For example, heater drain valves flash the drain water tothe condenser.

Critical Control Valve Selection Considerations

Basic Control Valve Selection Criteria

Because flashing is the result of the fluid pressure at the valve outlet fallingbelow the fluid’s vapor pressure, flashing is the result of system conditions

only. There is no control valve design that will prevent the occurrence of flashing. Therefore, the specifier’s objectives are to:

1. Identify and select control valve types that will provide resistance toflashing

damage.

2. Specify materials of construction that will resist flashing damage.

3. Consider the effects of choked flow on the valve sizing calculations.

4. Where possible, change system design parameters to minimize thenegative effects of flashing.

Erosion Resistant Control Valve Types

Angle Bodies Vs. Globe Valve Bodies - For flashing applications, angle bodies areoften the preferred body style. The features that support this preference areillustrated in Figure 88 and they are discussed below.

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Figure 88

Angle Body Construction

1. After the fluid flows through the trim, the flow path is parallel to valve andpipe walls; i.e., the angle at which the high-velocity vapor cavities and solidparticles (if present) impinge on critical surfaces is near zero.

2. Because the flow path of an angle valve is less tortuous than the flow path

of a standard globe valve, there are fewer areas of locally high or lowpressure. Consequently, the vena contracta is more likely to be well definedand located in an area that is downstream of the valve trim. If the venacontracta is downstream of the trim, fluid vaporization and flashing damage

is more likely to occur downstream of the valve trim.

3. The transit distance through an angle body valve is shorter than it is in astandard globe valve. Therefore, it is likely that the flashing fluid will not

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attain equilibrium (totally change state) untilafter the fluid has passed the

critical trim components.

4. In order to protect the downstream piping against flashing damage, angle

bodies are typically available with hardened outlet liners that can be easilyand economically replaced if necessary.

5. In many instances, the availability of reduced capacity trim (restricted trim)allows the selection of a line-size valve body instead of body size that is

smaller than the pipeline size. Bynot selecting a valve body size that is lessthan line size, the increased volume on the outlet side of the valve helps toreduce the outlet velocity.

Line-Of-Sight Valve Constructions - For medium to low pressure flashingapplications, an eccentric rotary plug valve is often a good selection. Refer to

Figure 89. In flashing or erosive applications, the valve is installed with the plugon the downstream side of the body. In this orientation, the flow restriction is

well defined and the vena contracta - and therefore the occurrence of flashing -is most likely to occur at a location that is downstream of the valve. A spoolpiece of sacrificial pipe is often installed downstream of the valve for the

purpose of absorbing flashing damage.

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Figure 89

Eccentric Rotary Plug Valve

Velocity Control - Generally speaking, specifiers should avoid valve and trimdesigns that include features that would obstruct flow or increase the fluid

velocity. For example, multi-stage anti-cavitation trims are generally avoidedbecause high-velocity flashing flows is likely to cause flashing erosion in theinterstage areas of the trim. Single stage trims may be applied when

necessary.

Materials Of Construction

Body Materials - For flashing applications, specifiers typically follow the same

guidelines that were discussed for erosive fluid applications. When anglebodies and eccentric rotary plug valves are selected, the erosive flows do notimpinge directly on the body walls and standard carbon steel bodies may

provide adequate life.

A listing of popular erosion-resistant body materials is shown in Figure 30 of

this Module.

Trim Materials - A listing of popular erosion-resistant trim materials was shownin Figure 31 of this Module.

System Design Considerations

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The location of a valve in a flashing application can have significant impact on the life of the

system. For example, Figure 90 shows a heater drain valve that flashes the drain water to a

condenser. In Installation A, flashing will occur in the long run of piping between the valveand the condenser. The pipe may have to be replaced periodically because of the flashing

damage that will occur. In installation B, the control valve is close coupled to the condenser.

In this installation, the flashing will occur within the condenser and without the potential for

valve and piping damage.



Figure 90

Example Of Changes In System Design To Prevent Flashing Damage

Valve Sizing Procedures

Because flashing is usually accompanied by choked flow, the valve sizing pressure drop

must be limited to the lesser of the ∆Pactual or the ∆Pchoked . The equation for calculatingchoked flow is:

∆Pallow = FL2(P1-r cPv)

where:

∆Pallow the maximum pressure drop that is effective in producing

flow

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FL ISA nomenclature for the control valve recovery

coefficient. Fisher nomenclature is Km where Km = FL2.

P1 Upstream fluid pressure.

r c The critical pressure ratio; 0.96-0.28(Pv/Pc) where Pv is

the fluid’s vapor pressure and Pc is the fluid’s criticalpressure.

Pv The fluid’s vapor pressure.

Flashing In Combination With Particle Erosion Or Corrosion

Flashing in combination with particle erosion exponentially increases thepotential for erosion damage. Flashing in corrosive fluids can hasten both theerosion and corrosion processes because of the removal of protective platings,

coatings, and naturally-occurring passive layers that provide corrosionresistance.

Importance Of Accurate Data

Sensitivity To Accurate Fluid Properties And Service Conditions - Because the

occurrence of flashing can only be predicted if the values that are given for Pv,

P2, T, and Pc are accurate, this information should be verified.

Information Sources - Accurate fluid properties (Pv and Pc) can be found inreference books that list physical properties of fluids and they can be obtained

from the Saudi Aramco Process Group. Accurate service conditions (thevalues of P1, P2, and T) can be obtained from process documentation andfrom operating personnel.

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SELECTING And sizing CONTROL VALVEs To Attenuate Aerodynamic

Control Valve Noise

Sources Of Control Valve Noise

Types Of Control Valve Noise

Mechanical Vibration - Mechanical vibration of valve components is a result of random pressure fluctuations within the valve body and/or fluid impingement

upon movable or flexible valve components. As shown in Figure 91, the mostprevalent source of noise that results from mechanical vibration is the lateralmovement of the valve plug relative to the plug guiding surfaces. The noise

that is generated by this type of vibration normally has a frequency of less than

1500 hertz, and it is often described as a metallic rattling. One particularlytroublesome form of mechanical vibration is observed in a valve component

that resonates at its natural (resonant) frequency. Resonant vibrationgenerates a sound that is a single-pitched tone, normally having a frequencybetween 3000 and 7000 hertz. This type of vibration generates high levels of

mechanical stress that can cause fatigue failure of the vibrating part. Valvecomponents that are susceptible to resonance include contoured valve plugs

with hollow skirts, and flexible members such as the metal seat ring of a ballvalve. The potential for physical damage to valve components is generally of greater concern than the noise that is emitted. Fortunately, the noise that is

generated by mechanical vibration has, for the most part, been eliminated with

improved valve designs.



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Figure 91

Mechanical Noise

Hydrodynamic Noise - Excessive noise levels can be generated by the collapseof bubbles in a cavitating flow. Refer to Figure 92. The noise that is generated

in a cavitating flow includes a broad range of frequencies. Hydrodynamic noiseis frequently described as the sound that would be generated if gravel wereflowing in the fluid stream. Excessive levels of hydrodynamic noise are

accompanied by an increased potential for valve and piping damage fromvapor cavity implosions (cavitation) and from low frequency vibration of thevalve and piping. As a result, high levels of hydrodynamic noise are generally

treated as a cavitation concern instead of a concern for excessive levels of noise in the environment.

Test results indicate that the noise that is produced by flashing liquids is rarelyexcessive; therefore, while flashing liquids are a concern because of the

potential for erosion damage, flashing liquids are not typically associated withunacceptably high levels of noise.



Figure 92

Hydrodynamic Noise

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Aerodynamic Noise - Aerodynamic noise is generated by the turbulence that isassociated with the control of gas, steam, or vapors. Major sources of aerodynamic noise are the stresses or shear forces that are present inturbulent flows. Some sources of turbulence are obstructions in the flow path,

rapid expansion or deceleration of high-velocity flows, and directional changesin the fluid stream as shown in Figure 93.



Figure 93

Sources Of Aerodynamic Noise

Aerodynamic noise is generally considered to be the primary control valvenoise problem. The focus on aerodynamic noise is supported by the following:

1. Aerodynamic noise has its highest energy components at the frequencieswhere the human ear is most sensitive - between 1,000 and 8,000 hertz.

