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Contents Page No. Abbreviations and Terminology................................................. 5 1 Introduction ………………………………………………………….. 6 2 Basic Valve Components............................................................ 8 3 Block Valves................................................................................. 11 3.1 Gate Valves........................................................................ 11 3.2 Ball Valves......................................................................... 19 3.3 Plug Valves........................................................................ 21 4 Flow Control (Throttle) Valves.................................................... 22 4.1 Globe Valves..................................................................... 23 4.2 Butterfly Valves................................................................. 27 4.3 Diaphragm Valves............................................................. 29 4.4 Needle Valves.................................................................... 32 5 Non-return (Check) Valves.......................................................... 34 5.1 Swing Check Valves......................................................... 35 5.2 Lift Check Valves.............................................................. 36 5.3 Piston Check Valves......................................................... 38 5.4 Ball Check Valves............................................................. 38 5.5 Stop Check Valve.............................................................. 39 6 Pressure Control Valves............................................................. 40 6.1 Pressure Reducing Valves............................................... 40 6.2 Pressure Relief Valves (PRVs)......................................... 43 6.3 Pressure Safety Valves (PSVs)........................................ 44 6.4 Rupture Discs.................................................................... 46 7 Valve Actuators........................................................................ 49 7.1 Manual Actuators.............................................................. 50 7.2 Electric Motor Actuators (MOVs)..................................... 52 7.3 Electric Solenoid Actuators (SOVs)................................ 53 7.4 Pneumatic Actuators........................................................ 55 7.5 Hydraulic Actuators.......................................................... 57 8 P&ID Valve Symbols.................................................................... 58 9 Valve Applications....................................................................... 62 10 Valve Maintenance.................................................................. 63 11 Summary................................................................................ 66 12 Glossary................................................................................. 67 Appendix A................................................................................... 68 Appendix B................................................................................... 71 Exercises 1-7................................................................................ 74

1 Introduction Valves were mentioned in the earlier module Pipework, where they were listed with other pipe fittings. Because they are the most important part of any piping system, they are described more fully in this module. The gas liquefaction process uses a lot of valves. ADGAS uses many different types of valves in a large range of sizes. Valves control the flow of fluids through pipes by: • starting and stopping flow—to control process or isolate part of a pipeline • changing the flow rate—allowing more or less fluid to flow • re-directing flow from one line to another at a pipeline junction • allowing flow in one direction only • reducing fluid pressure • keeping the pressure in a container or pipeline below a fixed maximum • preventing accidents by relieving overpressure in a container or pipeline It is very important to use the correct: • valve type—to suit the task it performs, as described above • size—to suit the pipe size and the flow rate required • material—to suit the fluid passing through it and to avoid corrosion

The valve body is the main part of the valve. All other parts fit onto the body. It is usually cast or forged and the shape varies with the type of valve. Inlet and outlet pipes fit onto the valve body through threaded, bolted (flanged) or welded joints. The fluid passes through the valve body when the valve is open. The valve body must be strong enough to take the maximum pressure of the process fluid. It must also be made of a material that is not attacked by the fluid. The valve bonnet is a removable cover fitted to the body. Some bonnets support the moving parts of the valve. Others just close the hole in the body through which the moving parts pass for assembly and dismantling. The valve trim is the name give to the parts inside a valve. This normally includes: • the opening/closing element—closes the fluid path through the valve body • the valve stem—connects the actuator to the closing element • the valve seat—makes a seal with the closing element when the valve is closed • sleeves—sometimes used to guide the stem Valve packing was described in detail in the earlier module in this course: Gland Packing. It allows the valve stem to pass into the valve body without loss of fluid or fluid pressure from the valve. It forms a dynamic seal between the valve stem and the bonnet. The actuator operates the stem and closing element assembly. The simplest actuator is the manually operated handwheel shown in Figure 2.1. Other actuators may be operated by: • electric motor—motor operated valve (MOV) • electric solenoid—solenoid operated valve (SOV) • air—pneumatically operated valve • oil—hydraulically operated valve Valve actuators are described in Section 7 of this module.

