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Valves Types


Dear engineers, If you are searching for "Valves" or "Valves Types" the article below present all the information about valves types, installation and operation.

Gate Valves

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

  1. They have good shutoff characteristics.

  2. They are bidirectional.

  3. 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:

  1. 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.

  2. Gate valves require large space envelope for installation, operation, and maintenance.

  3. 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.

  4. Some designs of gate valves are susceptible to thermal or pressure binding, depending upon the application.

  5. 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.

  6. 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.

Wedge Type

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.

Double-Disc Type

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

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.

Pressure Binding

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.

Globe Valves

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:

  1. Good shutoff capability

  2. Moderate to good throttling capability

  3. Shorter stroke (compared to a gate valve)

  4. Available in tee, wye, and angle patterns, each offering unique capabilities

  5. Easy to machine or resurface the seats

  6. 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:

  1. Higher pressure drop (compared to a gate valve)

  2. Requires greater force or a larger actuator to seat the valve (with pressure under the seat)

  3. 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:

  1. Cooling water systems where flow needs to be regulated

  2. Fuel oil system where flow is regulated and leak tightness is of importance.

  3. High-point vents and low-point drains when leak tightness and safety are major considerations.

  4. Feed water, chemical feed, condenser air extraction, and extraction drain systems.

  5. Boiler vents and drains, main steam vents and drains, and heater drains.

  6. Turbine seals and drains.

  7. Turbine lube oil system and others.

Needle Valves

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

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:

  1. 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.

  2. Each type of check valve has limitations on its installation configurations.

  3. 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  1. 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.

Application Considerations

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

 

Valves Guide

What are Valves?

Valves are mechanical devices that controls the flow and pressure within a system or process. They are essential components of a piping system that conveys liquids, gases, vapors, slurries etc..

Different types of valves are available.. gate, globe, plug, ball, butterfly, check, diaphragm, pinch, pressure relief, control valves etc. Each of these types has a number of models, each with different features and functional capabilities. Some valves are self-operated while others manually or with an actuator or pneumatic or hydraulic is operated.


Functions from Valves are..

  • Stopping and starting flow
  • Reduce or increase a flow
  • Controlling the direction of flow
  • Regulating a flow or process pressure
  • Relieve a pipe system of a certain pressure


There are many valve designs, types and models, with a wide range of industrial applications. All satisfy one or more of the functions identified above. Valves are expensive items, and it is important that a correct valve is specified for the function, and must be constructed of the correct material for the process liquid.

Regardless of type, all valves have the following basic parts.. the body, bonnet, trim (internal elements), actuator, and packing. The basic parts of a valve are illustrated in the image on the right.


Valve Body

The valve body, sometimes called the shell, is the primary boundary of a pressure valve. He serves as the main element of a valve assembly because it is the framework that holds all the parts together.

The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints.

The valve-body ends are designed to connect the valve to the piping or equipment nozzle by different types of end connections, such as butt or socket welded, threaded or flanged.

Valve bodies are cast or forged in a variety of forms and each component have a specific function and constructed in a material suitable for that function.



Valve Body

Valve Body


Valve Bonnet

Valve Bonnet


Valve Bonnet

The cover for the opening in the body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are in many designs and models available.

A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is commonly connected to the body by a threaded, bolted, or welded joint. During manufacture of the valve, the internal components, such as stem, disk etc., are put into the body and then the bonnet is attached to hold all parts together inside.

In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts. Valve bonnets, although a necessity for most valves, represent a cause for concern. Bonnets can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost, and are a source for potential leakage.


Valve Trim

The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as Valve trim. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim.

A Valve's trim performance is determined by the disk and seat interface and the relation of the disk position to the seat. Because of the trim, basic motions and flow control are possible. In rotational motion trim designs, the disk slides closely past the seat to produce a change in flow opening. In linear motion trim designs, the disk lifts perpendicularly away from the seat so that an annular orifice appears.