2. Large amounts of energy can be converted to aerodynamic noise without

damaging the valve. Today, with increasing focus on environmental issues,agencies and corporations have established guidelines and standards thatlimit the amount of noise that a valve can emit in the workplace.

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3. Extremely high levels of aerodynamic noise can cause mechanical damage

to the valve.

Mechanics Of Aerodynamic Noise Generation And Transmission

Researchers who study aerodynamic noise address four distinct phases of noise generation

and transmission. The four phases are shown in Figure 94.



Figure 94

Mechanics Of Noise Generation And Transmission

Noise Generation - Aerodynamic noise is generated in a control valve as a resultof turbulence and shear forces as previously described. The valve body is arigid casting with thick walls, an irregular shape, and substantial reinforcingand therefore does not vibrate with sufficient intensity to transmit noise to the

environment. The noise of concern is the noise that propagates to thedownstream piping.

Propagation - The noise that is generated in the control valve propagates intothe downstream piping as a result of fluid flow through the system.

Coupling - The noise within the piping causes the pipewall to vibrate. The

degree to which the sound energy is converted to pipewall vibrations isdetermined by many complex variables including:

• The resonant or natural frequency of the piping versus the predominantfrequency of the valve generated noise.

• The mass of the pipewall.

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• The mass of the flowing media.

• The stiffness of the pipewall.

Radiation - The vibration of the pipewall radiates sound pressure waves into theenvironment that can be detected by the human ear as noise, or sound.

Quantifying Noise Intensity

Measurement Parameters

A sound wave is a pressure wave that travels through a media (air, gas, liquid, solid, etc.)

with a fixed amplitude and frequency. As shown in Figure 95, noise is a random mixture of

sound waves of various amplitudes and frequencies. To fully characterize noise, one must

evaluate both the amplitude (intensity) and the frequency (pitch) of the noise.



Figure 95

Amplitude And Frequency Of Noise

Measurement Units And Scales

SPL and Lp - The intensity of sounds is often described in terms of sound

pressure level . Sound pressure level is commonly abbreviated with the term

SPL (sound pressure level) and with the term Lp (sound pressure). The termsSPL and Lp are identical in meaning. The abbreviation SPL has been popular

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in the past; however, emerging international standards have endorsed the

abbreviation Lp. To remain consistent with current Saudi Aramco standards,

the term SPL will be used throughout this module.

dB - As illustrated in Figure 96, all sound is generated from a form of mechanical, radiant energy that is transmitted by longitudinal pressure waves

in a material medium such as air. The pressure waves that produce mostsounds are relatively small in magnitude; accordingly, the pressures aremeasured in very small pressure units such as micropascals (10-6

Newton/m2). The most common unit of measurement that is used to describethe sound pressure level is a decibel, abbreviated dB. A decibel is equal to 20times the logarithm to the base ten of the ratio of a measured sound pressure

to a reference sound pressure of 20 micropascals.



1 2020

10dB LogMeasured Sound essure micropascals

micropascals=

Pr ,

Figure 96

Pressure Waves Measured In dBA

Frequency Spectrum Of Aerodynamic Noise - Aerodynamic control valve noise istypically generated over a broad range of frequencies. Figure 97 shows atypical frequency spectrum distribution for a typical valve. Note that the highest

SPL’s are generated in the frequency range of 1 to 8 KHz.

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Figure 97

Typical Frequency Distribution Of Aerodynamic Noise

dBA stands for A weighted decibel . A-weighting is designed to bias the noisefrequency spectrum to correspond with the frequency response (the

sensitivity) of the human ear. For example, consider a noise with an SPL of 100 dB at a frequency of 1,000 hertz. If the intensity of the noise remainsconstant and the frequency changes to 200 hertz, the noise will sound to the

human ear like a noise with an SPL of 90 dB. To account for the sensitivity of the human ear, we describe the sound pressure level of the sound at 200 hertzas 90 dBA. The plot in Figure 98 shows the corrections that are made at

various frequencies to achieve A-weighting. Referring to Figure 97, note that A-weighting does not have a significant impact on most aerodynamic noisemeasurements.



Figure 98

Corrections For A-Weighting At Various Frequencies

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Representative Noise Levels - The decibel scale ranges from zero for the least

perceptible sound, to about 130 dBA for the threshold of physical pain. The

chart in Figure 99 shows the approximate sound pressure level, in dBA, of familiar environments.



Figure 99

Representative Noise Levels

Measurement Techniques

Noise measurements can be made with meters that measure the sound pressure level at a

given location.

Line Source Measurement - For line sources (such as pipelines), noise

measurements are taken at a point that is 1 meter downstream of the valve

and 1 meter from the pipeline surface as shown in Figure 100. For linesources, equal noise levels will be measured on an imaginary cylinder for

which the axis is the pipe centerline. As an observer moves away from thepipeline, the SPL decreases inversely with the increase in the surface area of the imaginary cylinder.

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Figure 100

Line Source Noise Measurement

Line Source Distance Doubling - Each time the distance from a line source isdoubled, the apparent sound pressure level is reduced by approximately 3

dBA, as shown in Figure 101.



Figure 101

Line Source: SPL Attenuation From Distance Doubling = Approximately 3

dBA

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SPL At Any Distance - If an SPL measurement is taken 1 meter downstream of

the valve and 1 meter from the pipeline, the apparent dBA at any given

distance can be determined with the following equation:

SPL F Logr

R r= +

++

101

where:

r pipe radius in meters based on thepipe outside diameter

R distance in meters from the pipe

surface

F noise level at 1 meter from the pipe

surface

To illustrate the utility of the equation, assume that the SPL at a point that is 1

meter downstream of the valve and 1 meter away from the pipeline is 95 dBA. Assume also that the pipeline is 12 inches in diameter (a radius of 0.16 meter).To calculate the SPL at a point that is 32 meters from the pipeline, the equation

is solved as follows:

LpA

LpA

LpA

= (95) + 10 Log1+ 0.1

3 + 0.16

= 95 - 1

= 80.57 dBA

6

2

4 43.

Downstream Pipeline Attenuation - Downstream of the noise source, the noisethat propagates through the medium and the pipeline tends to diminish over distance. Though there is no absolute guideline for quantifying the amount of

noise attenuation at points downstream of the noise source, a commonguideline suggests that there will be a 1 dBA of noise attenuation for every 100feet of distance downstream of the source.

Point Source Measurement - For a point source (such as a vent), sound pressuremeasurements are routinely taken 3 meters from the source. As an observer

moves away from a point source, the SPL decreases inversely with theincrease in the surface area of an imaginary sphere, as shown in Figure 102.

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Figure 102

Point Source Noise Measurement

Point Source Distance Doubling - Each time the distance from a point source isdoubled, the apparent sound pressure level is reduced by approximately 6

dBA, as shown in Figure 103.



Figure 103

Point Source: Distance Doubling = Approximately 6 dBA Attenuation

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SPL At Any Distance - If an SPL measurement is taken at a point that is 3

meters away from a point source, the apparent dBA at any given distance can

be determined with the following equation:

SPL F LogR

= +203

where:

R Distance in meters from the source

F Sound pressure level at 3 meters fromthe source

For example, assume that the SPL at 3 meters from a vent is 95 dBA. Tocalculate the SPL at a distance of 24 meters from the pipeline, the equation is

solved as follows:

LpA

LpA

LpA

= (95) + 20 Log

= 95 -

= dBA

3

24

18 06

76 94

.

.

Vibration Measurement Vs. SPL Measurement - Noise levels can also be

determined by measuring the intensity of the pipewall vibrations with anaccelerometer and, then, converting the pipewall vibration measurements to anSPL value or to a frequency spectrum distribution. This technique is useful

when direct SPL measurements would be corrupted by ambient noise in theenvironment. Additional insights to a noise problem can often be gained byinterpreting a spectrum analysis that is developed from vibration

measurements. For example, excessive noise levels that have been attributedto control valve noise are sometimes found to occur at frequencies that matchthe operating frequency - or multiples of the operating frequency - of an

upstream compressor or other prime mover. In this instance, efforts to limitcontrol valve noise would not be productive because the origin of the noise is

the upstream device rather than the control valve.