Valves can be divided into four classes: • block valves—stop and start flow • throttle valves—control flow rate • non-return (or check) valves—prevent flow reversal • pressure control valves—prevent fluid pressure exceeding a set maximum

3 Block Valves Block valves either allow full flow or stop flow completely. They should only be operated in the fully open or fully closed position. If they are only partly opened, they offer a lot of resistance to flow. Fluid friction and turbulence cause a loss of pressure in the fluid and can cause vibration. Block valves are not meant to control flow rate. There are four main types of block valve used on the plant: • gate valves • slide valves • ball valves • plug valves

3.1 Gate Valves Most valves in the ADGAS plant are gate valves. They are used to start or stop a flow completely. They should not be used to control flow rate. Using a gate valve in a partially open position can damage the valve. Fluid flow across the gate causes erosion to the gate making it impossible to seal well against its seat. Fluid can flow through most gate valves in either direction. The closing element in a gate valve is a wedge-shaped disc or gate attached to the end of the stem, The gate fits into a wedge-shaped seat in the valve body to stop flow through the valve.

Turning the handwheel raises and lowers the gate. When the gate valve is fully closed, the gate fills the passage and stops the flow through the valve completely. When the valve is fully opened, the gate is positioned above the passage in the valve body. This allows full flow through the valve, with little or no obstruction. There is very little pressure drop across the valve. Gate valves are classed as linear-motion valves as the closing element moves in a straight line (e.g. down and up) to close and open the valve. Gate valves can have rising or non-rising stems. The valve shown in Figure 3.1 has a rising stem. The stem moves up and down with the gate. A rising stem is fixed to the gate and can not turn in it. The upper part of the stem is threaded and screws into a mating thread in a bushing. The bushing is held in a yoke located at the top of the bonnet as shown in Figure 3.2. The actuator turns the bushing in the yoke, screwing the stem into or out of the valve body.

Non-rising stems are threaded at the bottom. This thread mates with a thread in the gate as shown in Figure 3.3. Left-hand threads allow clockwise rotation of the handwheel to lower the gate and close the valve. The stem is fixed to the actuator and turns with it, as shown in Figure 3.4. The stem can rotate in its housing but does not move axially.

An open gate valve allows anything that can pass through the pipeline to pass through the valve. Sometimes it is necessary to send solid objects along a pipeline. The object sent is called a pig and the process is called pigging. A pipeline is pigged to flush pipes, clear blockages or for inspection purposes. Gate valves allow these operations. Figure 3.5 shows a cutaway section of pipe containing a pig. The gate, or disc design may be: • solid wedge • flexible wedge • split wedge • parallel disc Most gate valves have solid-wedge discs. A solid wedge is cast or forged in one piece as shown in Figure 3.6(a). This is the simplest and strongest type of disc. The flexible-wedge is also made in one piece. It has a groove cut around its perimeter that allows it to bend a little to fit the shape of the seat more easily. These discs may also have recesses cast into them to increase flexibility, as shown in Figure 3.6(b). Flexible-wedge discs are used for valves in steam lines. When the temperature of a closed valve rises, solid-wedge discs can expand and stick in their seats. Flexiblewedge discs can compress mere easily and are less likely to distort. Figure 3.7 shows a rising-stem, flexible-wedge gate valve. Split-wedge discs are made in two separate halves. This allows the wedge angle between their outer faces to adjust to fit the seat. This is especially useful if a solid particle is stuck between the disc and its seat. Split-wedge discs are used for gases, especially corrosive gases. Parallel slide valves also have split discs. Their faces are parallel, not wedge shaped, as shown in Figure 3.8. A spring between the disc halves pushes them against their seats. When the valve is closed, the disc on the outlet side is also pushed against its seat by the fluid pressure on the inlet side. As the valve opens and closes, the sliding action keeps the disc faces clean but causes wear to discs and seats. When fully open, the discs are completely clear of the bore giving no obstruction to flow through the valve. Figure 3.8:

Gate valve seats may be integral with the valve body or separate seat rings. Integral seats are cut into the valve body and are part of the body. These seats can not be replaced. They can be repaired by lapping with grinding paste. Seat rings may be pressed or screwed into the body. These can be of a different material and can be replaced when worn or damaged. Knife gate valves, have a simple, one-piece closing element. It is a parallel-sided plate that may move clear of the flow path to open or may have a hole that moves into the flow path. These two types are shown in Figure 3.9(a) and (b).

