Valves – Their Components, Classification, and Functions
Valves – Their Components, Classification, and Functions
Fluid power is controlled primarily through the use of control devices called valves. Valves are integral components in piping systems. In addition to the function of ‘conducting the fluid medium’, they are the primary method of controlling the flow, pressure, and direction of the fluid. They can be needed to operate continuously (e.g., control valves), or they can be operated intermittently (e.g., isolation valves), or they can be installed to operate rarely if ever (e.g., safety valves).
Valves are mechanical devices which control the flow of fluid and pressure within a system or process. They regulate, direct, or control the fluid flow by opening, closing, or partially obstructing various passage ways. They control system or process fluid flow and pressure by performing any of these functions like (i) stopping and starting of fluid flow, (ii) varying (throttling) the quantity of the fluid flow, (iii) controlling the direction of the fluid flow, (iv) regulating down-stream pressure of a system or a process, and (v) relieving over-pressure of a component or a piping. There are several valve designs and types available which satisfy one or more of these functions. A multitude of valve types and designs safely accommodate a wide variety of industrial application of the valves.
Valves are components in piping systems which, in addition to the function of ‘conducting the medium’ (directing, changing the nominal width), also have the functions of blocking or regulating the rate of flow and pressure. Depending on use, different materials are normally used for valve construction. Valves can be distinguished according to their functional features, basic design, and type of connection.
Valves can be extremely simple and a low-cost item in the piping system, or they can be extremely complicated and expensive items. They are essential components of a piping system which conveys liquids, gases, vapours, and slurries etc. They are needed to operate under varying operating conditions which can be corrosive, fluids with high temperatures, and fluids carrying dust. Hence, in the design of the piping system, valves probably need more engineering effort than any other component of the piping system.
The selection of these valves involves not only the type but also the size, actuating technique, and remote-control capability. Valve type selection takes into account (i) needed functions, (ii) service conditions, (iii) fluid type and its condition, (iv) fluid characteristics, (v) frequency of operation, (vi) isolation requirements, (vii) maintenance requirements, (viii) environmental considerations, (ix) past experience in comparable conditions, (x) weight and size, and (xi) cost.
Valves are available with a wide variety of valve bodies in different styles, materials, connections, and sizes. Selection is primarily dependent on the service conditions, the task, and the load characteristics of the application. Each valve type has its advantages and limitations. The basic requirements and selection depend on the ability of the valves to perform specific functions such as (i) ability to throttle or control the rate of flow, (ii) lack of turbulence or resistance to flow when fully open since turbulence reduces head pressure, (iii) quick opening and closing mechanism since rapid response is several times needed in an emergency or for safety, (iv) tight shut off since it prevents leaks against high pressure, (v) ability to allow flow in one direction only since it prevents fluid return, (vi) opening at a pre-set pressure since this procedure control to prevent equipment damage, and (vii) ability to handle abrasive fluids as hardened material prevents rapid wear.
Heavy industries such as iron and steel, oil and gas, chemical, and power etc. are heavily dependent on valves. There are several piping systems in these industries, which are to keep the process variables (e.g., flow, pressure, temperature, and level etc.) under control within a required operating range to ensure that a quality end product is produced.
Components of a valve
A valve is basically an assembly of valve components. The basic components of a valve assembly have common nomenclature regardless of the type of valve such as valve body, bonnet, disk (pressure boundary), seat, stem, and yoke. Valve trim is a collective name for the replaceable parts, in a valve. A typically trim design includes a disk (also spelled disc), seat, stem, and sleeves needed to guide the stem. Fig 1 shows basic components of a valve.
Fig 1 Basic components of a valve
Valve body – The valve body, sometimes called the shell, is the primary boundary of a valve. It serves as the main element of a valve assembly since it is the framework which 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, forged, or fabricated in a variety of forms and each component have a specific function and constructed in a material suitable for that function.
Although a sphere or a cylinder is theoretically be the most economical shape to resist fluid pressure when a valve is open, there are several other considerations. For example, several valves need a partition across the valve body to support the seat opening, which is the throttling orifice. With the valve closed, loading on the body is difficult to determine. The valve end connections also distort loads on a simple sphere and more complicated shapes. Ease of manufacture, assembly, and costs are additional important considerations. Hence, the basic form of a valve body typically is not spherical, but ranges from simple block shapes to highly complex shapes in which the bonnet, a removable piece to make assembly possible, forms part of the pressure resisting body. Narrowing of the fluid passage (venturi effect) is also a common method for reducing the overall size and cost of a valve. In other cases, large ends are added to the valve for connection into a larger line.
Valve bonnet – The cover for the opening in the valve body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are available in several designs and models. In some designs, the body itself is split into two sections which bolt together. Like valve bodies, bonnets vary in design. Some bonnets function simply as valve covers, while others support valve internals and accessories such as the stem, disk, and actuator. During manufacture of the valve, the internal components, such as stem, and disk etc., are put into the body and then the bonnet is attached to hold all the components together inside.
The bonnet is cast, or forged of the same material as the body and is connected to the body by a threaded, bolted, or welded joint. In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the weld joint or bolts which connect the bonnet to the body are pressure-retaining parts. Valve bonnets, although a necessity for the majority of the valves, can also represent a cause for concern since bonnets can complicate the manufacture of valves, increase valve size, represent a considerable cost portion of valve cost, and are a source for potential leakage.