Combining Measurements From Independent Systems - The combined SPL of twonoise sources is determined bydifference between the sound pressure level of

the two sources. The chart in Figure 104 shows the DSPL (the number of dBthat must be added to the louder of two noise sources) for different values of

SPL1 - SPL2.

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Two examples of combining SPL measurements are illustrated in Figure 104.

For the two identical levels of SPL, the combined SPL is equal to the SPL of a

single source + 3 dBA. In the example where the sound pressure level of onesource is 65 dBA and the sound pressure level of a second source is 95 dBA,

the total difference is greater than 12 dBA; therefore, the combined SPL is(approximately) the louder of the two sources.



Figure 104

Combining Noise Sources

Consequences Of Control Valve Noise

The concern for high levels of noise is a twofold issue. First, noise is a human factors issue.

Second, research indicates that high levels of noise can cause the total failure of valves and

other piping system components.

Human Factors - Prolonged exposure to high levels of noise generally results inhearing loss. Because the hearing loss results from mechanical damage to theear, the losses are cumulative and permanent.

SAES-A-105 is dedicated to the subject of noise limits and noise control at allSaudi Aramco sites. Figure 105 shows the exposure limits that are established

in Section 4.3.1 of SAES-A-105.

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Figure 105

dBA Versus Exposure Guidelines

Section 5.8 of SAES-J-700 relates noise concerns as they apply specifically tocontrol valves. This standard gives the following maximum LpA limits for control valve noise:

• 90 dBA, measured at a distance of 1 meter downstream of the valve, for

valves normally in operation (throttling). This limit is intended to apply to avalve that is in continuous operation.

• 95 dBA for recycle valves and other control valves which are normallyclosed (such as manual valves). This limit is intended to apply to a valvethat is intermittently operated.

• 105 dBA (before any acoustic treatment) for any valve in any application.

This limit is absolute limit and is notto be exceeded under anycircumstances.

The first two guidelines are based on environmental and human factors issues.The third guideline limits noise to a level that is below the threshold of equipment damage.

Equipment Damage - High levels of noise is an indication that there has been a

transformation of energy within the fluid stream to mechanical vibration. These

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vibrations are generally of a fairly high frequency and can cause wear and

fatigue of internal valve components. For example, a vibrating plug can cause

wear of guiding surfaces. If the plug and cage become worn, the increasedclearance between the plug and guiding surface can allow high intensity

vibrations that result in total failure of the stem-to-plug connection.

As a result of laboratory tests and experiential factors, valve manufacturershave established a maximum upper noise limit of 110 dBA to preserve themechanical integrity of the valve and other piping system components. As

shown in Figure 106, the threshold for valve damage is actually greater than110 dBA for larger valves; however, the common 110 dBA limit is broadlyapplied to all valve sizes.



Figure 106

SPL Versus Valve Damage In Various Valve Sizes

Flags For Excessive Noise And Common Noise Applications

SPL> 90 dBA For A Standard Valve

Whenever any standard control valve with standard trim and with standard downstream

piping generates an SPL that is in excess of 90 dBA, the specifier must evaluate the

application in terms of excessive noise generation.

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Outlet Velocity Greater Than 0.3 Mach

A valve outlet velocity that is greater than 0.3 Mach indicates the potential for excessive

noise generation. 0.3 Mach is the approximate boundary at which the noise that is propagated into the downstream piping becomes greater than the noise that is generated in

any valve, including those valves with quiet trim options.

P1/P2 > 5 For Dry Gas And Superheated Steam Services

Significant potential for excessive noise generation exists whenever the pressure ratio

(P1/P2) in psia of dry gas and superheated steam is 5.0 or greater. Pressure ratios of 5.0 or

greater can create the high outlet velocities that are associated with high levels of

aerodynamic noise.

SPL > Limits That Are Established By Saudi Aramco Engineering Standards

Saudi Aramco Engineering Standards SAES-J-700 and SAES-A-105 that were previouslydiscussed define absolute SPL limits for various applications and conditions.

Specific Applications

In addition to the flags that are described above, many services are known, through

experience, to present the potential for excessive levels of aerodynamic noise. Control valve

applications that commonly present high potential for noise generation include:

• Compressor bypass valves.

• Atmospheric vent valves.

• Gas injection valves.

Predicting Control Valve Noise

Introduction

The SPL of a valve that is being considered for a given application can be predicted with the

use of various noise prediction equations that have been developed by valve manufacturers,

by standard organizations such as the ISA and the IEC, and by academia.

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Influences On Noise Generation And Transmission

The common noise prediction methods are based on broadly varying approaches to acoustic

theory, experience, and laboratory research. Most techniques could be made more precise if more complete system and operating conditions could be made available. However, the

common prediction techniques are reasonably accurate in view of the information that is

commonly available to the specifier. The parameters that are typically included in prediction

methods are discussed below.

Pressure Drop (∆∆P) - The pressure drop across the valve is a representation of the total energy that is available to be converted into sound energy.

Flow Rate - Mass flow, in conjunction with other factors, helps to quantify the

total stream power that can be converted to noise.

Pressure Drop Ratio ( ∆∆P/P1) - The pressure drop ratio serves to account for fluid

velocity. The impact on velocity is as follows:

Impact On Fluid VelocityP P

≈−

1

1 1∆ /

Downstream Pressure P 2 - The downstream pressure influences the fluid densityand therefore the fluid velocity at the valve outlet. Downstream pressure also

influences the degree of coupling that occurs.

Valve Acoustic Efficiency Factors - Acoustic efficiency is a measure of how muchof the total flow stream energy will be converted to sound (the ratio of thestream power that is converted into sound to the stream power). Acoustic

efficiency is a complex function of the valve’s pressure recovery coefficient,the number and size of flow passages, other valve design factors, and therelative pressures at the inlet, vena contracta, and the valve outlet.

Downstream Pipe Size - The size of the downstream piping has a direct effect onfluid velocity in the downstream system.

Downstream Pipe Schedule - The mass and the acoustic characteristics of the

downstream piping influence the degree of acoustic coupling and thetransmission losses that occur at the pipewall.

Valve Noise Peak Frequency Vs. Pipe Coincident Frequency - As the peakfrequency of the valve generated noise approaches the pipe coincidentfrequency (the natural resonant frequency of the pipe), a greater degree of

coupling occurs and more noise is transmitted to the environment. As thefrequency of the valve generated noise moves away from the pipe coincident

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frequency, coupling is less complete and less noise is transmitted to the

environment.

Fluid Temperature - The fluid temperature influences fluid density and therefore

fluid velocity.

Distance From Source - The measured SPL decreases as the point of measurement is moved further from the source.

Noise Prediction Equations

Differences In Nomenclature - The equations that have been developed by each

valve manufacturer and the equations that are endorsed in ISA/IEC standards

do not necessarily include terms for each of the influences previouslydescribed. Many noise prediction techniques account for multiple influenceswith a single term and minor influences are sometimes ignored. In addition,manufacturers often use entirely unique approaches that involve proprietary

coefficients that prevent the user from "reverse-engineering" the equations. Asa result, each manufacturer's prediction techniques are designed to provideaccurate results for that manufacturer's valves only.

Application - Generally speaking, several different methods may be used toapply the noise prediction techniques that have been developed by valve

manufacturers. Methods include:

• Direct calculation with the use of appropriate equations.

• Graphical methods in which one refers to a series of charts or tables to

determine the values of the various components of the total noise level;e.g., one may determine the SPL that is associated with the pressure drop,with the pressure ratio, with the downstream piping, etc. and, then, sum all

the components.

• Sizing software that calculates SPL levels at the same time the valve sizingequations are solved.

Of the three methods, the software approach is by far the most time-efficient

and the most preferred.

Fisher Noise Prediction Equations - Fisher Controls’ noise prediction equation is

as follows:

LpA = DLpADP+DLpACg+DLpADP/P1+DLpAK+DLpAP2 + ∆LpAM2

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Where:

LpA Sound pressure level (dBA)

DLpA∆P Base SPL as function of DP

DLpACg The correction for the required Cg

DLpA∆P/P1 The correction for the pressure drop ratio DP/P1abs

DLpAK The correction for the pipe size and schedule.

