Petrogas Training Center

Unit 1

T YPES OF VA L VES

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TYPES OF VALVES
The purpose of a valve is to start, stop, or regulate a flow of liquid or gas through a plant

system. Valves can be classified by disc arrangement, by function, by operating

conditions such as temperature and pressure, or by the way they attach to plant systems

(flanging, welding, threading, etc.). This unit covers the use of various types of valves, the

functions of basic valve components, and the arrangement of these components within the

valve body.

1.1 Purpose, Classification, and Components of Valves

OBJECTIVES:

 Explain the purpose of a valve in a plant system.

 List the components of a valve and describe the function of each component.

 Identify the ways that basic valves are classified.

 Explain what bridge wall markings and service markings are.

 List what different types of valves are made of.

1.1.1 Purpose of Valves

Valves are attached to piping or equipment to control the flow of a fluid or gas in a plant

system. An open valve permits flow through the system; a closed valve shuts off the

flow. Certain valves are also designed to regulate the flow of a fluid or gas through

the system. This is accomplished by maintaining the valve in a partially open

position. Figure 1.1-1 illustrates an open valve, a partially open valve, and a closed

valve.

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1. Types of Valves (continued)

(a) (b) (c)

The size of a valve often determines its ability to regulate or control flow within a system

, the smaller the valve, the finer the control. Valves vary in size from very small (.125

in or .31 cm) to very large (several feet or meters in diameter). Regardless of size,

however, all valves are built to withstand differences in flow, temperature, and pressure.

Otherwise, the components that make up various-sized valves are essentially the same.

1.1.2 Common Classifications of Valves

Although all types of valves have basically the same parts (disc, body, etc.), the

components do not always look the same. Moreover, valves are attached to systems in

a variety of ways. Therefore, valves are classified into categories that are descriptive of

their function and operation.

The most common way to classify a valve is by the arrangement or shape of the disc,

which is the part of the valve that controls the flow of fluid through the valve.

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In the gate valve (Figure 1.1-2), the disc arrangement is not designed to control the

rate of flow. Gate valves are used essentially

for on or off service, or for isolation.

The gate valve disc, referred to as a gate, is

usually wedge-shaped to fit between the

seats. Gate valves are most often found in

liquid systems. They are always either fully

open or fully closed.

Figure 1.1-2. Gate Valve Disc

Arrangement

The globe valve can be used for isolation or for the regulation

of flow within a system. When used for regulation, it is

BALL
DISC
often called a control valve. The control valve is

distinguished from a regular globe valve by the shape and

arrangement of the disc and seating areas. By its design, the

globe valve generally provides for a tight seal and has

good throttling characteristics. Figure 1.1-3A. Ball-

Shaped Disc

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There are four basic types of disc and seat

arrangements for globe valves:

(1) The ball-shaped disc (Figure 1.1-3A) fits on a

tapered, flat-sur- faced seat and is usually used on

relatively low-pressure, low-temperature systems.

(2) The composition disc (Figure 1.1-3B) is

renewable and can be adapted to varying

COMPOSITION DISC
types of flow.
Figure 1.1-3B.
Composition Disc

(3) The plug-type disc (Figure 1.1-3C) is also renewable,

along with its seat rings, and is very useful for

heavy duty throttling.

Plug
DISC

Figure 1.1-3C. Plug

Type Disc

(4) The needle point disc (Figure 1.1-3D), which is very

narrow and, therefore, best suited for close regulation of

flow.

NEEDLE
DISC

Figure 1.1-3D. Needle

Point Disc
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The butterfly valve is light in construction. It is

usually found on large installations

where pressure is not high. The disc in this type

of valve is attached to the stem, and it

swings open or closed, as illustrated in Figure 1.1-

4. The butterfly valve is used primarily for

isolation of flow.

Figure 1.1-4. Butterfly Valve Disc

Diaphragm valves are characterized by the

materials that comprise their discs. The discs

are made of flexible material, such as rubber,

and they provide an effective seal that

prevents leakage into other working parts

of the valve. These valves are often found in

systems carrying a flow of chemical fluids

or harmful gases, where any leakage could

be potentially dangerous.

Figure 1.1-5. Diaphragm Valve

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Diaphragm valves are also used in places where corrosion may be a problem.

Figure 1.1-5 shows a diaphragm valve in a closed position.

The preceding examples illustrate the most common way to classify a valve - by the

shape or arrangement of the disc and seating area. Other ways to classify valves are

by their function; by the conditions under which they operate, such as temperature and

pressure variations; and by the way they attach to systems, such as by flanging,

welding, or threading.

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1. Types of Valves (continued)

1.1.3 Valve Components

Figure 1.1-6 shows the parts of a valve. This diagram should be referred to as each part

is discussed. The use of the diagram will make it easier to become familiar with the

placement of each part and the way in which each part is attached to other parts of the

valve. It should be remembered, however, that the shape and arrangement of these

parts will vary with different types of valves.

At the bottom is the valve body. The body is the largest structural part of the valve. It

provides the means for attaching the valve to system components or piping.

