TYPES OF VALVES
Due to the various environments, system fluids, and system conditions in which
flow must be controlled, a large number of valve designs have been developed.
A basic understanding of the differences between the various types of valves, and
how these differences affect valve function, will help ensure the proper application
of each valve type during design and the proper use of each valve type during
operation.
a. Globe
b. Gate
c. Plug
d. Ball
e. Needle
f. Butterfly
g. Diaphragm
h. Pinch
i. Check
j. Safety/relief
k. Reducing
b. Gate
c. Plug
d. Ball
e. Needle
f. Butterfly
g. Diaphragm
h. Pinch
i. Check
j. Safety/relief
k. Reducing
Gate Valves
A gate valve is a linear motion valve used to start or stop fluid flow; however, it does not
regulate or throttle flow. The name gate is derived from the appearance of the disk in the flow
stream. Figure 4 illustrates a gate valve.
The disk of a gate valve is completely removed from the flow stream when the valve is fully
open. This characteristic offers virtually no resistance to flow when the valve is open. Hence,
there is little pressure drop across an open gate valve.
When the valve is fully closed, a disk-to-seal ring contact surface exists for 360°, and good
sealing is provided. With the proper mating of a disk to the seal ring, very little or no leakage
occurs across the disk when the gate valve is closed.
to percent of opening. This means that flow rate does not change evenly with stem travel.
Also, a partially open gate disk tends to vibrate from the fluid flow. Most of the flow change
occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual
leakage if used to regulate flow. For these reasons, gate valves are not used to regulate or
throttle flow.
A gate valve can be used for a wide variety of fluids and provides a tight seal when closed. The
major disadvantages to the use of a gate valve are:
It is not suitable for throttling applications.
It is prone to vibration in the partially open state.
It is more subject to seat and disk wear than a globe valve.
Repairs, such as lapping and grinding, are generally more difficult to accomplish.
Gate Valve Disk Design
Gate valves are available with a variety of disks. Classification of gate valves is usually made
by the type disk used: solid wedge, flexible wedge, split wedge, or parallel disk.
Solid wedges, flexible wedges, and split wedges are used in valves having inclined seats. Parallel
disks are used in valves having parallel seats.
Regardless of the style of wedge or disk used, the disk is usually replaceable. In services where
solids or high velocity may cause rapid erosion of the seat or disk, these components should
have a high surface hardness and should have replacement seats as well as disks. If the seats
are not replaceable, seat damage requires removal of the valve from the line for refacing of the
seat, or refacing of the seat in place. Valves being used in corrosion service should normally
be specified with replaceable seats.
Solid Wedge
The solid wedge gate valve shown in Figure 5 is the most commonly used disk because of its simplicity and strength. A valve with this type of wedge may be installed in any position and it is suitable for almost all fluids. It is practical for turbulent flow.
Flexible Wedge
The flexible wedge gate valve illustrated in Figure 6 is a one-piece disk with a cut around the perimeter to improve the ability to match error or change in the angle between the seats. The cut varies 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 at the center, which allows more flexibility but compromises strength.
Split Wedge
Split wedge gate valves, as shown in Figure 7, are of the ball and socket design. These are self-adjusting and selfaligning to both seating surfaces. The disk is free to adjust itself to the seating surface if one-half of the disk is slightly out of alignment because of foreign matter lodged between the disk half and the seat ring. This
type of wedge is suitable for handling noncondensing gases and liquids at normal temperatures, particularly
corrosive liquids. Freedom of movement of the disk in the carrier prevents binding even though the valve may
have been closed when hot and later contracted due to cooling. This type of valve should be installed with the stem in the vertical position.
Parallel Disk
The parallel disk gate valve illustrated in Figure 8 is designed to prevent valve binding due to thermal transients. This design is used in both low and high pressure applications. The wedge surfaces between the parallel face disk halves are caused to press together under stem thrust and spread apart the disks to seal against the seats. The tapered wedges may be part of the disk halves or they may be separate elements. The lower wedge may bottom out on a rib at the valve bottom so that the stem can develop seating force. In one version, the wedge contact surfaces are curved to keep the point of contact close to the optimum. In other parallel disk gates, the two halves do not move apart under wedge action. Instead, the upstream pressure holds the downstream disk against the seat. A carrier ring lifts the disks, and a spring or springs hold the disks apart and seated when there is no upstream pressure. Another parallel gate disk design provides for sealing only one port. In these designs, the high pressure side pushes the disk open (relieving the disk) on the high pressure side, but forces the disk closed on the low pressure side. With such designs, the amount of seat leakage tends to decrease as differential pressure across the seat increases. These valves will usually have a flow direction marking which will show which side is the high pressure (relieving) side. Care should be taken to ensure that these valves are not installed backwards in the system.