DLpAP2 The correction for valve outlet pressure, P2 (psig)

∆LpAM2 Correction to be usedonly when the valve outlet velocityis higher than the recommended outlet velocity

Valtek Noise Prediction Equations - Valtek’s noise prediction equation is as

follows:

SPL = Vs + Ps + Es + Ts + Gs + As

Where:

SPL Sound pressure level (dBA)

Vs flow factor - 6.95 Ln (Cv) + 4.8

Ps pressure factor - 9.03 Ln (P1) +17.2

Es pressure ratio factor - 30 Log (∆P/P1) + 25.24

Ts temperature correction factor - -7.68 Log (T1) + 20.78

Gs gas property correction factor - 7.26 Log (Mw) - 11

As pipewall attenuation factor - from Valtek Engineering Bltn. 3

Masoneilan Noise Prediction Equations - The equation that Masoneilan publishes

in the literature that address noise prediction is as follows:

SL = 10log [28CvCf P1P2D2ηT /t3] + SLg

Where:

SL Sound pressure level, dBA

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28 Units constant

Cv Actual required flow coefficient

Cf Critical flow factor (same as FL)

P1 Upstream pressure, psia

P2 Downstream pressure, psia

D2 Downstream nominal pipe diameter, inches

η Acoustical efficiency factor, dimensionless; determined graphicallyfrom a chart.

T Absolute temperature, degrees R

t Pipe wall thickness, inchesSLg Gas property factor, dBA; dimensionless; determined graphically

from a chart.

CCI Noise Prediction Equations - The equations that Masoneilan publishes in theliterature that address noise prediction are as follows:

Trim Noise (SPLt)

SPLt = dBw + dBp1 + dB 2 - dBp2 - dBnt - dBd2 - dBt2 - dBr + 63 - A

Pipe Noise (SPLp)

Inlet: SPLp1 = dBw + dB 1 + dBd1 - dBt1 - dBr - 104 - A

Outlet: SPLp2 = dBw + dB 2 + dBd2 - dBt2 - dBr - 104 - A

Where:

SPLt Total trim noise in dBA

dBw function of mass flow rate

dBp1 function of inlet pressure

dBρ2 function of outlet fluid density

dBp2 function of outlet pressure

dBd1 function of the inlet pipe I.D.

dBnt function of number of turns in the disk

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dBd2 function of the downstream pipeI.D.

dBt2 function of the downstream pipewall thickness

dBr function of near field distance

63 units constant

104 units constant

A thermal lagging; 5 dBA for steam and 0 dBA for no thermal lagging.

ISA Noise Prediction Equations - ISA Standard S75.17-1989 and Part 8, Section3 of IEC 65B/231/DIS describe a very thorough method for predicting the SPL

of a standard valve. The ISA/IEC noise prediction method requires one to

manually solve up to forty or more equations in order to calculate an estimatedSPL value and the method appliesonly to standard valves (those that do not

include quiet trim options). The technique involves the following major steps:

1. Determination of a regime. Every application will fall into a regime (Regime I

through Regime V) depending on the pressure conditions.

2. Determination of the acoustic efficiency factor for the regime. The acoustic

efficiency factor,η, is a measure of the flow stream energy that can be

converted to sound energy.

3. Calculation of the sound power level. The sound power level, Wa, iscalculated with the use of the acoustic efficiency factor and a correctedvalue that indicates the stream power.

4. Calculation of the valve internal sound pressure level, Lpi. Lpi is calculatedas a log function of a constant, the sound power (previously calculated), the

mass density, the speed of sound under downstream conditions, and theinside diameter of the downstream piping.

5. The transmission loss is calculated as a function of the pipe coincidentfrequency and the peak generated frequency of the control valve noise.

6. The final sound pressure level is calculated as a function of the internalsound pressure level that is corrected for transmission loss.

Because it is mathematically intensive, because its use is strictly limited to

outlet velocities that are equal to or less than 0.3 mach, and because it cannotbe used to predict the SPL of noise-abatement valves, specifiers rarely makeuse of the ISA/IEC equations. Specifiers do, on occasion, use the ISA/IEC

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equations to determine if the results that are obtained with a manufacturers

method are reasonably accurate.

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Control Valve Options For Attenuating Control Valve Noise

Source Treatments Vs. Path TreatmentsThe methods that are used to attenuate aerodynamic control valve noise can be categorized

as either source treatments or path treatments. Common source and path treatments are

shown in Figure 107 and they are discussed below.



Figure 107

Source Versus Path Treatments Of Aerodynamic Noise

Source Treatments - The term source treatment refers to the measures that aretaken to actually reduce the amount of noise that is generated by the valve.

Source treatments address the cause of the noise, rather than the symptom. As Illustrated in Figure 107, the most common source treatment is theselection of special control valve trim that is designed to reduce the level of

noise that is generated in the valve and propagated through the downstreampiping. Because source treatments address the problem rather than the

symptom, Saudi Aramco typically prefers source treatments over pathtreatments.

Path Treatments - The term path treatment refers to any measure that is takento prevent the noise that is generated within the valve and the piping fromreaching the environment. As shown in Figure 107, common examples of path

treatments include heavy-walled pipe, pipeline insulation, and equipment that isinserted into the pipeline that reduces the intensity of the sound that reachesthe environment.

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Valve Style Versus Noise Attenuation

Different valve styles (globe, angle, ball, butterfly) of the same capacity will produce

broadly differing levels of aerodynamic noise. When noise attenuation in a throttling

application is an objective, the selection of a globe or angle style valve is generally

recommended for the following reasons:

• Low efficiency valves (those with high K m or FL values) tend to limit the maximum

fluid velocity to a greater extent than high efficiency (ball and butterfly) valves.

• A broad range of special noise-abatement trims are routinely available for globe and

angle style valves whereas only a few noise-abatement options are available for rotary-

shaft (ball and butterfly) valves.

Body Options For Globe And Angle Valves

Valve bodies with enlarged flow areas and with expanded outlet connections are often usedto limit aerodynamic noise by reducing velocities. Refer to Figure 108. Enlarged bodies are

typically identified with a nomenclature such as “an 8x6 body”, indicating an 8-inch body

that includes 6-inch trim. A valve that is described as having a 6x8x8 body will have a 6-

inch inlet, an 8-inch nominal body size, and an 8-inch outlet connection.



Figure 108

Example Of An Expanded Flow Areas And Expanded Outlet Connection

Noise Abatement Trim Design Strategies

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Reducing Sound Power Levels - Most noise-abatement valve trims are designed

to separate the flowing fluid into many small flow streams. The division of the

large free jet into many small fluid streams reduces the scale of the shock cellshear and the intensity of the consequent noise. The strategy of breaking the

fluid stream into several small streams is effective because of the relationshipsbetween port area, sound power, and the sound pressure level. Theserelationships are illustrated in Figure 109.



Flow Flow

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For Flow

W A ∝ WB ∝ 8

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Sound 2 x WB,

LpA(A) LpA(B)

Figure 109

Capacity, Sound Power, And LpA For Single Port And Multi-Port Valve Trim

Minimizing Shock Cell Interaction - In order to preserve the benefits of breaking

the free jet (the flow stream that enters the valve) into many small streams, theindividual streams must not be allowed to recombine after exiting the trim.

Figure 110 shows that the streams grow and recombine as the pressure dropratio increases. If the streams recombine after exiting the trim, the noise levelswill increase. Figure 110 also shows two methods of preserving stream

separation.

• The use of smaller passages in the trim.

• Increased separation of the individual passages.

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Figure 110

Shock Cell Interaction

Shifting Frequencies - Another benefit of multiple-passage trims is that the

frequency of the sound that is generated by each small hole is much higher than the frequency of the sound that would be generated by a single large

passage. The frequency is often shifted to a frequency that is much greater than the pipe coincident frequency; therefore, much of total noise that is

generated in the valve does not couple to the pipewall and it is not radiated tothe environment. Refer to Figure 111.

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Figure 111

Valve Noise Frequency Versus The SPL That Is Transmitted To The

Environment

Commonly Available Noise Abatement Valve Options

Slotted Cages - Many manufacturers offer noise abatement trim that is based ona slotted cage design. Figure 112 shows a typical slotted cage that is similar tothe Fisher Whisper Trim I design. The slots separate the fluid stream and

reduce the amount of flow turbulence, thereby reducing the level of noise thatis generated as the fluid flows through the cage passages.

