Control valves.docx

7/27/2019 Control valves.docx

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Two-port valves

Globe valves

Globe valves are frequently used for control applications becauseof their suitability for throttling flow and the ease with which theycan be given a specific 'characteristic', relating valve opening toflow.

Two typical globe valve types are shown in Figure 6.1.1. Anactuator coupled to the valve spindle would provide valvemovement.

Fig.

6.1.1 Two differently shaped globe valves

The major constituent parts of globe valves are:

The body.

The bonnet.

The valve seat and valve plug, or trim.

The valve spindle (which connects to the actuator). The sealing arrangement between the valve stem and the

bonnet.

Figure 6.1.2 is a diagrammatic representation of a single seattwo-port globe valve. In this case the fluid flow is pushing againstthe valve plug and tending to keep the plug off the valve seat.


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Fig. 6.1.2 Flow

through a single seat, two-port globe valve

The difference in pressure upstream (P1) and downstream (P2) ofthe valve, against which the valve must close, is known as thedifferential pressure (DP). The maximum differential pressureagainst which a valve can close will depend upon the size and

type of valve and the actuator operating it.

In broad terms, the force required from the actuator may bedetermined using Equation 6.1.1.

Equation 6.1.1

Where:A = Valve seating area (m

2)

P = Differential pressure (kPa)F = Closing force required (kN)

In a steam system, the maximum differential pressure is usuallyassumed to be the same as the upstream absolute pressure. Thisallows for possible vacuum conditions downstream of the valve


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when the valve closes. The differential pressure in a closed watersystem is the maximum pump differential head.

If a larger valve, having a larger orifice, is used to pass greater

volumes of the medium, then the force that the actuator mustdevelop in order to close the valve will also increase. Where verylarge capacities must be passed using large valves, or where veryhigh differential pressures exist, the point will be reached where itbecomes impractical to provide sufficient force to close aconventional single seat valve. In such circumstances, thetraditional solution to this problem is the double seat two-portvalve.

As the name implies, the double seat valve has two valve plugson a common spindle, with two valve seats. Not only can thevalve seats be kept smaller (since there are two of them) but also,as can be seen in Figure 6.1.3, the forces are partially balanced.This means that although the differential pressure is trying to keepthe top valve plug off its seat (as with a single seat valve) it is alsotrying to push down and close the lower valve plug.


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Fig. 6.1.3

Flow through a double seat, two-port valve

However, a potential problem exists with any double seat valve.Because of manufacturing tolerances and differing coefficients of

expansion, few double seat valves can be guaranteed to givegood shut-off tightness.

Shut-off tightnessControl valve leakage is classified with respect to how much thevalve will leak when fully closed. The leakage rate across astandard double seat valve is at best Class III, (a leakage of 0.1%of full flow) which may be too much to make it suitable for certainapplications. Consequently, because the flow paths through thetwo-ports are different, the forces may not remain in balancewhen the valve opens.

Various international standards exist that formalise leakage rates


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in control valves. The following leakage rates are taken from theBritish Standard BS 5793 Part 4 (IEC 60534-4). For anunbalanced standard single seat valve, the leakage rate willnormally be Class IV, (0.01% of full flow), although it is possible to

obtain Class V, (1.8 x 10-5 x differential pressure (bar) x seatdiameter (mm). Generally, the lower the leakage rate the morethe cost.

Balanced single seat valvesBecause of the leakage problem associated with double seatvalves, when a tight shut-off is required a single seat valve should

be specified. The forces required to shut a single seat globe valveincrease considerably with valve size. Some valves are designedwith a balancing mechanism to reduce the closing forcenecessary, especially on valves operating with large differentialpressures. In a piston-balanced valve, some of the upstream fluidpressure is transmitted via internal pathways into a space abovethe valve plug, which acts as a pressure balancing chamber. Thepressure contained in this chamber provides a downforce on thevalve plug as shown in Figure 6.1.4, balancing the upstreampressure and assisting the normal force exerted by the actuator,to close the valve.