Some parallel disk gate valves used in high pressure systems are made with an integral bonnet vent and bypass line. A three-way valve is used to position the line to bypass in order to equalize pressure across the disks prior to opening. When the gate valve is closed, the three-way valve is positioned to vent the bonnet to one side or the other. This prevents moisture from accumulating in the bonnet. The three-way valve is
positioned to the high pressure side of the gate valve when closed to ensure that flow does not bypass the isolation valve. The high pressure acts against spring compression and forces one gate off of its seat. The three-way valve vents this flow back to the pressure source.
Split wedge gate valves, as shown in Figure 7, are of the ball and socket design. These are self-adjusting and selfaligning to both seating surfaces. The disk is free to adjust itself to the seating surface if one-half of the disk is slightly out of alignment because of foreign matter lodged between the disk half and the seat ring. This
type of wedge is suitable for handling noncondensing gases and liquids at normal temperatures, particularly
corrosive liquids. Freedom of movement of the disk in the carrier prevents binding even though the valve may
have been closed when hot and later contracted due to cooling. This type of valve should be installed with the stem in the vertical position.
The parallel disk gate valve illustrated in Figure 8 is designed to prevent valve binding due to thermal transients. This design is used in both low and high pressure applications. The wedge surfaces between the parallel face disk halves are caused to press together under stem thrust and spread apart the disks to seal against the seats. The tapered wedges may be part of the disk halves or they may be separate elements. The lower wedge may bottom out on a rib at the valve bottom so that the stem can develop seating force. In one version, the wedge contact surfaces are curved to keep the point of contact close to the optimum. In other parallel disk gates, the two halves do not move apart under wedge action. Instead, the upstream pressure holds the downstream disk against the seat. A carrier ring lifts the disks, and a spring or springs hold the disks apart and seated when there is no upstream pressure. Another parallel gate disk design provides for sealing only one port. In these designs, the high pressure side pushes the disk open (relieving the disk) on the high pressure side, but forces the disk closed on the low pressure side. With such designs, the amount of seat leakage tends to decrease as differential pressure across the seat increases. These valves will usually have a flow direction marking which will show which side is the high pressure (relieving) side. Care should be taken to ensure that these valves are not installed backwards in the system.
positioned to the high pressure side of the gate valve when closed to ensure that flow does not bypass the isolation valve. The high pressure acts against spring compression and forces one gate off of its seat. The three-way valve vents this flow back to the pressure source.
Gate Valve Stem Design
Gate valves are classified as either rising stem or nonrising stem valves. For the nonrising stem gate valve, the stem is threaded on the lower end into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type valve will almost always have a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. Figures 2 and 3 illustrate rising-stem gate valves and nonrising stem gate valves. The nonrising stem configuration places the stem threads within the boundary established by the valve packing out of contact with the environment. This configuration assures that the stem merely rotates in the packing without much danger of carrying dirt into the packing from outside to inside. Rising stem gate valves are designed so that the stem is raised out of the flowpath when the valve is open. Rising stem gate valves come in two basic designs. Some have a stem that rises through the handwheel while others have a stem that is threaded to the bonnet.
Gate Valve Seat Design
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. Small, forged steel, gate valves may have hard faced seats pressed into the body. In some series, this type of valve in sizes from 1/2 to 2 inches is rated for 2500 psig steam service. In large gate valves, disks are often of the solid wedge type with seat rings threaded in, welded in, or pressed in. Screwed in seat rings are considered replaceable since they may be removed and new seat rings installed.
Globe Valves
A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. A Z-body globe valve is illustrated in Figure 9. As shown in Figure 9, the globe valve disk can be totally removed from the flowpath or it can completely close the flowpath. The essential principle of globe valve operation is the perpendicular
movement of the disk away from the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow. Therefore, the globe valve may be used for both stopping and starting fluid flow and for regulating flow. When compared to a gate valve, a globe valve generally yields much less seat
leakage. This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk.
