Valves General for Dummies

8/2/2019 Valves General for Dummies

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VALVES

GENERAL FOR

DUMMIES

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2If you can't explain it simply, you don't understand it well enough.Albert Einstein

WHAT ARE VALVES

Valves are mechanical devices that controls the flow and pressure within a system or process. They

are essential components of a piping system that conveys liquids, gases, vapors, slurries etc..

Different types of valves are available: gate, globe, plug, ball, butterfly, check, diaphragm, pinch,

pressure relief, and control valves. Each of these types has a number of models, each with different

features and functional capabilities. Some valves are self-operated while others manually or with an

actuator or pneumatic or hydraulic is operated.

Functions from valves are:

Stopping and starting flow Reduce or increase a flow Controlling the direction of flow Regulating a flow or process pressure Relieve a pipe system of a certain pressureThere are many valve designs, types and models, with a wide range of industrial applications. All

satisfy one or more of the functions identified above.

Valves are expensive items, and it is important that a correct valve is specified for the function, and

must be constructed of the correct material for the process liquid.

CLASSIFICATION OF VALVES

The following are some of the commonly used valve classifications, based on mechanical motion:

Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm,pinch, and lift check valves, moves in a straight line to allow, stop, or throttle the flow.

Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, asin butterfly, ball, plug, eccentric- and swing check valves, the valves are called rotary motion

valves.

Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through90, motion of the stem to go to fully open from a fully closed position or vice versa.

Classification of valves based on motion

Valve types Linear motion Rotary motion Quarter turn

Gate Valve x

Globe valve x

Plug valve x x

Ball valve x x

Butterfly valve x x

Swing check valve x


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Diaphragm valve x

Pinch valve x

Safety valve x

Relief valve x

CLASS RATINGS

Pressure-temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-

Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three

types of classes: standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900,

1500, 2500, and 4500 valves.

VALVE BODY

The Valve body is the first boundary of a pressure valve.

He serves as the main element of a valve assembly

because it is the framework that holds all the parts

together. The valve-body ends are designed to connect

the valve to the piping or equipment nozzle by different

types of end connections, such as butt or socket welded,

threaded or flanged.

Valve bodies are cast or forged in a variety of forms and

each component have a specific function and constructed

in a material suitable for that function.

IMAGEof a standard Gate Valve.

VALVE BONNET

The cover for the opening in the body is the valve bonnet, and is the second most important boundary

of a pressure valve. Like valve bodies, bonnets are in many designs and models available.

A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is

commonly connected to the body by a threaded, bolted, or welded joint. During manufacture of the

valve, the internal components, such as stem, disk and actuator, are put into the body and then the

bonnet is attached to hold all parts together inside.

VALVE TRIM
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Valve's trim is a collective name for the replaceable parts, in a valve. A typically valve design includes

a disk, seat, stem, and sleeves needed to guide the stem.

VALVE DISK

The disc is the part which allows, throttles, or stops flow, depending on its

position. In the case of a plug or a ball valve, the disc is called plug or a ball.

The disk is the third most important primary pressure boundary. With the

valve closed, full system pressure is applied across the disk, and for this

reason, the disk is a pressure related component.

Disks are usually forged, and in some designs, hard surfaced to provide good

wear properties. Most valves are named, according to the design of their disks.

VALVE SEAT(S)

A valve may have one or more seats. In the case of a globe or a swing-check valve, there is usually

one seat, which forms a seal with the disc to stop the flow. In the case of a gate valve, there are two

seats; one on the upstream side and the other on the downstream side. A gate valve disc has two

seating surfaces that come in contact with the valve seats to form a seal for stopping the flow.

The seat ensure the seating surface for the disk. For a good sealing, a fine surface finish from the

seating area is necessary. In some designs, the body is machined to serve as the seating surface, in

other designs, forged seal rings are threaded or welded to the body. To improve the wear resistance

of the seat or seal rings, the surface is often hard faced.

VALVE STEM

The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing

the valve, and is responsible for the proper positioning of the disk. It is connected to the valve

handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe

valves, linear motion of the disc is needed to open or close the valve, while in plug, ball and butterfly

valves, the disc is rotated to open or close the valve.

Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent

leakage, in the area of the seal, a fine surface finish of the stem is necessary.

There are five types of valve stems:

Rising Stem with Outside Screw and YokeThe exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The

stem threads are isolated from the flow medium by the stem packing. Two different styles of these

designs are available; one with the handwheel attached to the stem, so they can rise together, and

the other with a threaded sleeve that causes the stem to rise through the handwheel. This type of

valve is indicated by "O. S. & Y." is a common design for NPS 2 and larger valves.

Rising Stem with Inside Screw


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The threaded part of the stem is inside the valve body, and the stem packing along the smooth

section that is exposed to the atmosphere outside. In this case, the stem threads are in contact

with the flow medium. When rotated, the stem and the handwheel to rise together to open the

valve.

Nonrising Stem with Inside ScrewThe threaded part of the stem is inside the valve and does not rise. The valve disc travels along

the stem, like a nut if the stem is rotated. Stem threads are exposed to the flow medium, and as

such, are subjected to the impact. That is why this model is used when space is limited to allow

linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem

material.

Sliding StemThis valve stem does not rotate or turn. It slides in and out the valve to open or close the valve.

This design is used in hand-operated lever rapid opening valves. It is also used in control valves

are operated by hydraulic or pneumatic cylinders.

Rotary StemThis is a commonly used model in ball, plug, and butterfly valves. A quarter-turn motion of the

stem open or close the valve.

VALVE STEM PACKING

For a reliable seal between the stem and the

bonnet, a gasket is needed. This is called a

Packing, and it is fitted with e.g. the following

components:

1. Gland follower, a sleeve whichcompresses the packing, by a gland into the

so called stuffing box.

2. Gland, a kind of bushing, whichcompressed de packing into the stuffing box.

3. Stuffing box, a chamber in which thepacking is compressed.

4. Packing, available in severalmaterials, like teflon, elastomeric material,

fibrous material etc..5. A backseat is a seating arrangement

inside the bonnet. It provides a seal between

the stem and bonnet and prevents system pressure from building against the valve pakking,

when the valve is fully open. Back seats are often applied in globe valves.


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An important aspect of the life time of a valve is the sealing assembly. Almost all valves, like standard

Ball, Globe, Gate, Plug and Butterfly valves have their sealing assembly based upon shear force,

friction and tearing.

Therefore valve packaging must be properly happen, to prevent damage to the stem and fluid or gas

loss. When a packing is too loose, the valve will leak. If the packing is too tight, it will affect the

movement and possible damage to the stem.

VALVE YOKE

A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the yoke holding

a yoke nut, stem nut, or yoke bushing and the valve stem passes through it. A yoke usually has

openings to allow access to the stuffing box, actuator links, etc.. Structurally, a yoke must be strong

enough to withstand forces, moments, and torque developed by the actuator.

VALVE YOKE NUT

A yoke nut is an internally threaded nut and is placed in the top of a yoke by which the stem passes.

In a gate valve e.g., the yoke nut is turned and the stem travels up or down. In the case of globe

valves, the nut is fixed and the stem is rotated through it.

VALVE ACTUATOR

Hand-operated valves are usually equipped with a handwheel attached to the valve's stem or yoke nut

which is rotated clockwise orcounter clockwise to close or open a valve. Globe and Gate Valves are

opened and closed in this way. Hand-operated, quarter turn valves, such as Ball, Plug or Butterfly, has

a lever for actuate the valve. There are applications where it is not possible or desirable, to actuate

the valve manually by handwheel or lever. These applications include:

Large valves that must be operated against high hydrostatic pressure Valves they must be operated from a remote location When the time for opening, closing, throttle or manually controlling the valve is longer, than

required by system-design criteria

These valves are usually equipped with an actuator. An actuator in the broadest definition is a device

that produces linear and rotary motion of a source of power under the action of a source of control.

Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating

valves are given a positioning signal to move to any intermediate position. There a many different

types of actuators, but the following are some of the commonly used valve actuators:

Gear Actuators Electric Motor Actuators Pneumatic Actuators Hydraulic Actuators Solenoid Actuators


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Cross section of a Piston Check Valve

Globe Valve

1. Body2. Bonnet3. Seat ring4. Disk5. Disk locknut6. Disk washer7. Stem8. Back seat9. Packing10. Gland11. Gland follower12. Set screw13. Stem nut14. Hand wheel

Cross section of a

Swing Check Valve


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Cross section of a Ball Valve Cross section of a TruSeal

Double Block and Bleed

Plug Valve

Cross section of a Butterfly Valve

(Lug Weaver type)


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GATE VALVE

Gate Valves are primarily designed to start or stop flow,

and when a straight-line flow of fluid and minimum flow

restriction are needed. In service, these valves generally

are either fully open or fully closed.

The disk of a gate valve is completely removed when the

valve is fully open; the disk is fully drawn up into the

valve bonnet. This leaves an opening for flow through the

valve at the same inside diameter as the pipesystem in

which the valve is installed.

A gate valve can be used for a wide range of liquids and

provides a tight seal when closed.

Advantages of using gate valves:

Good shutoff features Gate Valves are bidirectional and therefore theycan be used in two directions

Pressure loss through the valve is minimalThe major drawbacks to the use of a gate valve are:

They can not be quickly opened or closed Gate Valves are not suitable for regulate orthrottle flow

They are sensitive to vibration in the open stateCONSTRUCTION OF A GATE VALVE

Gate Valves consists of three main parts: body, bonnet, and trim. The body is generally connected to

other equipment by means of flanged, screwed or welded connections. The bonnet, which containing

the moving parts, is attached to the body, usually with bolts, to permit maintenance. The valve trim

consists of the stem, the gate, the disc or wedge and the seat rings.

DISKS OF A GATE VALVE

Gate Valves are available with different disks or wedges. Ranging of the gate valves is usually made

by the type of wedge used.

The most common were:

Solid wedge is the most commonly used disk by its simplicity and strength. A valve with this typeof wedge can be installed in each position and it is suitable for almost all liquids. The solid wedge is

a single-piece solid construction, and is practically for turbulent flow.


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Flexible wedge is a one-piecedisc with a cut around the perimeter to

improve the ability to correct mistakes or

changes in the angle between the seats.

The reduction will vary in size, shape and

depth. A shallow, narrow cut gives little

flexibility but retains strength.

A deeper and wider cut, or cast-in

recess, leaves little material in the

middle, which allows more flexibility, but

compromises strength.

Split wedge is self-adjusting andselfaligning to both seats sides. This

wedge type consists of two-piece

construction which seats between the

tapered seats in the valve body. This

type of wedge is suitable for the treatment of non-condensing gases and liquids at normal

temperatures, particularly corrosive liquids.

STEM OF A GATE VALVE

The stem, which connects the handwheel and disk with each other, is responsible for the proper

positioning of the disk. Stems are usually forged, and connected to the disk by threaded or other

techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.

Gate Valves are classified as either:

Rising stem Non rising stemFor a valve of the rising stem type, the stem will rise above the handwheel if the valve is opened. This

happens, because the stem is threaded and mated with the bushing threads of a yoke. A yoke is an

integral part from a rising stem valve and is mounted to the bonnet.

For a valve of the non rising stem type, there is no upward stem movement if the valve is opened. The

stem is threaded into the disk. As the handwheel on the stem is rotated, the disk travels up or down

the stem on the threads while the stem remains vertically stationary.

The two links (on the right above) to detailed (large) drawings of both stem types, tell you more, aswhat I can tell you here.

SEATS OF A GATE VALVE

Seats for gate valves are either provided integral with the valve body or in a seat ring type of

construction. Seat ring construction provides seats which are either threaded into position or are


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Z-Body Y-Body Angle-Body

pressed into position and seal welded to the valve body. The latter form of construction is

recommended for higher temperature service.

Integral seats provide a seat of the same material of construction as the valve body while the pressed-

in or threaded-in seats permit variation. Rings with hard facings may be supplied for the application

where they are required.

GLOBE VALVE

A globe valves is a linear motion valve and are primarily designed

to stop, start and regulate flow. The disk of a globe valve can be

totally removed from the flowpath or it can completely close the

flowpath.

The fundamental principle of the globe valve operation is the

perpendicular motion of the disk away from the seat. This ensures

that the ring-shaped space between the disk and seat ringgradually close as the valve is closed. This property gives a globe

valve reasonably good throttling capability. Therefore, the globe

valve can be used for starting and stopping flow and to regulate

flow.

Advantages of using globe valves:

Good shutoff capability Reasonably good throttling capabilityThe major drawbacks to the use of a globe valve are:

Higher pressure drop compared to a gate valve Large valve sizes require considerable power or a larger actuator to operate

BODY DESIGNS OF GLOBE VALVES


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There are three primary body designs for globe valves, namely: Z-body, Y-body and Angle body.

Z-body design is the most common body type, with a Z-shaped diaphragm.

The horizontal setting of the seat allows the stem and disk to travel perpendicular to the horizontal

line.

Y-body design is an alternative for the high pressure drop, inherent in globe valves.

Seat and stem are angled at approximately 45 degrees, what gives a straighter flowpath at full

opening.

Angle-body design is a modification of the basic Z-type globe valve.

The ends of this globe valve are at an angle of 90 degrees, and fluid flow occurs with a single 90

degrees turn.

DISKS OF A GLOBE VALVE

The most common disk designs for globe valves are: ball disk, composition disk and the plug disk.Ball disk design is used primarily in low pressure and low temperature systems. It is capable of

throttling flow, but in principle it is applied to stop and start flow.

Composition disk design uses a hard, non-metallic insert ring on the disk, which ensures a tighter

closure.

Plug disk design provides better throttling than ball or composition designs. They are available in

many different designs and they are all long and tapered.

STEM AND DISK CONNECTIONS OF A GLOBE VALVE

Globe valves uses two methods for connecting the disk and the stem: the T-slot and the disk nut

construction. In the T-slot design, the disk slides over the stem, while in the disk nut design, the disk

is screwed into the stem.

SEATS OF GLOBE VALVES

Globe valve seats are either integrated or screwed in to the valve body. Many globe valves have

backseats inside the bonnet. Back seats provides a seal between the stem and bonnet and prevents

system pressure from building against the valve pakking, when the valve is fully open. Back seats are

often applied in globe valves.

FLOW DIRECTION OF GLOBE VALVES

For applications with low temperature, globe valves are normally installed so that the pressure is

under the disc. This contributes an easy operation and helps protect the packing.

For applications with high temperature steam service, globe valves are installed so that the pressure is

above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat.
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BALL VALVE

A ball valve is a quarter-turn rotational motion valve that uses a ball-shaped disk to stop or start flow.

If the valve is opened, the ball rotates to a point where the hole through the ball is in line with the

valve body inlet and outlet. If the valve is closed, the ball is rotated so that the hole is perpendicular

to the flow openings of the valve body and the flow is stopped.

Advantages of using ball valves:

Quick quarter turn on-off operation Tight sealing with low torque Smaller in size than most other valvesDisadvantages of ball valves:

Conventional ball valves have poor throttling properties In slurry or other applications, the suspended particles can settle and become trapped in body

cavities causing wear, leakage, or valve failure.

TYPES OF BALL VALVES

Ball valves are basically available in three versions: full port, venturi port and reduced port.

The full-port valve has an internal diameter equal to the inner diameter of the pipe.

Venturi and reduced-port versions generally are one pipe size smaller than the line size.

