valves handbook

8/9/2019 Valves Handbook

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Seridium AG SWITZERLAND Piping Products and Systems, Engineering and EPC Contracting

D-U-N-S© (Dun & Bradstreet): 485622430www.seridium.com - email: [email protected]

VALVES QUICK REFERENCE HANDBOOK

Introduction to Valves - Only the Basics -

What are Valves?

Valves are mechanical devices that control 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 pressure

There 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, as in 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

through 90°, 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 YES NO NOGlobe YES NO NO

Plug NO YES YES

Ball NO YES YES

Butterfly NO YES YES

Swing Check NO YES NO

Diaphragm YES NO NO

Pinch YES NO NO


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Safety YES NO NO

Relief YES NO NO

Valve Types Linear Motion Rotary Motion Quarter Turn

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.

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

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


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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, 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 Yoke

The 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


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2 and larger Valves.

• Rising Stem with Inside Screw

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 Screw

The 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 Stem

This 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 Stem

This is a commonly used model in ball, plug, and Butterfly Valves. A quarter-turn motion of

the stem open or close the Valve.

In the main Menu "Valves" you will find some links to detailed (large) images of Rising and NON

Rising Stem Valves.

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 which compresses the packing, by a gland into the so called

stuffing box.

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

3. Stuffing box, a chamber in which the packing is compressed.4. Packing, available in several materials, 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.

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.

Typical sealing assembly


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1. Gland Follover 2. Gland 3. Stuffing Box with Packing 4. Back Seat

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.


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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

Summary

On this page are defined a number of basic information from Valves.

As you may have seen in the main Menu "Valves", you can find also information about several and

often applied Valves in Petro and chemical industry.

It can give you an impression, and good understanding of the differences between the various

types of Valves, and how these differences affect the Valve function. It will help to a proper

application of each type of Valve during the design and the proper use of each type of Valve during


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Introduction to Valves - Only the Basics - Gate Valves -Gate Valves

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 they can be used in two directions

• Pressure loss through the Valve is minimal The 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 or throttle flow

• They are sensitive to vibration in the open state

Construction 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 type of 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.

• Flexible wedge is a one-piece disc with a cut around the perimeter to improve the ability to

correct mistakes or changes in the angle between the seats.


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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 and selfaligning 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.

Most common wedges for Gate Valves

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 Stem

For 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.

In the main Menu "Valves" you will find links to detailed (large) drawings of both stem types .


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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

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.


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Introduction to Valves - Only the Basics - Globe Valves -Globe Valves 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 ring gradually 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 capability

The 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

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 fullopening.

• 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.


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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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Introduction to Valves - Only the Basics - Ball Valves -Ball Valves

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 Valves

Disadvantages 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

Valve Bonnet-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 is inserted 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

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

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,


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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


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Introduction to Valves - Only the Basics - Plug Valves -Plug Valves 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 Valves

Disadvantages 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.

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

is closed 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 the body 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 to 100 percent of the internal pipe area.

• Round port plugs have a round opening through the plug. If the port opening is the same

size or larger than the inside diameter of the pipe, a full port is meant. If the opening is


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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

restricted flow 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


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Introduction to Valves - Butterfly Valves -Butterfly Valves

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.

Types of Butterfly Valves

Butterfly Valves has a short circular body, a round disc, metal-to-metal or soft seats, top and

bottom shaft bearings, 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.

Butterfly valves possess many advantages over gate, globe, plug, and ball valves, especially for

large valve applications. Savings in weight, space, and cost are the most obvious advantages. The

maintenance costs are usually low because there are a minimal number of moving parts and there

are no pockets to trap fluids.

Butterfly valves are especially well-suited for the handling of large flows of liquids or gases

atrelatively low pressures and for the handling of slurries or liquids with large amounts

ofsuspended solids.