3.2 Ball Valves Ball valves start and stop flow by rotating a ball-shaped closing element. They are classed as rotational-motion valves. The ball has a hole through it of the same diameter as the pipeline. The valve is open when the hole lines up with the inlet and outlet of the valve body. Figure 3.10 shows a ball valve with part of the body cut away to show the closing element. The valve above is shown partially open to show the hole in the ball. This is not the normal valve position; a ball valve is normally only used in the fully closed or fully open positions. Figure 3.11 shows the same ball valve looking through the valve inlet. In Figure 3.11(a) the valve is in the closed position. In Figure 3.11(b) the valve is in the open position. The open valve leaves a clear path for flow with no obstruction. These valves can be pigged. The valve shown has a lever actuator that turns through 90o between the fully closed and fully open positions. The lever is in line with the pipeline when the valve is open. Figure 3.11: Ball Valve—End View

3.3 Plug Valves Operation of a plug valve is similar to the ball valve; they are also rotational-motion valves. The main difference is the shape of the closing element, which is a tapered plug of circular section. The plug has a hole called a port. Figure 3.12 shows a plug valve that is lined with PTFE to protect it from corrosion and allow lubricant-free operation. Single-port plug valves are used to start and stop flow. Multi-port plug valves redirect flow from one pipeline to another. Figure 3.13 shows an example of a multiport plug valve.

4 Flow Control (Throttle) Valves The control of flow rate by reducing the area of the flow path through a valve is called throttling. Throttling a fluid also reduces its pressure. Block valves should not be used to throttle flow. The pressure drop across them is too great and the flow becomes turbulent. Fluid flow can be either smooth (laminar), or not smooth (turbulent) as shown in Figure 4.1. Turbulent flow can cause many problems in pipelines and equipment. In a valve, it can erode the closing element and valve seat. Erosion was described in the earlier module in this course: Bearings. It is the slow wearing away of a solid material by a fluid passing over it. Turbulent flow increases the rate of wear. Figure 4.2 shows smooth and turbulent flow in rivers. Throttle valves are designed to operate partially opened with little pressure loss and turbulence. Throttle valves are also called regulating valves. There are four main types: • globe valves • butterfly valves • diaphragm valves • needle valves 4.1 Globe Valves Globe valves are linear-motion valves and can look very similar to gate valves from the outside. Globe valves have rising stems but, unlike gate valves, the actuator is fixed to the stem and rises with it. Figure 4.3 shows a globe valve in the fully closed and open positions. Globe valve design makes them good for flow regulation as well as starting and stopping flow. In most designs, the flow direction is as shown in Figure 4.4. Here, the fluid pressure helps to push the valve open. The packing is not under pressure when the valve is closed and this helps it to last longer. The flow direction is often marked on the valve body. Make sure that you fit the valve the correct way around. Globe valves can have three main types of body. • Z-type • angle • Y-type The valves shown in Figures 4.3 and 4.4 have Z-type bodies. The name is given because of the path the fluid has to take as it passes through the valve. It changes direction twice, like the letter Z. Z-type globe valves are used mainly for small-size, low-pressure applications. In large, high-pressure lines, the changes of flow direction cause a large pressure drop and turbulence that can damage the trim Figure 4.5 shows an angle-type globe valve. The flow changes direction only once and the pressure drop is less than for the Z-type. It can be used for medium-pressure applications. Figure 4.6 shows a Y-type globe valve. Having the seat at about 45o to the flow direction straightens the flow path and reduces the pressure drop. This type of valve can be used for high-pressure applications Most globe valves use one of three types of disc: • ball • plug • composition Ball discs have a curved lower surface. They seal on a tapered seat that has a flat surface, as shown in Figure 4.7(a). They are used mainly for low-pressure and lowtemperature applications Plug discs come in different shapes but are all tapered. The seat has a matching taper as shown in Figure 4.7(b). Composition discs have a hard backing piece with a soft face as shown in Figure 4.7(c). Hard particles trapped between the disc and the seat push into the soft face, maintaining a good seal. Composition discs are replaceable.