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Fig.

6.1.4 A steam control valve with piston balancing

Slide valves, spindle operatedSlide valves tend to come in two different designs; wedge gatetype and parallel slide type. Both types are well suited for isolating

fluid flow, as they give a tight shut-off and, when open, thepressure drop across them is very small. Both types are used asmanually operated valves, but if automatic actuation is required,the parallel slide valve is usually chosen, whether for isolation orcontrol. Typical valves are shown in Figure 6.1.5.


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Fig.6.1.5 Wedge gate valve and parallel slide valve (manual

operation)

The parallel slide valve closes by means of two spring loadedsliding disks (springs not shown), which pass across the flow-pathof the fluid, the fluid pressure ensuring a tight joint between thedownstream disk and its seat. Large size parallel slide valves areused in main steam and feedlines in the power and processindustries to isolate sections of the plant. Small-bore parallelslides are also used for the control of ancillary steam and waterservices although, mainly due to cost, these tasks are oftencarried out using actuated ball valves and piston type valves.

Top

Rotary type valves

Rotary type valves, often called quarter-turn valves, include plugvalves, ball valves and butterfly valves. All require a rotary motionto open and close, and can easily be fitted with actuators.

Eccentric plug valves
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Figure 6.1.6 shows a typical eccentric plug valve. These valvesare normally installed with the plug spindle horizontal as shown,and the attached actuator situated alongside the valve.

Plug valves may include linkages between the plug and actuatorto improve the leverage and closing force, and special positionersthat modify the inherent valve characteristic to a more usefulequal percentage characteristic (valve characteristics arediscussed in Tutorial 6.5).

Fig. 6.1.6 Side

view of an eccentric plug valve (shown in a partially open

position)

Ball valvesFigure 6.1.7 shows a ball valve consisting of a spherical balllocated between two sealing rings in a simple body form. The ballhas a hole allowing fluid to pass through. When aligned with thepipe ends, this gives either full bore or nearly full bore flow withvery little pressure drop. Rotating the ball through 90 opens andcloses the flow passage. Ball valves designed specifically forcontrol purposes will have characterized balls or seats, to give apredictable flow pattern.


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Fig. 6.1.7 Ball valve

(shown in a fully open position)

Ball valves are an economic means of providing control with tight

shut-off for many fluids including steam at temperatures up to250C (38 bar g, saturated steam). Above this temperature,special seat materials or metal-to-metal seatings are necessary,which can be expensive. Ball valves are easily actuated and oftenused for remote isolation and control. For critical controlapplications, segmented balls and balls with specially shapedholes are available to provide different flow characteristics.

Butterfly valvesFigure 6.1.8 is a simple schematic diagram of a butterfly valve,which consists of a disc rotating in trunnion bearings. In the openposition the disc is parallel to the pipe wall, allowing full flow


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through the valve. In the closed position it is rotated against aseat, and perpendicular to the pipe wall.

Fig. 6.1.8 Butterfly

valve (shown in its open position)

Traditionally, butterfly valves were limited to low pressures andtemperatures, due to the inherent limitations of the soft seatsused. Currently, valves with higher temperature seats or highquality and specially machined metal-to-metal seats are availableto overcome these drawbacks. Standard butterfly valves are nowused in simple control applications, particularly in larger sizes andwhere limited turndown is required.

Special butterfly valves are available for more demanding duties.


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A fluid flowing through a butterfly valve creates a low pressuredrop, in that the valve presents little resistance to flow when open.In general however, their differential pressure limits are lower than

those for globe valves. Ball valves are similar except that, due totheir different sealing arrangements, they can operate againsthigher differential pressures than equivalent butterfly valves.

OptionsThere are always a number of options to consider when choosinga control valve. For globe valves, these include a choice of

spindle gland packing material and gland packing configurations,which are designed to make the valve suitable for use on highertemperatures or for different fluids. Some examples of these canbe seen in the simple schematic diagrams in Figure 6.1.9. It isworth noting that certain types of gland packing produce a greaterfriction with the valve spindle than others. For example, thetraditional stuffing box type of packing will create greater frictionthan the PTFE spring-loaded chevron type or bellows sealed type.Greater friction requires a higher actuator force and will have anincreased propensity for haphazard movement.