Ball valves are manufactured in different body configurations and the most common are:

Top entry ball valves allow access to valve internals for maintenance by removal of the valvebonnet-cover. It is not required to be removed valve from the pipe system.

Split body ball valves consists of a two parts, where one part is smaller as the other. The ball isinserted in the larger body part, and the smaller body part is assembled by a bolted connection.

The valve ends are available as butt welding, socket welding, flanged, threaded and others.

MATERIALS OF BALLS AND SEATS

Balls are usually made of several metallics, while the seats are from soft materials like Teflon,

Neoprene, and combinations of these materials. The use of soft-seat materials imparts excellent


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sealing ability. The disadvantage of soft-seat materials (elastomeric materials) is, that they are not

can be used in high temperatures processes.

For example, fluorinated polymer seats can be used for service temperatures from 200 (and larger)

to 230C and higher, while graphite seats may be used for temperatures from ? to 500C and higher.

BALL VALVE STEM DESIGN

The stem in a ball valve is not attached to the ball. Usually it has a rectangular portion at the ball, and

that fits into a slot cut into the ball. The enlargement permits rotation of the ball as the valve is

opened or closed.

BALL VALVE BONNET

The bonnet of a ball valve is fastens to the body, which holds the stem assembly and ball in place.

Adjustment of the bonnet permits compression of the packing, which supplies the stem seal. Packing

material for ball valve stems is usually Teflon or Teflon-filled or O-rings instead of packing.

BALL VALVES APPLICATIONS

The following are some typical applications of ball valves:

Air, gaseous, and liquid applications Drains and vents in liquid, gaseous, and other fluid services Steam service

PLUG VALVE

A plug valve is a quarter-turn rotational motion valve that use a

tapered or cylindrical plug to stop or start flow. In the open

position, the plug-passage is in one line with the inlet and outlet

ports of the valve body. If the plug 90 is rotated from the open

position, the solid part of the plug blocks the port and stops flow.

Plug valves are similar to ball valves in operation.

Advantages of using plug valves:

Quick quarter turn on-off operation Minimal resistance to flow Smaller in size than most other valvesDisadvantages of plug valves:

Requires a large force to actuate, due to high friction. NPS 4 and larger valves requires the use of an actuator. Reduced port, due to tapered plug.


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TYPES OF PLUG VALVES AND SEALING

Plug valves are available in a nonlubricated or lubricated design and with several styles of port

openings. The port in the tapered plug is generally rectangular, but they are also available with round

ports and diamond ports.

Plug valves are also available with cylindrical plugs. The cylindrical plugs ensure greater port openings

equal to or larger than the pipe flow area.

Lubricated plug valves are provided with a cavity in the middle along there axis. This cavity isclosed at the bottom and fitted with a sealant-injection fitting at the top. The sealant is injected

into the cavity, and a check valve below the injection fitting prevents the sealant from flowing in

the reverse direction. The lubricant in effect becomes a structural part of the valve, as it provides

aflexible and renewable seat.

Nonlubricated Plug Valves contain an elastomeric body liner or a sleeve, which is installed in thebody cavity. The tapered and polished plug acts like a wedge and presses the sleeve against the

body. Thus, the nonmetallic sleeve reduces the friction between the plug and the body.

PLUG VALVE DISK

Rectangular port plugs are the most common port shape. The rectangular port represents 70 to100 percent of the internal pipe area.

Round port plugs have a round opening through the plug. If the port opening is the same size orlarger than the inside diameter of the pipe, a full port is meant. If the opening is smaller than the

inside diameter of the pipe, a standard round port is meant.

Diamond port plug has a diamond-shaped port through the plug and they are venturi restrictedflow types. This design is suitable for throttling service.

TYPICAL APPLICATIONS OF PLUG VALVES

A plug valve can be used in many different fluid services and they perform well in slurry applications.

The following are some typical applications of plug valves:

Air, gaseous, and vapor services Natural gas piping systems Oil piping systems Vacuum to high-pressure applications

BUTTERFLY VALVE

A butterfly valve is a quarter-turn rotational motion valve, that is used to stop, regulate, and start

flow. Butterfly valves are easy and fast to open. A 90 rotation of the handle provides a complete

closure or opening of the valve. Large Butterfly valves are usually equipped with a so-called gearbox,

where the handwheel by gears is connected to the stem. This simplifies the operation of the valve, but

at the expense of speed.


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Advantages of butterfly valves

Compact design requires considerably less space, compared to other valves Light in weight Quick operation requires less time to open or close Available in very large sizes Low-pressure drop and high-pressure recoveryDisadvantages of butterfly valves

Throttling service is limited to low differential pressure Cavitation and choked flow are two potential concerns Disc movement is unguided and affected by flow turbulence

TYPES OF BUTTERFLY VALVES

Butterfly valves has a short circular body, a round disc,

metal-to-metal or soft seats, top and bottom shaftbearings, and a stuffing box.

The construction of a butterfly valve body varies. A

commonly used design is the wafer type that fits between

two flanges. Another type, the lug wafer design, is held in

place between two flanges by bolts that join the two

flanges and pass through holes in the valve's outer casing.

Butterfly valves are even available with flanged, threaded

and butt welding ends, but they are not often applied.

SEAT DISK AND STEM OF A BUTTERFLY VALVE

Stopping flow is achieved by the valve disk sealing against a seat that is on the inside diameter

periphery of the valve body. Often an elastomeric seat material will be used. Disk and stem of a

butterfly valve consists of two parts, and there a two methods to be fastened together.

In the first method, the disk is bored through and secured to the stem with bolts or pins. The second

method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-

shaped stem. This method allows the disk to "float" and seek its center in the seat.

TYPICAL APPLICATIONS OF BUTTERFLY VALVES

A butterfly valve can be used in many different fluid services and they perform well in slurry

applications. The following are some typical applications of butterfly valves:

Cooling water, air, gases, fire protection etc. Slurry and similar services Vacuum service High-pressure and high-temperature water and steam services


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CHECK VALVE

Check valves are "automatic" valves that open with

forward flow and close with reverse flow. The pressure

of the fluid passing through a system opens the valve,

while any reversal of flow will close the valve. Exact

operation will vary depending on the type of check valve

mechanism. Most common types of check valves are

swing, lift (piston and ball), butterfly, stop and tilting-

disk.

IMAGEof a typical Check valve (swing type).

TYPES OF CHECK VALVES

Swing check valveA basic swing check valve consists of a valve body, a bonnet, and a disk that is connected to a

hinge. The disk swings away from the valve-seat to allow flow in the forward direction, and returns

to valve-seat when upstream flow is stopped, to prevent backflow.

The disc in a swing type check valve is unguided as it fully opens or closes. There are many disk

and seat designs available, in order to meet the requirements of different applications. The valve

allows full, unobstructed flow and automatically closes as pressure decreases. These valves are

fully closed when flow reaches zero, in order to prevent backflow. Turbulence and pressure drop in

the valve are very low.

Lift check valveThe seat design of a lift-check valve is similar to a globe valve. The disc is usually in the form of a

piston or a ball. Lift check valves are particularly suitable for high-pressure service where velocity

of flow is high. In lift check valves, the disc is precisely guided and fits perfectly into the dashpot.

Lift check valves are suitable for installation in horizontal or vertical pipe-lines with upward flow.

Flow to lift check valves must always enter below the seat. As the flow enters, the piston or ball is

raised within guides from the seat by the pressure of the upward flow. When the flow stops or

reverses, the piston or ball is forced onto the seat of the valve by both the backflow and gravity.
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BELLOW(S) SEAL(ED) VALVES

In this article, the author Mr. Satish Chidrawar (at the bottom of this page you will find more about

the author) first reviews the construction, design and operation of the bellow seal. He then provides

various examples of where bellow seal valves are use.