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Figure 112

Slotted Cage

Parallel Hole, Radial Flow Trims - Noise-abatement cages that are based on a

parallel hole, radial flow design are commonly available. Valtek’s MegaStreamtrim is shown in Figure 113 and Fisher Control’s Whisper Trim III cage design

is shown in Figure 114. To ensure that the small flow streams remainseparated as the fluid exits the cage, trim is typically available with varioushole sizes and hole spacing dimensions. In addition, the flow may be directed

through several stages of drilled hole components.

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Figure 113

Valtek’s MegaStream Cage Design



Figure 114

Fisher Control’s Whisper Trim III Cage Design

Tortuous Path Trims - The prime objective of noise-abatement trims that arebased on tortuous path designs is to introduce frictional losses that will reduce

the velocity of the fluid as it passes through the trim. Control Components’Drag trim is shown in Figure 115 and Valtek’s TigerTooth trim is shown inFigure 116.

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Figure 115

CCI’s Drag Trim Design



Figure 116

Valtek’s TigerTooth Trim

Axial Flow Trims - The axial flow valve and trim design that is shown in Figure117 includes a multiple-step plug and seat design. Because of the relatively

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large flow passages through the trim, this trim design is well suited to the

control of gasses that include entrained solids. This design is unique to

Masoneilan’s 77000 series LO-DB product.



Figure 117

Masoneilan’s 77000 Series LO-DB Trim Design

Characterizing Noise Abatement Trim

In many applications, excessive noise may be generated at one flow condition, moderate

noise may be generated at another flow condition, and, entirely acceptable levels of noise

may be generated at yet another flow condition. If a cage with small holes and wide hole

spacing is selected to ensure adequate noise attenuation at the worst-case flow condition, a

very large valve size may be required to achieve the needed flow capacity.

A characterized cage is often a viable option to the selection of a larger valve. As shown in

Figure 118, characterization is accomplished by designing a cage that provides theappropriate balance of noise attenuation and flow capacity that is needed over the rated

travel of the valve. For example, if considerable noise attenuation is needed at the minimum

flow condition, small, widely spaced holes may be located near the seat. If only moderate

noise attenuation is needed at mid-travel positions, larger holes with a wider spacing may

provide the needed noise attenuation while providing additional flow capacity. In some

applications, the chief concern at the maximum flow condition is flow capacity rather than

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noise attenuation. To achieve the needed capacity, the holes in the upper portion of the cage

may be large and closely spaced.

Characterized cages are designed for specific application requirements. To enable the designof an optimally characterized cage, specifiers must provide the valve manufacturer with the

pressure and flow conditions for as many operating points as possible



Figure 118

Characterized Noise Abatement Cage Design

Common Selection Problems And Specification Errors

Absence Of Industry Standards For Noise Prediction Equations

The prediction and abatement of control valve noise is an area for which there are few

universally accepted standards. As a result, specifiers must remain aware of the

ramifications of the broadly varying methods that are used to predict aerodynamic noise and

of the valve vendor’s interests in winning bid awards on the basis of low cost.

Vendor Tendency To Under-Predict Noise - Although most manufacturer’s noise

prediction techniques are based on sound engineering principles and up-to-date acoustic theories, it is only logical that any method of noise predictionwould include an accuracy limit of at least plus or minus 5 dBA. However, it is

in the manufacturer’s best interest to be as optimistic as possible; i.e.,manufacturers may take advantage of rounding, push the performance limits of specific valves, employ prediction techniques other than their published

techniques, or calculate the SPL at working distances rather than at standarddistances.

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Limited User Ability To Evaluate Submitted Bids - Most noise prediction

techniques include some factors for which the derivation is known only to the

manufacturer. As a result, there is no common basis for evaluating all thevarious prediction methods or the amount of noise attenuation that is provided

by various noise abatement trim options. Confidence in a particular manufacturer’s claims can only be gained through experience.

Specifier's Failure To Identify Worst Case Service Conditions

In order for vendors to submit bids for products that will provide the desired noise

attenuation, specifiers must provide complete and accurate data and they must fully

document the worst-case service conditions.

Worst-Case Scenarios - To ensure proper performance, specifiers must define all

worst case scenarios. Worst-case scenarios for noise generation include theconditions that occur during startup, during shutdown, during emergency

situations, and during periods of increased or decreased throughput.

Changes In Service Conditions

Because of changes in process design, changes in daily throughput, or

changes in fluid composition, the valve SPL can change dramatically.Whenever possible, specifiers should anticipate such changes in operatingconditions and provide all pertinent data.

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Work Aid 1: Fluid Compatibility Information That Is Used To S elect Control

Valves For Corrosive Fluid Applications

Work Aid 1A: NACE Compliant Materials Of Construction

Remarks/

A

p

p

l

i

c

a

t

io

n

s

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Requiress

tr ess

r elievi

ng

and

post-weld

heattr eatm

ent

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Requiresp

ost-weld

heat

tr eatment

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Post-weldh

eattr eatmentn

otr equir ed

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Cast formi

s

not N

ACE

app

r oved;ther ef o

r e,bodies

must

be

f or ged

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May beh

ar df aced

with

Alloy

6

f or

incr eased

dur a

bility

Excellentr esis

t

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Moderatei

ncr ease

in

ha

r dness

over

S31600,butless

r esistantto

g

e

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Highlye

r osion

r esist

ant

Highlyer osion

r esistantse

atr ings

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NACEa

ppr oved

butlo

w

str ength.May

r equir e

lar g

er

stem

diame

t

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Muchs

tr onger

than

S31600.

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NACEa

ppr oved.

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NACEC

lass

III-

Bol

ting

is

exposed

to

atmosphe

r e

and

ther ef

o

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NACEC

lass

II-

Bolt

ing

is

exposed

to

H

2S

becau

se

of insulati

o

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Commonm

ater ialf or

pr

essur e

r egulator

spr ings

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Bellevilles

pr ings

in

ext

er nally

loaded

packing

desi

gns

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Work Aid 1B: Recommended Materials Of Construction For Seawater And Brine

Services

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Work Aid 1C: Valve And Material Selection Guidelines For Amine (DGA) LetdownApplications

Refer to the following:

• The compatibility table (Table I) that is located in SAES-L-008.

• Fisher Controls PS Sheet 59:042(A) Application Guideline - Rich Amine Letdown Valve

(located in the Addendum of This Module).

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Work Aid 2: Hierarchical Listings Of Erosion Resistant Valve Styles And

Construction Materials

Work Aid 2A: Hierarchy Of Erosion Resistant Valve Styles That Is Used To Select

Control Valves For Erosive Fluid Applications

Valve

S

t

y

l

e

Comme

n

t

Cage-

gui

ded

va

lve

s

Potential

f or

th

e

p

lug

bi

ndi

ng

in

th

e

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c

age.

Sui

ta

bl

e

wh

en

the

vo

lum

e

r

at

io

of p

ar

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ti

cula

te

is

v

er y

low

or

wh

en

the

e

r os

ive

m

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ed

ia

i

s

a

f

la

shin

g

l

iqu

id

(wi

th

no

so

lid

s)

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.

Cage-gu

ide

d

a

ng

le

val

ves

Anglebo

dy

r ed

uc

es

bod

y

d

ama

ge.

Post-

gui

de

d

v

alv

e

Post

gui

di

ng

r ed

u

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s ce

s

pl

ug

bi

nd

ing.

Des

ign

s

w

ith

pr

ot

ect

ed

bu

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sh

ings

of

f e

r

incr

eas

ed

pr o

tec

tio

n.

Post-

Gui

ded

Angle

bod

y

m

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A

ngle

Va

lv

es

in

imiz

es

bo

dy

dam

age

.

Post-Gu

ide

d

A

ng

l

e

Val

ves

Liner r e

duc

es

er

o

si

on

to

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w

ith

har

de

ne

d,r e

pla

cea

ble

ou

tle

tl

ine

r

va

lve

out

le

ta

nd

d

own

str

eam

pi

pin

g.

