The emphasis is firmly on the practicalities of repairing and restoring, so technical content is kept to a minimum, and always explained in a way that can be followed by readers with no background in electronics. Those who have a good grounding in electronics, but wish to learn more about the practical aspects, will benefit from the emphasis given to hands-on repair work, covering mechanical as well as electrical aspects of servicing.
Repair techniques are also illustrated throughout. Full coverage of valve amplifiers will add to its appeal to all audio enthusiasts who appreciate the sound quality of valve equipment.
A practical manual for collectors, owners, dealers and service engineers Essential information for all radio and audio enthusiasts Valve technology is a hot topic. There are two main types of check valves, lift and swing. The picture below shows a lift check valve. Check valves, such as the one shown below to the right, prevent fluid from flowing through until pressure builds up and forces the valve to open. Check valves allow fluid flow in one preferred direction and prevent back flow, or flow in the opposite direction.
These kinds of valves begin to close as the pressure in a pipeline drops and the fluid momentum slows down. When the flow direction reverses, the check valve closes completely. In the schematic below to the left, the fluid travels from left to right. The gate prevents any backflow from occurring. The pictures below show a cutaway view of a swing check valve. The fluid enters from the left, forcing the valve open. When the fluid stops flowing or when there is backflow, the valve will swing closed.
Copyright Cameron, Houston, TX. Lift check valves are typically used in vertical flowlines. Either a spring or the force of gravity keeps the gate in the closed position. If the fluid pressure is great enough, the gate will open and the fluid will flow up through the pipe.
Check valves are used in applications that can't allow any backflow. The picture below shows a check valve that is in place to prevent any backflow into the feed pump. Pressure relief valves, commonly known as PRVs, prevent system breakdown by releasing pressure. Pressure relief valves open when the system pressure becomes too high or too low, and reclose once the system pressure has been restored to a safe level. Opening the valve decreases pressure in the system by allowing fluid to discharge.
All of these allow the system to stay below a set pressure limit. The pressure that the valves will open to if exceeded can be set or calibrated. Pressure relief valves can be grouped into four categories: direct-loaded and pressure-actuated, pilot-operated and pressure-actuated, temperature-actuated, and power-actuated.
The most common PRVs are direct-loaded and pilot-operated. Direct-loaded pressure relief valves involve a spring-loaded disc. Valves of this type are meant to lower system pressure by discharging fluid from the process.
This valve is incorporated into the process such that the fluid exerts a force on the disc. The disc is held in place by a force provided by a spring in the opposite direction. At a specific fluid pressure the force the fluid exerts on the disc is greater than that of the spring holding the disc in place.
The disc then rises and some fluid is allowed to discharge from the system. The pressure at which the fluid overcomes the spring can be set by adjusting the spring. In high back-pressure situations, a balanced-bellows design is preferred over the conventional design described above. In this design, bellows present allow for compression and expansion before pressure reaches the spring. Also, there is a vent in the bonnet to ensure exposure to the atmosphere, which guarantees that the relief will open regardless of the back-pressure applied.
Below is a picture of a direct-loaded pressure valve where the arrows show the direction the fluid would flow when the disc is raised. In pilot-operated pressure relief valves, such as the one shown below, the system fluid is responsible for causing the valve to open and close. This is unlike the direct-loaded valve type where the spring was responsible for valve closing.
The valve consists of a piston, a dome, and a pilot. The piston is the movable part of the valve that will open if the pressure limit is exceeded. The dome is the open space above the piston.
The pilot acts as a connecting piece between the valve inlet and the dome. When fluid is flowing at a safe pressure, the fluid pushes against the piston. However the fluid also flows through the pilot and into the dome, pushing the piston from the opposite direction with the same pressure.
The piston is cleverly designed to have a larger surface area on the dome side so that even though the same pressure is exerted on both sides of the piston, there is a larger force on the dome side due to the larger area. This produces a net force that seals the piston, preventing flow past the piston. The pilot itself is a three-way connecting valve that is self-actuated.
Under safe pressure the pilot connects the inlet to the dome as described above. If pressure exceeds a certain limit the pilot will switch and connect the inlet to a discharge stream.
Now the system fluid is only pushing against the piston from one side and the pilot opens. The valve outlet discharges into the same stream as the pilot discharge allowing for very fast pressure relief. Note these valves may also be spring operated. The picture to below, to the left, is that of a direct-operated PRV. The main function of a pressure relief valve is for safety considerations. Alfa Laval , Richmond, VA. AMRI, Inc. Cameron , Houston, TX.