Leakage at various points in pipelines found in chemical plants creates emissions. All such leakage

points can be detected using various methods and instruments and should be noted by the plant

engineer. Critical leakage points include flanged gasket joints and the valve / pump gland packing,

etc. Today the chemical process industry is gearing itself towards safer technology for better

environmental protection and it has become every process engineer's responsibility to design plants

that limit damage to the environment through the prevention of leakage of any toxic chemicals.

Leakage from the valve gland or stuffing box is normally a concern for the maintenance or plant

engineer. This leakage means:

a) Loss of material b) Pollution to the atmosphere c) Dangerous for plant employees.

Bellow Sealed Gate Valve


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For example, take the case of a steam leakage through the valve gland. At 150 PSI, a clearance of

just 0.001" through the gland will mean a leak at the rate of 25 lb/hour. This equates to a loss of USD

1.2 per eight hour shift, or USD 1,100 per year. Similarly, a tiny drop of 0.4 mm diameter per secondresults in a waste of about 200 litres per year of costly oil or solvent. This leakage can be reduced

considerably by using the bellow seal valve. This article will now consider the construction and

operation of the bellow seal.

Bellow construction

The bellow cartridge is welded to both the valve bonnet and the valve stem. The bellow cartridge has

a number of convolutions and these convolutions become compressed or expanded depending upon

the movement of valve stem. (Scientifically speaking the bellow gets compressed when the valve is in

the open position and expanded when the valve is in the closed condition). It is important to properly

install the valve bodies. The bellow can be sealed to the valves in two different ways. Firstly, the

bellow can be welded to the valve stem at the top and the valve body on the bottom. In this case the

process fluid is contained inside the bellow or in second method the bellow is welded to the valve stem

at the bottom and the body on the top. In this case the process fluid is contained in the annular region

between the valve bonnet and bellow (from the outside).

The bellow is a critical component and forms the heart of the bellow seal valves. To avoid any twisting

of the bellow the valve must have a stem with linear movement only. This can be achieved using a so-

called sleeve-nut at the yoke portion of the valve bonnet. A handwheel is fitted onto the sleeve-nut

which effectively transfers a rotary motion of the handwheel into a linear motion in the valve stem.

Bellow types

There are two main types of bellow: the Forged Bellow and the Welded Bellow. Formed-type bellows

are made from rolling a flat sheet (thin wall foil) into a tube which is then longitudinally fusion welded.

This tube is subsequently mechanically or hydrostatically formed into a bellow with rounded and

widely spaced folds. The welded leaf type bellow is made by welding washer-like plates of thin metal

together at both the inner and outer circumference of the washers - like plates. A welded leaf bellow

has more folds per unit length as compared to forged bellows. Thus, for the same stroke length,

forged bellows are two to three times longer than their welded leaf counterparts.

Reportedly, mechanically forged bellows fail at random spots, while the welded leaf usually fails at or

near a weld. To ensure full penetration of bellow ends and end coller welding it is advisable to

fabricate using micro plasma welding.

Bellow design

The multi-ply bellow design is preferred for handling higher pressure fluids (generally two or three

plies of the metal wall). A two ply bellow can increase its pressure rating by 80% to 100% as


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compared to a single ply bellow of the same thickness. Alternatively, if a single ply bellow of a

thickness equivalent to a pressure rating of a two ply bellow is used, the stroke length is reduced.

Thus, a multi-ply bellow design offers a distinct advantage over a single ply bellow. It is clear that the

bellow is subject to metal fatigue and this fatigue can induce weld failure. The bellow fatigue life is

affected by the material of construction, fabrication technique, stroke length and stroke frequency, in

addition to the usual parameters such as fluid temperature and pressure.

Bellow materials

The most popular stainless steel bellow material is AISI 316Ti which contain Titanium to withstand

high temperatures. Alternatively, Inconel 600 or Inconel 625 improve fatigue strength and corrosion

resistance as compared with stainless steel bellows. Similarly, Hastalloy C-276 offers greater corrosion

resistance and fatigue strength than Inconel 625. Fatigue resistance can be improved by using a

multiply bellows system and reducing the stroke length; this can significantly increase the bellow

service life.

Valve options

The most common valve types to be fitted with bellow seals are the gate and globe designs (see

Figure 1).These are very suited for use with bellows due to their internal construction and axial

movement of the valve stem.

Based on available information, it seems that current bellow seal valves range in size from 3 mm NB

to 650 mm NB. Pressure ratings are available in from ANSI 150# to 2500#. Material options for the

valves include carbon steel, stainless steel and exotic alloys.

Applications

Heat transfer media: hot oil is commonly used in industries such as synthetic fibres / POY (Partially

Oriented Yarn). However, there is always a risk of fire due to hot oil spillage on highly inflammable

chemicals. Here, bellow seal valves can stop the leakage.

Vacuum / ultra high vacuum: some applications require a vacuum pump to continually extract air from

a pipeline. Any conventional valves installed on the pipeline can allow external air to enter the pipeline

thorough the valve stuffing box. Hence the bellow seal valve is the only solution to prevent air from

passing through the stuffing box.

Highly hazardous fluids: for media such as chlorine (see Figure 2), hydrogen, ammonia and phosgene,

the bellow seal valve is an ideal design as leakage through the gland is totally eliminated.

Nuclear plant, heavy water plant: in instances where radiation leakage is to be prevented at all times,

the bellow seal valve is the ultimate choice.

Costly fluids: in some applications leaks need to be avoided simply because of the high cost of the

fluid. Here, an economic assessment often favours the use of bellow seal valves.

Environmental standards: around the world, standards regarding emissions and the environment are

getting more stringent day by day. It can therefore be difficult for companies to expand within existing


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premises. With the use of bellow seal valves, expansion without additional environmental

damage is possible.

INTRODUCTION TO PRESSURE SEAL VALVES

Pressure seal construction is adopted for valves for high pressure service, typically in excess of above

170 bar. The unique feature about the pressure seal bonnet is that the body-bonnet joints seals

improves as the internal pressure in the valve increases, compared to other constructions where the

increase in internal pressure tends to create leaks in the body-bonnet joint.

Pressure seal design

A/B. Bonnet tendency to move up or down as pressure changes C. System pressure D. Sealing forces due to pressure

The higher the internal pressure, the greater the sealing force. Easy dismantling is made possible

by dropping the bonnet assembly into the body cavity and driving out the four-segmental thrust rings

by means of a push pin.

Relying on fairly simple design principles, pressure seal valves have proven their capability to

handle increasingly demanding fossil and combined-cycle steam isolation applications, as designers


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continue to push boiler, HRSG, and piping system pressure/temperature envelopes. Pressure seal

valves are typically available in size ranges from 2 inches to 24 inches and ASME B16.34 pressure

classes from #600 to #2500, although some manufacturers can accommodate the need for larger

diameters and higher ratings for special applications.

Pressure seal valves are available in many material qualities such as A105 forged and Gr.WCB

cast, alloy F22 forged and Gr.WC9 cast; F11 forged and Gr.WC6 cast, austenitic stainless F316 forged

and Gr.CF8M cast; for over 500C, F316H forged and suitable austenitic cast grades.

The pressure seal design concept can be traced back to the mid-1900s, when, faced with ever

increasing pressures and temperatures (primarily in power applications), valve manufacturers began

designing alternatives to the traditional bolted-bonnet approach to sealing the body/bonnet joint.

Along with providing a higher level of pressure boundary sealing integrity, many of the pressure seal

valve designs weighed significantly less than their bolted bonnet valve counterparts.

BOLTED BONNETS VS. PRESSURE SEALS

To better understand the pressure seal design concept, let's contrast the body-to-bonnet

sealing mechanism between bolted bonnets and pressure seals. Figure 1 depicts the typical Bolted

Bonnet Valve. The body flange and bonnet flange are joined by studs and nuts, with a gasket of

suitable design/material inserted between the flange faces to facilitate sealing. Studs/nuts/bolts are

tightened to prescribed torques in a pattern defined by the manufacturer to affect optimal sealing.