Butterfly valves are built on the principle of a pipe damper. The flow control element is a disk of

approximately the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis. When the disk lies parallel to the piping run, the valve is fully

opened. When the disk approaches the perpendicular position, the valve is shut. Intermediate

positions, for throttling purposes, can be secured in place by handle-locking devices.

Butterfly Valve Seat Construction

Stoppage of flow is accomplished by the valve disk sealing against a seat that is on the inside

diameter periphery of the valve body. Many butterfly valves have an elastomeric seat against

which the disk seals. Other butterfly valves have a seal ring arrangement that uses a clamp-ring

and backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings.

In early designs, a metal disk was used to seal against a metal seat. This arrangement did not

provide a leak-tight closure, but did provide sufficient closure in some applications (i.e., water


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distribution lines).

Butterfly Valve Body Construction

Butterfly valve body construction varies. The most economical is the wafer type that fits between

two pipeline flanges. Another type, the lug wafer design, is held in place between two pipe flanges

by bolts that join the two flanges and pass through holes in the valve's outer casing. Butterfly

valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded

end construction.

Seat Disk and Stem of a Butterfly Valve

The stem and disk for a butterfly valve are separate pieces. The disk is bored to receive the stem.

Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned.

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

alternate 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.

Uniform sealing is accomplished and external stem fasteners are eliminated. This method of

assembly is advantageous in the case of covered disks and in corrosive applications.

In order for the disk to be held in the proper position, the stem must extend beyond the bottom of

the disk and fit into a bushing in the bottom of the valve body. One or two similar bushings are

along the upper portion of the stem as well. These bushings must be either resistant to the media

being handled or sealed so that the corrosive media cannot come into contact with them.

Stem seals are accomplished either with packing in a conventional stuffing box or by means of O-

ring seals. Some valve manufacturers, particularly those specializing in the handling of corrosive

materials, place a stem seal on the inside of the valve so that no material being handled by the

valve can come into contact with the valve stem. If a stuffing box or external O-ring is employed,

the fluid passing through the valve will come into contact with the valve stem.

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

Advantages of Butterfly Valves

• Compact design requires considerably less space, compared to other Valves

• Light in weight


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• Quick operation requires less time to open or close

• Available in very large sizes

• Low-pressure drop and high-pressure recovery

Disadvantages 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

Shipment & Storage

• Position discs at 10% open so that they are unseated.

• The faces of each valve should be covered to prevent damage to the seat face, disc edge,

or valve interior.

• Store indoors, preferably with ambient temperatures between 5°C and 30°C.

• Open and close the valves every 3 months.

• Ship and store valves so that no heavy loads are applied to the bodies.

Valve Location

• Butterfly valves should be installed if possible a minimum of 6 pipe diameters from other

line elements, i.e. elbows, pumps, valves, etc. Sometimes this is not feasible, but it is

important to achieve as much distance as possible.

• Where the butterfly valve is connected to a check valve or pump, keep enough space

between them to ensure the disc does not interfere with the adjacent equipment.

Valve Orientation

As a rule of thumb, butterfly valves be installed with the stem in the vertical position with the

actuator mounted vertically directly above it, however, there are some applications where the stem

should be horizontal. The .pdf file below tells you why the stem somtimes must be positioned

horizontally.

Installation Procedures

1. Make sure the pipeline and flange faces are clean. Any foreign material such as metal filings, pipe scale, welding slag, welding rods, etc. can limit disc movement or damage the

disc or seat.

2. Gaskets are not required on resilient seated valves because they extend to both faces of

the valve.

3. Align the pipe-work, and spread the flanges enough to allow the valve body to be easily

inserted between the flanges without contacting the pipe flanges.

4. Check that the valve disc has been set to about 10% open so it doesn't become jammed in

the fully seated position.

5. Insert the valve between the flanges as shown, taking care not to damage the seat faces.

Always lift the valve by the locating holes or by using a nylon sling on the neck or the body.

Never lift the valve by the actuator or operator mounted on the valve.