Gate valves are classified as either rising stem or nonrising stem valves. For the nonrising stem gate valve, the stem is threaded on the lower end into the gate. As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type valve will almost always have a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. Figures 2 and 3 illustrate rising-stem gate valves and nonrising stem gate valves. The nonrising stem configuration places the stem threads within the boundary established by the valve packing out of contact with the environment. This configuration assures that the stem merely rotates in the packing without much danger of carrying dirt into the packing from outside to inside. Rising stem gate valves are designed so that the stem is raised out of the flowpath when the valve is open. Rising stem gate valves come in two basic designs. Some have a stem that rises through the handwheel while others have a stem that is threaded to the bonnet.
Gate Valve Seat Design
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. Small, forged steel, gate valves may have hard faced seats pressed into the body. In some series, this type of valve in sizes from 1/2 to 2 inches is rated for 2500 psig steam service. In large gate valves, disks are often of the solid wedge type with seat rings threaded in, welded in, or pressed in. Screwed in seat rings are considered replaceable since they may be removed and new seat rings installed.
A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. A Z-body globe valve is illustrated in Figure 9. As shown in Figure 9, the globe valve disk can be totally removed from the flowpath or it can completely close the flowpath. The essential principle of globe valve operation is the perpendicular
movement of the disk away from the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow. Therefore, the globe valve may be used for both stopping and starting fluid flow and for regulating flow. When compared to a gate valve, a globe valve generally yields much less seat
leakage. This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk.
Globe valves can be arranged so that the disk closes against or in the same direction of fluid
flow. When the disk closes against the direction of flow, the kinetic energy of the fluid impedes
closing but aids opening of the valve. When the disk closes in the same direction of flow, the
kinetic energy of the fluid aids closing but impedes opening. This characteristic is preferable
to other designs when quick-acting stop valves are necessary.
Globe valves also have drawbacks. The most evident shortcoming of the simple globe valve is
the high head loss from two or more right angle turns of flowing fluid. Obstructions and
discontinuities in the flowpath lead to head loss. In a large high pressure line, the fluid dynamic
effects from pulsations, impacts, and pressure drops can damage trim, stem packing, and
actuators. In addition, large valve sizes require considerable power to operate and are especially
noisy in high pressure applications.
Other drawbacks of globe valves are the large openings necessary for disk assembly, heavier
weight than other valves of the same flow rating, and the cantilevered mounting of the disk to
the stem.
Globe Valve Body Designs
The three primary body designs for globe valves are Z-body, Y-body, and Angle.
Z-Body Design
The simplest design and most common for water applications is the Z-body. The Z-body is illustrated in Figure 9. For this body design, the Z-shaped diaphragm or partition across the globular body contains the seat. The horizontal setting of the seat allows the stem and disk to travel at right angles to the pipe axis. The stem passes through the bonnet which is attached to a large opening at the top of the valve body. This provides a symmetrical form that simplifies manufacture, installation, and repair.
Y-Body Design
Figure 10 illustrates a typical Y-body globe valve. This design is a remedy for the high pressure drop inherent in globe valves. The seat and stem are angled at approximately 45°.
The angle yields a straighter flowpath (at full opening) and provides the stem, bonnet, and packing a relatively pressureresistant envelope.
Y-body globe valves are best suited for high pressure and other severe services. In small sizes for intermittent flows, the pressure loss may not be as important as the other considerations favoring the Y-body design. Hence, the flow passage of small Y-body globe valves is not as carefully streamlined as that for larger valves.
The angle body globe valve design, illustrated in Figure 11, is a simple modification of the basic globe valve. Having ends at right angles, the diaphragm can be a simple flat plate. Fluid is able to flow through with only a single 90° turn and discharge downward more symmetrically than the discharge from an ordinary globe. A particular advantage of the angle body design is that it can function as both a valve and a piping elbow.
For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe. The angle valve's discharge conditions are favorable with respect to fluid dynamics and erosion.
Most globe valves use one of three basic disk designs: the ball disk, the composition disk, and the plug disk.
Ball Disk
The ball disk fits on a tapered, flat-surfaced seat. The ball disk design is used primarily in relatively low pressure and low temperature systems. It is capable of throttling flow, but is primarily used to stop and start flow.
Composition Disk
The composition disk design uses a hard, nonmetallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications. They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable.
Plug Disk
Because of its configuration, the plug disk provides better throttling than ball or composition designs. Plug disks are available in a variety of specific configurations. In general, they are all long and tapered.
Globe Valve Disk and Stem Connections
Globe valves employ two methods for connecting disk and stem: T-slot construction and disk
nut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, the
disk is screwed into the stem.