However, as system pressure increases, the potential for leakage through the body/bonnet joint also

increases.


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Now let's look at the pressure seal joint detailed in Figure 2. Note the differences in the

respective body/bonnet joint configurations. Most pressure seal designs incorporate "bonnet take-up

bolts" to pull the bonnet up and seal against the pressure seal gasket. This in turn creates a seal

between the gasket and the inner diameter (I.D.) of the valve body.

A segmented thrust ring maintains the load. The beauty of the pressure seal design is that as system

pressure builds, so does the load on the bonnet and, correspondingly, the pressure seal gasket.

Therefore, in pressure seal valves, as system pressure increases, the potential for leakage through the

body/bonnet joint decreases.

This design approach has distinct advantages over bolted bonnet valves in main steam,

feedwater, turbine bypass, and other power plant systems requiring valves that can handle the

challenges inherent in high-pressure and temperature applications.

But over the years, as operating pressures/temperatures increased, and with the advent of

peaking plants, this same transient system pressure that aided in sealing also played havoc with

pressure seal joint integrity.

PRESSURE SEAL GASKETS

One of the primary components involved in sealing the pressure seal valve is the gasket itself.

Early pressure seal gaskets were manufactured from iron or soft steel. These gaskets were

subsequently silver-plated to take advantage of the softer plating material's ability to provide a tighter

seal. Due to the pressure applied during the valve's hydrotest, a "set" (or deformation of the gasket

profile) between the bonnet and gasket was taken. Because of the inherent bonnet take-up bolt and

pressure seal joint elasticity, the potential for the bonnet to move and break that "set" when subjected

to system pressure increases/ decreases existed, with body/bonnet joint leakage the result.

This problem could be effectively negated by utilizing the practice of "hot torquing" the bonnet

take-up bolts after system pressure and temperature equalization, but it required owner/user

maintenance personnel to do so after plant startup. If this practice was not adhered to, the potential

for leakage through the body/bonnet joint existed, which could damage the pressure seal gasket, the

bonnet and/or the I.D. of the valve body, as well as creating compounding problems and inefficiencies

that the steam leakage could have on plant operations. As a result, valve designers took several steps

to address this problem.

Figure 2 shows a combination of live-loaded bonnet take-up bolts (thus maintaining a constant

load on the gasket, minimizing the potential for leakage) and the replacement of the iron/soft steel,

silverplated pressure seal gasket with one made of die-formed graphite. The gasket design shown in

Figure 3 can be installed in pressure seal valves previously supplied with the traditional type gasket.

The advent of graphite gaskets has further solidified the dependability and performance of the

pressure seal valve in most applications and for even daily start/stop operating cycles.

Although many manufacturers still recommend "hot torquing," the potential for leakage when

this is not done is greatly diminished. The seating surfaces in pressure seal valves, as in many power

plant valves, are subjected to, comparatively speaking, very high seating loads. Seat integrity is


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maintained as a function of tight machining tolerances on component parts, means of providing the

requisite torque to open/close as a function of gears or actuation, and selection/ application of proper

materials for seating surfaces.

Cobalt, nickel, and iron-based hardfacing alloys are utilized for optimal wear resistance of the

wedge/disc and seat ring seating surfaces. Most commonly used are the CoCr-A (e.g., Stellite)

materials. These materials are applied with a variety of processes, including shielded metal arc, gas

metal arc, gas tungsten arc, and plasma (transferred) arc. Many pressure seal globe valves are

designed having integral hardfaced seats, while the gate valve and check valves typically have

hardfaced seat rings that are welded into the valve body.

VALVING TERMINOLOGY

If you have dealt with valving for any length of time, you've probably noticed valve manufacturers are

not overly creative with the terms and vernacular used in the business. Take for example, "bolted

bonnet valves." The body is bolted to the bonnet to maintain system integrity. For "pressure seal

valves," system pressure aids the sealing mechanism. For "stop/check valves," when the valve stem is

in the closed position, flow is mechanically stopped, but when in the open position, the disc is free to

act to check a reversal of flow. This same principle applies to other terminology used for design, as

well as valve types and their component parts.


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Face to Face and End to End dimensions of RF valves