4.2 Butterfly Valves Butterfly valves are rotational-motion valves. Like ball and plug valves, they need only a quarter turn (90o) to fully open or close them. They can start, stop and regulate flow, although they are not very good at completely stopping flow. Figure 4.8 shows a typical butterfly valve. The lever is in line with the pipeline when the valve is open. The closing element is a circular disc of a similar diameter to the ID of the pipe. The disc turns to open and close the valve. The disc or seat may be made of a polymer (plastic) to give a better seal. Butterfly valves are simple and take up little space. This makes them especially good for use in large pipelines or where there is not much space. Operating a butterfly valve can take a lot of force as you have to push it against the fluid pressure. Larger valves usually have geared actuators to make operation easier, as shown in Figure 4.9 Most butterfly discs turn on a stem that passes through the centre of the disc along a diameter. When the valve is closed, fluid pressure pushes equally on both sides of the stem: half the force is pushing in the closing direction and half in the opening direction, as shown in Figure 4.10(a). If the closing mechanism failed and the disc became free to turn on its own, there is an equal chance of the valve swinging open or closed Some valves have a stem that is offset from the centre. With the stem offset as shown in Figure 4.10(b), the fluid force is greater on the side that opens the valve. This makes opening easier and closing more difficult. If the mechanism fails and the disc becomes free it will automatically open. This type of valve can be used in fail open applications. If the stem is offset to the other side, as shown in Figure 4.10(c), the valve is easy to close but difficult to open. If the mechanism fails and the disc becomes free it will automatically close. This type of valve can be used in fail closed applications. Make sure that you fit offset-stem valves so that flow is in the correct direction. If not, a fail-open valve will become fail-closed and a fail-closed valve becomes failopen. Figure 4.11 shows a large butterfly valve with an offset stem. 4.3 Diaphragm Valves The closing element of a diaphragm valve is not a solid disc. Instead, it has a sheet of flexible material called a diaphragm. This diaphragm completely separates the valve trim from the fluid flowing through the valve. This means that the fluid does not contact the trim and the stem does not need any gland packing. Figure 4.12 shows an exploded view of a typical diaphragm valve. Diaphragm valves are rising-stem, linear-motion valves. As the actuator turns, the stem screws into or out of the sleeve attached to the actuator. The stem of the valve in the figure can not be seen from outside. You can see the position of the valve from a position indicator that rises and falls with the stem. A compressor is attached to the bottom of the stem. Its job is to push down on the diaphragm to close the valve. A stud locates the compressor on the diaphragm. Figure 4.13 shows a diaphragm valve in the fully closed and open positions. This valve has no position indicator as you can see the valve stem. The diagram in Figure 4.14 shows a diaphragm valve in the closed, throttling and open positions 4.4 Needle valves Needle valves are linear-motion valves. They can make very small adjustments to flow rate. Their name comes from the long, tapered shape of the bottom of the spindle that forms the closing element. Figure 4.15 shows a typical needle valve.

5 Non-return (Check) Valves Non-return valves, also called check valves, stop flow reversal in a pipe. They only allow fluid to flow in one direction. The pressure of the fluid passing through the valve in the correct direction opens it automatically. If the flow tries to reverse, the valve closes automatically. They have an arrow on the body that shows the correct flow direction, as shown in Figure 5.1. Make sure that you mount non-return valves the correct way round. There are a number of designs of non-return valve. Some rely on the weight of the closing element and fluid flow only to close them. Others have a spring to help close them. 5.1 Swing Check Valves In this type, the valve disc is hinged at the top. When there is no flow, the weight of the disc closes the valve. Figure 5.2 shows a typical swing-check valve in open and closed positions. This valve must be mounted in a horizontal pipeline, with the disc hinge at the top to allow gravity to close it. If it can not be mounted in this way, another type of check valve should be used, or a swing-type with an external counterbalance, as shown in Figure 5.3 The counterbalance arm clamps onto the hinge pin in a position that closes the disc.

5.2 Lift Check Valves These valves have a similar valve body and seating arrangement to globe valves. Figure 5.4 shows the body of a lift check valve. Flow must enter from under the seat to lift the closing element, as shown in Figure 5.5(a). Flow in the reverse direction pushes the closing element against its seat, as shown in Figure 5.5(b). The closing element may be free to fall under its own weight, as shown in Figure 5.5 or it may be helped by a spring, as shown in Figure 5.6.

5.3 Piston Check Valves Piston check valves are similar to lift check valves. Instead of a valve disc there is a piston that slides in a cylinder. This gives a smoother motion during operation. Figure 5.7 shows an example of this type of valve 5.4 Ball Check Valves These have a spherical (ball-shaped) closing element. Like the other check valves, the closing element may operate by gravity or the flow pressure or it may be springloaded. Figure 5.8 shows examples of ball check valves 5.5 Stop Check Valves A stop check is a non-return globe valve. It is similar to a globe valve but the valve disk is free to slide on the stem. With the valve stem raised, it acts as a lift check valve allowing flow only from below the disc, as shown in Figure 5.9. If there is no flow, or if flow reverses, the disc drops into the position shown in the figure. When the stem is lowered to the closed position, the disc can not lift and flow is stopped in both directions.