Valve trim – The internal elements of a valve are collectively referred to as a valve’s trim. The trim typically includes a disk, seat, stem, and sleeves needed to guide the stem. The performance of a valve 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 disk – The disc is that part of the valve which allows, throttles, or stops the fluid flow, depending on its position. For a valve having a bonnet, the disk is the third primary principal pressure boundary. The disk provides the capability for permitting and prohibiting fluid flow. With the disk closed, full system pressure is applied across the disk if the outlet side is depressurized. For this reason, the disk is a pressure-retaining part.
In the case of a plug or a ball valve, the disk 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 normally forged, and in some designs, hard surfaced to provide good wear properties. A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed. Majority of the valves are named, in part, according to the design of their disks.
Valve seat(s) – The seat or seal rings provide the seating surface for the disk. A valve can have one or more seats. In the case of a globe or a swing-check valve, there is normally 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 is on the up-stream side and the other is on the down-stream side. A gate valve disk has two seating surfaces which come in contact with the valve seats to form a seal for stopping the flow.
In some designs, the valve 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. For improving the wear-resistance of the seal rings, the surface is frequently 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 normally considered pressure boundary parts since the body has sufficient wall thickness to withstand design pressure without relying upon the thickness of the seal rings.
Valve stem – The valve stem, which connects the actuator and the disk, is responsible for positioning the disk. It provides the necessary movement to the disk, 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 disk is needed to open or close the valve, while in plug, ball and butterfly valves, the disk is rotated to open or close the valve. Stems are normally forged, and connected to the disk by threaded or welded joints or other techniques.
For valve designs needing 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 can 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.
There are five types of valve stems. The first type is the rising stem with outside screw and yoke (OS&Y). The exterior of this 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 which causes the stem to rise through the handwheel. This type of valve is a common design for 60 mm OD (outside diameter)pipe size and larger valves.
The second type is the rising stem with inside screw. The threaded part of the stem is inside the valve body, and the stem packing along the smooth section which 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 rise together to open the valve.
The third type is the 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. Hence, this type of stem 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.
The fourth type is the 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 which are operated by hydraulic or pneumatic cylinders.
The fifth type is the rotary stem. This is a normally used stem model in ball, plug, and butterfly valves. A quarter-turn motion of the stem open or close the valve.
Valve stem packing – Majority of the valves use some form of packing to prevent leakage from the space between the stem and the bonnet. Packing is normally a fibrous material (such as flax) or another compound (such as Teflon) which forms a seal between the internal parts of a valve and the outside where the stem extends through the body. Valve packing is to be properly compressed to prevent fluid loss and damage to the valve’s stem. If a valve’s packing is too loose, the valve leaks, which is a safety hazard. If the packing is too tight, it impairs the movement and possibly damage the stem.
For a reliable seal between the stem and the bonnet, a gasket is needed. This is called a packing, and it is fitted with several components namely (i) gland follower, a sleeve which compresses the packing, by a gland into the so called stuffing box, (ii) gland, a kind of bushing, which compressed the packing into the stuffing box, (iii) stuffing box, a chamber in which the packing is compressed, (iv) packing, available in several materials, like Teflon, elastomeric material, or fibrous material etc., and (v) a back seat 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 packing, when the valve is fully open. Back seats are frequently 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. Hence, valve packaging is to be properly done to prevent damage to the stem and loss of fluid or gas. When a packing is too loose, the valve leaks. If the packing is too tight, it affects the movement and possible damage to the stem.
Valve yoke – A yoke connects the valve body or bonnet with the actuating mechanism. The top of the yoke holds a yoke nut, stem nut, or yoke bushing and the valve stem passes through it. A yoke normally has openings to allow access to the stuffing box, and actuator links etc. Structurally, a yoke is to 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, for example, 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 – The actuator operates the stem and disk assembly. The valve operation of the stem and disk assembly can be done by 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.
Hand-operated valves are normally 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 valves, have a lever for actuating the valve. There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include (i) large valves which are to be operated against high hydrostatic pressure, (ii) valves which are to be operated from a remote location, and (iii) when the time for opening, closing, throttling, or manually controlling the valve is longer, than needed by system-design criteria. These valves are normally equipped with an actuator.
The actuator, in the broadest definition, is a device which produces linear and rotary motion with a source of power under the action of a source of a control mechanism. 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 are several different types of actuators. The commonly used valve actuators are (i) gear actuators, (ii) electric motor actuators, (iii) pneumatic actuators, (iv) hydraulic actuators, and (v) solenoid actuators.
Classification of valves
There are three basic types of valves. These are (i) directional control valves, (ii) pressure control valves, and (iii) flow control valves. Directional control valves determine the path through which a fluid traverses a given circuit. For example, they establish the direction of motion of a hydraulic cylinder or motor. This control of the fluid path is accomplished primarily by check valves, shuttle valves, and two-way, three-way, and four-way directional control valves. Pressure control valves protect the system against over-pressure, which can occur because of excessive actuator loads or because of the closing of a valve. In general, pressure control is accomplished by pressure relief, pressure reducing, sequence, unloading, and counter-balance valves. In addition, fluid flow rate is to be controlled in different pipe lines of a hydraulic circuit. For example, the control of actuator speed depends on flow rates. This type of control is accomplished through the use of flow control valves. Non-compensated flow control valves are used where precise speed control is not needed since flow rate varies with pressure drop across a flow control valve. Pressure-compensated flow control valves automatically adjust to changes in pressure drop to produce a constant flow rate.
Further, valves can be classified in several ways. Some of the valve classification methods are described below.