Valves may be installed into a system in a variety of ways. When they are welded in, the

inlet and outlet stubs of the valve body are smooth. The flange is another common type

of connection. When it is used, the inlet and outlet stubs are flanged and the valve is

bolted to system piping flanges. Valves are often attached to low-pressure systems by

threading. In this case, both the inlet and outlet stubs are threaded.

System flow passes through the body of the valve. The design of the valve body can

allow for straight-through flow, or it can change the direction of flow. Figure 1.1-7 shows

a globe valve body with straight-through flow, and Figure 1.1-8 illustrates an angled

globe valve. Notice that the direction of flow is changed by the construction of the valve

body.

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1. Types of Valves (continued)

Figure 1.1-6. Valve Components

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1. Types of Valves (continued)

Figure 1.1-7. Straight-Through Flow Figure 1.1-8. Angled Globe Valve

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The seating area is located inside of the valve body; it is an immovable part of the

valve. This is the area where the disc closes on the valve body to stop flow through the

valve. To do this, the seating areas of the disc and seat must be smooth, and they

must fit together perfectly. The seat can be threaded, press-fit, or welded into the

body of the valve; however, in some cases it is cast as part of the valve body. In

high-temperature, high-pressure systems, a combination of threading and welding is used

to prevent leakage between the valve body and the seat. Figure 1.1-9 illustrates

how the seat may be threaded (Figure 1.1-gA), welded (Figure 1.1-gB), or threaded

and welded (Figure 1.1-9C) to the valve body.

THREADED

WELDED

WELDED

THREADED

Figure 1.1-9. Threaded and Welded Seat Attachment

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The materials used for seat construction will vary, depending on where tile valve is

installed. For example, for low-pressure, low-temperature systems, such as service water,

the valve seating area may be made of bronze, or a Teflon-type material. In systems

requiring high temperatures arid pressures, the seating area must be very strong.

Stellite (a commercial for an extremely hard metal) is often used in such places. This

metal is highly resistant to cutting damage due to leakage of steam, and usually forms a

layer, or face, over the metal that comprises the seating area of the valve.

The disc is the part of the valve that closes against the seat to stop flow. When the

disc is fully raised off of the seat, it is in the open position; when it is pressed against

the seat, it is closed, The partially open position of the disc is called the "throttled

position." The throttled position is what allows the valve to regulate flow.

Many disc designs are available. The double-

disc arrangement (Figure 1.1-10) is designed

for special purposes. It is used to equalize the

pressure difference across the disc. In this

way, the valve can be operated with a relatively

small remote operator. (This valve will be

discussed in more detail in Section 1.5.)

Figure 1.1-10. Double-Disc

Arrangement

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Most valve discs are attached to a stem that

connects to the handwheel, or valve operator. The

stem transmits the motion of the handwheel, or

operator, to the internal disc, opening and closing

the valve. The arrangement is shown in Figure

1.1-11.

Figure 1.1-11. Handwheel,

Rotating Stem and Disc

The disc is attached to the stem in a variety of ways. In the slip-type joint, the dic slips

over the end of the stem. The valve body prevents the disc from slipping loose from the

stem. Threading is another method used to connect the stem and disc, and on some

valves, the stem and disc are manufactured as one piece that cannot be separated.

Pins or cotter keys are often used as an added means to prevent the disc from detaching

from the stem.

The rest of the valve parts are supported by the bonnet (Figure 1.1-12). The bonnet is

attached to the valve body by bolting, threading, or welding. The shape of the bonnet is

determined by the type and shape of the disc, since it provides housing for the disc when it

is raised up from the valve seat.

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1. Types of Valves (continued)

Figure 1.1-12. (a) Bolted Bonnet (b) Threaded Bonnet (c) Welded Bonnet

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At the point where the stem goes through the

bonnet to connect with the handwheel, or

operator, there is a recessed area. This

recessed area is called the stuffing box

(Figure 1.1-13). As the name implies, the

stuffing box is filled with a packing. The

purpose of the packing is to prevent leakage

through the bonnet to the atmosphere,

while allowing the stem to move when

the hand-wheel, or operator, is turned. The

packing is made of a material that can be

compressed to form a seal around the stem

and allow movement at the same time.

To hold the packing in place, a packing

gland is attached. The packing is

compressed by the packing gland to prevent

leakage by the stem and through the bonnet. Packing glands are usually bolted

or threaded to the valve bonnet.

Movement of the disc inside the valve is controlled by the handwheel, or operator, via

the stem. A handwheel is generaly turned manually, while an operator is controlled by an

electric, pneumatic, or hydraulic motor.

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Useful information is provided on the outside of the valve body. This information,

called abridge wall markings," indicates how the internal parts of the valve are arranged

and, thus, how the disc closes on the seat of the valve. Figure 1.1-14(A) shows this type

of information. An arrow Figure 1.1-14(B) or a set of letters Figure 1.1-14(C) will show

the direction of flow within the valve.

Figure 1.1-14. Bridge Wall Markings

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In addition to bridge wall markings, service markings are placed on the outside of the

valve body. These markings indicate the maximum allowable pressure that can safely

be placed on the valve for a certain type of service. However, service markings

typically do not indicate maximum ratings for steam pressure.