Globe Valve Seats
Globe valve seats are either integral with or screwed into the valve body. Many globe valves
have backseats. A backseat is a seating arrangement that provides a seal between the stem and
bonnet. When the valve is fully open, the disk seats against the backseat. The backseat design
prevents system pressure from building against the valve packing.
Globe Valve Direction of Flow
For low temperature applications, globe and angle valves are ordinarily installed so that pressure
is under the disk. This promotes easy operation, helps protect the packing, and eliminates a
certain amount of erosive action to the seat and disk faces. For high temperature steam service,
globe valves are installed so that pressure is above the disk. Otherwise, the stem will contract
upon cooling and tend to lift the disk off the seat.
Ball Valves
A ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow.
The ball, shown in Figure 12, performs the same function as the disk in the globe valve. When
the valve handle is turned to open the valve, the ball rotates to a point where the hole through
the ball is in line with the valve body inlet and outlet. When the valve is shut, the ball is rotated
so that the hole is perpendicular to the flow openings of the valve body and the flow is stopped.
Most ball valve actuators are of the quick-acting type, which require a 90° turn of the valve
handle to operate the valve. Other ball valve actuators are planetary gear-operated. This type
of gearing allows the use of a relatively small handwheel and operating force to operate a fairly
large valve.
Some ball valves have been developed with a spherical surface coated plug that is off to one side
in the open position and rotates into the flow passage until it blocks the flowpath completely.
Seating is accomplished by the eccentric movement of the plug. The valve requires no
lubrication and can be used for throttling service.
A ball valve is generally the least expensive of any valve configuration and has low maintenance costs. In addition to quick, quarter turn on-off operation, ball valves are compact, require no lubrication, and give tight sealing with low torque.
Disadvantages
Conventional ball valves have relatively poor throttling characteristics. In a throttling position, the partially exposed seat rapidly erodes because of the impingement of high velocity flow.
Port Patterns
Ball valves are available in the venturi, reduced, and full port pattern. The full port pattern has a ball with a bore equal to the inside diameter of the pipe.
Valve Materials
Balls are usually metallic in metallic bodies with trim (seats) produced from elastomeric (elastic materials resembling rubber) materials. Plastic construction is also available. The resilient seats for ball valves are made from various elastomeric material. The most common seat materials are teflon (TFE), filled TFE, Nylon, Buna-N, Neoprene, and combinations of these materials. Because of the elastomeric materials, these valves
cannot be used at elevated temperatures. Care must be used in the selection of the seat material to ensure that it is compatible with the materials being handled by the valve.
Ball Valve Stem Design
The stem in a ball valve is not fastened to the ball. It normally has a rectangular portion at the
ball end which fits into a slot cut into the ball. The enlargement permits rotation of the ball as
the stem is turned.
Ball Valve Bonnet Design
A bonnet cap fastens to the body, which holds the stem assembly and ball in place. Adjustment
of the bonnet cap permits compression of the packing, which supplies the stem seal. Packing for
ball valve stems is usually in the configuration of die-formed packing rings normally of TFE,
TFE-filled, or TFE-impregnated material. Some ball valve stems are sealed by means of O-rings
rather than packing.
Ball Valve Position
Some ball valves are equipped with stops that permit only 90° rotation. Others do not have
stops and may be rotated 360°. With or without stops, a 90° rotation is all that is required for
closing or opening a ball valve.
The handle indicates valve ball position. When the handle lies along the axis of the valve, the
valve is open. When the handle lies 90° across the axis of the valve, the valve is closed. Some
ball valve stems have a groove cut in the top face of the stem that shows the flowpath through
the ball. Observation of the groove position indicates the position of the port through the ball.
This feature is particularly advantageous on multiport ball valves.
Plug Valves
A plug valve is a rotational motion valve used to stop or start fluid flow. The name is derived
from the shape of the disk, which resembles a plug. A plug valve is shown in Figure 13. The
simplest form of a plug valve is the petcock. The body of a plug valve is machined to receive
the tapered or cylindrical plug. The disk is a solid plug with a bored passage at a right angle to
the longitudinal axis of the plug.
Plug valves are available in either a lubricated or nonlubricated design and with a variety of styles of port openings through the plug as well as a number of plug designs.
Plug Ports
An important characteristic of the plug valve is its easy adaptation to multiport construction. Multiport valves are widely used. Their installation simplifies piping, and they provide a more convenient operation than multiple gate valves. They also eliminate pipe fittings. The use of a multiport valve, depending upon the number of ports in the plug valve, eliminates the need of as many as four conventional shutoff valves.