Valve classification based on the material of the valve – Under this classification valves can be cast iron valve, nodular cast iron valve, steel valve (cast, forged, or fabricated), stainless steel valve, alloy steel valve, brass valve, bronze valve, aluminum valve, copper valve, Monel valve, Hastelloy valve, and plastic valve etc.
Valve classification based on mechanical motion – Under this classification valves can be (i) linear motion valve, (ii) rotary motion valve, and (iii) quarter turn valve. Linear motion valve is the valve in which the closure member, such as in gate, globe, diaphragm, pinch, and lift check valve, moves in a straight line to allow, stop, or throttle the flow. Rotary motion valve is the valve in which the valve-closure member travels along an angular or circular path, such as in butterfly, ball, plug, eccentric- and swing check valve. Quarter turn valve is the valve in which the rotary motion of the valve needs around quarter turn (i.e., 0-degree through 90-degree) motion of the stem to go to fully open position from a fully closed position or vice versa.
Classification based on the methods of controlling flow through the valve – Under this classification valves can be (i) valve in which there is a movement of a disk, or plug into or against an orifice such as in globe or needle type valve, (ii) valve in which there is a slide of a flat, cylindrical, or spherical surface across the orifice, (iii) valve in which there is a rotation of a disk or ellipse about a shaft extending across the diameter of an orifice, and (iv) valve in which a flexible material is moved into the flow passage.
Classification based on the valve operation – Under this classification valves can be (i) an on and off valve, (ii) a pressure relief valve, (iii) a check valve, and (iv) a control valve. The on and off valve stops the flow, provides tight shut-off when being closed, and provides low pressure drops when being fully opened. Majority of the control valves can be used for on-off duty, especially ball valves. Gate valves are frequently used in on-off service. Pressure relief valve is designed to protect a system from being over pressurized (pressure – less than 70 MPa, temperature – less than 550 deg C). Types of the pressure relief valve can be direct loaded relief valve, or pilot operated relief valve. Check valve prevents reversal of flow. It opens with forward flow and closes against reverse flow. Types of check valves are lift check valve, swing check valve, and tilting-disk check valve etc. Control valve is used to regulate the flow automatically to any desired amount and for high pressure drop. Types of control valves are globe valve, ball valve, butterfly valve, and plug valve. The selection of the control valve is based on the characteristics of each type of control valve, pressure of the system, temperature of the system, and the type of flowing fluid.
Classification based on the valve function – Under this classification valves can be (i) an isolating valve, and (ii) a regulating valve. Isolating valves are basically intended for shutting off lines. Because of their construction they are not suitable for flow regulation, or only to a limited extent. For the isolation function only the ‘fully open’ or ‘closed’ valve positions are permissible. With regulating valves on the other hand, all intermediary positions are also admissible.
Classification based on the valve actuation mechanism – Some type of actuator is necessary to allow for the positioning of a valve. Actuators vary from simple manual handwheels to relatively complex electrical and hydraulic manipulators. Actuators of the valve can be (i) manual handwheel, (ii) manual lever, (iii) electric motor, (iv) pneumatic, (v) hydraulic, and (vi) solenoid. Valve actuators are selected based upon a number of factors including torque necessary to operate the valve and the need for automatic actuation. All actuators except manual handwheel and lever are adaptable to automatic actuation.
Classification based on end connection of the valve – Under this classification valves can have (i) threaded connection, (ii) welded connection, and (iii) flanged connection. Threaded ends are used for small applications up to 100 mm. They are cheap but can be stripped and leak and for this reason they are used where leakage is not a problem, Threaded end valves are not to be used with corrosive fluids, since threads can either fall or become inseparable. Welded end valves are used when zero leakage is needed for environmental, safety, or any efficiency reasons. The piping can be welded to the valve and hence providing one piece construction. Several valve users insist that high pressure application needs a permanent end especially if the valves involved high temperatures. Flanged ends are the most expensive but are the best from the installation and removal stand-point. The main advantage of flanged connection is that the valve can be removed easily from the pipe line.
Classification based on the corrosion protection of the valve – Under this classification valves can be (i) a galvanized (zinc coated) valve, (ii) an epoxy coated valve, and (iii) an enamel coated valve.
Classification based on special valve applications – Under this classifications valves which are included include (i) emergency shut down valve, (ii) high integrity pressure protection system (HIPPS) valve, (iii) automatic blow down valve, (iv) multi-port valves, (v) bellow sealed valves, (vi) cryogenic service valves, (vii) vent valves, (viii) chimney valves, (ix) valves for low temperature and high temperature service, vacuum service, clean service, dirty service, hazard service, fouling / scaling service, abrasive service, slurry service, solid handling, and corrosive service, and (x) fire hydrants.
Commonly used types of valves
There are a large variety of valves and valve configurations to suit different services and conditions. Because of the diversity of the types of systems, fluids, and environments in which valves are to operate, a vast range of valve types have been developed. The services and conditions in which valves are to operate include different uses (on / off, and control), different fluids (liquid, and gas, etc. which can be combustible, toxic, and corrosive etc.), different materials, and different pressure and temperature conditions. Valves are used for starting or stopping fluid flow, regulating or throttling flow, preventing back flow, or relieving and regulating pressures in liquid or gaseous handling applications.