The letters "w," "o," or "G" on the valve body indicate the type of service the valve is

designed for. The `W" indicates that the valve should be used for water service; the "0'

for oil service; and the "G for gas service. (Valves with `W," `0," and "0' are typically for

low temperature service. Other letter designations on valves indicate other types of

service. The manufacturers instructions will describe these as they appear.)

1.2 Purpose and Function of Gate Valves

OBJECTIVES:

 Explain the purpose of a gate valve.

 Describe the operation of a gate valve.

 Explain the difference between a rising stem and a non-rising stem.

Gate valves are relatively simple in design and function. They are used in many places,

primarily for on or off service and for isolation of a flow. Normally, gate valves are

placed where straight free flow is desired and where an immediate shutoff~ of flow may

be necessary.

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The wedge-shaped disc (Figure 1.2-1) fits between

the seats and forms a tight shutoff of flow when
closed. It is directed by a set of guides that insure
proper alignment when the disc is closed on the
seating area. The disc is wedged against the seat
when the closing point is reached. The wedge-
shaped disc is sometimes split down the center,
resulting in two halves that rest independently on
opposite sides of the valve seat.

2. Purpose and Function of Gate Valves Figure 1.2-1. Wedge-Shaped Disc

(continued)

The other type of disc commonly found on a gate

valve is the double disc. It is made of two separate
discs with a spring mechanism attached. The
disc is held in place on the seat by the spring while the
valve is in operation. The double disc closes cleanly
on its seat; there is no jamming or wedging as it
seals the valve. In addition, the double disc is
seated by system pressure, making it applicable in
high-temperature systems. Changes in temperature
will not cause jamming. When the valve is
closed, system pressure is applied to one side. It
will compress the spring, pushing the one disc off
of its seat. This puts system pressure on the
other disc. The higher the system pressure, the Figure 1.2-2. Double Disc
more tightly the disc is seated. Figure 1.2-2 with Spring
shows a double disc with spring action.

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The seats on a gate valve are either cast as part of the valve body or are installed, which

makes them replaceable. Small valves generally have fixed seats, since they are

usually threaded or flanged into a system. It is more economical to replace these valves

than to repair them when they are worn or damaged. Large valves, on the other hand, are

generally welded in place and are equipped with replaceable seats. With a large valve, it

is less expensive to replace the seat than the entire valve.

Gate valve stems also come in two varieties: rising or non-rising.

Some rising stems (Figure 1.2-3) are threaded though the handwheel and yoke

bushing. Then, as the handwheel is rotated, the stem will

rise. The handwheel is keyed to the bushing and does not

rise. Other rising stems are bolted or keyed to the

handwheel and threaded through the stem bushing.

Then, as the handwheel is rotated, the stem will rise. The

rising stem has one advantage over the non-rising stem.

The amount of stem protruding from the handwheel

indicates the position of the disc, that is, whether it is

open or closed.

Figure 1.2-3. Rising

Stem

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On gate valves with non-rising stems (Figure 1.2-4), the stem and hand-wheel are bolted

or keyed to rotate together. The disc is

generally threaded to the stem, and it then

threads itself up the stem as the handwheel is

rotated. The yoke bushing in this arrangement

has no threads, so the stem is kept from rising

when the handwheel is rotated. It is used where

space is a problem. An indicator should be

attached to the valve to show the position of the

stem and the disc. The non-rising stem valve is

well suited for mechanical operation, since

allowances for stem movement in and out are not

necessary.
Figure 1.2-4 the stem
and hand wheel

Many valves have electric or hydraulic motors attached, which can be remotely operated.

A limit switch (Figure 1.2-5) may be used to indicate the position of the valve. It may also

be used to insure that power is stopped when the valve is in the fully closed position. In

this way, operators do not have to personally inspect the valve to determine whether it is

open or closed.

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Figure 1.2-5. Limit Switch

Gate valves may be made of various materials, depending on where and how they are

used in plant systems. For example, valves attached to low-pressure, low-temperature

systems are normally made of bronze or brass. Cast iron valves will be found on low-

pressure steam or lubricating systems. Valves used in high-pressure, high-temperature

systems are made of special alloy metals. Stainless steel is commonly used where there

is a possibility of corrosion to the valve.

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1.3 Globe, Butterfly, Diaphragm, and Check Valves

OBJECTIVES:

 Identify the valves discussed in this section.

 Describe the disc shape and the seating area arrangement of each type of

valve.

 Explain how each type of valve regulates a system flow.

1.3.1 Globe Valves

Globe valves are constructed in several ways. The seating surface, the body type,

and the disc arrangement differ according to the

design and function of the valve.

For example, an angled globe valve (Figure 1.3-1)

will change the direction of flow. This design elminates

the necessity of added joints, normally required to

change the direction of flow when other valves are

used. The possibility of leakage when using the angled

globe valve is reduced, since joint connections are

potential sources of leakage. Figure 1.3-1. Angled Globe

Valve

Globe valve discs come in four basic varieties: (1) ball-type, (2) compcsition, (3) plug

type, and (4) needle.

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1.3 Globe, Butterfly, Diaphragm, and Check Valves (Continued)

The ball-type (or globe-type) disc (Figure 1.3-2A) seats against a tapered, flat-surfaced
seat. It is generafly used In a fully open or shut position, but it may be employed for
moderate throttling of a flow. These valves are normally found in low-pressure, low-
temperature systems.