Spring-loaded packing re-adjusts itself as it wears. This reducesthe need for regular manual maintenance. Bellows sealed valvesare the most expensive of these three types, but provide minimalfriction with the best stem sealing mechanism. As can be seen inFigure 6.1.9, bellows sealed valves usually have another set oftraditional packing at the top of the valve spindle housing. This will

act as a final defence against any chance of leaking through thespindle to atmosphere.


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Fig.

6.1.9 Alternative gland packings

Valves also have different ways of guiding the valve plug insidethe body. One common guidance method, as depicted in Figure6.1.10, is the 'double guided' method, where the spindle is guidedat both the top and the bottom of its length. Another type is the'guided plug' method where the plug may be guided by a cage ora frame. Some valves can employ perforated plugs, whichcombine plug guidance and noise reduction.

Fig. 6.1.10 Guiding arrangements

Summary of two-port valves used for automatic controlBy far the most widely used valve type for the automatic control of


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steam processes and applications is the globe valve. It isrelatively easy to actuate, it is versatile, and has inherentcharacteristics well suited to the automatic control needs ofsteam.

It should also be said that two-port automatic control valves arealso used within liquid systems, such as low, medium and hightemperature hot water systems, and thermal oil systems. Liquidsystems carry an inherent need to be balanced with regard tomass flows. In many instances, systems are designed where two-port valves can be used without destroying the balance ofdistribution networks.

However, when two-port valves cannot be used on a liquidsystem, three-port valves are installed, which inherently maintaina balance across the distribution system, by acting in a divertingor mixing fashion.

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Three-port valves

Three-port valves can be used for either mixing or divertingservice depending upon the plug and seat arrangement inside thevalve. A simple definition of each function is shown in Figure6.1.11.
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Fig.

6.1.11 Three-port valve definition

There are three basic types of three-port valve:

Piston valve type.

Globe plug type. Rotating shoe type.

Piston valvesThis type of valve has a hollow piston, (Figure 6.1.12), which is


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moved up and down by the actuator, covering andcorrespondingly uncovering the two-ports A and B. Port A andport B have the same overall fluid transit area and, at any time,the cumulative cross-sectional area of both is always equal. For

instance, if port A is 30% open, port B is 70% open, and viceversa. This type of valve is inherently balanced and is powered bya self-acting control system. Note: The porting configuration maydiffer between manufacturers.

Globe type three-port valves (also called 'lift and lay')Here, the actuator pushes a disc or pair of valve plugs betweentwo seats (Figure 6.1.13), increasing or decreasing the flow

through ports A and B in a corresponding manner.

Fig.

6.1.13 Globe type three-port valves

Note: A linear characteristic is achieved by profiling the plug skirt(see Figure 6.1.14).


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Fig. 6.1.14 Plug

skirt modified to give a linear characteristic

Rotating shoe three-port valveThis type of valve employs a rotating shoe, which shuttles acrossthe port faces. The schematic arrangement in Figure 6.1.15illustrates a mixing application with approximately 80% flowingthrough port A and 20% through port B, 100% to exit through port

AB.

Fig. 6.1.15 Rotating shoe on

a mixing application

Using three-port valvesNot all types can be used for both mixing and diverting service.Figure 6.1.16 shows the incorrect application of a globe valvemanufactured as a mixing valve but used as a diverting valve.


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Fig. 6.1.16 Three-portmixing valve used incorrectly as a diverting valve

The flow entering the valve through port AB can leave from eitherof the two outlet ports A or B, or a proportion may leave fromeach. With port A open and port B closed, the differential pressureof the system will be applied to one side of the plug.