6 Pressure Control Valves Pressure control valves can be divided into three main types: • pressure reducing • pressure relief • pressure safety Pressure reducing valves operate where a pressure drop is needed between two parts of a process. Pressure relief valves maintain fluid pressure below a maximum allowable value for a process. Pressure safety valves protect the plant from damage caused by overpressure. These last two valves do similar jobs and are similar in construction. 6.1 Pressure Reducing Valves Reducing valves automatically reduce liquid or gas pressure to a pre-set value. One common use is to control the pressure of gas leaving gas bottles and vessels. You will see pressure reducing valves on gas welding equipment Their construction can be quite complicated but a simple, non-adjustable valve is shown in Figure 6.1 The operation of this valve depends on the balance between the fluid pressures acting above and below a piston, and a spring force. When the force of the low pressure fluid       plus the spring force           pushing down on the piston is more than the force of the high pressure supply fluid          pushing up, the piston closes the valve.

When the force of the low pressure fluid drops, the new lower pressure plus the spring force pushing down on the piston becomes less than the force of the high pressure fluid pushing up and the piston opens the valve

During operation, the valve continuously opens and closes to maintain a flow of fluid at the reduced pressure. The only way to change the outlet pressure in the simple valve is to change the spring to a stronger or weaker one. Some reducing valves have an adjusting screw to change the spring force. This allows you to change the output fluid pressure easily. Many reducing valves have more than one piston and may also use diaphragms to improve their performance. Figure 6.2 show an example of a steam pressure reducing valve.

6.2 Pressure Relief Valves (PRVs) Pressure relief valves are used mainly to relieve overpressure of liquids. This often happens when a liquid in a closed container or pipeline expands as its temperature increases. Under normal operating conditions, a spring holds the PRV closed. Fluid pressure pushes against the spring to open the valve. The fluid pressure needed to push the valve open is called the setpoint pressure. The setpoint pressure is usually the maximum normal operating pressure of the liquid. An adjustment screw changes the spring force for different setpoint pressures. When the liquid pressure exceeds the setpoint pressure, the valve opens slowly. It releases just enough liquid to bring the pressure down to the normal operating pressure. The spring then closes the valve slowly so that normal operations can continue. The outlet from the valve is connected back into the inlet of the equipment so that no liquid is lost. Figure 6.3 shows a typical PRV. Notice that the valve outlet diameter is greater than the inlet. This allows fluid to escape quickly to bring pressure down to normal.

6.3 Pressure Safety Valves (PSVs) Pressure safety valves are used mainly to relieve overpressure of gases and vapours (e.g. steam). The setpoint pressure is greater than the maximum normal operating pressure of the process fluid but less than the maximum safe working pressure of the equipment. When the fluid pressure exceeds the setpoint pressure, the valve pops fully open. This happens very quickly to release overpressure as quickly as possible. The pressure at which the valve closes again is lower than the opening setpoint pressure. The difference between opening and closing pressures is called the blowdown. Blowdown is given as a percentage of opening setpoint pressure. For example, a valve may open at 15bar with a blowdown of 10%. 10% of 15bar is 1.5bar the valve will close at a pressure that is 1.5bar lower than 15bar 15bar – 1.5bar = 13.5bar. PSVs on gas processing systems normally vent to flare—the valve outlet is connected to the flare system where the gas burns off. PSVs on steam systems vent to atmosphere—the steam is released into the air. Figure 6.4 shows a typical PSV.

The PSV outlet diameter is greater than the inlet for the same reason as for the PRV. PSVs often have an external operating lever. This is used to manually check the operation of the valve. All valves should be handled with care and kept clean but this is especially important for PSVs. They protect the plant and their failure to operate correctly can cause a lot of expensive damage and injury to personnel. Make sure that all protective plugs and wrappings are in place when you receive a PSV from stores. Before fitting, check that all wrappings and plugs are removed and that the seals protecting the valve settings are in place and unbroken. Check the information on the nameplate and identification tags against the work order. Make sure that inlet and outlet ports and the pipes going to them are clean. Valves removed from service should be tested in the workshop. The opening setpoint pressure and blowdown are tested and their values recorded. Figure 6.5 shows a PSV being tested on a special rig.