Valves are available with a wide variety of valve bodies in different styles, materials, connections and sizes. Selection is primarily dependent on the service conditions, the task, and the load characteristics of the application. The most common types include ball valves, butterfly valves, check valves, diaphragm valves, globe valves, gate valves, knife gate valves, parallel slide valves, pinch valves, piston valves, plug valves, safety valves, and sluice valve etc. 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, some works well in corrosive systems, while others handle high pressure fluids. Each valve type has certain inherent advantages and disadvantages. Understanding these differences and how they affect 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. There are normally four methods of controlling flow through a valve namely (i) move a disk, or plug into or against an orifice (for example, globe or needle type valve), (ii) slide a flat, cylindrical, or spherical surface across an orifice (for example, gate and plug valves), (iii) rotate a disk or ellipse about a shaft extending across the diameter of an orifice (for example, a butterfly or ball valve), and (iv) move a flexible material into the flow passage (for example, diaphragm and pinch valves). Each method of controlling flow has characteristics which makes it the best choice for a given application of function.
Gate valve – 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. The disk of a gate valve is completely removed from the flow stream when the valve is fully open. This characteristic offers virtually no resistance to flow when the valve is open. Hence, there is little pressure drop across an open gate valve. When the valve is fully closed, a disk-to-seal ring contact surface exists for 360-degree, and good sealing is provided. With the proper mating of a disk to the seal ring, very little or no leakage occurs across the disk when the gate valve is closed.
On opening the gate valve, the flow path is enlarged in a highly non-linear 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. Majority of the flow change occurs near shut-off 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 (i) It is not suitable for throttling applications, (ii) it is prone to vibration in the partially open state, and (iii) it is more subject to seat and disk wear than a globe valve. Repairs, such as lapping and grinding, are normally more difficult to accomplish.
Gate valves are available with a variety of disks. Classification of gate valves is normally made by the type disk used such as 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 normally replaceable. In services, where solids or high velocity can cause rapid erosion of the seat or disk, these components are to have a high surface hardness and are to have replacement seats as well as disks. If the seats are not replaceable, seat damage needs 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 are normally to be specified with replaceable seats. A gate valve is shown in Fig 2.
Fig 2 Gate valve
The solid wedge gate valve shown has the most commonly used disk because of its simplicity and strength. A valve with this type of wedge can be installed in any position and it is suitable for almost all fluids. It is practical for turbulent flow. The flexible wedge gate valve has 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 centre, 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 forces 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 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 can cause the valve body neck to rupture, the bonnet to lift off, or the seat ring to collapse. Following correct warming procedures, these problems can be prevented.
Split wedge gate valves are of the ball and socket design. These are self-adjusting and self-aligning 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 non-condensing gases and liquids at normal temperatures, particularly corrosive liquids. Freedom of movement of the disk in the carrier prevents binding even though the valve can have been closed when hot and later contracted because of cooling. This type of valve is to be installed with the stem in the vertical position.
The parallel disk gate valve is designed to prevent valve binding because of thermal transients. This design is used in both low-pressure 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 can be part of the disk halves or they can be separate elements. The lower wedge can 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 quantity of seat leakage tends to decrease as differential pressure across the seat increases. These valves normally have a flow direction marking which shows which side is the high-pressure (relieving) side. Care is to 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 by-pass line. A three-way valve is used to position the line to by-pass 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 non-rising stem valves. For the non-rising stem gate valve, the stem is threaded on the lower end into the gate. As the handwheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve almost always has a pointer-type indicator threaded onto the upper end of the stem to indicate valve position. The non-rising 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 flow path when the valve is open. Rising stem gate valves come in two basic designs. One design has a stem which rises through the handwheel while others have a stem which 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 desired 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 can be used for the application where they are needed. Small, forged steel, gate valves can have hard faced seats pressed into the body. In some series, this type of valve in sizes from 15 mm to 25 mm is rated for 17.5 MPa steam service. In large gate valves, disks are frequently of the solid wedge type with seat rings threaded in, welded in, or pressed in. Screwed in seat rings are considered replaceable since they can be removed and new seat rings installed
Globe valve – A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. The globe valve disk can be totally removed from the flow path or it can completely close the flow path. 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. Hence, the globe valve can be used for both stopping and starting fluid flow and for regulating flow.
When compared to a gate valve, a globe valve normally yields much less seat leakage. This is since 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 flow path leads 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 need 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 three primary body designs for globe valves are Z-body, Y-body, and angle (Fig 3). The simplest design and most common for water applications is the Z-body. In 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 which simplifies manufacture, installation, and repair.
Fig 3 Globe valve
Y-body globe valve design is a remedy for the high-pressure drop inherent in globe valves. The seat and stem are angled at around 45 degrees. The angle yields a straighter flow path (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 is not as important as the other considerations favouring 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 is a simple modification of the basic globe valve. Having ends at right angles, the diaphragm can be a simple flat plate. Fluid is able to flow through with only a single 90-degree turn and discharge down-ward more symmetrically than the discharge from an ordinary globe. A particular advantage of the angle body design is that it can function as both a valve and a piping elbow. For moderate conditions of pressure, temperature, and flow, the angle valve closely resembles the ordinary globe. The angle valve’s discharge conditions are favourable with respect to fluid dynamics and erosion.
Majority of the globe valves use one of three basic disk designs namely (i) the ball disk, (ii) the composition disk, and (iii) 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, non-metallic insert ring on the disk. The insert ring creates a tighter closure. Composition disks are primarily used in steam and hot water applications. They resist erosion and are sufficiently resilient to close on solid particles without damaging the valve. Composition disks are replaceable. Plug disk, because of its configuration, 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 namely (i) T-slot construction, and (ii) 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. Several globe valves have back-seats. A back-seat is a seating arrangement which provides a seal between the stem and bonnet. When the valve is fully open, the disk seats against the back-seat. The back-seat 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 quantity 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 contracts upon cooling and tend to lift the disk off the seat.