When port A is closed, port B is open, and differential pressure

will be applied across the other side of the plug. At someintermediate plug position, the differential pressure will reverse.This reversal of pressure can cause the plug to move out ofposition, giving poor control and possible noise as the plug'chatters' against its seat.

To overcome this problem on a plug type valve designed fordiverting, a different seat configuration is used, as shown in Fig.

6.1.17. Here, the differential pressure is equally applied to thesame sides of both valve plugs at all times.


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Fig. 6.1.17 Plug type divertingvalve

In closed circuits, it is possible to use mixing valves or divertingvalves, depending upon the system design, as depicted in Figures6.1.18 and 6.1.19.

In Figure 6.1.18, the valve is designed as a mixing valve as it hastwo inlets and one outlet. However, when placed in the returnpipework from the load, it actually performs a diverting function,as it diverts hot water away from the heat exchanger.


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Fig.

6.1.18 Mixing Valve installed on the return pipework

Consider the mixing valve used in Figure 6.1.18, when the heat

exchanger is calling for maximum heat, perhaps at start-up, port Awill be fully open, and port B fully closed. The whole of the waterpassing from the boiler is passed through the heat exchanger andpasses through the valve via ports AB and A. When the heat loadis satisfied, port A will be fully closed and port B fully open, andthe whole of the water passing from the boiler bypasses the loadand passes through the valve via ports AB and B. In this sense,the water is being diverted from the heat exchanger in relation to

the requirements of the heat load.

The same effect can be achieved by installing a diverting valve inthe flow pipework, as depicted by Figure 6.1.19.


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Fig.

6.1.19 Diverting valve installed on the flow pipework

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Control valvesarevalvesused within industrial plants andelsewhere to control operating conditions such astemperature,pressure,flow,andliquidlevel by fully or partially opening orclosing in response to signals received from controllers that
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compare a "setpoint" to a "process variable" whose value isprovided bysensorsthat monitor changes in such conditions.

[1]

The opening or closing of control valves is done by means of

electrical,hydraulicorpneumaticsystems.

Types of control valve

The different types of control valve may be categorized as shownbelow:

Conventional Valve

Severe Service Valve

Types of control valve bodies

The different types of control valve bodies may be categorized asshown below:[2]

Globe control valve with the pneumatic actuator and smartpositioner
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Angle Valves

Cage-style Valve bodies

DiskStack style Valve bodies

Angle seat piston valves

Globe Valves

Single-Port Valve Bodies

Balanced-Plug Cage-Style Valve Bodies

High Capacity, Cage-Guided Valve Bodies

Port-Guided Single-Port Valve Bodies

Double-Ported Valve Bodies

Three-Way Valve Bodies

Rotary ValvesButterfly ValveBodies

V-Notch Ball Control Valve Bodies

Eccentric-Disk Control Valve Bodies

Eccentric-Plug Control Valve Bodies

7.0 Locating the Control Valve

Care should be taken in selecting the location of the control valve in

the pipeline. The following points should be considered.

A. Access for Maintenance

Valves should be installed in location where they can be conveniently

and safely operated and maintained. In selecting the position for the
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valve, it is advisable to check for the availability of convenient points

for installation of lifting hoists so the actuator and valve components

can be removed.

B. Configuration of Associated PipeFor efficient operation of the control valve, the associated pipe

configuration must be taken into account. Failure to do so could

adversely affect the capacity or the flow control performance of the

valve.

Valves should be installed with a minimum of ten straight pipe

diameters upstream of the valve and five straight pipe diameters

downstream before going into a pipe bend. Ideally, there should be

no other valves or fittings in these lengths of straight piping.

When considering the pipe installation, care should be taken in

locating T-pieces on the upstream and downstream side of the

control valve as these can generate shockwaves that can filter back

to the valve and adversely affect its performance.

C. Installation of Isolating Valves

If it is essential to install isolating valves adjacent to the control valve,these valves should be of the same bore as the control valve so to

avoid the generation of a nozzle discharge effect into the control

valve inlet, which can adversely affect the stability and performance

of the control valve.