EccentricRo

Straightth

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ta

r y

P

lug

V

al

ves

r o

ugh

f lo

w

pa

th

m

ini

miz

es

imp

ing

em

ent

on

cr

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it

ical

par

ts

.R

atin

gs

lim

ite

d

t

o

A

NS

ICl

ass

6

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00

.

Sweep

Flo

w

(

Ve

ntu

r iS

tyl

e)

Ang

le

Va

l

ve

s

Very

r ug

ged

c

ons

tr u

cti

on.

Swe

ep

f l

o

w

des

ign

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d

ir ec

ts

f l

ow

awa

y

f

r om

cr

iti

cal

su

r f a

ces

.R

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at

ings

to

A

NS

ICla

ss

900

.

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A standardm

ater ial.May

b

e

selected

f or mildly

er osiv

e

application

s

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Muchg

r eater er osion

r esistance

than

car bon

ste

el

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Superior e

r osion

r esist

ance

in

f lashing

applicatio

ns

Hierarchy Of Erosion Resistant Trim Materials

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Remarks

Good corrosionresistancebut,in itsbasic

form,offer slittleerosionresistance.

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Typically heat-treat

ed toHRC38.Gooderosionresistance butlacksgene

ralcorr osionresistance.

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Typically heat-treat

edusingH1075(HRC32)for standardservice

andwithH1150(HRC33)for NACE.Goodstrength,hardness, anderosionresistance.

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Hardfacing onplug

tips,plugguidingsurfaces,andseatringsprovidesexcellent

resistance toerosionandcorr osion.

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Can behard

enedto56-60HRC.Veryhardanderosionresistant

innon-corr osiveapplications.suchasboiler feedwater andsteam.Verysusceptible toSCC.

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Very toughmate

rialwithsuperior erosionresistance.Corr odesrapidly in

thepresenceof someboiler feedwater corr osioninhibitors(hydrazines).

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Superior erosi

onandwear resistance;however,thebindersthathold

thetungstencarbidearesusceptible tocorr osion insomeapplicationsincludinghydr azine-treatedboiler feed

water andammonia.

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Work Aid 3: Procedures That Are Used To Select Control Valve Options

For High Temperature Fluid Applications

Body And Bonnet Material Selection

Refer to any source of ANSI pressure/temperature ratings for bodies and bonnets. One

source is Fisher Specification Bulletin 59.1:021.

Trim Material Selection

Refer to the appropriate specification bulletin and locate the manufacturers

recommendations for trim packages that are compatible with the body and bonnet material.

Select trim that is rated for the maximum operating temperature. Also ensure that the trim

will provide the necessary resistance to corrosion and erosion.

Gasket Material SelectionRefer to the manufacturers temperature ratings for both flat sheet gaskets and spiral wound

gaskets.

Packing Material Selection

Refer to Section 4.1.5 of SAES-J-700 for packing material guidelines. Standard PTFE is to

be selected for temperatures up to 400 degrees F. Above 400 degrees F, graphite packing

materials are to be selected.

Bonnet Type Selection

Refer to Section 4.1.5 of SAES-J-700 for bonnet selection guidelines. At temperatures above

400 degrees F, extended bonnets are to be considered.

Thermal Cycling Considerations

Observe all notes in the manufacturer’s product literature. In general, the following are to be

avoided:

• Threaded bonnets

• Threaded seat rings

In addition, the materials of construction of spiral wound gaskets should be closely

evaluated.

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Work Aid 4: Procedures That Are Used To Select And Size Control Valves

For Cavitating Fluid Applications

Work Aid 4A: Procedures That Are Used To Perform Basic Selection And Sizing With

The Use Of Control Component’s Inc. Sizing Software

Preliminary Entries

1. Change to the appropriate directory. The default is c:\cci.

2. Launch the program by typing the name of the executive file: VALSIZ.

3. When prompted to select a method of data input option, select Enter New Data.

4. When prompted for a run descriptor, enter any appropriate name.

5. When prompted to select a valve style, select the desired valve type.

6. When prompted to enter a pipe geometry factor option, select Computer To Calculate.

Enter the upstream pipe size. Enter the downstream pipe size.7. When prompted to select a nominal valve size option, select Computer To Calculate.

8. When prompted to select a noise option, select Noise Level Not Calculated.

9. When prompted to select a percent over-capacity margin, enter 10 percent.

10. When prompted, enter the number of flow conditions.

Entering Fluid Properties And Service Conditions

Note: The procedures in this section will be repeated for each flow condition.

1. When prompted, enter the inlet pressure and select the appropriate units.

2. When prompted, enter the outlet pressure and select the appropriate units.

3. When prompted, select the fluid type.

4. When prompted, enter the fluid temperature, select the appropriate units, and very thefluid state.

5. When prompted, select either volumetric or mass flow units. Enter the flow rate and

select the units for the flow rate.

Repeat items 1 through 5 immediately above for each flow condition.

Design Information

1. When prompted, enter the design pressure (the shutoff pressure) and select the

appropriate units.

2. When prompted, enter the design temperature (a temperature that will provide some

safety margin; e.g., a temperature that is 25 percent higher than the normal operating

temperature. Select the appropriate units.

Change Menu

The change menu displays all the information that has been entered. As the change menu

screens are displayed for review, the specifier may select entries to change by placing the

cursor on the entry to be changed and, then, pressing the space bar. When all of the change

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menu screens have been displayed, the items that have been selected will be listed and they

can be changed at that time.

Calculation Results

1. After all changes have been made, several screens display the results of the sizing

calculations.

2. The first screen shows the valve Cv that is required. Other valve indices and ANSI Class

body rating information is also displayed.

3. The screen that is titled “Trim Exit Velocity Analysis” lists the trim outlet velocities for

various trims. The trim that will provide an outlet velocity that is less than 100 feet per

second will be indicated by color coding.

4. The next several screens display the results of the calculations for each of the service

conditions. The number of turns that are required to prevent cavitation damage at that

flow condition are shown on each screen.5. When prompted to view application information, select NO.

6. When prompted to select an option to proceed, select the appropriate response.

Work Aid 4B: Procedures That Are Used To Perform Basic Selection And Sizing With

The Use Of Valtek’s Sizing Software

Preliminary Entries

1. Change to the appropriate directory. The default is c:\valtek.

2. Launch the program by typing the name of the executive file: QQ.

3. When prompted, select Valve Sizing.

Project Identification

The entries in the boxed area in the upper right corner of the screen identify the project.

They are optional entries. For the purpose of these exercises, press the cursor down arrow

until the cursor is on the first entry field of the boxed area on the left hand side of the screen.

Valve Selection

To identify the selected valve style and options, move the cursor with the use of the up

arrow and the down arrow. For each entry field, a sub-menu will appear on the screen.

Select the option that is desired by typing the number that precedes the option.

Valve Sizing

1. Press either the F2 key or the Page Down key to display the valve sizing.2. Enter the appropriate values in all the entry fields that are highlighted. Entries are not

required for the fields that are titled Required Cv.

3. To select the fluid, move the cursor to the entry field that is titled “Fluid” and, then, press

the space bar. Select the appropriate fluid from the list.

4. Press the F3 key to calculate the valve size information. A description of the selected

valve is displayed in the lower left hand corner of the screen.

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To exit the program, press the F10 key several times and follow the instructions that are

given in the prompt.

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Work Aid 4C: Procedures That Are Used To Perform Basic Selection And Sizing With

The Use Of Fisher Control’s Sizing Program

Preliminary Entries

1. Change to the appropriate directory. The default is c:\fsp.

2. Launch the program by typing the name of the executive file: FSP. When the title screen

appears, press the Enter key.

3. From the menu that appears, select Valve.

4. From the menu that appears, select Fisher Water.

5. From the menu that appears, select Valve Sizing and LpA.

Setting Options

Press the F3 key and ensure that the options are set as follows:

Solve for Cg, Cs, or Cv.

LpA (SPL) OFF

Cavitation Check ON

Calculate SG

Pipe Size/Sched

Warnings ON

To change an option, place the cursor on the option and press enter. When all options have

been set, press the ESCAPE key to return to the program.