Ball valve – A ball valve is a rotational motion valve which uses a ball-shaped disk to stop or start fluid flow. The ball 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. Majority of the ball valve actuators are of the quick-acting type, which need a 90-degree turn of the valve handle to operate the valve. Other ball valve actuators are planetary gear-operated. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve.
Some ball valves have been developed with a spherical surface coated plug which is off to one side in the open position and rotates into the flow passage until it blocks the flow path completely. Seating is accomplished by the eccentric movement of the plug. The valve needs no lubrication and can be used for throttling service.
A ball valve is normally 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, need 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 normally metallic in metallic bodies with trim (seats) produced from elastomeric (elastic materials resembling rubber) materials. Plastic construction is also available. The resilient seats for ball valves are made from various elastomeric material. The most common seat materials are Teflon (TFE), filled TFE, nylon, Buna-N, neoprene, and combinations of these materials. Because of the elastomeric materials, these valves cannot be used at high temperatures. Care is be taken 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. Packing for ball valve stems is usually in the configuration of die-formed packing rings normally of TFE, TFE-filled, or TFE-impregnated material. Some ball valve stems are sealed by means of O-rings rather than packing.
Some ball valves are equipped with stops which permit only 90-degree rotation. Others do not have stops and can be rotated 360-degree. With or without stops, a 90-degree rotation is all which is needed for closing or opening a ball valve. The handle indicates valve ball position. When the handle lies along the axis of the valve, the valve is open. When the handle lies 90-degree across the axis of the valve, the valve is closed. Some ball valve stems have a groove cut in the top face of the stem which shows the flow path 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. Fig 4 shows a ball valve.
Fig 4 Ball valve
Plug valve – A plug valve is a rotational motion valve which is used to stop or start the fluid flow. The name is derived from the shape of the disk, which resembles a plug. 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. When the plug is turned 90-degree from the open position, the solid part of the plug blocks the ports and stops fluid flow. Fig 5 shows a plug valve.
Fig 5 Plug valve
Plug valves are available in either a lubricated or non-lubricated 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 multi-port construction. Multi-port 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 multi-port valve, depending upon the number of ports in the plug valve, eliminates the need of as many as four conventional shut-off valves. Plug valves are normally used in non-throttling, on-off operations, particularly where frequent operation of the valve is necessary. These valves are not normally recommended for throttling service since, like the gate valve, a high percentage of flow change occurs near shut-off at high velocity. However, a diamond-shaped port has been developed for throttling service.
Multi-port valves are particularly advantageous on transfer lines and for diverting services. A single multi-port valve can be installed in lieu of three or four gate valves or other types of shut-off valve. A disadvantage is that several multi-port valve configurations do not completely shut off the flow. In majority of the cases, one flow path is always open. These valves are intended to divert the flow of one line while shutting off flow from the other lines. If complete shut-off of flow is a requirement, it is necessary that a style of multi-port valve be used which permits this, or a secondary valve is to be installed on the main line ahead of the multi-port valve to permit complete shut-off of the flow. In some multi-port configurations, simultaneous flow to more than one port is also possible. High care is to be taken in specifying the particular port arrangement needed to ensure that proper operation is possible.
Plugs are either round or cylindrical with a taper. They can have different types of port openings, each with a varying degree of area relative to the corresponding inside diameter of the pipe. The most common port shape is the rectangular port. The rectangular port represents at least 70 % of the corresponding pipe’s cross-sectional area. Round port plug is a term which describes a valve that has a round opening through the plug. If the port is the same size or larger than the inside diameter of the pipe then it is referred to as a full port. If the opening is smaller than the inside diameter of the pipe then the port is referred to as a standard round port. Valves having standard round ports are used only where restriction of flow is not important. A diamond port plug has a diamond-shaped port through the plug. This design is for throttling service. All diamond port valves are venturi restricted flow type. Clearances and leakage prevention are the main considerations in plug valves.
Several plug valves are of all metal construction. In these versions, the narrow gap around the plug can allow leakage. If the gap is reduced by sinking the taper plug deeper into the body, actuation torque climbs rapidly and galling can occur. To remedy this condition, a series of grooves around the body and plug port openings is used with grease prior to actuation. Applying grease lubricates the plug motion and seals the gap between plug and body. Grease injected into a fitting at the top of the stem travels down through a check valve in the passage-way, past the plug top to the grooves on the plug, and down to a well below the plug. The lubricant is required to be compatible with the temperature and nature of the fluid. All manufacturers of lubricated plug valves have developed a series of lubricants which are compatible with a wide range of media. Their recommendation is to be followed as to which lubricant is best suited for the service. The most common fluids controlled by plug valves are gases and liquid hydrocarbons. Some water lines have these valves, provided that lubricant contamination is not a serious danger. Lubricated plug valves can be as large as 600 mm and have pressure capabilities up to 40 MPa. Steel or iron bodies are available. The plug can be cylindrical or tapered.
As regards non-lubricated plugs, there are two basic types of non-lubricated plug valves namely (i) lift-type and elastomer sleeve, or (ii) plug coated. Lift-type plug valves provide a means of mechanically lifting the tapered plug slightly to disengage it from the seating surface to permit easy rotation. The mechanical lifting can be accomplished with a cam or external lever. In a common, non-lubricated, plug valve having an elastomer sleeve, a sleeve of TFE completely surrounds the plug. It is retained and locked in place by a metal body. This design results in a primary seal being maintained between the sleeve and the plug at all times regardless of position. The TFE sleeve is durable and inert to all but a few rarely encountered chemicals. It also has a low coefficient of friction and is, hence, self-lubricating.