The composition disc (Figure 1.3-2B) is renewable. It is available in a variety of materials

that are designed for different types of service, such as high- and low-temperature
water, air, or steam. The seating surface is often formed by a rubber "o" ring or washer.

A. Ball-Type Disc B. Composition Disc

Figure 1.3-2. Globe Valves Showing Four Types of Discs

C. Plug-Type Disc D. Needle Point Disc

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1.3 Globe, Butterfly, Diaphragm, and Check Valves (Continued)

The plug-type disc (Figure 1.3-2C) is cone shaped, and it fits into a cone-shaped seat.

This design provides for a wide seating area and allows for excellent throttling
characteristics. The valve can be used for all pressure and teiiperature conditions.

The needle point valve (Figure 1.3-20) is an excellent example of a modified plug disc
design. The diameter of the seat opening is very narrow, and the disc descends well
below it and into the orifice formed by the seat. The needle point valve is used when
very close or delicate regulation of flow is required.

Many globe valves, especially those used for steam service, are built with a back seat. A
back seat is a seating arrangement that provides a seal between the stem and the
bonnet. The seating area of the bonnet is located at the point where the stem passes
through the bonnet (Figure 1.3-3). On a valve with a back seat, the stem is manufactured
with a seating area that is similar to a globe valve disc. When the valve is fully open, the
back seat on the stem seats with the bonnet seat.

The back seat design prevents system pressure

from building against the valve packing. In its
fully open position, the back seat of the disc
prevents leakage into the upper part of the valve.

Figure 1.3-3. Back Seat

Design

Globe valves are usually constructed with a rising stem, similar to that found on many gate
valves. The stem is threaded into the bonnet or yoke bushing. As with gate valves, the
specific function of a globe valve determines the material used in its construction. Bronze,
cast iron, and steel are widely used materials. For special services, globe valves are
made of nickel-bearing iron or steel, stainless steel, titanium, or other alloys.

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3. Globe, Butterfly, Diaphragm, and Check Valves (Continued)

1.3.2 Plug Valves and Ball Valves

The plug valve and the ball valve are distinguished from other types of valves by the fact

that they are not made with discs that rise and descend from the valve seating area. The

discs on these valves open by rotation. When the valve is actuated, the disc makes a one-

quarter turn, which stops the flow through the valve very quickly. This design does not

provide the same tight sealing quality for high-pressure service as the globe valve.

However, it does have excellent sealing qualities at lower pressures. For this reason, plug

valves and ball valves are not usually found in systems requiring a tight shutoff. They are

generally used in lubricating systems.

A plug valve (Figure 1.3-4A) or a ball valve (Figure

1.3-48) may be constructed with a single port or

with more than one port. Dual-port plug or bail

valves allow the pump suction to be shifted from

one supply source to another without a loss of flow.

The seats are often made of plastic-coated, self-

Figure 1.3-4A. Plug Valve
lubricating material. The plug valve or ball valve is

very adaptable to systems carrying slurry through

the valve. (Slurry is a fluid that often contains

large solid particles.)

Figure 1.3-4B. Ball Valve

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3. Globe, Butterfly, Diaphragm, and Check Valves (Continued)

1.3.3 Butterfly Valve

Butterfly valves are normally found in low-pressure, low-temperature systems. They are

useful on large volume systems where space is limited.

The butterfly valve is operated by a disc that

turns 90 degrees from the fully open position to

the fully closed position when actuated. The

disc is always the same diameter as the piping

on which the valve is attached. This gives

smoother flow with less pressure drop across

the valve. Figure 1.3-5 shows a butterfly

valve with the disc fully open. The position

of the butterfly valve can easily be observed.

When the operating lever is in line with the

Figure 1.3-5. Butterfly Valve
piping it is attached to, the valve is open.

When the valve operator is perpendicular to the piping, the valve is closed.

Most butterfly valves are constructed with a resilient, natural gum rubber seat that provides

a firm fit and a tight seal when the disc closes on it. In addition to the shutoff function,

butterfly valves can be used for throttlinc applications. However, this application is

not used frequently, because of the extremely poor throttling characteristics of a

butterfly valve.

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1.3.4 Diaphragm Valves

Another type of valve with disc and seat seating surfaces made of flexible material is the
diaphragm valve. Diaphragm valve seats are usually made of rubber, neoprene, or soft
plastic, all of which provide very effective control and sealing qualities to prevent
abrasion or corrosion of the components of the valve.

Figure 1.3-6 shows the basic diaphragm valve design.

Although it is somewhat different in appearance, the
handwheel on the valve works like those found on gate
or globe valves. The bonnet, as well, is similar to the
bonnet on a gate valve. It is large enough to house the
diaphragm compressor when the valve is fully open.
Unlike other types of valves, however, a compressor is
connected to the stem of the diaphragm valve. As it is
raised or lowered, the coiressor operates against
the seating area of the valve's diaphragm.

Figure 1.3-6. Diaphragm Valve

The diaphragm is flexible and, when pressed against

the seating surface of the valve, it will stop flow. This
design gives an excellent seal, even when slurries are
carried through the valve. Figure 1.3-7 shows three
different types of diaphragms.