D. Pipe Size

It is not considered good practice to install valves into pipe that ismore than two sizes larger than the valve size, but if doing so is

necessary to achieve reasonable velocities in the piping, consider a

two-stage expansion of the pipe with a short recovery distance

between the two expanders.


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E. Actuator Mounting

Wherever possible, install the valve with the actuator mounted

vertically above the valve. If it is necessary to install the actuator in

any other plane, consult Copes-Vulcan.F. Pipe Supports

Pipe supports should be located in the vicinity of the valve in order to

support the valve weight and to absorb any pipe loads. Except when

mounting a large actuator in other than the vertical plane, no supports

should be attached to the valve actuator.

G. Fluid Velocity Limitation

Consideration should be given to the velocity of fluid through the

pipeline. If noise is a concern, Copes-Vulcan recommends that the

exit velocity of the valve fluid not exceed one-third sonic for gas or

steam applications.

8.0 Installation

Care taken during the installation of a control valve will provide

substantial benefits in terms of trouble-free service. The most

common causes of problems with control valves are a result of

incorrect installation, transference if pipe stresses to the valve body,

or ingress of foreign matter into the valve trim, which causes the

valve to stick and damages the valve trim.

Avoid locating the control valve at a point where large end loads may

occur.

E. Precautions for Pipeline Flushing

Flushing which is used to remove solid particles from the piping

system, can lead to problems with the control valve. The flushed

particles can get trapped in the e valve trim, where they may score

the components, damage the valve seats, and cause the valve to

stick.


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If flushing is to be carried out with the valve in line, it is recommended

that he valve trim be removed and replaced with a special set of

flushing trim or stuffing box plug. Copes- Vulcan can supply special

flushing trim that can be used during flushing and chemical cleaning.

This trim normally consists of a temporary stem and cage.

F. Precautions for Chemical Cleaning

Chemical cleaning of pipeline can cause problems similar to those

encountered after pipeline flushing, especially if the cleaning

solutions are not neutralized after cleaning is complete and are

allowed to remain in the valve body. Some acids will etch seating or

gasket surfaces or may attack other parts of the valve trim. Copes-

Vulcan strongly recommends that the valve trim be removed prior tochemical cleaning and replaced with a special set of flushing trim or

stuffing box plug. Consult Copes-Vulcan if a set of flushing trim is

needed.

The heart of the control valve is the trim, especially the mating parts thathrottle the stream to the demands of the controller. These valve partsare subjected to the most taxing service, and selecting the best type foan application involves many considerations. Because the trim in rotarymotion valves is vastly different in design from that in linear motion


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valves, body choice is also affected.

Aside from being the right size, the most important consideration isprobably serviceability - will the trim do a proper job for an acceptable

length of time? The corrosive and erosive nature of the fluid are importaelements in the analysis, as are extremes of temperature or pressuredifferential. The risk of cavitation or excessive aerodynamic noiserequires appraisal, as does the importance of flow characteristics on thloop stability. The trim design affects the forces that are demanded of tactuator. Hence, dynamic and actuator cost are involved.

Special considerations for valve trim include a need for tight shutoff and

regulatory requirements, such as those governing sanitary service or thdesirability of having a quick-change facility for easy cleaning orreplacement.

The following are the main considerations for choosing a valve trim:

1. Service life. Is fluid corrosive or erosive?2. Special considerations. Sanitary service, extreme temperature, no

stem packing leakage is allowed.

3. Noise. Can service conditions lead to cavitation or aerodynamicnoise problems?

4. Serviceability. Does valve trim have to be removed frequently forcleaning and the like?

5. Actuator requirements. Static forces, dynamic stability.6. Leakage considerations. Is tight shutoff required?7. Flow characteristic. What should be the installed control valve's

gain?

Selection of Valve Trim Material

The overwhelming majority of valve trim material is AISI type 316stainless steel. This is a good all-around choice for general service forabout -320F (-195C) to 750F (+400C) and moderately corrosivefluids. More specialized material choices are dictated by special erosive


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corrosive, or temperature requirements. However, such trim is usually othe custom variety, and increased cost and delivery cycles are to beexpected.