DeZurik, Inc. Emerson Process Management. Flowserve Corporation , Irving, TX. Kurt J. Lesker Company , Clairton, PA. The Swagelok Company. Valtorc International , Kennesaw, GA. Crowl, Daniel A. New York, NY: Institute, Hoop, Emily. August Print. Hustchison, J. Instrument Society of America, Ilg, Edmond. Lyons, Jerry L.
Lyons' Encyclopedia of Valves. New York: Van Nostrand Reinhold, Merrick, Ronald C. Valve Selection and Specification Guide. New York: Van Nostrand Reinhold, , , , Mukherjee, Siddhartha. Pressure-Relief System Design.
Perry, Robert H. Perry's Chemical Engineers' Handbook. Skousen, Philip L. Smith, E. An Introductory Guide to Valve Selection. London, Mechanical Engineering Publications Limited, , , , Torzewski, Kate. Ulanski, Wayne. Valve and Actuator Technology. New York: McGraw-Hill, , Valve Manufacturers Association of America. Zappe, R. Valve Selection Handbook. Houstan, Texas: Gulf, Visual Encyclopedia of Chemical Engineering.
Transport Storage. Valves Valves regulate the flow of fluids and isolate piping or equipment for maintenance without interrupting the other connected units. They categorize valves into four main types: Linear motion valves, Rotary valves, Self-actuated valves, and Specialty valves Linear Motion Valves Linear motion valves include any valve that closes by the linear sliding motion of a structure within the valve. Copyright DeZurik, Inc. Suitable for high temperature and high pressure situations.
Low pressure drop when fully open. Tight seal when fully closed. Relatively free of buildup. For this reason, the disk is a pressure-retaining part. Disks are typically forged and, in some designs, hard-surfaced to provide good wear characteristics.
A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed. Most valves are named, in part, according to the design of their disks. The seat or seal rings provide the seating surface for the disk. In some designs, the body is machined to serve as the seating surface and seal rings are not used.
In other designs, forged seal rings are threaded or welded to the body to provide the seating surface. 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. The stem , which connects the actuator and disk, is responsible for positioning the disk. Stems are typically forged and connected to the disk by threaded or welded joints.
For valve designs requiring stem packing or sealing to prevent leakage, a fine surface finish of the stem in the area of the seal is necessary. Typically, a stem is not considered a pressure boundary part. Connection of the disk to the stem can allow some rocking or rotation to ease the positioning of the disk on the seat. Alternately, the stem may be flexible enough to let the disk position itself against the seat.
However, constant fluttering or rotation of a flexible or loosely connected disk can destroy the disk or its connection to the stem. Two types of valve stems are rising stems and nonrising stems. Illustrated in Figures 2 and 3, these two types of stems are easily distinguished by observation.
For a rising stem valve, the stem will rise above the actuator as the valve is opened. This occurs because the stem is threaded and mated with the bushing threads of a yoke that is an integral part of, or is mounted to, the bonnet. There is no upward stem movement from outside the valve for a nonrising stem design. For the nonrising stem design, the valve disk is threaded internally and mates with the stem threads. The actuator operates the stem and disk assembly.
An actuator may be a manually operated handwheel, manual lever, motor operator, solenoid operator, pneumatic operator, or hydraulic ram. In some designs, the actuator is supported by the bonnet. In other designs, a yoke mounted to the bonnet supports the actuator. Except for certain hydraulically controlled valves, actuators are outside of the pressure boundary. Yokes, when used, are always outside of the pressure boundary.
Most valves use some form of packing to prevent leakage from the space between the stem and the bonnet. Packing is commonly a fibrous material such as flax or another compound such as teflon that forms a seal between the internal parts of a valve and the outside where the stem extends through the body. Valve packing must be properly compressed to prevent fluid loss and damage to the valve's stem. If a valve's packing is too loose, the valve will leak, which is a safety hazard.
If the packing is too tight, it will impair the movement and possibly damage the stem. Because of the diversity of the types of systems, fluids, and environments in which valves must operate, a vast array of valve types have been developed. Examples of the common types are the globe valve, gate valve, ball valve, plug valve, butterfly valve, diaphragm valve, check valve, pinch valve, and safety valve.
Each type of valve has been designed to meet specific needs. Some valves are capable of throttling flow, other valve types can only stop flow, others work well in corrosive systems, and others handle high pressure fluids.
Each valve type has certain inherent advantages and disadvantages. Understanding these differences and how they effect the valve's application or operation is necessary for the successful operation of a facility. Although all valves have the same basic components and function to control flow in some fashion, the method of controlling the flow can vary dramatically. In general, there are four methods of controlling flow through a valve. Each method of controlling flow has characteristics that makes it the best choice for a given application of function.