Another feature of the diaphragm valve that

distinguishes it from other types of valves is that it
has no packing. Packing is not necessary, since the
diaphragm forms a boundary against the leakage of
fluid by the stem and bonnet union. Figure 1.3-7. Types of Diaphragms

Two basic body designs are used for diaphragm valves: the straight-through and the
weir.

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3. Globe, Butterfly, Diaphragm, and Check Valves (Continued)

As shown in Figure 1.3-8, the straight-through body

has no obstructions where the diaphragm seats. The

bore of the valve is the same as that of the piping to

which the valve is connected. This is very useful where

a severe pressure drop across the valve is

unacceptable.

Figure 1.3-8. Straight-Through

Diaphragm Valve

The weir (Figure 1.3-9) has a raised surface where

the diaphragm seats on the valve body. This allows

for the use of stiffer diaphragm material, but it

reduces the area through which flow may pass,

thus causing a large pressure drop across the

valve.

Figure 1.3-9. Weir Diaphragm

Valve

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A great many combinations of material can be used in the construction of a diaphragm

valve. This makes the diaphragm valve highly suitable for use in systems handling

corrosive solutions, as well as systems with moderate differences in temperature and

pressure. Although most diaphragm valve designs are not intended for precision throttling,

they can maintain good throttling qualities, and some types have relatively low pressure-

drop features. Diaphragm valves are widely used throughout the plant, but the are

particularly well suited for systems carrying a flow of chemicals or harmful gases.

1.3.5 Check Valves

The check valve is designed to permit flow in one direction only. It will prevent any
backflow of fluid within the piping to which it is connected. Most check valves cannot be
used as isolation valves, because they are built to close only if the flow is stopped or if,
for some reason, the system to which they are connected tries to reverse the direction of
flow.

There are three basic types of check valves: (1) swing,

(2) lift, and (3) Stop check. The swing-type check valve is
shown in Figure 1.3-10. on this valve, the disc is
attached to a pivoting arm or hinge pin. Flow through the
system keeps the disc raised and fully open. If flow is
stopped or reversed, the disc will close on the seat
immediately. The valve body is equipped with a cap that
provides access to the disc for maintenance.

Figure 1.3-10. Swing Type

Check Valve

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On the lift-type check valve (Figure 1.3-11), the disc is

not pivoted by an arm. it is raised when system pressure
is greater than the weight of the disc. Then flow is
permitted through the valve. The disc on this type of
valve is shaped like a globe. Guides attached to the
disc keep it in the proper position, preventing the disc
from jamming against the seat when the valve is opened
or closed. Lift-type check valves axe well suited for
service in systems with a fluctuating flow, such as
steam, air, and gases, because they do not damage as
easily as swing check valves.

The ball check is a variation of the lift check, This

disc looks like a ball. Ball checks are commonly used
on gauge glass isolation valves.

The stop check valve (Figure 1.3-12) resembles a

modified globe valve and can be used as an isolation
valve. When the stem is completely down, the disc
is on the seat and the valve is closed. However, when
the stem is raised, the disc operates like the disc on a
lift-type check valve. As with the lift-type check valve, the
stop check responds to system pressure for opening
and closing. As system pressure increases to overcome
the weight of the disc, the disc is lifted, and the flow Figure 1.3-12. Stop Check
can be carried through the valve. Valve

Check valves always operate automatically in response to system conditions, provided

they are in proper working order. Most check valves have replaceable seats, discs, and
caps. Check valves can be made of bronze, cast iron, or steel; however, for special uses,
they may be made of stainless steel, nickel, or other corrosion-resistant materials.

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1.4 Handwheels and Mechanical Operations

OBJECTIVES:

 List the types of mechanical operators discussed in this section.

 Explain how each type of mechanical operator activates a valve to open or

close.

1.4.1 Handwheel Operation

A handwheel is operated manually to open or close a valve. The handwheel should be

located so that proper force can be applied by the valve operator to turn the wheel, but, in

some systems, this is impossible. Valves often must be located in relatively inaccessible

places. In these cases, adapters are attached to the handwheels to operate the valves.

An adapter may be a chain running through a pulley or an extension arm attached to the

handwheel. Figure 1.4-1 illustrates these two types of adapters.

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4. Handwheels and Mechanical Operations (continued)

Proper manual operation of a valve requires certain specific procedures to ensure that the

valve will not be damaged. For example, two hands should be used when opening a valve,

one on each side of the handwheel. In this way, pressure or torque can be applied evenly

to the valve.

If the valve has a single operator, such as the type

found on but terfly valves and illustrated in Figure

1.4-2, or a wrench operator, one hand is used to

pull the operator while the other puts pressure

against the stem in the opposite direction. This

method of opening a butterfly or other single

operator valve will ensure that ensure that forces

Figure 1.4-2. Single-Operator
on the valve are evenly distributed.
Butterfly Valve

The application of evenly distributed force when opening a valve is essential in

preventing damage to the valve. If uneven force is applied, the valve stem can be twisted,

bent, or broken. The stem bushing receives unnecessary wear if uneven force is applied

when opening a valve. To keep valves in good working condition and system operations

running smoothly, care should always be taken to apply even pressure when manually

opening a valve. The position of a valve is always checked in the closed direction. This is

done by trying to close the valve. Checking the position in this manner will prevent an

undesired flow from the system through the valve.