When installing plug valves, care is to be taken to allow room for the operation of the handle, lever, or wrench. The manual operator is normally longer than the valve, and it rotates to a position parallel to the pipe from a position 90-degree to the pipe. The gland of the plug valve is equivalent to the bonnet of a gate or globe valve. The gland secures the stem assembly to the valve body. There are three normal types of glands. These are (i) single gland, (ii) screwed gland, and (iii) bolted gland. For ensuring a tight valve, the plug is to be seated at all times. Gland adjustment is to be kept tight enough to prevent the plug from becoming unseated and exposing the seating surfaces to the live fluid. Care is to be exercised not to overtighten the gland, which results in a metal-to-metal contact between the body and the plug. Such a metal-to-metal contact creates an additional force which needs extreme effort to operate the valve.
Diaphragm valve – A diaphragm valve is a linear motion valve which is used to start, regulate, and stop fluid flow. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal. Diaphragm valves are, in effect, simple ‘pinch clamp’ valves. A resilient, flexible diaphragm is connected to a compressor by a stud moulded into the diaphragm. The compressor is moved up and down by the valve stem. Hence, the diaphragm lifts when the compressor is raised. As the compressor is lowered, the diaphragm is pressed against the contoured bottom in the straight through valve or the body weir in the weir-type valve as shown in Fig 6.
Fig 6 Diaphragm valves
Diaphragm valves can also be used for throttling service. The weir-type is the better throttling valve but has a limited range. Its throttling characteristics are essentially those of a quick opening valve because of the large shut-off area along the seat. A weir-type diaphragm valve is available to control small flows. It uses a two-piece compressor component. Instead of the entire diaphragm lifting off the weir when the valve is opened, the first increments of stem travel raise an inner compressor component which causes only the central part of the diaphragm to lift. This creates a relatively small opening through the centre of the valve. After the inner compressor is completely open, the outer compressor component is raised along with the inner compressor and the remainder of the throttling is similar to the throttling which takes place in a conventional valve.
Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries, radio-active fluids, or other fluids which are to remain free from contamination. The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline. Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism. Several fluids which clog, corrode, or gum up the working parts of most other types of valves, pass through a diaphragm valve without causing problems. Conversely, lubricants used for the operating mechanism cannot be allowed to contaminate the fluid being handled. There are no packing glands to maintain and no possibility of stem leakage.
There is a wide choice of available diaphragm materials. Diaphragm life depends upon the nature of the material handled, temperature, pressure, and frequency of operation. Some elastomeric diaphragm materials can be unique in their excellent resistance to certain chemicals at high temperatures. However, the mechanical properties of any elastomeric material are lowered at the higher temperature with possible destruction of the diaphragm at high pressure. Hence, the manufacturer is to be consulted when they are used in high temperature applications. All elastomeric materials operate best below 65 deg C. Some function at higher temperatures. Viton, for example, is noted for its excellent chemical resistance and stability at high temperatures. However, when fabricated into a diaphragm, Viton is subject to lowered tensile strength just as any other elastomeric material behaves at high temperatures. Fabric bonding strength is also lowered at high temperatures, and in the case of Viton, temperatures can be reached where the bond strength becomes critical.
Fluid concentrations is also a consideration for diaphragm selection. Several of the diaphragm materials show satisfactory corrosion resistance to certain corrodents up to a specific concentration and / or temperature. The elastomer can also have a maximum temperature limitation based on mechanical properties which can be in excess of the allowable operating temperature depending upon its corrosion resistance. This is to be checked from a corrosion table.
Diaphragm valves have stems which do not rotate. The valves are available with indicating and non-indicating stems. The indicating stem valve is identical to the non-indicating stem valve except that a longer stem is provided to extend up through the handwheel. For the non-indicating stem design, the handwheel rotates a stem bushing which engages the stem threads and moves the stem up and down. As the stem moves, so does the compressor which is pinned to the stem. The diaphragm, in turn, is secured to the compressor.
Some diaphragm valves use a quick-opening bonnet and lever operator. This bonnet is inter-changeable with the standard bonnet on conventional weir-type bodies. A 90-degree turn of the lever moves the diaphragm from full open to full closed position. Diaphragm valves can also be equipped with chain wheel operators, extended stems, bevel gear operators, air operators, and hydraulic operators. Several diaphragm valves are used in vacuum service. Standard bonnet construction can be employed in vacuum service through 100 mm in size. On valves 100 mm and higher, a sealed, evacuated, bonnet is to be used. This is desired to guard against premature diaphragm failure. Sealed bonnets are supplied with a seal bushing on the non-indicating types and a seal bushing plus O-ring on the indicating types.
Construction of the bonnet assembly of a diaphragm valve is shown in Fig 6. This design is desired for valves which are handling dangerous liquids and gases. In the event of a diaphragm failure, the hazardous materials are not to be released to the atmosphere. If the materials being handled are extremely hazardous, it is desired that a means be provided to permit a safe disposal of the corrodents from the bonnet.