Due to the various environments, system fluids, and system conditions in which flow must be controlled, a large number of valve designs have been developed. A basic understanding of the differences between the various types of valves, and how these differences affect valve function, will help ensure the proper application of each valve type during design and the proper use of each valve type during operation.
A gate valve is a linear motion valve used to start or stop fluid flow; however, it does not regulate or throttle flow. The name gate is derived from the appearance of the disk in the flow stream. Figure 4 illustrates a gate valve. The disk of a gate valve is completely removed from the flow stream when the valve is fully open. This characteristic offers virtually no resistance to flow when the valve is open. Hence, there is little pressure drop across an open gate valve.
With the proper mating of a disk to the seal ring, very little or no leakage occurs across the disk when the gate valve is closed. On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening. This means that flow rate does not change evenly with stem travel.
Also, a partially open gate disk tends to vibrate from the fluid flow. Most of the flow change occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage if used to regulate flow. For these reasons, gate valves are not used to regulate or throttle flow.
A gate valve can be used for a wide variety of fluids and provides a tight seal when closed. The major disadvantages to the use of a gate valve are:. Gate valves are available with a variety of disks. Classification of gate valves is usually made by the type disk used: solid wedge, flexible wedge, split wedge, or parallel disk. Solid wedges, flexible wedges, and split wedges are used in valves having inclined seats.
Parallel disks are used in valves having parallel seats. Regardless of the style of wedge or disk used, the disk is usually replaceable.
In services where solids or high velocity may cause rapid erosion of the seat or disk, these components should have a high surface hardness and should have replacement seats as well as disks.
If the seats are not replaceable, seat damage requires removal of the valve from the line for refacing of the seat, or refacing of the seat in place. Valves being used in corrosion service should normally be specified with replaceable seats. The solid wedge gate valve shown in Figure 5 is the most commonly used disk because of its simplicity and strength.
A valve with this type of wedge may be installed in any position and it is suitable for almost all fluids. It is practical for turbulent flow. The flexible wedge gate valve illustrated in Figure 6 is a one-piece disk with a cut around the perimeter to improve the ability to match error or change in the angle between the seats. The cut varies in size, shape, and depth. A shallow, narrow cut gives little flexibility but retains strength. A deeper and wider cut, or cast-in recess, leaves little material at the center, which allows more flexibility but compromises strength.
A correct profile of the disk half in the flexible wedge design can give uniform deflection properties at the disk edge, so that the wedging force applied in seating will force the disk seating surface uniformly and tightly against the seat.
Gate valves used in steam systems have flexible wedges. The reason for using a flexible gate is to prevent binding of the gate within the valve when the valve is in the closed position. When steam lines are heated, they expand and cause some distortion of valve bodies.
If a solid gate fits snugly between the seat of a valve in a cold steam system, when the system is heated and pipes elongate, the seats will compress against the gate and clamp the valve shut. This problem is overcome by using a flexible gate, whose design allows the gate to flex as the valve seat compresses it.
The major problem associated with flexible gates is that water tends to collect in the body neck. Under certain conditions, the admission of steam may cause the valve body neck to rupture, the bonnet to lift off, or the seat ring to collapse. Following correct warming procedures prevent these problems. Split wedge gate valves, as shown in Figure 7, are of the ball and socket design. These are self-adjusting and selfaligning to both seating surfaces.
The disk is free to adjust itself to the seating surface if one-half of the disk is slightly out of alignment because of foreign matter lodged between the disk half and the seat ring. This type of wedge is suitable for handling noncondensing gases and liquids at normal temperatures, particularly corrosive liquids.
Freedom of movement of the disk in the carrier prevents binding even though the valve may have been closed when hot and later contracted due to cooling. This type of valve should be installed with the stem in the vertical position. The parallel disk gate valve illustrated in Figure 8 is designed to prevent valve binding due to thermal transients.
This design is used in both low and high pressure applications. The wedge surfaces between the parallel face disk halves are caused to press together under stem thrust and spread apart the disks to seal against the seats. The tapered wedges may be part of the disk halves or they may be separate elements.
The lower wedge may bottom out on a rib at the valve bottom so that the stem can develop seating force. In one version, the wedge contact surfaces are curved to keep the point of contact close to the optimum.
In other parallel disk gates, the two halves do not move apart under wedge action. Instead, the upstream pressure holds the downstream disk against the seat. A carrier ring lifts the disks, and a spring or springs hold the disks apart and seated when there is no upstream pressure.