6.4 Rupture Discs Rupture discs, also called bursting discs, are a simple and cheap form of pressure safety device. They have no moving parts and break when a particular pressure is reached, allowing fluid to escape very quickly. A rupture disc is chosen that will burst as soon as the maximum allowable system pressure is exceeded. Once the disc has burst, it is replaced with a new disc of the correct bursting pressure. Figure 6.6 shows rupture discs before and after bursting.

Most discs are circular with a concave dished surface, as shown in Figure 6.7. The disc is fitted between flanges in a special assembly as shown in Figure 6.7.

When the disc bursts, the fluid may vent to a holding vessel, straight to atmosphere or through an exhaust device like the one shown in Figure 6.7(b). In some systems a rupture disc is fitted just before a PSV. This is done where the fluid being contained is very corrosive and could damage the valve so that it does not operate properly. The disc forms a barrier between the system fluid and the PSV. This arrangement is shown in Figure 6.8. Rupture

7 Valve Actuators An actuator is the part of a valve system that operates the valve. Valve operation may be: • manual—operated directly by a person • semi-automatic—operated by another source of power that is switched on by a person • automatic—operated by another source of power that is switched on by a signal from a sensing device There are six main types of actuator: • manual • electric motor • electric solenoid • pneumatic • hydraulic • self-actuated—check valves, PRVs and PSVs (described in the last section) The type of actuator used depends mainly on whether or not automatic operation is needed and how much torque is needed to operate the valve. Automatically operated valves need a source of power to operate them: electric, pneumatic or hydraulic. If this power source fails, the valve must be left in a safe position—it must fail safe. This may be open, closed or in the position it was operating in before the power failure. Automatically operated valves may be: • fail open • fail closed • fail locked

7.1 Manual Actuators The most common manual actuators are: • handwheel—for linear motion valves: gate valves, globe valves, etc. • lever—for rotational motion valves: ball valves, butterfly valves, etc. Most of the valves shown earlier in this module have manual actuators. Figure 7.1 shows another example of handwheel and a lever-actuated valves.

Some larger handwheel-actuated valves have hammer handwheels. These turn freely on the spindle for part of a turn. The wheel has lugs that hit against a bar (the secondary wheel) that is fixed to the spindle, as shown in Figure 7.2.

The hammering action of the lugs on the secondary wheel helps the operator to open a valve when it is stuck. A gear system is used on bigger valves to increase the actuating torque. Figure 7.3(a) shows a typical gear-actuated valve

The handwheel is fixed to a small gear that engages with a bigger gear on the valve stem, as shown in Figure 7.3(b). The gear ratio: • reduces the speed of operation—the valve stem turns more slowly than the handwheel • increases the torque—the torque at the valve stem is more than the torque you use at the handwheel

7.2 Electric Motor Actuators (MOVs) Motor operated valves (MOVs) allow semi-automatic and automatic operation. The motor turns the valve stem through a gear train. This increases the motor torque at the valve stem. Figure 7.4(a) shows a typical MOV and Figure 7.4(b) shows an example of a gear train that allows motor or manual operation. The handwheel gear can be disengaged for automatic operation.

The motor can be reversed for opening or closing the valve. These valves can control flow but motor actuators are mainly used to fully open and close the valve. Limit switches stop the motor when the valve is fully open or closed

7.3 Electric Solenoid Actuators (SOVs) When current passes through a coil of wire, the coil acts like a magnet. It attracts magnetic materials like iron. We say that the coil is energised. An electric solenoid uses this idea to move an iron armature. The magnetic field pulls the armature into the coil when the current is switched on. Solenoid operated valves (SOVs) use this motion to operate a valve. Because the motion is linear, an SOV operates directly on the valve stem to open and close it. This is different from the MOV, which rotates the stem in the normal way. SOVs are mainly used on linear motion valves. A spring pushes the valve in one direction • open for fail open valves • closed for fail closed valves When the current is switched off (the solenoid is de-energised), or when power fails, the spring operates the valve. The solenoid moves the valve in the other direction when the current is switched on (the solenoid is energised). The solenoid can be energised automatically, or semi-automatically with a manual switch. Figure 7.5 shows the basic parts of an SOV.

Solenoid valves are often used to control the air supply to larger, pneumatic valve actuators. Figure 7.6 shows a small solenoid air valve.