While there are no hard-and-fast rules governing what trim material tospecify, FCI standard 65-1 may be consulted [Ref. 1].

Guidelines published by various manufacturers are of use, but oftencontradictory and vague. Table 9-1 is typical of such guidelines. In theselection process, one has to make a reasonable compromise betweeninitial cost and expected service life. For example, if a solid Stellite trimwill extend the service life by only 20% but doubles the cost of the valve

then it may be a poor choice. Reliability, downtime, and labor costs mube considered. Pressure drop alone is not necessarily an indication ofwear. A valve operating at choked flow (PFkXTPPI) on gasesexperiences the same velocity whether the pressure drop is 15 or 5000psi. Associated vibration caused by high throttling noise, on the otherhand, may cause substantial wear on guide surfaces.

Hardened Trim

Hardened trim is usually specified for high pressure drop service for oththan clean, dry gases. Although solid hard plugs are the economicalchoice for valves below 1 inch in size (type 440-C or 17-4 PH stainlesssteel, alloy 6, and tungsten carbide are typical), a hard material overlay(usually a cobalt alloy of 38 to 60 RChardness) is more economical forlarger sizes. For abrasive or slurry surfaces, it is wise to use a fully

coated or solid, hardened closure member and seat ring. But hardsurfacing of the seating surfaces may be sufficient to improve the lifeexpectancy of the seat tightness on services such as superheated steatwo-phase flow, or high temperatures over 600F (315C). An alertmanufacturer will select a material combination that offers at least 150Brinell hardness difference between the closure member and the seat


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ring whenever severe service is encountered. For more details onmaterials refer to Chapter 11.

Home

Diaphragm valves(or membrane valves) consists of a valvebody with two

or more ports, a diaphragm, and a "saddle" or seat upon which thediaphragm closes the valve. The valve is constructed from eitherplasticorsteel.

Originally, the diaphragm valve was developed for use in non-hygienicapplications. Later on the design was adapted for use in the bio-pharmaceutical industry by using compliantmaterialsthat can withstandsanitizing and sterilizing methods.

There are two main categories of diaphragm valves: one type seals over a

"weir" (saddle) and the other (sometimes called a "straight-way" valve)seals over a seat. The main difference is that a saddle-type valve has itstwo ports in line with each other on the opposite sides of the valve,whereas the seat-type has the in/out ports located at a90 degree anglefrom one another. The saddle type is the most common in processapplications and the seat-type is more commonly used as a tank bottomvalve but exists also as a process valve. While diaphragm valves usuallycome in two-port forms, they can also come with three ports and evenmore. When more than three ports are included, they generally require

more than one diaphragm; however, special dual actuators can handlemore ports with onemembrane.

Diaphragm valves can be manual or automated. Their application isgenerally as shut-off valves in process systems within the food andbeverage,pharmaceuticalandbiotechindustries. The older generation ofthese valves is not suited for regulating and controlling process flows,
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however newer developments in this area have successfully tackled thisproblem.

In addition to the well known, two way shut off diaphragm valve, other typesinclude: three way zero deadleg valve,sterileaccess port, block and bleed,valbow and tank bottom valve just to name a few.

[edit]Actuators

Diaphragm valves can be controlled by various types ofactuators.Themost common diaphragm valves usepneumatic actuators;in this type ofvalve, air pressure is applied through aSchrader valvewhich raises thediaphragm and opens the valve. This type of valve is extremely quick andas such is one of the more common valves used in operations where valve

speed is a necessity.

Hydraulicdiaphragm valves also exist for higher pressure and lower speedoperations. Some diaphragm valves are also controlled manually.