Another parallel gate disk design provides for sealing only one port. In these designs, the high pressure side pushes the disk open relieving the disk on the high pressure side, but forces the disk closed on the low pressure side. With such designs, the amount of seat leakage tends to decrease as differential pressure across the seat increases. These valves will usually have a flow direction marking which will show which side is the high pressure relieving side. Care should be taken to ensure that these valves are not installed backwards in the system.
Some parallel disk gate valves used in high pressure systems are made with an integral bonnet vent and bypass line. A three-way valve is used to position the line to bypass in order to equalize pressure across the disks prior to opening.
When the gate valve is closed, the three-way valve is positioned to vent the bonnet to one side or the other. This prevents moisture from accumulating in the bonnet. The three-way valve is positioned to the high pressure side of the gate valve when closed to ensure that flow does not bypass the isolation valve. The high pressure acts against spring compression and forces one gate off of its seat.
The three-way valve vents this flow back to the pressure source. Gate valves are classified as either rising stem or nonrising stem valves. For the nonrising stem gate valve, the stem is threaded on the lower end into the gate.
As the hand wheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type valve will almost always have a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. Figures 2 and 3 illustrate rising-stem gate valves and nonrising stem gate valves. The nonrising stem configuration places the stem threads within the boundary established by the valve packing out of contact with the environment.
This configuration assures that the stem merely rotates in the packing without much danger of carrying dirt into the packing from outside to inside. Rising stem gate valves are designed so that the stem is raised out of the flowpath when the valve is open. Rising stem gate valves come in two basic designs. Some have a stem that rises through the handwheel while others have a stem that is threaded to the bonnet.
Seats for gate valves are either provided integral with the valve body or in a seat ring type of construction. Seat ring construction provides seats which are either threaded into position or are pressed into position and seal welded to the valve body. The latter form of construction is recommended for higher temperature service. Integral seats provide a seat of the same material of construction as the valve body while the pressed-in or threaded-in seats permit variation. Rings with hard facings may be supplied for the application where they are required.
Small, forged steel, gate valves may have hard faced seats pressed into the body. In large gate valves, disks are often of the solid wedge type with seat rings threaded in, welded in, or pressed in.
Screwed in seat rings are considered replaceable since they may be removed and new seat rings installed. A globe valve is a linear motion valve used to stop, start, and regulate fluid flow.
A Z-body globe valve is illustrated in Figure 9. As shown in Figure 9, the globe valve disk can be totally removed from the flowpath or it can completely close the flowpath. The essential principle of globe valve operation is the perpendicular movement of the disk away from the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow.
Therefore, the globe valve may be used for both stopping and starting fluid flow and for regulating flow. When compared to a gate valve, a globe valve generally yields much less seat leakage.
This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk. Globe valves can be arranged so that the disk closes against or in the same direction of fluid flow. When the disk closes against the direction of flow, the kinetic energy of the fluid impedes closing but aids opening of the valve. When the disk closes in the same direction of flow, the kinetic energy of the fluid aids closing but impedes opening.
This characteristic is preferable to other designs when quick-acting stop valves are necessary. Globe valves also have drawbacks. The most evident shortcoming of the simple globe valve is the high head loss from two or more right angle turns of flowing fluid.
Obstructions and discontinuities in the flowpath lead to head loss. In a large high pressure line, the fluid dynamic effects from pulsations, impacts, and pressure drops can damage trim, stem packing, and actuators. In addition, large valve sizes require considerable power to operate and are especially noisy in high pressure applications. Other drawbacks of globe valves are the large openings necessary for disk assembly, heavier weight than other valves of the same flow rating, and the cantilevered mounting of the disk to the stem.
The simplest design and most common for water applications is the Z-body. The Z-body is illustrated in Figure 9. For this body design, the Z-shaped diaphragm or partition across the globular body contains the seat. The horizontal setting of the seat allows the stem and disk to travel at right angles to the pipe axis. The stem passes through the bonnet which is attached to a large opening at the top of the valve body.
This provides a symmetrical form that simplifies manufacture, installation, and repair. Figure 10 illustrates a typical Y-body globe valve. This design is a remedy for the high pressure drop inherent in globe valves. The angle yields a straighter flowpath at full opening and provides the stem, bonnet, and packing a relatively pressure-resistant envelope.
Y-body globe valves are best suited for high pressure and other severe services. In small sizes for intermittent flows, the pressure loss may not be as important as the other considerations favoring the Y-body design. Hence, the flow passage of small Y-body globe valves is not as carefully streamlined as that for larger valves.
