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Gate valves are primarily designed to serve as isolation valves. In service, these valves generally are either fully open or fully closed. When fully open, the fluid or gas flows through the valve in a straight line with very little resistance. Gate valves should not be used in the regulation or throttling of flow because accurate control is not possible. Furthermore, high-flow velocity in partially opened valves may cause erosion of the discs and seating surfaces. Vibration may also result in chattering of the partially opened valve disc. An exception to the above are specially designed gate valves that are used for low-velocity throttling; for example, guillotine gate valves for pulp stock.
Advantages of Gate Valves
They have good shutoff characteristics.
They are bidirectional.
The pressure loss through the valve is minimal.
Disadvantages of Gate Valves
The following are some of the disadvantages of gate valves that must be considered when selecting a gate valve for an application:
Gate valves are not quick opening or closing valves. Full-stem travel to open or close a gate valve requires many turns of its handwheel or an actuator.
Gate valves require large space envelope for installation, operation, and maintenance.
The slow movement of the disc near the full-closed position results in high-fluid velocities, causing scoring of seating surfaces, referred to as wire drawing. It also causes galling of sliding parts.
Some designs of gate valves are susceptible to thermal or pressure binding, depending upon the application.
In systems experiencing high-temperature fluctuations, wedge-gate valves may have excessive leakage past the seats due to changes in the angular relationship between the wedge and the valve seats caused by piping loads on the valve ends.
Repair or machining of valve seats in place is difficult.
Construction of a Gate Valve
Gate valves consist of three major components: body, bonnet, and trim. The body is generally connected to the piping by means of flanged, screwed, or welded connections. The bonnet, containing the moving parts, is joined to the body, generally with bolts, to permit cleaning and maintenance. The valve trim consists of the stem, the gate, the wedge, or disc, and the seat rings.
Two basic types of gate valves are the manufactured-wedge type and the double-disc type, and there are several variations within each of these types. A third type of gate valve, called conduit valve, is shown in Fig. A10.5.
There are four types of wedges: solid, hollow, split, and flexible wedge. The solid wedge is a single-piece solid construction. It does not compensate for changes in seat alignment due to pipe end loads or thermal fluctuations. As such it is most susceptible to leakage. Except for NPS 2 (DN 50) and smaller, solid-wedge discs are generally not recommended for use in applications having temperatures in excess of 250°F (121°C). Solid-wedge gate valves are considered the most economical. Almost all small, NPS 2 (DN 50) and smaller, gate valves are solid-wedge gate valves. Solid-wedge gate valves are generally used in moderate to lower pressure-temperature applications. It is common practice to use cast iron or ductile iron solid-wedge gate valves in cold or ambient water lines. A hollow wedge is a variation of solid wedge with the exception of a hole in the center. The hollow wedge travels along the stem when the threaded stem isrotated, thus opening or closing the valve port.
The flexible wedge is also one-piece construction like a solid wedge, but areas behind the seating surfaces are hollowed out to provide flexibility. This construction compensates for changes in seat alignment for improved seating while maintaining the strength of a solid wedge in the middle. This design offers better leaktightness and improved performance in situations with potential for thermal binding.
The split wedge consists of two-piece construction which seats between the tapered seats in the valve body. The two pieces of split wedge seat flat against the valve seats as the stem is moved downward, and they move away from the valve seats when the stem is pulled upward.
In the wedge or disc-wedge types either a tapered solid or tapered split wedge is used. In the rising stem valves (Fig. A10.1), the operating threads are out of direct contact with the fluid or gas. The nonrising stem type (Fig. A10.2) is preferred where space is limited and where the fluid passing through the valve will not corrode or erode the threads or leave deposits on the threads. Also, the nonrising stem valve is preferred for buried service.
When the valve is closed, the gate disc is wedged on both sides against the seat. In split-wedge gate valves (Fig. A10.6), the two-piece wedge disc is seated between matching tapered seats in the body. This type is preferred where the body seats might be distorted due to pipeline strain.