Valve trim parts may be constructed of assorted materials because of the different properties needed to withstand different forces and conditions. Bushings and packing glands do not experience the same forces and conditions as do the valve disc and seat(s).

Flow-medium properties, chemical composition, pressure, temperature, flow rate, velocity and viscosity are some of the important considerations in selecting suitable trim materials. Trim materials may or may not be the same material as the valve body or bonnet.


Valve Disk and Seat(s)

Disc The disc is the part which allows, throttles, or stops flow, depending on its position. In the case of a plug or a ball valve, the disc is called plug or a ball. The disk is the third most important primary pressure boundary. With the valve closed, full system pressure is applied across the disk, and for this reason, the disk is a pressure related component. Disks are usually forged, and in some designs, hard surfaced to provide good wear properties. Most valves are named, the design of their disks.


Seat(s) The seat or seal rings provide the seating surface for the disk. A valve may have one or more seats. In the case of a globe or a swing-check valve, there is usually one seat, which forms a seal with the disc to stop the flow. In the case of a gate valve, there are two seats; one on the upstream side and the other on the downstream side. A gate valve disc has two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow. To improve the wear-resistance of the seal rings, the surface is often hard-faced by welding and then machining the contact surface of the seal ring. A fine surface finish of the seating area is necessary for good sealing when the valve is closed. Seal rings are not usually considered pressure boundary parts because the body has sufficient wall thickness to withstand design pressure without relying upon the thickness of the seal rings.


Valve Stem

The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball and Butterfly valves, the disc is rotated to open or close the valve.

Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.

There are five types of valve stems..

  • Rising Stem with Outside Screw and Yoke
    The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are isolated from the flow medium by the stem packing. Two different styles of these designs are available; one with the handwheel attached to the stem, so they can rise together, and the other with a threaded sleeve that causes the stem to rise through the handwheel. This type of valve is indicated by "O. S. and Y." is a common design for NPS 2 and larger valves.
  • Rising Stem with Inside Screw
    The threaded part of the stem is inside the valve body, and the stem packing along the smooth section that is exposed to the atmosphere outside. In this case, the stem threads are in contact with the flow medium. When rotated, the stem and the handwheel to rise together to open the valve.
  • Non Rising Stem with Inside Screw
    The threaded part of the stem is inside the valve and does not rise. The valve disc travels along the stem, like a nut if the stem is rotated. Stem threads are exposed to the flow medium, and as such, are subjected to the impact. That is why this model is used when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem material.
  • Sliding Stem
    This valve stem does not rotate or turn. It slides in and out the valve to open or close the valve. This design is used in hand-operated lever rapid opening valves. It is also used in control valves are operated by hydraulic or pneumatic cylinders.
  • Rotary Stem
    This is a commonly used model in ball, plug, and Butterfly valves. A quarter-turn motion of the stem open or close the valve.

Below you will find some links to detailed (large) images of Rising and NON Rising Stem valves.


Valve Stem Packing

For a reliable seal between the stem and the bonnet, a gasket is needed. This is called a Packing, and it is fitted with e.g. the following components..

  • Gland follower, a sleeve which compresses the packing, by a gland into the so called stuffing box.
  • Gland, a kind of bushing, which compressed de packing into the stuffing box.
  • Stuffing box, a chamber in which the packing is compressed.
  • Packing, available in several materials, like Teflon®, elastomeric material, fibrous material etc..
  • A backseat is a seating arrangement inside the bonnet. It provides a seal between the stem and bonnet and prevents system pressure from building against the valve pakking, when the valve is fully open. Back seats are often applied in gate and globe valves.

An important aspect of the life time of a valve is the sealing assembly. Almost all valves, like standard Ball, Globe, Gate, Plug and Butterfly valves have their sealing assembly based upon shear force, friction and tearing.

Therefore valve packaging must be properly happen, to prevent damage to the stem and fluid or gas loss. When a packing is too loose, the valve will leak. If the packing is too tight, it will affect the movement and possible damage to the stem.