The angle body globe valve design, illustrated in Figure 11, is a simple modification of the basic globe valve. Having ends at right angles, the diaphragm can be a simple flat plate. A particular advantage of the angle body design is that it can function as both a valve and a piping elbow. For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe.
The angle valve's discharge conditions are favorable with respect to fluid dynamics and erosion. Most globe valves use one of three basic disk designs: the ball disk, the composition disk, and the plug disk.
The ball disk fits on a tapered, flat-surfaced seat. The ball disk design is used primarily in relatively low pressure and low temperature systems. It is capable of throttling flow, but is primarily used to stop and start flow. The composition disk design uses a hard, nonmetallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications.
They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable. Because of its configuration, the plug disk provides better throttling than ball or composition designs.
Plug disks are available in a variety of specific configurations. In general, they are all long and tapered. Globe valves employ two methods for connecting disk and stem: T-slot construction and disk nut construction. In the T-slot design, the disk slips over the stem. In the disk nut design, the disk is screwed into the stem.
Globe valve seats are either integral with or screwed into the valve body. Many globe valves have backseats. A backseat is a seating arrangement that provides a seal between the stem and bonnet.
When the valve is fully open, the disk seats against the backseat. The backseat design prevents system pressure from building against the valve packing.
For low temperature applications, globe and angle valves are ordinarily installed so that pressure is under the disk. This promotes easy operation, helps protect the packing, and eliminates a certain amount of erosive action to the seat and disk faces. For high temperature steam service, globe valves are installed so that pressure is above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat. A ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow.
The ball, shown in Figure 12, performs the same function as the disk in the globe valve. When the valve handle is turned to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet.
When the valve is shut, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body and the flow is stopped. Other ball valve actuators are planetary gear-operated.
This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. Some ball valves have been developed with a spherical surface coated plug that is off to one side in the open position and rotates into the flow passage until it blocks the flowpath completely. Seating is accomplished by the eccentric movement of the plug. The valve requires no lubrication and can be used for throttling service.
A ball valve is generally the least expensive of any valve configuration and has low maintenance costs. In addition to quick, quarter turn on-off operation, ball valves are compact, require no lubrication, and give tight sealing with low torque.
Conventional ball valves have relatively poor throttling characteristics. In a throttling position, the partially exposed seat rapidly erodes because of the impingement of high velocity flow. Ball valves are available in the venturi, reduced, and full port pattern.
The full port pattern has a ball with a bore equal to the inside diameter of the pipe. Balls are usually metallic in metallic bodies with trim seats produced from elastomeric elastic materials resembling rubber materials. Plastic construction is also available. The resilient seats for ball valves are made from various elastomeric material. Because of the elastomeric materials, these valves cannot be used at elevated temperatures. Care must be used in the selection of the seat material to ensure that it is compatible with the materials being handled by the valve.
The stem in a ball valve is not fastened to the ball. It normally has a rectangular portion at the ball end which fits into a slot cut into the ball. The enlargement permits rotation of the ball as the stem is turned. A bonnet cap fastens to the body, which holds the stem assembly and ball in place.
Adjustment of the bonnet cap permits compression of the packing, which supplies the stem seal. Some ball valve stems are sealed by means of O-rings rather than packing. The handle indicates valve ball position. When the handle lies along the axis of the valve, the valve is open. Some ball valve stems have a groove cut in the top face of the stem that shows the flowpath through the ball.
Observation of the groove position indicates the position of the port through the ball. This feature is particularly advantageous on multiport ball valves. A plug valve is a rotational motion valve used to stop or start fluid flow. The name is derived from the shape of the disk, which resembles a plug.
A plug valve is shown in Figure The simplest form of a plug valve is the petcock. The body of a plug valve is machined to receive the tapered or cylindrical plug. The disk is a solid plug with a bored passage at a right angle to the longitudinal axis of the plug. In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve body. Plug valves are available in either a lubricated or nonlubricated design and with a variety of styles of port openings through the plug as well as a number of plug designs.
An important characteristic of the plug valve is its easy adaptation to multiport construction. Multiport valves are widely used. Their installation simplifies piping, and they provide a more convenient operation than multiple gate valves.
They also eliminate pipe fittings. The use of a multiport valve, depending upon the number of ports in the plug valve, eliminates the need of as many as four conventional shutoff valves. Plug valves are normally used in non-throttling, on-off operations, particularly where frequent operation of the valve is necessary.
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