7.4 Pneumatic Actuators Pneumatic actuators are operated by air pressure. The air pushes on a diaphragm that moves the stem up or down. Some actuators are moved both ways by air pressure but most use air to operate in one direction and have a spring that pushes in the other direction. In Figure 7.7(a) you can see the spring above the diaphragm, pushing down to operate the valve. The section shown in Figure 7.7(b) shows the spring below the diaphragm, pushing up to operate the valve. Pneumatic actuators are mainly used to operate linear motion valves. They can be operated automatically, or semi-automatically with a manually-energized solenoid valve on the air line. If the spring opens the valve when air pressure is removed, the valve will fail open and it is called a direct-acting valve. If the spring closes the valve when air pressure is removed, the valve will fail closed and it is called a reverse-acting valve. Figure 7.7(a) shows a pneumatically activated gate valve. The position indicator shows that the valve is open (O) when raised by air pressure below the diaphragm, and shut (S) by the spring. Figure 7.7(b) shows a pneumatically activated globe valve. The valve is open when lowered by air pressure above the diaphragm and closed by the spring. Both these valves are fail-closed, reverse-acting valves. Valves that have air pressure on both sides of the diaphragm are called duplex valves. They are operated by adjusting the pressure above and below the diaphragm. Some pneumatic actuators use a piston instead of a diaphragm. This allows the greater motion needed to operate rotational motion valves through a lever system.

Hydraulic actuators perform in a similar way to pneumatic actuators. They use oil, or sometimes water, to move a piston. The hydraulic fluid is pumped to one side of the piston to operate the valve in one direction. If a spring operates the valve in the opposite direction, it will be fail-open or fail-closed, depending on which way the spring operates the valve. Some hydraulic actuators use double-acting cylinders that operate by pumping oil to either side of the piston. Pumping to one side opens the valve and pumping to the other side closes the valve. If power is lost to a doubleacting actuator, the valve normally stays where it is at the time of failure: it is fail locked. Figure 7.8 shows two different types of hydraulic valve actuators.

8 P&ID Valve Symbols Many of the symbols used on P&IDs were described in the Basic Maintenance Course module, Drawings and Diagrams. Symbols used on ADGAS P&IDs were also given in the module on Pipework in this course. Table 8.1 shows the symbols used for the valves described in this module.

9 Valve Applications Using the correct valve for any application is very important. Different types of valve perform different jobs on the plant: blocking flow (start/stop); throttling (flow rate control); pressure control; flow direction control; safety. The size and pressure rating of a valve is important to allow the correct flow rate and process pressure to be safely maintained. This information is usually marked on the valve body. The valve material must be suitable for the type of fluid being handled. Often the valve trim is of a different material than the bonnet and body and these are specified separately. All of these requirements are listed in the ADGAS Piping Specification. Appendix A contains ADGAS Specification A as an example. Appendix B shows the BS standard materials and dimensions for class 150, 300 and 600 gate valves. Table 9.1 contains a guide to the use of the more common valves.

10 Valve Maintenance Discs and seats are the parts of a valve that are most likely to become worn or damaged. Damage to these causes the valve to leak internally. They can be damaged by corrosion and erosion. In some valves the seats can be repaired by lapping with grinding paste. You will see how this is done and do it yourself in the practical Exercises in this module. Some valves have separate seats that you can remove and replace when they are damaged. External leakage is caused by failure of a gasket or packing. A little leakage from packing is acceptable and is often needed to lubricate the stem. Sometimes, you can reduce leakage by tightening down the gland follower. If this does not work you need to replace the packing as described in the module Gland Packing.

Gasket leaks may also be stopped by tightening flange bolts but this is not recommended. You must be careful not to exceed the recommended stud torque or you may shear a stud. This would cause even more leakage and possibly complete failure of the gasket. Wherever possible, the valve should be isolated and removed from service for repairs. If this is not possible for operational reasons, a leaking valve can be temporarily repaired by injecting a sealant. Permanent repairs can then be done during a shutdown period. Sealant injection for different parts of a valve is shown in Figure 10.1.

For seat repairs, the valve body is drilled and sealant injected into the area of the seat as shown in Figure 10.1(a) and (b). Gland sealing is done in a similar way, by drilling the packing gland and injecting sealant into the stuffing box, as shown in Figure 10.1(c). To repair the bonnet gasket, a clamp is fitted around the joint and sealant injected through a hole in the clamp, as shown in Figure 10.1(d). To repair flange and body leaks the leaking part is enclosed in a jacket. The jacket is then injected with sealant as shown in Figures 10.1(e) and (f). Other parts of a valve that you should check for damage are bushings in the yoke or bo