Typical sealing assembly



1 Gland Follover

2 Gland

3 Stuffing Box with Packing

4 Back Seat





Valve Yoke and Yoke Nut

Yoke

A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the Yoke holding a Yoke nut, stem nut, or Yoke bushing and the valve stem passes through it. A Yoke usually has openings to allow access to the stuffing box, actuator links, etc.. Structurally, a Yoke must be strong enough to withstand forces, moments, and torque developed by the actuator.

Yoke Nut

A Yoke nut is an internally threaded nut and is placed in the top of a Yoke by which the stem passes. In a Gate valve e.g., the Yoke nut is turned and the stem travels up or down. In the case of Globe valves, the nut is fixed and the stem is rotated through it.


Valve Actuator

Hand-operated valves are usually equipped with a handwheel attached to the valve's stem or Yoke nut which is rotated clockwise or counter clockwise to close or open a valve. Globe and gate valves are opened and closed in this way.

Hand-operated, quarter turn valves, such as Ball, Plug or Butterfly, has a lever for actuate the valve.

There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include..

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

These valves are usually equipped with an actuator.
An actuator in the broadest definition is a device that produces linear and rotary motion of a source of power under the action of a source of control.

Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There a many different types of actuators, but the following are some of the commonly used valve actuators..

  • Gear Actuators
  • Electric Motor Actuators
  • Pneumatic Actuators
  • Hydraulic Actuators
  • Solenoid Actuators

For more information about Actuators see main Menu 'Valves'


Classification of Valves

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

  • Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm, pinch, and lift Check Valves, moves in a straight line to allow, stop, or throttle the flow.
  • Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, as in butterfly, ball, plug, eccentric- and Swing Check Valves, the valves are called rotary motion valves.
  • Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through 90°, motion of the stem to go to fully open from a fully closed position or vice versa.

Classification of Valves based on Motion

Valve Types Linear Motion Rotary Motion Quarter Turn Gate YES NO NO Globe YES NO NO Plug NO YES YES Ball NO YES YES Butterfly NO YES YES Swing Check NO YES NO Diaphragm YES NO NO Pinch YES NO NO Safety YES NO NO Relief YES NO NO

Class Ratings

Pressure-temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes.. standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves.


Different types of valves are used in process plants, so there are many standards for valves. Below are the most common..

ASME B16.10 covers Face-to-Face and End-to-End Dimensions of Ferrous Valves

API STANDARDS

API 6D is used to design a valve that is used in the transportation pipeline.

API 600 is used for Bolted Bonnet Steel Gate Valves for Petroleum and Natural Gas Industries

API STD 602 is used to design Compact Steel Gate Valves – Flanged, Threaded, Welding, and Extended-Body Ends.

API STD 594 covers the design, material, face-to-face dimensions, pressure-temperature ratings, examination, inspection, and test requirements for Check Valves with Flanged, Lug, Wafer, and Butt-welding type ends.

API STD 599 covers design for Metal Plug Valves with Flanged, Threaded, and Welding Ends, in sizes NPS 1 through NPS 24.

API STD 609 is for Butterfly Valves with Double Flanged, Lug- and Wafer-Type.

API STD 526 covers Flanged Steel Pressure Relief Valves.

API RP 520 Part 1 overs Sizing and Selection, and Part 2 covers Installation of Pressure-Relieving Devices in Refineries.

API STD 598 covers inspection, supplementary examination, and pressure test requirements for both resilient-seated and metal-to-metal seated gate, globe, plug, ball, check, and butterfly valves at the valve manufacturer’s plant.

Most of the standards above, are used for dimensional inspection of piping components. Some are also used for component design.


Summary

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

As you may have seen in the main Menu "Valves", you can find also information about several and often applied valves in petro and chemical industry.
It can give you an impression, and good understanding of the differences between the various types of valves, and how these differences affect the valve function. It will help to a proper application of each type of valve during the design and the proper use of each type of valve during operation.


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