Plug valves are normally used in non-throttling, on-off operations, particularly where frequent operation of the valve is necessary. These valves are not normally recommended for throttling service because, like the gate valve, a high percentage of flow change occurs near shutoff at high velocity. However, a diamond-shaped port has been developed for throttling service.
Multiport Plug Valves
Multiport valves are particularly advantageous on transfer lines and for diverting services. A single multiport valve may be installed in lieu of three or four gate valves or other types of shutoff valve. A disadvantage is that many multiport valve configurations do not completely shut off flow.
In most cases, one flowpath is always open. These valves are intended to divert the flow of one line while shutting off flow from the other lines. If complete shutoff of flow is a requirement, it is necessary that a style of multiport valve be used that permits this, or a secondary valve should be installed on the main line ahead of the multiport valve to permit complete shutoff of flow.
In some multiport configurations, simultaneous flow to more than one port is also possible. Great care should be taken in specifying the particular port arrangement required to guarantee that proper operation will be possible.
Plug Valve Disks
Plugs are either round or cylindrical with a taper. They may have various types of port openings, each with a varying degree of area relative to the corresponding inside diameter of the pipe.
Rectangular Port Plug
The most common port shape is the rectangular port. The rectangular port represents at least 70% of the corresponding pipe's cross-sectional area.
Round Port Plug
Round port plug is a term that describes a valve that has a round opening through the plug. If the port is the same size or larger than the pipe's inside diameter, it is referred to as a full port. If the opening is smaller than the pipe's inside diameter, the port is referred to as a standard round port. Valves having standard round ports are used only where restriction of flow is unimportant.
Diamond Port Plug
A diamond port plug has a diamond-shaped port through the plug. This design is for throttling service. All diamond port valves are venturi restricted flow type.
Lubricated Plug Valve Design
Clearances and leakage prevention are the chief considerations in plug valves. Many plug valves
are of all metal construction. In these versions, the narrow gap around the plug can allow
leakage. If the gap is reduced by sinking the taper plug deeper into the body, actuation torque
climbs rapidly and galling can occur. To remedy this condition, a series of grooves around the
body and plug port openings is supplied with grease prior to actuation. Applying grease
lubricates the plug motion and seals the gap between plug and body. Grease injected into a
fitting at the top of the stem travels down through a check valve in the passageway, past the plug
top to the grooves on the plug, and down to a well below the plug. The lubricant must be
compatible with the temperature and nature of the fluid. All manufacturers of lubricated plug
valves have developed a series of lubricants that are compatible with a wide range of media.
Their recommendation should be followed as to which lubricant is best suited for the service.
The most common fluids controlled by plug valves are gases and liquid hydrocarbons. Some
water lines have these valves, provided that lubricant contamination is not a serious danger.
Lubricated plug valves may be as large as 24 inches and have pressure capabilities up to 6000
psig. Steel or iron bodies are available. The plug can be cylindrical or tapered.
Nonlubricated Plugs
There are two basic types of nonlubricated plug valves: lift-type and elastomer sleeve or plug
coated. Lift-type valves provide a means of mechanically lifting the tapered plug slightly to
disengage it from the seating surface to permit easy rotation. The mechanical lifting can be
accomplished with a cam or external lever.
In a common, nonlubricated, plug valve having an elastomer sleeve, a sleeve of TFE completely
surrounds the plug. It is retained and locked in place by a metal body. This design results in
a primary seal being maintained between the sleeve and the plug at all times regardless of
position. The TFE sleeve is durable and inert to all but a few rarely encountered chemicals. It
also has a low coefficient of friction and is, therefore, self-lubricating.
Manually Operated Plug Valve Installation
When installing plug valves, care should be taken to allow room for the operation of the handle,
lever, or wrench. The manual operator is usually longer than the valve, and it rotates to a
position parallel to the pipe from a position 90° to the pipe.
Plug Valve Glands
The gland of the plug valve is equivalent to the bonnet of a gate or globe valve. The gland
secures the stem assembly to the valve body. There are three general types of glands: single
gland, screwed gland, and bolted gland.
To ensure a tight valve, the plug must be seated at all times. Gland adjustment should be kept
tight enough to prevent the plug from becoming unseated and exposing the seating surfaces to
the live fluid. Care should be exercised to not overtighten the gland, which will result in a
metal-to-metal contact between the body and the plug. Such a metal-to-metal contact creates an
additional force which will require extreme effort to operate the valve.