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4. Handwheels and Mechanical Operations (continued)

The knocker valve, shown in Figure 1.4-3, is equipped with

a modified common handwheel. This handwheel is

designed for heavy-duty service. The handwheel on the

knocker valve is weighted, it has a special impact surface,

and the handwheel has about one quarter turn of "free

play."

Figure 1.4-3. Knocker Valve

The knocker valve is opened or closed in the following manner. First, the handwheel is

turned in a direction opposite to the' desired rotation, approximately 1/4 of a turn. Then it

is turned as forcefully as possible in the desired direction. This action will slam the stem of

the valve intomotion. Thus, the disc inside the valve is either raised or lowered from the

seat.

1.4.2 Motor-Driven Valve Operators

In addition to handwheels, motor-driven valve operators are also used to open or close

valves. Electric, hydraulic, or pneumatic motors provide ease in handling and allow for

remote operation. Electric solenoids are also used for remote operation.

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The electric motors connected to the valve stem are

usually high-powered, but they operate at relatively low

speeds. When the motor is energized, the stem begins

its rotation to raise or lower the disc. A limit switch

stops the motor when the valve is fully open or closed.

Figure 1.4-4 illustrates this process.

Figure 1.4-4. Electric Valve

Operator

Pneumatic and hydraulic motors operate in a similar manner. The stem of the valve is

rotated by pistons, which are activated by air or hydraulic oil, In this way, the disc is

raised or lowered from its seat.

There are some common variations to this process. For

example, pneumatic and hydraulic operators may be

applied to a single piston or to a piston-operated rack and

pinion to produce movement. If pressure is applied under

the piston, the disc is raised to open the valve. The top side

of the piston is vented, allowing the piston and valve to

Figure 1.4-5. Single make a complete stroke.
Piston Pneumatic
Operator

The reverse takes place when the valve is closed. Figure 1.4-5 shows a single piston

pneumatic operator.

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4. Handwheels and Mechanical Operations (continued)

An electric solenoid valve, shown in Figure 1.4-6, is

often used to control the flow of air through a system.

This valve is always either fully open or fully closed,

and it is not used for throttling. A magnetic field is

formed when the solenoid operator is energized.

The magnetic field draws a metal slug into it, which

opens or closes the valve.

Figure 1.4-6. Electric
Solenoid Valve

The diaphragm operator illustrated in Figure 1.4-7 is

moved by air pressure. The diaphragm is raised or
lowered by variations in applied air pressure.

The diaphragm operator is commonly designed to operate

in the following way. The diaphragm separates two
chambers within the operator body. A spring under the
diaphragm holds the valve open or closed when no air
pressure is being exerted on the valve. By applying
Figure 1.4-7. Diaphragm
and varying air pressure from the opposite side of the operator
diaphram, the disc movement is varied.

A vent on the side of the diaphragm where air pressure is being supplied to the valve

keeps the air vented and ensures that the valve will not become bound, which would

prevent the disc from moving. As air pressure decreases, the valve is opened or closed by

the spring, ensuring direct seating. Figure 1.4-8 illustrates this process.

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The diaphragmoperated valve, like other

valves, may be deesigned for reverse

seating.

The disc arrangevent is shown in Figure 1.4-

9. When the valve stem is raised, the valve will

close.

Although there are several variations of this

type of valve, all diaphragm operators allow

flow to be automatically regulated as system

conditions change.

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1.5 Variations on Common Valves and Flow Characteristics

OBJECTIVES:

 Give an example of the application of each of the following valves: pinch, boot,

cryogenic.

 Explain how flow through a valve varies with disc design and what can be done

to change the flow characteristics of a valve.

1.5.1 Variations on Common Valves

In addition to the valves that are commonly found in industrial facilities, there are also

valves designed for special applications. These valves may be found where corrosive or

caustic chemicals are used, where cleanliness is an important consideration, or where

precise flow control is necessary. This section describes certain special purpose valves

and explores some of the difficulties encountered in operating valves under extreme

conditions.

1.5.1.1 Variations of the Diaphragm Valve

Two specialized variations of the diaphragm valve

are the pinch valve and the boot valve.

The pinch valve, illustrated in Figure 1.5-1,is

equipped with a flexible tube that goes through

the valve body and forms flange gaskets between

the valve and the connecting pipes.

Figure 1.5-1. Pinch Valve
Cutaway

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5. Variations on Common Valves and Flow Characteristics (continued)

The flexible tube is made of a material that has good abrasion-resistent properties.

These features enable the valve to handle and control a flow containing slurries,

chemicals, processed food, or pharmaceutical materials. In addition, the flexible tube

on a pinch valve serves as a protective barrier, or sleeve, to form a seal that prevents

dirt or other contaminants from entering or leaving the system.

A compressor arrangement, or disc, on the pinch valve is designed to close the valve

pinching off the flow with a tight seal. The flexible material, along with system pressure

and flow, then forces the sleeve open as the stem and compressor are rotated to the

open position. The pinch valve may be actuated manually or through a mechanical

operator.