In the rising-stem type of valve, the upper part of the stem is threaded and a nut is fastened solidly to the handwheel and held in the yoke by thrust collars. As the handwheel is turned, the stem moves up or down. In the nonrising stem valve, the lower end of the stem is threaded and screws into the disc, vertical motion of the stem being restrained by a thrust collar. The rising-stem valve requires a greater amount of space when opened. However, it is generally preferred because the position of the stem indicates at once whether the valve is open or closed. Nonrising stem valves are some times provided with an indicator for this purpose.
In the double-disc parallel-seat valves (Figs. A10.7a, A10.7b, and A10.7c), the discs are forced against the valve seats by a wedging mechanism as the stem is tightened. Some double-disc parallel- seat valves employ a design which depends mainly upon the fluid pressure exerted against one side of the disc or the other for its tightness. The major advantage of this type is that the disc cannot be jammed into the body, an action that might make it difficult to open the valve. This is particularly important where motors are used for opening and closing the valve.
Unlike the wedge in a wedge-gate valve, which only comes into contact with the seat rings when the valve is nearly closed, each disc in the parallel-seat valve slides against its seat while the valve is being opened or closed. Consequently, these components must be made of metals, which do not gall or tear when in sliding contact with each other. The double-disc parallel-seat gate valve is often favored for high-temperature steam service because it is less likely to stick in the closed position as a result of change in temperature.
Conduit Gate Valve
It is also referred to as a slide valve or parallel slide. The disc surfaces are always in contact with the body seats. Like the double-disc or parallel-seated gate valve, its disc seats against the downstream seat, depending on the flow direction. The inside diameter of a conduit gate valve is equal to the inside diameter of the connecting pipe. These valves are used in pipelines where pigs are run through the piping to perform cleaning of builtup deposits or debris. The typical applications of conduit valves include dirty river water with suspended solids or water with sludge or debris.
Conduit gate valves require a large-space envelope because of their longer disc proportions to accommodate both the blank and the spacer halves of the disc assembly. The valve is closed by moving the blank half downward to block the valve port. The spacer is accommodated in the sump part of the valve body. Refer to Fig. A10.5.
Conduit valves with Teflon (PTFE) seats can be used for low to intermediate temperatures (to 450°F or 232°C). Metal-seated valves may be used for temperatures up to 1000°F (538°C).
Thermal binding occurs when a valve is tightly shut off while the high temperature system is in operation. Later when the system is shut down and allowed to cool, thermal contraction of the valve seats move inward more than the wedge shrinkage. This can bind the wedge and seats tight enough to not allow the wedge to unseat or move when the handwheel or the valve actuator is activated to open the valve. Parallel seated gate valves are most suitable for applications having potential for thermal binding. Split-wedge or flexible-wedge type gate valves are expected to perform better than solid-wedge gate valves when thermal binding is a concern.
Sometimes in high-temperature applications, the flow medium, such as water or steam, is trapped in the valve bonnet area when the valve is closed for system shutdown. The valves that do not permit this trapped liquid or the condensate to reenter the piping either upstream or downstream may experience excessive pressures in the bonnet cavity when the system returns to operating temperature. This built-up pressure in the bonnet cavity can prevent the valve from opening and may cause damage to valve parts. See Fig. A10.8a.
Pressure binding may not occur if the leakage past the upstream seat is adequate to prevent over pressurization of the valve bonnet cavity. The following options offer solutions to this problem:
Drill a small hole on the upstream side of the disc. See Fig. A10.8b.
Install a small manual stop valve between the valve bonnet-neck and the upstream end of the valve. This valve shall be opened during startup.
Install a small relief valve in the bonnet.
Edward valves offer a new valve called ACEVE to solve this problem.
Typical Gate Valve Applications
Socket or butt-welding end-gate valves in air, fuel gas, feedwater, steam, lube oil, and other systems are typical applications. Threaded-end gate valves may be used in air, gaseous, or liquid systems. Con-cern for leakage from threaded connection can be addressed by seal welding the threaded connection or by using thread sealants, as appropriate. In low-pressure and low-temperature systems such as fire protection systems’ water piping or water distribution pipelines, flanged gate valves are commonly used.