6. Place the valve between the flanges, centre it, insert the bolts and hand-tighten them.Carefully open the disc, making sure the disc does not contact the inside of the adjacent

pipes.

7. Very slowly close the valve disc to ensure disc edge clearance from the adjacent pipe

flange.

8. Fully open the disc and tighten all flange bolts as shown.

9. Repeat a full close to full open rotation of the disc to ensure proper clearances.


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Introduction to Valves - Only the Basics - Check Valves -Check Valves

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.

Types of Check Valves

• Swing Check Valve

A 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 Valve

The 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.


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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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Introduction to Valves - Only the Basics - Double Block and Bleed -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.

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.


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

that is created between

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 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


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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.

True meaning of Double Block and Bleed

Rudy Garza, Mechanical Lead-Static Equipment Engineering Group at ExxonMobil Development

Company, gave a presentation at the VMA Technical Seminar in San Antonio entitled "Isolation

Philosophies" in which he asserted that many people take the term "Double Block & Bleed" (DBB)

to mean the same thing as Double Positive Isolation" (DPI).

It's time to do maintenance on a section of process. You don't want to shut down the entire facility,

so you decide to block off and depressurize just the section you're working on. Just upstream is a

double block and bleed Valve - a trunnion-mounted Ball Valve with self-relieving seals and a bleed

Valve to vent the cavity. You close the Ball Valve and open the bleeder. Now you can de-

pressurize the line downstream and open it up to work on it.


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Introduction to Valves - Only the Basics - Bellow Sealed Valves -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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Bellow Sealed Gate Valve

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

second results 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


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

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

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

damage is possible.

About the Author

Mr. Satish Chidrawar is CEO of Valvola Corporation in Mumbai, India. He has more than 35 years

of diversified Engineering experience, including more than 21 years in "Valve Engineering". Mr.

Chidrawar is also responsible for developing & maintaining technical methods used in bellow

sealed Valves for various critical conditions. He holds a diploma in Mechanical Engineering from

Board of Technical Education - Maharashtra, India and has authored various papers on bellow

seal Valves.


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Introduction to Valves - Only the Basics - Pressure Seal Valves -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 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 500°C, 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


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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

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

also increases.

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.


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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.

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

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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Introduction to Valves - Only the Basics - Pressure Relief Valves -

Pressure Relief Valves

A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system


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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

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 Valve

The 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 overpressure allowance 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.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,


33/51



therefore, will have no effect on set pressure. Back pressure may, however, affect flow.

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 automatically such 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 automatically such 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.

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 Valve which 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 which incorporates 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.


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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

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.

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 operating conditions within the system to be

protected.

• Set pressure

is the gauge pressure at which under operating conditions direct loaded safety Valves commence to lift.

• Test pressure

is the gauge pressure at which under test stand conditions (atmospheric backpressure)

direct loaded safety Valves commence to lift.

• Opening pressure

is the gauge pressure at which the lift is sufficient to discharge the predetermined 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 of the

system to be protected.

• Opening pressure difference

is the pressure rise over the set pressure necessary for a lift suitable 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 operating pressure.

• 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 from the

minimum diameter which is used to calculate the flow capacity without any deduction for


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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 sectional area equal to the flow

area of the safety Valve without regard to flow losses of the Valve.

• Actual flowing capacityis 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 and actual 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 fluid at 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 pressure relief device is

measured.• Leak test pressure

is the specified inlet static pressure at which a quantitative seat leakage test is performed in

accordance with a standard procedure.• Measured relieving capacity

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

• Rated relieving capacity

is that portion of the measured relieving capacity permitted by the applicable 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, usually expressed

as a percentage of set pressure.

• Popping pressure

is the value of increasing static inlet pressure of a pressure Relief Valve at which 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 working pressure plus the accumulation as established by

reference to the applicable codes for operating or fire contingencies.

Storage handling and transportation of Safety Valves

Storage and handling

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


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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


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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 Val

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