Data Entry And Sizing Calculations

1. Enter the appropriate data in the Service Conditions portion of the screen.

2. Enter the appropriate data in the Valve Specifications portion of the screen.

To determine the value of K m for the initially selected valve, locate the Fisher Catalog

10 page for the initially selected valve. Browse through the K m values that are listed and

select a typical value. Enter this value.

To determine the value of K c for the initially selected trim, refer to the Help Screens by

performing the following procedures:

Press the F1 key twice to view an index of Help Screens.

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Press the "K" key to navigate to the topics that begin with the letter 'K'.

Select "K c Table" from the index of topics

Press the PAGE DOWN key until the Help Screens for the selected valve is displayed.

Determine the value of K c.

Press the Escape key to return to the sizing screen.

Enter the value of K c.

3. Press the F2 key to calculate and display the valve sizing information.

4. To enter data for the minimum flow condition and for the maximum flow conditions, the

data that has been entered for the normal flow condition can be copied. To copy data

from the normal flow screen to the minimum or maximum flow screens, perform thefollowing:

• Press the ESCAPE key.

• With the use of the left arrow key or the right arrow key, move the cursor to the

condition to which values are to be copied.

• Press and hold the ALT key, and, then, press the C key.

• Enter the number of the flow condition that is to be copied to the selected flow

condition.

• To copy the information to the new condition and to view the calculation

screen, press the ENTER key.

• Change the sizing inputs that are different for this flow condition

(P1, dP, and Q).

• Press the F2 key to calculate the sizing information.

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Display The Calculated Results

Press the F9 key to display a table of calculated values. Ensure that the valve K c is greater

than the value of Ar . If the value of Ar is greater than the K c of the selected valve, select avalve trim with a higher value of K c and repeat the sizing procedures.

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Work Aid 5: Guidelines For Valve Style And Material Selection And

Procedures That Are Used To Size Control Valves For

Flashing Fluid Applications

Work Aid 5A: Procedures That Are Used To Size Control Valves For Flashing Fluid

Applications

1. Ensure the values that are given as the fluid properties and the service conditions are

accurate. A slight error in these values can cause an application to be erroneously

interpreted as flashing, cavitating, or neither flashing or cavitating.

2. Because flashing is usually accompanied by choked flow, the valve sizing pressure drop

must be limited to the lesser of the ∆Pactual or the ∆Pchoked . The equation for calculating

choked flow is:

∆Pallow = FL2(P1-r cPv)

where:

∆Pallow the maximum pressure drop that is effective in producing flow

FL ISA nomenclature for the control valve recovery coefficient.Fisher nomenclature is Km where Km = FL2.

P1 Upstream fluid pressure.

r c The critical pressure ratio; 0.96-0.28(Pv/Pc) where Pv is the

fluid’s vapor pressure and Pc is the fluid’s critical pressure.

Pv The fluid’s vapor pressure.

Work Aid 5B: Guidelines For Valve Style And Material Selection That Are Used To

Select Control Valves For Flashing Fluid Applications

Valve Style Selection Guidelines

1. For flashing fluid applications, specifiers should select control valve styles according to

same guidelines that are applied to erosive flows. Refer to Work Aid 2A of this Module.

2. Because flashing tends to occur downstream of the control valve, specifiers should

consider the use of outlet liners.

3. If flashing and cavitation can occur in the same valve, specifiers should avoid multi-

stage anti-cavitation trim if possible. Interstage flashing can damage multi-stage anti-

cavitation trim.

Body and Trim Material Selection Guidelines

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1. For flashing fluid applications, specifiers should select control valve materials of

construction according to same guidelines that are applied to erosive flows. Refer to

Work Aid 2B of this Module.2. Materials of construction must also be selected on the basis of their compatibility with

the process fluid (corrosion resistance) and on the basis of their temperature ratings.Other Considerations

1. If outlet liners are not available, a spool piece of heavy, sacrificial piping can be installed

downstream of the control valve.

2. In some instances, flashing can be avoided by changing the system design parameters.

Any change in system design that will help to maintain the value of P vc above the value

of Pv should be pursued.

3. If the valve discharges to a tank or vessel, it may be possible to mount the valve directly

on the tank and direct the flashing into the vessel where it will not cause damage.

Specifiers should consult with system design personnel to ensure the feasibility of thisapproach.

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Work Aid 6: Procedures That Are Used To Select And Size Control Valves

To Attenuate Aerodynamic Control Valve Noise

Work Aid 6A: Procedures That Are Used To Select And Size Noise Attenuating Control

Valves With The Fisher Sizing Program

Preliminary Entries

1. Change to the appropriate directory. The default is c:\fsp.

2. Launch the program by typing the name of the executive file: FSP. When the title screen

appears, press the Enter key.

3. From the menu that appears, select Valve.

4. From the menu that appears, select Fisher Vapor.

5. From the menu that appears, select Valve Sizing and LpA.

Setting Options

Press the F3 key and ensure that the options are set as follows:

Solve for Cg, Cs, or Cv.

LpA (SPL) ON

Pipe Size/Sched

Warnings ON

Diffuser: Manual Sizing

To change an option, place the cursor on the option and press enter. When all options have

been set, press the ESCAPE key to return to the program.

Data Entry And Sizing Calculations

1. Enter the appropriate data in the Service Conditions portion of the screen.

2. Under the heading Valve Specifications, enter an estimated value of C1 for the selected

valve type. An estimated value of C1 may be determined by browsing through the C1

column on the Fisher Catalog 10 page for the selected valve and identifying a value

of C1 that is typical for the type and size of the selected valve. Alternatively, manyspecifiers perform initial sizing with the C1 values that are listed below.

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Typical C1 Values

That Are

Used For

Initial Sizing

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3. With the cursor on Valve Type, press the F4 key and select the valve type from the

drop down menu.

4. Enter the pipe size and schedule.

5. Press the F2 key to calculate the valve sizing and noise prediction information.

6. To enter data for the minimum flow condition and for the maximum flow conditions,

the data that has been entered for the normal flow condition can be copied. To copy

data from the normal flow screen to the minimum or maximum flow screens, performthe following:

a. Press the ESCAPE key.

b. With the use of the left arrow key or the right arrow key, move the cursor

to the condition to which values are to be copied.

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c. Press and hold the ALT key, and, then, press the C key.

d. Enter the number of the flow condition that is to be copied to theselected flow condition.

e. To copy the information to the new condition and to view the calculation

screen, press the ENTER key.

f. Change the sizing inputs that are different for this flow condition

(P1, dP, and Q).

g. Press the F2 key to calculate the sizing information.Display The Calculated Results

Press the F9 key to display a table of calculated values.

Work Aid 6B: Procedures That Are Used To Select And Size Noise Attenuating Control

Valves With Control Components Sizing Software

Preliminary Entries

1. Change to the appropriate directory. The default is c:\cci.

2. Launch the program by typing the name of the executive file: VALSIZ.

3. When prompted to select a method of data input option, select Enter New Data.

4. When prompted for a run descriptor, enter any appropriate name.

5. When prompted to select a valve style, select the desired valve type.

6. When prompted to enter a pipe geometry factor option, select Computer To Calculate.Enter the upstream pipe size. Enter the downstream pipe size.

7. When prompted to select a nominal valve size option, select Computer To Calculate.

8. When prompted to select a noise option, select User To Select Downstream Pipe.

9. When prompted to select a percent over-capacity margin, enter 10 percent.

10. When prompted, enter the number of flow conditions.

Entering Fluid Properties And Service Conditions

Note: The procedures in this section will be repeated for each flow condition.

When prompted, each of the values that is requested.

Design Information1. When prompted, enter the design pressure (the shutoff pressure) and select the

appropriate units.

2. When prompted, enter the design temperature (a temperature that will provide some

safety margin; e.g., a temperature that is 25 percent higher than the normal operating

temperature. Select the appropriate units.

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Change Menu

The change menu displays all the information that has been entered. As the change menu

screens are displayed for review, the specifier may select entries to change by placing the

cursor on the entry to be changed and, then, pressing the space bar. When all of the change

menu screens have been displayed, the items that have been selected will be listed and they

can be changed at that time.

Calculation Results

1. After all changes have been made, several screens display the results of the sizing

calculations. While viewing these screens, record the pertinent data.