Conventional globe valves may be used for isolation and throttling services. Although these valves exhibit slightly higher pressure drops than straight- through valves (e.g., gate, plug, ball, etc.), they may be used where the pressure drop through the valve is not a controlling factor. Also, wye-pattern (Fig. A10.9) and angle-pattern (Fig. A10.10) globe valves exhibit improved flow characteristics over the tee-pattern (Fig.
A10.11) globe valve. Because the entire system pressure exerted on the disc is transferred to the valve stem, the practical size limit for these valves is NPS 12 (DN 300). Globe valves larger than NPS 12 (DN 300) are an exception rather than the rule. Larger valves would require that enormous forces be exerted on the stem to open or close the valve under pressure. Globe valves in sizes up to NPS 48 (DN 1200) have been manufactured and used.
Globe valves are extensively employed to control flow. The range of flow control, pressure drop, and duty must be considered in the design of the valve to avert premature failure and to assure satisfactory service. Valves subjected to high-differential pressure-throttling service require specially designed valve trim. Generally the maximum differential pressure across the valve disc should not exceed 20 percent of the maximum upstream pressure or 200 psi (1380 kPa), whichever is less. Valves with special trim may be designed for applications exceeding these differential pressure limits.
Types of Globe Valves
Tee Pattern globe valves have the lowest coefficient of flow and higher pressure drop. They are used in severe throttling services, such as in bypass lines around a control valve. Tee-pattern globe valves may also be used in applications where pressure drop is not a concern and throttling is required. Refer to Fig. A10.11.
Wye Pattern globe valves, among globe valves, offer the least resistance to flow. They can be cracked open for long periods without severe erosion. They are extensively used for throttling during seasonal or startup operations. They can be rod through to remove debris when used in drain lines that are normally closed. Refer to Fig. A10.9.
Angle Pattern globe valves turns the flow direction by 90 degrees without the use of an elbow and one extra weld. They have a slightly lower coefficient of flow than wye-pattern globe valves. They are used in applications that have periods of pulsating flow because of their capability to handle the slugging effect of this type of flow. Refer to Fig. A10.10.
Construction of a Globe Valve
A typical large globe valve with flanged ends is illustrated in Fig. A10.11, and a large wye-pattern globe is illustrated in Fig. A10.9. Globe valves usually have rising stems, and the larger sizes are of the outside screw-and-yoke construction. Components of the globe valve are similar to those of the gate valve. This type of valve has seats in a plane parallel or inclined to the line of flow.
Maintenance of globe valves is relatively easy, as the discs and seats are readily refurbished or replaced. This makes globe valves particularly suitable for services which require frequent valve maintenance. Where valves are operated manually, the shorter disc travel offers advantages in saving operator time, especially if the valves are adjusted frequently.
The principal variation in globe-valve design is in the types of discs employed. Plug-type discs have a long, tapered configuration with a wide bearing surface. This type of seat provides maximum resistance to the erosive action of the fluid stream. In the composition disc, the disc has a flat face that is pressed against the seat opening like a cap. This type of seat arrangement is not as suitable for high differential pressure throttling.
The conventional disc, in contrast to the plug type, provides a thin contact between the taper of the conventional seat and the face of the disc. This narrow contact area tends to break down hard deposits that may form on the seats and facilitates pressure-tight closure. This arrangement allows for good seating and moderate throttling.
In cast-iron globe valves, disc and seat rings are usually made of bronze. In steel-globe valves for temperature up to 750°F (399°C), the trim is generally made of stainless steel and so provides resistance to seizing and galling. The mating faces are normally heat-treated to obtain differential hardness values. Other trim materials, including cobalt-based alloys, are also used.
The seating surface is ground to ensure full-bearing surface contact when the valve is closed. For lower pressure classes, alignment is maintained by a long disc locknut. For higher pressures, disc guides are cast into the valve body. The disc turns freely on the stem to prevent galling of the disc face and seat ring. The stem bears against a hardened thrust plate, eliminating galling of the stem and disc at the point of contact.
Advantages of a Globe Valve
The following summarizes the advantages of globe valves:
Good shutoff capability
Moderate to good throttling capability
Shorter stroke (compared to a gate valve)
Available in tee, wye, and angle patterns, each offering unique capabilities
Easy to machine or resurface the seats
With disc not attached to the stem, valve can be used as a stop-check valve.