[edit]Body materials

Brass

Steel type:o

Cast Irono Ductile Irono Carbon Steelo Stainless Steel

Plastic type:o ABS (Acrylonitrile butadiene styrene)o PVC-U (Polyvinyl chloride,unplasticized) also known as PVCu

or uPVCo PVC-C (Polyvinyl chloride, post chlorinated) also known as

PVCc or cPVCo PP (Polypropylene)o PE (Polyethylene)also known as LDPE, MDPE and HDPE (see

note)o PVDF (Polyvinylidene fluoride)

[edit]Body lining materials
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Depending on temperature, pressure and chemical resistance, one of thefollowing is used:

Unlined type Rubber lined type:

o NR/Hard Rubber/Ebonite,o BR/Soft rubbero EPDM

Fluorine plastic lined typeo FEP/F46o PFAo PO

[edit]Diaphragm materials

Unlined or Rubber Lined Type:o NR/Natural Rubbero NBR/Nitrile/Buna-No EPDM

o FKM/Vitono SI/Silicone rubber

Fluorine Plastic Type:o FEP/F46,with EPDM backo PTFE/F4,with EPDM backo PFA,with EPDM back

A pneumatic actuatorconverts energy (in the form ofcompressed air,typically) into motion. The motion can be rotary or linear, depending on thetype of actuator. Some types of pneumatic actuators include:

Tie rod cylinders Rotary actuators Grippers Rodless actuators with magnetic linkage or rotary cylinders Rodless actuators with mechanical linkage
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Pneumatic artificial muscles Speciality actuators that combine rotary and linear motionfrequently

used for clamping operations Vacuum generators

A Pneumatic actuator mainly consists of a piston, a cylinder, and valves orports. The piston is covered by adiaphragm,or seal, which keeps the air inthe upper portion of the cylinder, allowing air pressure to force thediaphragm downward, moving the piston underneath, which in turn movesthevalve stem,which is linked to the internal parts of theactuator.Pneumatic actuators may only have one spot for a signal input, top orbottom, depending on action required. Valves require little pressure tooperate and usually double or triplethe input force. The larger the size ofthe piston, the larger the output pressure can be. Having a larger piston

can also be good if air supply is low, allowing the same forces with lessinput. These pressures are large enough to crush object in the pipe. On100 kPa input, you could lift a small car (upwards 1,000 lbs) easily, and thisis only a basic, small pneumatic valve. However, the resulting forcesrequired of the stem would be too great and cause thevalve stemto fail.

This pressure is transferred to the valve stem, which is hooked up to eitherthe valve plug (seeplug valve),butterfly valveetc. Larger forces arerequired in high pressure or high flow pipelines to allow the valve toovercome these forces, and allow it to move the valves moving parts to

control the material flowing inside.

Valves input pressure is the "control signal." This can come from a variety

of measuring devices, and each different pressure is a different set point for

a valve. A typical standard signal is 20100 kPa. For example, a valve

could be controlling the pressure in a vessel which has a constant out-flow,

and a varied in-flow (varied by the actuator and valve). A pressure

transmitter will monitor the pressure in the vessel and transmit a signal

from 20100 kPa. 20 kPa means there is no pressure, 100 kPa meansthere is full range pressure (can be varied by the transmiters calibration

points). As the pressure rises in the vessel, the output of the transmitter

rises, this increase in pressure is sent to the valve, which causes the valve

to stroke downard, and start closing the valve, decreasing flow into the
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vessel, reducing the pressure in the vessel as excess pressure is

evacuated through the out flow. This is called a direct acting process

Fisher GX control valve

When handwheel is operated , two levers associated with handwheel

and connected to stem connector apply force on stem connector

thereby moving stem . Generally handwheel is at neutral position and

valve is operating pneumatically . Upper mechanical stop provided on

an air to close valve . Valve can be moved from full open

position(siemens positioner value is 0% ) to full closed

position(siemens positioner value is 100% ) by handwheel .When

handwheel is moved from neutral to closing direction , a resistance ifs

felt and sound is produced when valve is being moved . When

handwheel is tried to be moved in open direction from full open

position to open it more , no resistance is felt and valve does not move

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