Diaphragm Valves
A diaphragm valve is a linear motion valve that is used to start, regulate, and stop fluid flow.
The name is derived from its flexible disk, which mates with a seat located in the open area at
the top of the valve body to form a seal. A diaphragm valve is illustrated in Figure 14.
connected to a compressor by a stud molded into the diaphragm. The compressor is moved up
and down by the valve stem. Hence, the diaphragm lifts when the compressor is raised. As the
compressor is lowered, the diaphragm is pressed against the contoured bottom in the straight
through valve illustrated in Figure 14 or the body weir in the weir-type valve illustrated in
Figure 15.
Diaphragm valves can also be used for throttling service. The weir-type is the better throttling
valve but has a limited range. Its throttling characteristics are essentially those of a quickopening
valve because of the large shutoff area along the seat.
A weir-type diaphragm valve is available to control small flows. It uses a two-piece compressor
component. Instead of the entire diaphragm lifting off the weir when the valve is opened, the
first increments of stem travel raise an inner compressor component that causes only the central
part of the diaphragm to lift. This creates a relatively small opening through the center of the
valve. After the inner compressor is completely open, the outer compressor component is raised
along with the inner compressor and the remainder of the throttling is similar to the throttling that
takes place in a conventional valve.
Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries,
radioactive fluids, or other fluids that must remain free from contamination.
Diaphragm Construction
The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline.
Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism.
Many fluids that would clog, corrode, or gum up the working parts of most other types of valves
will pass through a diaphragm valve without causing problems. Conversely, lubricants used for
the operating mechanism cannot be allowed to contaminate the fluid being handled. There are
no packing glands to maintain and no possibility of stem leakage. There is a wide choice of
available diaphragm materials. Diaphragm life depends upon the nature of the material handled,
temperature, pressure, and frequency of operation.
Some elastomeric diaphragm materials may be unique in their excellent resistance to certain
chemicals at high temperatures. However, the mechanical properties of any elastomeric material
will be lowered at the higher temperature with possible destruction of the diaphragm at high
pressure. Consequently, the manufacturer should be consulted when they are used in elevated
temperature applications.
Viton, for example, is noted for its excellent chemical resistance and stability at high
temperatures. However, when fabricated into a diaphragm, Viton is subject to lowered tensile
strength just as any other elastomeric material would be at elevated temperatures. Fabric
bonding strength is also lowered at elevated temperatures, and in the case of Viton, temperatures
may be reached where the bond strength could become critical.
Fluid concentrations is also a consideration for diaphragm selection. Many of the diaphragm
materials exhibit satisfactory corrosion resistance to certain corrodents up to a specific
concentration and/or temperature. The elastomer may also have a maximum temperature
limitation based on mechanical properties which could be in excess of the allowable operating
temperature depending upon its corrosion resistance. This should be checked from a corrosion
table.
Diaphragm Valve Stem Assemblies
Diaphragm valves have stems that do not rotate. The valves are available with indicating and
nonindicating stems. The indicating stem valve is identical to the nonindicating stem valve
except that a longer stem is provided to extend up through the handwheel. For the nonindicating
stem design, the handwheel rotates a stem bushing that engages the stem threads and moves the
stem up and down. As the stem moves, so does the compressor that is pinned to the stem. The
diaphragm, in turn, is secured to the compressor.
Diaphragm Valve Bonnet Assemblies
Some diaphragm valves use a quick-opening bonnet and lever operator. This bonnet is
interchangeable with the standard bonnet on conventional weir-type bodies. A 90° turn of the
lever moves the diaphragm from full open to full closed. Diaphragm valves may also be
equipped with chain wheel operators, extended stems, bevel gear operators, air operators, and
hydraulic operators.
Many diaphragm valves are used in vacuum service. Standard bonnet construction can be
employed in vacuum service through 4 inches in size. On valves 4 inches and larger, a sealed,
evacuated, bonnet should be employed. This is recommended to guard against premature
diaphragm failure.
Sealed bonnets are supplied with a seal bushing on the nonindicating types and a seal bushing
plus O-ring on the indicating types. Construction of the bonnet assembly of a diaphragm valve
is illustrated in Figure 15. This design is recommended for valves that are handling dangerous
liquids and gases. In the event of a diaphragm failure, the hazardous materials will not be
released to the atmosphere. If the materials being handled are extremely hazardous, it is
recommended that a means be provided to permit a safe disposal of the corrodents from the
bonnet.