Disadvantages of a Globe Valve
The following are some shortcomings inherent in globe valves:
Higher pressure drop (compared to a gate valve)
Requires greater force or a larger actuator to seat the valve (with pressure under the seat)
Throttling flow under the seat and shutoff flow over the seat
Typical Applications of Globe Valves
The following are some of the typical applications of globe valves:
Cooling water systems where flow needs to be regulated
Fuel oil system where flow is regulated and leak tightness is of importance.
High-point vents and low-point drains when leak tightness and safety are major considerations.
Feed water, chemical feed, condenser air extraction, and extraction drain systems.
Boiler vents and drains, main steam vents and drains, and heater drains.
Turbine seals and drains.
Turbine lube oil system and others.
Needle valves generally are used for instrument, gauge, and meter line service. Very accurate throttling is possible with needle valves and, therefore, they are extensively used in applications that involve high pressures and/or high temperatures. In needle valves (Fig. A10.12), the end of the stem is needle point.
Check valves are designed to pass flow in one direction with minimum resistance and to prevent reverse or back flow with minimal leakage. The principal types of check valves used are the tee-pattern lift check, the swing check, the tilting-disc check, the wye-pattern lift check, and the ball check, illustrated in Figs. A10.13 to A10.17, respectively.
Construction of a Check Valve
A basic check valve consists of a valve body, bonnet or cover, and a disc which is attached to a hinge and swings away from the valve seat to allow fluid to flow in the forward direction, as in a swing or tilting-disc check valve, and returns to valve seat when upstream flow is stopped. Thus, reverse flow is prevented. In folding- disc check valves, the disc consists of two halves attached in the middle.
The two halves fold backward when upstream flow is initiated. Activated by a spring, the two halves quickly close the flow path when upstream flow ceases. In the case of lift-check valves, the disc is in the form of a piston which is moved out of the flow path by upstream flow and returns to the valve seat by gravity to stop back flow. Ball-check valves have a disc in the form of a ball.Check valves are available in sizes from NPS ¹⁄₄ (DN 6) through NPS 72 (DN 1800). Other sizes may be made available to meet specific size requirements. Depending upon the design requirements of a piping system, a check valve may have butt welding, socket welding, threaded, or flanged ends.
Advantages of Check Valves
They are self-actuated and require no external means to actuate the valve either to open or close. They are fast acting.
Disadvantages of Check Valves
The following are some of the disadvantages that are attributed to check valves:
Since all moving parts are enclosed, it is difficult to determine whether the valve is open or closed. Furthermore, the condition of internal parts cannot be assessed.
Each type of check valve has limitations on its installation configurations.
Valve disc can stick in open position.
Types of Check Valves
There are several types of check valves having varying body configurations. The following are some commonly used types of check valves:
Swing Check Valve. In swing check valves, the disc is unguided when it moves to fully open position or to fully closed position. Many different disc and seat designs are available to satisfy requirements of varying applications. Soft-seated– swing check valves provide improved leak tightness compared to metal-to- metal seating surfaces. Combination seats consisting of a metal seat ring with resilient insert also offer better leak tight characteristics. The seating angle, the angle between the seat and the vertical plane, may vary from 0 to 45 degrees. Vertical seats have a 0° angle. Larger seat angles reduce the disc travel, resulting in quick closing, thus minimizing the possibility of water hammer. Usually the seat angles are in the range of 5 to 7 degrees.
Lift Check Valve. Lift check valves are particularly adapted for high-pressure service where velocity of flow is high. In lift check valves, the piston disc is accurately guided by long contact and a close sliding fit with the perfectly centered dash pot. The walls of the piston and dash pot are of approximately equal thickness. Large steam jackets are located outside of the dash pot and inside the piston to eliminate sticking because of differential expansion. The seat ring is of a barrel- type design of heavy uniform cross-section. It is normally screwed in and seal welded. The flow opening is full port size. Refer to Figs. A10.13 and A10.16. 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. The ball-lift check valves are used in highly viscous fluid service. These valves have superior leaktight characteristics to those of swing- check valves. The piston type lift check valves have a tendency to stick in the open position when service fluid has sediment trapped above the piston. Large lift check valves are furnished with an equalizer line between the chamber above the disc and the downstream side of the valve.