according to ASME B16.10 class 150

NPS

GateSolid Wedge

andDouble Disc

GateConduit

PlugShort

Pattern

PlugRegular

Pattern

Globeand

Lift Check

SwingCheck

BallLong

Pattern

BallShort

Pattern

A1/2 108 108 108 108 108

3/4 117 117 117 117 1171 127 140 127 127 127 127

1 140 140 140 140 140

1 165 165 165 165 165 165

2 178 178 178 203 203 178 178

2 190 190 190 216 216 190 190

3 203 203 203 241 241 203 203

4 229 229 229 305 292 292 229 229

5 254 254 381 356 330

6 267 267 267 394 406 356 394 267

8 292 292 292 457 495 495 457 292

10 330 330 330 533 622 622 533 330

12 356 356 356 610 698 698 610 356

14 381 381 686 787 787 686 381

16 406 406 762 914 864 762 406

18 432 432 864 978 864

20 457 457 914 978 914

22 508 1067

24 508 508 1067 1295 1067

26 559 559 1295

28 610 610 1448

30 610 660 1524

32 711

34 762 1016

36 711 813 1956


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NPS

GateSolid WedgeDouble Discand Conduit

PlugShort andVenturiPattern

PlugRegularPattern

Globeand

Lift Check

SwingCheck

BallLong

Pattern

BallShort

Pattern

A1/2 140 (1) 152 140 140

3/4 152 (1) 178 152 1521 165 (1) 159 (6) 203 216 165 165

1 178 (1) 216 229 178 178

1 190 190 (6) 229 241 190 190

2 216 216 267 267 216 216

2 241 241 292 292 241 241

3 282 282 318 318 282 282

4 305 305 356 356 305 305

5 381 400 400

6 403 403 403 444 444 403 403

8 419 419 502 559 533 502 419

10 457 457 568 622 622 568 457

12 502 502 711 711 711 648 502

14 762 762 (4) 762 838 762 572

16 838 838 (4) 838 864 838 61018 914 914 (4) 914 978 914 660

20 991 991 (4) 991 1016 991 711

22 1092 1092 (4) 1092 1118 1092

24 1143 1143 (4) 1143 1346 1143 813

26 1245 1245 (4) 1245 1346 1245

28 1346 1346 (4) 1346 1499 1346

30 1397 1397 (4) 1397 1594 1397

32 1524 1524 (4) 1524 1524

34 1626 1626 (4) 1626 1626

36 1727 1727 (4) 1727 2083 1727


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NPS

GateSolid WedgeDouble Discand Conduit

PlugShort andVenturiPattern

PlugRegularPattern

Globeand

Lift Check

SwingCheck

BallLong

Pattern

BallShort

Pattern

A1/2 140 (1) 152 140 140

3/4 152 (1) 178 152 1521 165 (1) 159 (6) 203 216 165 165

1 178 (1) 216 229 178 178

1 190 190 (6) 229 241 190 190

2 216 216 267 267 216 216

2 241 241 292 292 241 241

3 282 282 318 318 282 282

4 305 305 356 356 305 305

5 381 400 400

6 403 403 403 444 444 403 403

8 419 419 502 559 533 502 419

10 457 457 568 622 622 568 457

12 502 502 711 711 711 648 502

14 762 762 (4) 762 838 762 572

16 838 838 (4) 838 864 838 61018 914 914 (4) 914 978 914 660

20 991 991 (4) 991 1016 991 711

22 1092 1092 (4) 1092 1118 1092

24 1143 1143 (4) 1143 1346 1143 813

26 1245 1245 (4) 1245 1346 1245

28 1346 1346 (4) 1346 1499 1346

30 1397 1397 (4) 1397 1594 1397

32 1524 1524 (4) 1524 1524

34 1626 1626 (4) 1626 1626

36 1727 1727 (4) 1727 2083 1727


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NPS

GateSolid WedgeDouble Discand ConduitLong Pattern

PlugRegular

and VenturiPattern

GlobeLift Check

Swing CheckLong Pattern

BallLong

Pattern

A1/2 165 (1) 165 165

3/4 190 (1) 190 190

1 216 216 (3) 216 216

1 229 229 (3) 229 229

1 241 241 241 241

2 292 292 292 292

2 330 330 330 330

3 356 356 356 356

4 432 432 432 432

5 508 508

6 559 559 559 559

8 660 660 660 660

10 787 787 787 787

12 838 838 838 838

14 889 889 889 (5) 88916 991 991 991 (5) 991

18 1092 1092 (4) 1092 (5) 1092

20 1194 1194 (4) 1194 (5) 1194

22 1295 1295 (4) 1295 (5) 1295

24 1397 1397 (4) 1397 (5) 1397

26 1448 1448 (4) 1448 (5) 1448

28 1549 1600 (5) 1549

30 1651 1651 (4) 1651 (5) 1651

32 1778 (2) 1778 (4) 1778

34 1930 (2) 1930 (4) 1930

36 2083 (2) 2083 (4) 2083 (5) 2083


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NPS


PlugRegular

and VenturiPattern

GlobeLift Check


BallLong

Pattern

A3/4 229

1 254 (1) 254 (3) 254 254

1 279 (1) 279 (3) 279 279

1 305 (1) 305 (3) 305 305

2 368 368 (3) 368 368

2 419 419 (3) 419 419

3 381 381 (3) 381 381

4 457 457 (4) 457 457

5 559 559

6 610 610 610 610

8 737 737 737 737

10 838 838 838 838

12 965 965 965 965

14 1029 1029 1029

16 1130 1130 (4) 1130 (5) 113018 1219 1219 (5) 1219

20 1321 1321 (4) 1321 (5) 1321

22

24 1549 1549 (5) 1549


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NPS


PlugRegular

and VenturiPattern

GlobeLift Check


BallLong

Pattern

A1/2 216 (7)

3/4 229

1 254 (1) 254 (3) 254

1 279 (1) 279 (3) 279

1 305 (1) 305 (3) 305

2 368 368 (3) 368 368

2 419 419 (3) 419 419

3 470 470 (3) 470 470

4 546 546 (4) 546 546

5 673 673

6 705 705 705 705

8 832 832 832 832

10 991 991 991 991

12 1130 1130 1130 1130

14 1257 1257 125716 1384 1384 (4) 1384 (5) 1384

18 1537 1537 (5)

20 1664 1664 (5)

22

24 1943 1943 (5)

General notes:

Dimensions are in millimeters unless otherwise indicated. (1) = Solid wedge only. (2) = Double disc and conduit only. (3) = Regular pattern only. (4) = Venturi pattern only. (5) = Swing Check only. (6) = Short pattern only. (7) = Globe and Lift Check only. The face-to-face dimension for flanged valves is the distance between the extreme ends which are

the gasket contact surfaces.


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End-to-End dimensions apply to flanged valves where the gasket contact surfaces are not locatedat the extreme ends of the valve. The distance between the extreme ends is described as the end-

to-end dimension and applies to flanged valves like: Ring Joint, large or small female and large or

small groove

DOUBLE BLOCK AND BLEED SYSTEMS

The primary function of a double block and bleed system is for isolation and the secondary function is

for intervention.

Under certain conditions double block and bleed systems are needed to prevent product contamination

or where it is necessary to remove essential equipment from service for cleaning or repairs while the

unit continues in operation.

Of course, such equipment must be provided with a spare or it must be possible to bypass it

temporarily without shutting down the unit.

The nature of the fluid, its pressure and temperature, and many other factors must be considered

when determining the need for double block and bleed systems.

Generally, block valves should be considered for the onstream

isolation of equipment if the fluid is flammable or otherwise

hazardous, or if the fluid is in high-pressure or high-temperature

service. Where double block valves are used, a NPS or larger

bleed valve should be installed between the block valves.

The purpose of the bleed valve is twofold. First, the bleed

ensures that the upstream valve is in fact tight before slipping in

a blind off the downstream block valve. The bleed connection

also permits the safe withdrawal of moderate leakage from the upstream valve to again assure the

tight shutoff of the downstream valve.

Depending on the service conditions, it may be possible to use a single block valve with a body

bleed to provide double block and bleed provisions for onstream isolation of equipment.

Gate valves with flexible wedges and with body or bonnet bleed valve can serve this purpose if

specifically tested in accordance with API-598 for double block and bleed quality valves.

Some ball valves and nonlubricated plug valves, when equipped with a valve body bleed between the

seats, can also be satisfactory substitutes for double block valves.

Testing for double block and bleed quality valves requires the pressure-testing of each seat, with

leakage measured through the valve body bleed as a means of substantiating the independent leak

tightness of both the upstream and downstream seats of the valve.


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DOUBLE BLOCK AND BLEED VALVES

The Double Block and Bleed Valve or a DBBV can perform the tasks of 3 separate valves (2 separate

isolations and 1 drain valve) which apart from being hugely space saving can also save on weight and

time due to installation and maintenance practices requiring much less work and the operator being

able to locate and operate all 3 valves in one location.

Double block and bleed valves operate on the principle that isolation can be achieved from both the

upstream and downstream process flow / pressures.

This is achieved by two ball, gate, globe, needle, etc. valves placed back to back, with a third

"isolatable" valve in the centre cavity.

Once isolation has been achieved in one or more of the main process isolation valves, the cavity that

is created between these isolations can be drained. This is useful for flow diverting, sampling or

injection situations, and for maintenance and or integrity check situations where seat leakage can be

monitored through the third "bleed" valve.

The image on the left gives you a good

impression, how a DBB valve is

constructed.

In this image example, three balls are

mounted. 2 large balls that serve as a

block (both are closed), and the small

ball serve as the bleed (ball is in open

position).

Image comes from www.habonim.com.

It is a DBB valve in the dual-Safe

series. For more information about

Habonim click the PDF icon below.

ISOLATION (STOP) VALVES IN PRESSURE RELIEF PIPING

The article below (text) comes from the American Petroleum Institute (API)

Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part II-Installation

API recommended practice 520 fifth edition

Isolation (Stop) Valves in Pressure-Relief Piping

Isolation block valves may be used for maintenance purposes to isolate a pressure-relief device from

the equipment it protects or from its downstream disposal system. Since improper use of an isolation

valve may render a pressure-relief device inoperative, the design, installation, and administrative

controls placed on these isolation block valves should be carefully evaluated to ensure that plant

safety is not compromised. A pressure-relief device shall not be used as a block valve to provide

positive isolation.


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Inlet Isolation Valves

a. Valves shall be full bore. ASME Section VIII Appendix M recommends the use of full area isolation

(stop) valves. Mandatory paragraph UG-135 (b)(1), of ASME Section VIII, requires that the opening

through all pipe and fittings between a pressure vessel and its pressure-relief valve shall have the

area of the pressure-relief device inlet. It is therefore recommended that the minimum flow area in

the isolation valve be equal to or greater than the inlet area of the pressure-relief valve. The minimum

flow area of the isolation valve and the inlet area of the pressurerelief valve can be obtained from the

isolation valve manufacturer and the pressure-relief valve manufacturer.

b. Valves shall be suitable for the line service classification.

c. Valves shall have the capability of being locked or carsealed open.

d. When gate valves are used, they should be installed with stems oriented horizontally or, if this is

not feasible, the stem could be oriented downward to a maximum of 45 from the horizontal to keep

the gate from falling off and blocking the flow.

e. A bleed valve should be installed between the isolation valve and the pressure-relief device to

enable the system to be safely depressurized prior to performing maintenance. This bleed valve can


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also be used to prevent pressure build-up between the pressure-relief device and the closed outlet

isolation valve.

f. Consideration should be given to using an interlocking system between the inlet and outlet isolation

valves to assist with proper sequencing.

g. Consideration should be given to painting the isolation valve a special color or providing other

identification. When placing the pressure-relief device into service, it is recommended to gradually

open the isolation valve. This ramping up of system pressure can help prevent unwanted opening of a

valve seat due to the momentum of the fluid. The inlet valve must be open fully.