Reducing valve – Reducing valve automatically reduces supply pressure to a pre-selected pressure as long as the supply pressure is at least as high as the selected pressure. As shown in Fig 7, the main parts of the reducing valve are the main valve, an upward-seating valve which has a piston on top of its valve stem, an upward-seating auxiliary (or controlling) valve, a controlling diaphragm, and an adjusting spring and screw.
Fig 7 Reducing valves
Reducing valve operation is controlled by high pressure at the valve inlet and the adjusting screw on top of the valve assembly. The pressure entering the main valve assists the main valve spring in keeping the reducing valve closed by pushing upward on the main valve disk. However, some of the high pressure is bled to an auxiliary valve on top of the main valve. The auxiliary valve controls the admission of high pressure to the piston on top of the main valve. The piston has a larger surface area than the main valve disk, resulting in a net downward force to open the main valve. The auxiliary valve is controlled by a controlling diaphragm located directly over the auxiliary valve.
The controlling diaphragm transmits a downward force which tends to open the auxiliary valve. The downward force is exerted by the adjusting spring, which is controlled by the adjusting screw. Reduced pressure from the main valve outlet is bled back to a chamber beneath the diaphragm to counter-act the downward force of the adjusting spring. The position of the auxiliary valve, and ultimately the position of the main valve, is determined by the position of the diaphragm. The position of the diaphragm is determined by the strength of the opposing forces of the downward force of the adjusting spring against the upward force of the outlet reduced pressure.
Other reducing valves work on the same basic principle, but can use gas, pneumatic, or hydraulic controls in place of the adjusting spring and screw. Non-variable reducing valves replace the adjusting spring and screw with a pre-pressurized dome over the diaphragm. The valve stem is connected either directly or indirectly to the diaphragm. The valve spring below the diaphragm keeps the valve closed. As in the variable valve, reduced pressure is bled through an orifice to beneath the diaphragm to open the valve. Valve position is determined by the strength of the opposing forces of the downward force of the pre-pressurized dome against the upward force of the outlet-reduced pressure.
Non-variable reducing valves eliminate the need for the intermediate auxiliary valve found in variable reducing valves by having the opposing forces react directly on the diaphragm. Hence, non-variable reducing valves are more responsive to large pressure variations and are less susceptible to failure than are variable reducing valves.
Pinch valve – The relatively inexpensive pinch valve (Fig 8) is the simplest in any valve design. It is simply an industrial version of the pinch cock used in the laboratory to control the flow of fluids through rubber tubing. Pinch valves are suitable for on-off and throttling services. However, the effective throttling range is normally between 10 % and 95 % of the rated flow capacity. Pinch valves are ideally suited for the handling of slurries, liquids with large amounts of suspended solids, and systems which convey solids pneumatically. Since the operating mechanism is completely isolated from the fluid, these valves also find application where corrosion or metal contamination of the fluid can be a problem.
Fig 8 Pinch valve
The pinch control valve consists of a sleeve moulded of rubber or other synthetic material and a pinching mechanism. All of the operating portions are completely external to the valve. The moulded sleeve is referred to as the valve body. Pinch valve bodies are manufactured of natural and synthetic rubbers and plastics which have good abrasion resistance properties. These properties permit little damage to the valve sleeve, thereby providing virtually unimpeded flow. Sleeves are available with either extended hubs and clamps designed to slip over a pipe end, or with a flanged end having standard dimensions.
Pinch valves have moulded bodies reinforced with fabric. Pinch valves normally have a maximum operating temperature of 120 deg C. At 120 deg C, maximum operating pressure varies normally from 0.69 MPa for a 25 mm diameter valve and decreases to 0.1 MPa for a 300 mm diameter valve. Special pinch valves are available for temperature ranges of -75 deg C to 300 deg C and operating pressures of 2 MPa.
Majority of the pinch valves are supplied with the sleeve (valve body) exposed. Another style fully encloses the sleeve within a metallic body. This type controls flow either with the conventional wheel and screw pinching device, hydraulically, or pneumatically with the pressure of the liquid or gas within the metal case forcing the sleeve walls together to shut off the flow. Majority of the exposed sleeve valves have limited vacuum application because of the tendency of the sleeves to collapse when vacuum is applied. Some of the encased valves can be used on vacuum service by applying a vacuum within the metal casing and hence preventing the collapse of the sleeve.
Butterfly valve – A butterfly valve (Fig 9) is a rotary motion valve which is used to stop, regulate, and start fluid flow. Butterfly valves are easily and quickly operated because a 90-degree rotation of the handle moves the disk from a fully closed to fully opened position. Larger butterfly valves are actuated by handwheels connected to the stem through gears which provide mechanical advantage at the expense of speed. Butterfly valves possess several advantages over gate, globe, plug, and ball valves, especially for large valve applications. Savings in weight, space, and cost are the most obvious advantages. The maintenance costs are normally low since there are a minimal number of moving parts and there are no pockets to trap fluids.
Fig 9 Butterfly valve
Butterfly valves are especially well-suited for the handling of large flows of liquids or gases at relatively low pressures and for the handling of slurries or liquids with large quantities of suspended solids. Butterfly valves are built on the principle of a pipe damper. The flow control element is a disk of around the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis. When the disk lies parallel to the piping run, the valve is fully opened. When the disk approaches the perpendicular position, the valve is shut. Intermediate positions, for throttling purposes, can be secured in place by handle-locking devices.