Tilting Disc Check Valve. The tilting-disc check valve is designed to overcome some of the weaknesses inherent in conventional swing check valves. A combination of design features enables the valve to open fully and remain steady at lower flow velocities and to close quickly upon cessation of forward flow. The dome-shaped disc floats in the flow with fluid on both bottom and top of its surfaces, thus it has minimum dashpot effect. It performs well in pulsating, turbulent, and high-velocity flows. These attributes prolong the valve’s lift and reduce flow-induced dynamic loads on the piping system. Refer to Fig. A10.15.
Folding Disc Check Valves. This valve is also referred to as double-disc or split- disc check valve. Refer to Fig. A10.18. It is manufactured in wafer-body pattern and is available with soft or hard seats. It is very popular in low-pressure liquid and gaseous services. Its lightweight compact construction makes it a preferable check valve when space and convenience are important.
Vertical or In-Line Check Valve. These valves are available in two configurations: in-line ball check and fully guided disc with soft or hard seats. In-line ball check valves can be used in both vertical and horizontal lines. The fully guided disc inline check valves must be provided with a spring-assist closure when used in horizontal lines. In vertical lines, the guided disc in-line check valves may or may not be provided with spring-assist closure. The spring-assist closure not only assists in closing the valve quickly, it minimizes the possibility of water hammer by preventing flow reversal. They can be used in applications having pulsating flows, such as in a discharge line of a reciprocating compressor. Because they are compact in size, they are ideal for application in tight spaces.
Stop Check Valve. A stop check valve can either be used as a unidirectional check valve or as an isolation (stop) valve like a gate or globe valve. During normal operation of a system, these valves are used as a regular check valve; however, when needed, these valves can be closed with the help of a screw-down stem which is not fastened to the valve disc. The stem, when fully screwed down, holds the free-floating disc against the valve seat, just as in a gate or a globe valve. These valves are available in tee-pattern, wye-pattern, angle-pattern, and inclined pattern. The swing-and-piston lift-disc design check valves are commonly used as stop check valves. Refer to Figs. A10.19a and A10.19b.
The force of gravity plays an important role in the functioning of a check valve and, therefore, the location and orientation of the check valve must always be given consideration. Lift and ball check valves must always be placed so that the direction of lift is vertical. Swing checks must be located to ensure that the disc will always be closed freely and positively by gravity.
The flow velocity of the fluid through the valve has a significant effect on the life of the check valve. The valve should be sized such that the fluid velocity under normal conditions is sufficient to keep the disc fully open and pressed against the stop. This minimizes disc fluttering, which is the primary cause of valve failure.
Also, a check valve should not be located immediately downstream of a source of turbulence, such as a pump, elbow, control valve, or a tee-branch connection. It is recommended that manufacturer’s recommendations be followed to provide the required straight run of pipe upstream of the check valve. Some manufacturers recommend 8-to-10 pipe-diameter length of straight run of pipe upstream of the valve. Sometimes, the layout and the space available may not allow compliance to manufacturer’s recommendations. Alternatives must be evaluated and the most reasonable and feasible approach be implemented. A swing check valve may be used in the vertical run of a pipe only when the flow is upward. In addition, the flow velocity and the fluid pressure must be adequate to overcome the disc weight and swing it to the fully open position. In-line ball check valves are suitable for application in horizontal or vertical lines.When the flow is suspected to be pulsating and low, use of a swing check valve is not recommended. Due to the continuous flapping of the swing disc against the seat, valves suffer considerable damage, and at times the swing discs can come loose. Table A10.9 summarizes preliminary application guidelines for selection of a suitable type of check valve. The user must evaluate specific application featuresto determine the right valve for the application.
Typical Applications of Check Valves.
Table A10.9 provides a brief summary of different types of check valves and their typical applications. The preliminary guidelines of this table may be used to determine the suitable check valve for an application, considering the specifics of the application. #Little P.Eng