Outlet Isolation Valves

a. Valves shall be full bore. ASME Section VIII Appendix M recommends the use of full area isolation

(stop) valves. To help minimize the built-up back pressure, it is recommended that the minimum flow

area in the outlet isolation valve be equal to or greater than the outlet area of the pressure-relief

valve. The minimum flow area of the outlet isolation valve and the outlet area of the pressure-relief

valve can be obtained from the isolation valve manufacturer and the pressure-relief valve

manufacturer respectively.

b. Valves shall be suitable for line service classification.

c. Valves shall have the capability of being locked or carsealed open. This outlet isolation shall never

be closed while the vessel is in operation without using an inlet isolation valve that has first been

closed with the space between the inlet isolation valve and the pressure-relief valve adequately

depressured.

d. A bleed valve should be installed between the outlet isolation valve and pressure-relief device to

enable the system to be safely depressurized prior to performing maintenance. This bleed valve can

also be used to prevent pressure build-up between the pressure-relief device and the closed outlet

isolation valve.

e. Consideration should be given to using an interlocking system between the inlet and outlet isolation

valves to assist with proper sequencing.

f. Consideration should be given to painting the isolation valve a special color or providing other

identification. When the outlet isolation valve is used in conjunction with an inlet isolation valve, upon

commissioning the pressurerelief device, the outlet isolation valve shall be opened fully prior to the

inlet isolation valves.


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PRESSURE RELIEF VALVE

A pressure relief valve is a safety device designed to protect a pressurized vessel or system during an

overpressure event.

An overpressure event refers to any condition which would cause pressure in a vessel or system to

increase beyond the specified design pressure or maximum allowable working pressure (MAWP).

The primary purpose of a pressure relief valve is protection of life and property by venting fluid from

an overpressurized vessel.

Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such

as pressure, temperature and flow. Each of these systems requires a power source of some type, such

as electricity or compressed air in order to operate. A pressure relief valve must be capable of

operating at all times, especially during a period of power failure when system controls are

nonfunctional.

The sole source of power for the pressure relief valve, therefore, is the process fluid.

Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous

level, the pressure relief valve may be the only device remaining to prevent a catastrophic failure.

Since reliability is directly related to the complexity of the device, it is important that the design of the

pressure relief valve be as simple as possible.

The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a

specified overpressure, and close when the system pressure has returned to a safe level. Pressure

relief valves must be designed with materials compatible with many process fluids from simple air and

water to the most corrosive media. They must also be designed to operate in a consistently smooth

and stable manner on a variety of fluids and fluid phases.

SPRING LOADED PRESSURE RELIEF VALVEThe basic spring loaded pressure relief valve has been developed to meet the need for a simple,

reliable, system actuated device to provide overpressure protection.

The image on the right shows the construction of a spring loaded pressure relief valve.

The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disc held against

the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed,

and a body/bonnet to contain the operating elements. The spring load is adjustable to vary the

pressure at which the valve will open.

When a pressure relief valve begins to lift, the spring force increases. Thus system pressure must

increase if lift is to continue. For this reason pressure relief valves are allowed an overpressureallowance to reach full lift. This allowable overpressure is generally 10% for valves on unfired

systems. This margin is relatively small and some means must be provided to assist in the lift effort.

Most pressure relief valves, therefore, have a secondary control chamber or huddling chamber to

enhance lift. As the disc begins to lift, fluid enters the control chamber exposing a larger area of the

disc to system pressure.


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This causes an incremental change in force which overcompensates for the increase in spring force

and causes the valve to open at a rapid rate. At the same time, the direction of the fluid flow is

reversed and the momentum effect resulting from the change in flow direction further enhances lift.

These effects combine to allow the valve to achieve maximum lift and maximum flow within the

allowable overpressure limits. Because of the larger disc area

exposed to system pressure after the valve achieves lift, the

valve will not close until system pressure has been reduced to

some level below the set pressure. The design of the control

chamber determines where the closing point will occur.

The difference between the set pressure and the closing point

pressure is called blowdown and is usually expressed as a

percentage of set pressure.

BALANCED BELLOWS VALVES AND BALANCED PISTON

VALVES

When superimposed back pressure is variable, a balanced bellows

or balanced piston design is recommended. A typical balanced

bellow is shown on the right. The bellows or piston is designed

with an effective pressure area equal to the seat area of the disc.

The bonnet is vented to ensure that the pressure area of the

bellows or piston will always be exposed to atmospheric pressure

and to provide a telltale sign should the bellows or piston begin to

leak. Variations in back pressure, therefore, will have no effect on

set pressure. Back pressure may, however, affect flow.

OTHER DESIGNS OF RELIEF VALVES

Safety Valve. A safety valve is a pressure relief valve actuated

by inlet static pressure and characterized by rapid opening or pop action. (It is normally used for

steam and air services.)

Low-Lift Safety Valve. A low-lift safety valve is a safety valve in which the disc lifts automaticallysuch that the actual discharge area is determined by the position of the disc.

Full-Lift Safety Valve. A full-lift safety valve is a safety valve in which the disc lifts automaticallysuch that the actual discharge area is not determined by the position of the disc.

Relief Valve. A relief valve is a pressure relief device actuated by inlet static pressure having a

gradual lift generally proportional to the increase in pressure over opening pressure. It may be

provided with an enclosed spring housing suitable for closed discharge system application and is

primarily used for liquid service.


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Safety Relief Valve. A safety relief valve is a pressure relief valve characterized by rapid opening or

pop action, or by opening in proportion to the increase in pressure over the opening pressure,

depending on the application and may be used either for liquid or compressible fluid.

Conventional Safety Relief Valve. A conventional safety relief valve is a pressure relief valvewhich has its spring housing vented to the discharge side of the valve. The operational

characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by

changes of the back pressure on the valve.

Balanced Safety Relief Valve. A balanced safety relief valve is a pressure relief valve whichincorporates means of minimizing the effect of back pressure on the operational characteristics

(opening pressure, closing pressure, and relieving capacity).

Pilot-Operated Pressure Relief Valve. A pilotoperated pressure relief valve is a pressure relief valve

in which the major relieving device is combined with and is controlled by a self-actuated auxiliary

pressure relief valve.

Power-Actuated Pressure Relief Valve. A poweractuated pressure relief valve is a pressure relief

valve in which the major relieving device is combined with and controlled by a device requiring an

external source of energy.

Temperature-Actuated Pressure Relief Valve. A temperature-actuated pressure relief valve is a

pressure relief valve which may be actuated by external or internal temperature or by pressure on the

inlet side.

Vacuum Relief Valve. A vacuum relief valve is a pressure relief device designed to admit fluid to

prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after

normal conditions have been restored.

CODES, STANDARDS AND RECOMMENDED PRACTICES

Many Codes and Standards are published throughout the world which address the design and

application of pressure relief valves. The most widely used and recognized of these is the ASME Boiler

and Pressure Vessel Code, commonly called the ASME Code.

Most Codes and Standards are voluntary, which means that they are available for use by

manufacturers and users and may be written into purchasing and construction specifications. The

ASME Code is unique in the United States and Canada, having been adopted by the majority of state

and provincial legislatures and mandated by law.