Due to the smooth straight flow path through a

pinch valve, there is a minimum pressure drop

across the valve, as iilustrated in Figure 1.5-2.

Figure 1.5-2. Straight Flow Through

a pinch valve Body

The boot valve is another variation on the common diaphragm valve. It is similar to the

pinch valve in that it also has a smooth, flexible tube running through it.

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5. Variations on Common Valves and Flow Characteristics (continued)

There are two compressors that flex the boot to

open and close, and also provide throttling action.

The boot valve has good throttling characteristics; it

is used for the same basic purposes as the pinch

valve. Figure 1.5-3 illustrates a boot valve.

Figure 1.5-3. Boot Valve

The double compressor is used to close the boot valve, stopping flow through it. With
each side of the boot flexing to close the valve, wear on each individual side is reduced
thus giving added life to the valve.

1.5.1.2 Cryogenic Valves

Cryogenics is the term used for the storage and handling of substances such as liquid
nitrogen or oxygen at very low temperatures (temperatures in the minus 200 degree
centigrade range). This type of storage and handling poses special problems for the
design characteristics of the cryogenic valve. For example, liquified gases are
extremely cold, and they will cause most valve materials and components to become
brittle rapidly.

Another problem involved in the use of valves in liquid gas systems is heatgain, because
of the great temperature difference between the lines carrying and storing liquid gas
and the surrounding atmosphere of the plant. Heat gain can be expensive, since
additional machinery is required to minimize a temperature increase. Even with the use of
heavy insulation, there are always certain portions of a system that are exposed to the
plant atmosphere; this is especially true of areas around valves.

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5. Variations on Common Valves and Flow Characteristics (continued)

Thus, an effective cryogenic valve must be made to withstand extremely cold

temperatures with a minimum of heat gain from the surrounding plant atmosphere.

The disc and seating arrangement of a

cryogenic valve is quite similar to that of a
common globe valve. However, there are
some differences in the stem and packing
arrangement. The stem extends through a
long, hollow operating tube to connect the
disc to the operator. Around the outside of
the hollow tube and valve body is a vacuum
jacket, which acts as an insulator. Thermal
foam is placed at the top of the hollow tube
shaft to provide insulation. The valve is
packed at the top, or "warm," end of the
valve with chevron packing or an `0" ring
to ensure that there is no leakage from the
valve stem. This arrangement is illustrated
in Figure 1.5-4. Figure 1.5-4. A Cryogenic Valve
Arrangement of the Stem and Packing
Gland

Gases in a liquid state are very expensive to maintain and to store, and leakage must be

prevented. In addition, certain gases, such as hydrogen, are highly explosive and can

create great hazards to plant personnel and equipment if they are accidentally leaked from

their enclosures. Other gases may replace the oxygen in the air. In such cases,

personnel would need special breathing apparatus to enter the area to repair the leak.

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5. Variations on Common Valves and Flow Characteristics (continued)

Cryogenic valves are usually constructed from materials such as bronze and austenitic

stainless steel, because these materials do not generally become brittle. Carbon steel, on

the other hand, which is used in the manufacture of many common varieties of industrial

valves, is not used for cryogenic valves, since it will tend to become brittle easily when

exposed to extremely low te.~eratures. While the liquid gases are being cooled, they are

also contained at very high pressures. If the pipes or valves were to become brittle, they

would burst.

1.5.1.3 "Vee" Ball Valves

The °vee'1 ball valve is finding wider use in industrial facilities, because, like the pinch

valve, it has very low pressure drop characteristics and is designed to maintain the rate of

flow through it. It is also highly resistant to clogging, which makes it adaptable to a

wide variety of flow materi als.

The "vee" ball operates Just like the standard ball

valve, but slurries, such as pulp, coal, or ash can

also be controlied through the vee ball valve.

Control is applied by a wedge shearing action. As

the ball is rotated to close, any material that may clog

the passage is sheared off. The vee' ball valve is

illustrated in Figure 1.5-5.

Figure 1.5-5. "Vee" Ball Valve

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1.5.1.4 Piston Balance Valves

When a single disc valve is put into operation, all pressure drop across the valve is

exerted on the area of the valve seat. This force of pressure must be overcome by the

valve actuator. On high-pressure systems, this means that the valve actuator must be

large enough to combat the force of system pressure bearing on it through the valve.

To overcome this problem, certain valves are designed with a double disc arrangement.

This design is especially servicable where system pressure is very high or where constant

adjustments must be made to control a flow through the valve. One type of valve with a

double disc design is the piston balance valve.

A piston balance valve allows the force exerted

across the seating area to be as equal as

possible, while being directed in opposite

directions. The double disc arrangoment in

Figure 1.5-6 has the flow coming in from the

left. In the closed position, the fluid pressure

would press against the bottom of the top disc

and against the top of the bottom disc -

equalizing flow pressure. Even when the valve

is open, there is an equalizing force, which

gives the valve a smooth operation. Figure 1.5-6 Piston Balance Valve

Showing Double Disc Arrangement

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1.5.2 Flow Characteristics Regarding Valve Design

Host valves discussed in these two units on valve maintenance are designed with a single
disc. As has been noted, this design is adequate for most plant systems. The double disc
design can be used to equalize the pressure across the disc, and it can be varied to
change flow characteristics through the valve or to change the operation of the valve, such
as by decreasing the response time of the valve's action. Often, double disc valves used
for flow control will be designed for special flow characteristics.