2. When prompted to view application information, select NO.

3. When prompted to select an option to proceed, select the appropriate response.

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GLOSSARY

∆∆Pcav

The pressure drop at which a particular valve will become

susceptible to cavitation damage.

A-weighting The adjustment to a sound pressure measurement that

compensates for the frequency sensitivity of the human ear.

aerodynamic noise The noise that is associated with high speed, turbulent gas

flows.

ambient noise The background sound pressure level of a given environment.

application ratio (Ar) The ratio of the system pressure drop to the pressure

differential between P1 and Pv that is used to provide an index

of the susceptibility of a system to cavitate.

austenitic A family of stainless steels that include 18 percent chromium

and 8 percent nickel.cavitation In liquid service, the noisy and potentially damaging

phenomenon that accompanies vapor bubble formation and

collapse in the flowstream. Cavitation is most commonly

encountered in high pressure and high pressure drop services.

creep The loss of elasticity that occurs over time at elevated

temperatures.

crevice corrosion Corrosion that occurs in areas where access to oxygen is

restricted.

Cv see flow coefficient

dB see decibel

dBA A-weighted decibeldecibel A unit that expresses the ratio of two sound pressure levels;

i.e., 1 dB = 20 log10 Ps/Po, where Ps is the measured sound

pressure and Po is a reference pressure.

diffuser A noise abatement device that is essentially a downstream,

fixed restriction, the purpose of which is to reduce the pressure

drop across both the valve and the diffuser to reduce

aerodynamic noise.

dynamic unbalance The net force produced on the valve stem in any given open

position by the fluid pressure acting on the closure member

and stem within the pressure retaining boundary, with the

closure member at a stated opening and with stated flowingconditions.

elasticity The ability of a material to return to its initial form after being

exposed to stress.

erosion The damage that results from the impingement of particles or

vapor droplets on critical valve surfaces.

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erosion The damaging effects of flashing or abrasive media impinging

on component surfaces. Erosion may be forestalled with

hardened materials or with valve designs that separate theflowstream from critical valve components.

erosion corrosion A form of corrosion that occurs when erosive particles erode

the protective passive layer and the base material is attacked

by the environment.

extension bonnet A bonnet with a packing box that is extended above the bonnet

joint of the valve body so as to maintain the temperature of the

packing above or below the temperature of the process fluid.

flashing Phenomenon observed in liquid service when the pressure of

the fluid falls below its vapor pressure and when it does not

recover to a pressure above the vapor pressure.

flow characteristic Indefinite term, see inherent flow characteristic and installed flow characteristic

flow coefficient A constant (Cv), related to the geometry of a valve, for a given

valve opening, that can be used to predict flow rate. See

ANSI/ISA S75.01 "Control Valve Sizing Equations" and

ANSI/ISA S75.02 "Control Valve Capacity Test Procedure".

(The number of U.S. gallons of water at 60 degree F that will

flow through a valve with a one pound per square inch

pressure drop in one minute.)

fluid Substance in a liquid, gas, or vapor state.

frequency spectrum A plot of sound pressure level versus frequency.

Hertz The measure of frequency, or cycles per second.

high-recovery valve A valve design that, due to streamlined internal contours and

minimal flow turbulence, dissipates relatively little flow-stream

energy.

hydrodynamic noise The noise that is associated with cavitation. It sounds like

gravel flowing through the valve and associated piping.

IEC International Electrotechnical Commission

incipient cavitation The onset of cavitation, observed when the first vapor cavities

begin to form in the liquid stream.

inherent flow

characteristic

The relationship between the flow rate through a valve and the

travel of the closure member as the closure member is movedfrom the closed position to rated travel with constant pressure

drop across the valve.

installed flow

characteristic

The relationship between the flow rate through a valve and the

travel of the closure member as the closure member is moved

from the closed position to rated travel when the pressure drop

across the valve varies as influenced by the system in which

the valve is installed.

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intergranular corrosion A form of corrosion that occurs along the grain boundaries of a

material as a result of sensitization.

ISA Instrument Society of America.K c Control valve damage index that is used to describe a control

valve's relative susceptibility (due to its pressure recovery

characteristics and its materials of construction) to cavitation

damage.

K m The pressure recovery coefficient for a control valve. K m is

determined by valve manufacturers and published in sizing

catalogs. K m is used to calculate the ∆Pallow (choked flow

pressure drop) for valve sizing purposes. The value of K m may

also be used to predict cavitation damage.

line source A noise source from which equal noise levels are measured on

an imaginary cylinder with the line source as the axis of the

imaginary cylinder. A pipeline is a typical line source.

low-recovery valve A valve design that dissipates, due to the turbulence that is

created by the contours of the flow path, a considerable

amount of flowstream energy.

LpA An A-weighted sound pressure level; see sound pressure.

mach number The ratio of the fluid speed to the speed of sound in the fluid at

the local conditions.

martensitic A family of stainless steels that includes 12 percent chromium.

microjets Microscopic, high velocity fluid streams produced as a result

of vapor bubble collapse in cavitating liquids.

Micropascal A unit of pressure measurement for very small pressures. One

micropascal is equal to 10-6 Newton/m2.

NACE National Association of Corrosion Engineers.

noise Any sound that is considered unpleasant or unwanted. The

sound that is generated by the fluid leaving the control valve is

considered noise because of its intensity and because of its

high-frequency, broad-band spectrum.

octave band One of the established frequency groupings in which the

highest frequency in the grouping is twice the lowest (such as

the band 2000 to 4000 Hertz). Frequencies are grouped so that

filters can be constructed to measure the sound pressure levelover the bandwidth.

outgassing The action of dissolved gasses coming out of solution as a

result of pressure reduction or agitation.

passive layer A naturally occurring deposit of tough, adherent oxides that

form on the surface of a material.

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point source A noise source from which equal noise levels are measured on

an imaginary sphere, the center of which is the point source. A

vent is a typical point source.Pv The vapor pressure of a fluid.

Pvc Pressure at the vena contracta.

rebound The successive collapse, regrowth, and collapse of vapor

bubbles in a cavitating liquid.

recovery A relative term that describes the difference in pressure

between the valve vena contracta and the downstream system.

restricted trim* Control valve trim which has a flow are less than the full flow

area for that valve.

SCC Stress corrosion cracking.

sensitization A process in which exposure to high temperature causes

corrosion resistant alloys to precipitate out of the materialmatrix, leaving a zone at the grain boundary that is not

protected from corrosion attack.

silencer A device that removes acoustic energy from the flow stream.

There are two methods of silencer construction. The

dissipative or packed silencer removes the acoustic energy by

dissipating it into heat in the sound absorbing material lining

the structure. The reactive or packless silencer provides an

impedance mismatch to the acoustic energy such that the

acoustic energy is reflected back to the source and prevented

from traveling downstream.

sound An auditory sensation that is caused by pressure oscillations in

the ambient atmosphere due to the vibration that is created in

an elastic medium by a change in pressure, stress, or

displacement.

sound intensity The average rate of sound power that is transmitted in a

specified direction through a unit area.

sound level meter An instrument that includes a microphone, an amplifier, an

output meter, and usually frequency weighting networks for

the measurement of sound pressure.

sound power The measurement of total sound energy per unit of time that

radiates from a source. No meters are available to directlymeasure sound power.

sound pressure The force per unit area that is caused by a sound wave.

source The media where vibration is created due to a change in its

pressure, stress, or displacement.

SPL Sound pressure level, generally expressed in terms of dB or

dBA. SPL is being replaced by the term LpA.

SSC Sulfide stress cracking

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thermal cycling Descriptive of a process in which the operating temperature

repeatedly cycles over an arbitrary but broad range of

temperatures.trim The internal parts of a valve which are in flowing contact with

the controlled fluid.

trim, anti-cavitation Trim that is specifically designed to eliminate or reduce

cavitation and cavitation damage in a control valve. Common

designs stage the total pressure drop across one or several

specially designed restrictions.

vapor pressure (Pv) The pressure at which a given liquid begins to vaporize, given

a constant temperature.

vena contracta The location where the cross-sectional area of the flowstream

is at its minimum size, where fluid velocity is at its maximum

value, and where local fluid pressure is at its lowest value. Thevena contracta normally occurs downstream of the actual

physical restriction in a control valve.

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