The ASME Code provides rules for the design and construction of pressure vessels. Various sections of

the Code cover fired vessels, nuclear vessels, unfired vessels and additional subjects, such as welding

and nondestructive examination. Vessels manufactured in accordance with the ASME Code are

required to have overpressure protection. The type and design of allowable overpressure protection

devices is spelled out in detail in the Code.


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Terminology

The following definitions are taken from DIN 3320 but it should be noted that many of the terms and

associated definitions used are universal and appear in many other standards. Where commonly used

terms are not defined in DIN 3320 then ASME PTC25.3 has been used as the source of reference. This

list is not exhaustive and is intended as a guide only; it should not be used in place of the relevant

current issue standard:

Operating pressure (working pressure) is the gauge pressure existing at normal operatingconditions within the system to be protected.

Set pressure is the gauge pressure at which under operating conditions direct loaded safetyvalves commence to lift.

Test pressure is the gauge pressure at which under test stand conditions (atmosphericbackpressure) direct loaded safety valves commence to lift.

Opening pressure is the gauge pressure at which the lift is sufficient to discharge thepredetermined flowing capacity. It is equal to the set pressure plus opening pressure difference.

Reseating pressure is the gauge pressure at which the direct loaded safety valve is re-closed. Built-up backpressure is the gauge pressure built up at the outlet side by blowing. Superimposed backpressure is the gauge pressure on the outlet side of the closed valve. Backpressure is the gauge pressure built up on the outlet side during blowing (built-up

backpressure + superimposed backpressure).

Accumulation is the increase in pressure over the maximum allowable working gauge pressure ofthe system to be protected.

Opening pressure difference is the pressure rise over the set pressure necessary for a liftsuitable to permit the predetermined flowing capacity.

Reseating pressure difference is the difference between set pressure and reseating pressure. Functional pressure difference is the sum of opening pressure difference and reseating pressure

difference.

Operating pressure difference is the pressure difference between set pressure and operatingpressure.

Lift is the travel of the disc away from the closed position. Commencement of lift (opening) is the first measurable movement of the disc or the perception

of discharge noise.

Flow area is the cross sectional area upstream or downstream of the body seat calculated fromthe minimum diameter which is used to calculate the flow capacity without any deduction for

obstructions.

Flow diameter is the minimum geometrical diameter upstream or downstream of the body seat. Nominal size designation of a safety valve is the nominal size of the inlet.


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Theoretical flowing capacity is the calculated mass flow from an orifice having a cross sectionalarea equal to the flow area of the safety valve without regard to flow losses of the valve.

Actual flowing capacity is the flowing capacity determined by measurement. Certified flowing capacity is actual flowing capacity reduced by 10%. Coefficient of discharge is the ratio of actual to the theoretical discharge capacity. Certified coefficient of discharge is the coefficient of discharge reduced by 10% (also known as

derated coefficient of discharge).

The following terms are not defined in DIN 3320 and are taken from ASME PTC25.3:

Blowdown (reseating pressure difference) - difference between actual popping pressure andactual reseating pressure, usually expressed as a percentage of set pressure or in pressure units.

Cold differential test pressure the pressure at which a valve is set on a test rig using a test fluidat ambient temperature. This test pressure includes corrections for service conditions e.g.

backpressure or high temperatures.

Flow rating pressure is the inlet static pressure at which the relieving capacity of a pressurerelief device is measured.

Leak test pressure is the specified inlet static pressure at which a quantitative seat leakage testis performed in accordance with a standard procedure.

Measured relieving capacity is the relieving capacity of a pressure relief device measured at theflow rating pressure.

Rated relieving capacity is that portion of the measured relieving capacity permitted by theapplicable code or regulation to be used as a basis for the application of a pressure relieving

device.

Overpressure is a pressure increase over the set pressure of a pressure relief valve, usuallyexpressed as a percentage of set pressure.

Popping pressure is the value of increasing static inlet pressure of a pressure relief valve atwhich there is a measurable lift, or at which the discharge becomes continuous as determined by

seeing, feeling or hearing.

Relieving pressure is set pressure plus overpressure. Simmer is the pressure zone between the set pressure and popping pressure. Maximum operating pressure is the maximum pressure expected during system operation. Maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at

the top of a completed vessel in its operating position for a designated temperature.

Maximum allowable accumulated pressure (MAAP) is the maximum allowable workingpressure plus the accumulation as established by reference to the applicable codes for operating or

fire contingencies.


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STORAGE HANDLING AND TRANSPORTATION OF SAFETY VALVES

Storage and handling

Because cleanliness is essential to the satisfactory operation and tightness of a safety valve,

precautions should be taken during storage to keep out all foreign materials. Inlet and outlet

protectors should remain in place until the valve is ready to be installed in the system. Take care to

keep the valve inlet absolutely clean. It is recommended that the valve be stored indoors in the

original shipping container away from dirt and other forms of contamination.

Safety valves must be handled carefully and never subjected to shocks. Rough handling may alter the

pressure setting, deform valve parts and adversely affect seat tightness and valve performance.

The valve should never be lifted or handled using the lifting lever.

When it is necessary to use a hoist, the chain or sling should be placed around the valve body and

bonnet in a manner that will insure that the valve is in a vertical position to facilitate installation.

Installation

Many valves are damaged when first placed in service because of failure to clean the connection

properly when installed. Before installation, flange faces or threaded connections on both the valve

inlet and the vessel and/or line on which the valve is mounted must be thoroughly cleaned of all dirt

and foreign material.

Because foreign materials that pass into and through safety valves can damage the valve, the systems

on which the valves are tested and finally installed must also be inspected and cleaned. New systems

in particular are prone to contain foreign objects that inadvertently get trapped during construction

and will destroy the seating surface when the valve opens. The system should be thoroughly cleaned

before the safety valve is installed.

The gaskets used must be dimensionally correct for the specific flanges. The inside diameters must

fully clear the safety valve inlet and outlet openings so that the gasket does not restrict flow.

For flanged valves, draw down all connection studs or bolts evenly to avoid possible distortion of the

valve body. For threaded valves, do not apply a wrench to the valve body. Use the hex flats provided

on the inlet bushing.

Safety valves are intended to open and close within a narrow pressure range. Valve installations

require accurate design both as to inlet and discharge piping. Refer to International, National and

Industry Standards for guidelines.

Inlet pipingConnect this valve as direct and close as possible to the vessel being protected.

The valve should be mounted vertically in an upright position either directly on a nozzle from the

pressure vessel or on a short connection fitting that provides a direct, unobstructed flow between the

vessel and the valve. Installing a safety valve in other than this recommended position will adversely

affect its operation.


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The valve should never be installed on a fitting having a smaller inside diameter than the inlet

connection of the valve.

Discharge piping

Discharge piping should be simple and direct. A "broken" connection near the valve outlet is preferred

wherever possible. All discharge piping should be run as direct as is practicable to the point of final

release for disposal. The valve must discharge to a safe disposal area. Discharge piping must be

drained properly to prevent the accumulation of liquids on the downstream side of the safety valve.

The weight of the discharge piping should be carried by a separate support and be properly braced to

withstand reactive thrust forces when the valve relieves. The valve should also be supported to

withstand any swaying or system vibrations.

If the valve is discharging into a pressurized system be sure the valve is a "balanced" design. Pressure

on the discharge of an "unbalanced" design will adversely affect the valve performance and set

pressure.

Fittings or pipe having a smaller inside diameter than the valve outlet connections must not be used.

The bonnets of balanced bellows safety valves must always be vented to ensure proper functioning of

the valve and to provide a telltale in the event of a bellows failure. Do not plug these open vents.

When the fluid is flammable, toxic or corrosive, the bonnet vent should be piped to a safe location.

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