In order to better understand the flow control features of valve design, a comparison of
flow characteristics through a valve can be made between a regular valve and a valve
designed for special flow requirements.

The graph presented in Figure 1.5-7 illustrates

the general flow characteristic curve for standard
design valves. Flow versus valve opening is
plotted on the graph. The valve position is
plotted on the horizontal axis, and the percent
of flow is plotted along the vertical axis.

Figure 1.5-7 Flow Characteristic

Curve for a Disc Valve

When the standard valve is fully closed, there is, of course, zero flow. As the valve is
opened, flow begins to increase rapidly through it. Soon, the amount of increase in flow
begins to change so that when the valve is 3/4 open, there is almost no additional
increase as the valve is opened all the way.

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5. Variations on Common Valves and Flow Characteristics (continued)

The method of flow control is suitable for many industrial applications. However, when fine

flow control is required, such as during controlled chemical additions or for boiler feed flow,

a precise control of flow through the valve as it opens is required. This control can be

achieved by a valve designed to have linear flow characteristics. An exacte of this type of

valve is one designed with a double disc arrangement and used for the control of water

going to a boiler.

The graph presented in Figure 1.5-8 shows

the characteristics of linear flow through a

double-disc valve. As with the standard valve

graph, the comparison is made between valve

opening and percent of flow through the

valve. The valve position is plotted along the

horizontal axis and the percent of full flow is

plotted along the vertial axis. For flow to be Figure 1.5-8 Linear Flow
Characteristics of a Double Disc Valve
linear, the characteristic curve must be a

straight line; hence, the name "linear".

The graph shows that for each percent that the valve is opened or closed, the flow will

change by the same percent. If the valve position is changed from 1/8 open to 1/4 open,

flow will increase by the same amount – from 1/8 to 1/4.

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Unit 1
GLOSSARY
This glossary contains terms pertinent to the study of standard valves designed for

industrial use and gives the meanings of the terms in that context.

Back seat
- A seating arrangement that pro-vides a seal between the stem and the
bonnet and prevents system pressure from building against the valve
packing. It is a machined area or additional disc on the stem, just above
the disc, that seats on the bonnet. The back seat area on the stem or disc
only seals when the valve is in the fully open position.

Bonnet
- A component attached to the valve body that supports the internal parts of
the valve not supported by the body and pro-vides housing for the disc when it
is raised from the seating area.

Bridge wall markings

- Information located on the outside of the valve body that indicates how the
internal parts of the valve are arranged and the direction of flow.

Classification
- Reference to the way in which valves are classified: by disc arrangement,
by function, by operating conditions such as temperature and pressure, or
by the way they attach to plant systems (flanging, welding, threading,
etc.).

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Cryogenics
- The term used for the storage and handling of substances at very low
temperatures (temperatures in the minus centigrade degree range), such
as liquid nitrogen or liquid oxygen.

Diaphragm
- A disc made of flexible material, such as rubber, neoprene, or soft plastic,
which provides an effective seal to. Prevent leakage into other working
parts of a valve.

Disc
- /\ movable internal component of a valve, which, when actuated, lifts from or
closes on the seating area to start, stop, or regulate a system flow.

Double-disc arrangement (globe valve)

- A type of disc that is designed to reduce the pressure difference across the
valve while regulating a system flow.

Handwheel
- The external component of a valve that can be manually operated to control
the movement of the disc within the valve.

Limit switch
- Similar to a remote control device; it indicates the position of a valve on panels
in other parts of the plant so that operators do not have to inspect the valve to
tell whether it is open or closed. It may also stop the current or air flow that is
powering a valve when the valve reaches the end of its travel.

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Mechanical operator
- The external component of a valve that controls the movement of the
disc within a valve by an electric, pneumatic, or hydraulic motor.

Packing
- A material that can be compressed in the stuffing box to form a seal
around the stem, preventing leakage while, at the same time allowing
rotation of the valve stem.

Packing gland
- A part attached to the bonnet that holds the packing in place and
compresses it to prevent leakage through the stem and bonnet.

Pressure drop
- Refers to a decrease in pressure as a fluid moves through a valve: the
difference between inlet pressure and outlet pres sure.

Seating area
- An immovable part located inside of the valve body on which the disc
seats to stop flow through the valve.

Service markings
- Markings located on the outside of the valve body that indicate the allowable
pressure that can safely be placed on the valve for a certain type of service.

Stem
- A rod attached to the disc that connects the disc to the valve operator. The
stem transmits the rotation of the operator to the disc.

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Stuffing box
- A recessed area at the point where the stem goes through the bonnet where
the packing is instal led or housed.

Throttled position
- Any position of the disc other than fully open or fully closed.

Valve
- A hand-operated or mechanical operated device that starts, stops, or
regulates a liquid or gaseous flow through a system or component.

Valve body
- The largest structural part of a valve, which provides the means for
attaching the valve to system components or piping. It also houses the
internal parts of the valve.

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