Stoppage of flow in the butterfly valves is accomplished by the valve disk sealing against a seat which is on the inside diameter periphery of the valve body. Several butterfly valves have an elastomeric seat against which the disk seals. Other butterfly valves have a seal ring arrangement which uses a clamp-ring and backing-ring on a serrated edged rubber ring. This design prevents extrusion of the O-rings. In early designs, a metal disk has been used to seal against a metal seat. This arrangement has not provided a leak-tight closure, but has provided sufficient closure in some applications (i.e., water distribution lines).
The construction of the body of the butterfly valve varies. The most economical is the wafer type which fits between two pipeline flanges. Another type, the lug wafer design, is held in place between two pipe flanges by bolts which join the two flanges and pass through holes in the outer casing of the valve. Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded end construction.
Stem and disk for a butterfly valve are separate pieces. The disk is bored to receive the stem. Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned. In the first method, the disk is bored through and secured to the stem with bolts or pins. The alternate method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-shaped stem. This method allows the disk to ‘float’ and seek its centre in the seat.
Uniform sealing is accomplished and external stem fasteners are eliminated. This method of assembly is advantageous in the case of covered disks and in corrosive applications. In order for the disk to be held in the proper position, the stem is required to extend beyond the bottom of the disk and fit into a bushing in the bottom of the valve body. One or two similar bushings are along the upper portion of the stem as well. These bushings are to be either resistant to the media being handled or sealed so that the corrosive media cannot come into contact with them.
Stem seals are accomplished either with packing in a conventional stuffing box or by means of O-ring seals. Some valve manufacturers, particularly those specializing in the handling of corrosive materials, place a stem seal on the inside of the valve so that no material being handled by the valve can come into contact with the valve stem. If a stuffing box or external O-ring is used, the fluid passing through the valve comes into contact with the valve stem.
Needle valve – A needle valve is used to make relatively fine adjustments in the quantity of fluid flow. The distinguishing characteristic of a needle valve is the long, tapered, needle like point on the end of the valve stem. This ‘needle’ acts as a disk. The longer part of the needle is smaller than the orifice in the valve seat and passes through the orifice before the needle seats. This arrangement permits a very gradual increase or decrease in the size of the opening. Needle valves are frequently used as component parts of other, more complicated valves. For example, they are used in some types of reducing valves. One type of body design for a needle valve is the bar-stock body. Bar stock bodies are common, and, in globe types, a ball swivelling in the stem provides the necessary rotation for seating without damage.
Majority of the constant pressure pump governors have needle valves to minimize the effects of fluctuations in pump discharge pressure. Needle valves are also used in some components of automatic combustion control systems where very precise flow regulation is necessary. Needle valves are frequently used as metering valves. Metering valves are used for extremely fine flow control. The thin disk or orifice allows for linear flow characteristics. Hence, the number of handwheel turns can be directly correlated to the quantity of flow. A typical metering valve has a stem with 16 threads per 10 mm. Needle valves normally use one of two styles of stem packing namely (i) an O-ring with TFE backing rings, or (ii) a TFE packing cylinder. Needle valves are frequently equipped with replaceable seats for ease of maintenance. Fig 10 shows a needle valve and a valve with bar-stock body.
Fig 10 Needle valve
Check valve – Check valves are designed to prevent the reversal of flow in a piping system. These valves are activated by the flowing material in the pipeline. The pressure of the fluid passing through the system opens the valve, while any reversal of flow closes the valve. Closure is accomplished by (i) the weight of the check mechanism, (ii) by the back pressure, (iii) by a spring, or (iv) by a combination of these means. The normal types of check valves are swing valve, tilting-disk valve, lift check valve, piston valve, butterfly valve, and stop valve (Fig 11).
Fig 11 Check valves
The swing check valve allows full, unobstructed flow and automatically closes as pressure decreases. These valves are fully closed when the flow reaches zero and prevent back flow. Turbulence and pressure drop within the valve are very low. A swing check valve is normally recommended for use in systems employing gate valves because of the low pressure drop across the valve. Swing check valves are available in either Y-pattern or straight body design.
A straight check valve is shown in Fig 11. In either style, the disk and hinge are suspended from the body by means of a hinge pin. Seating is either metal-to-metal or metal seat to composition disk. Composition disks are normally desired for services (i) where dirt or other particles can be present in the fluid, (ii) where noise is objectionable, or (iii) where positive shut-off is needed. Straight body swing check valves contain a disk which is hinged at the top. The disk seals against the seat, which is integral with the body. This type of check valve normally has replaceable seat rings. The seating surface is placed at a slight angle to permit easier opening at lower pressures, more positive sealing, and less shock when closing under higher pressures.
Swing check valves are normally installed in conjunction with gate valves since they provide relatively free flow. They are desired for lines having low velocity flow and are not to be used on lines with pulsating flow when the continual flapping or pounding is destructive to the seating elements. This condition can be partially corrected by using an external lever and weight.
The tilting disk check valve is similar to the swing check valve. Like the swing check, the tilting disk type keeps fluid resistance and turbulence low because of its straight-through design. Tilting disk check valves can be installed in horizontal lines and vertical lines having upward flow. Some designs simply fit between two flange faces and provide a compact, lightweight installation, particularly in larger diameter valves.
The disk lifts off of the seat to open the valve. The air foil design of the disk allows it to ‘float’ on the flow. Disk stops built into the body position the disk for optimum flow characteristics. A large body cavity helps minimize flow restriction. As flow decreases, the disk starts closing and seals before reverse flow occurs. Back-pressure against the disk moves it across the soft seal into the metal seat for tight shut-off without slamming. If the reverse flow pressure is insufficient to cause a tight seal, the valve can be fitted with an external lever and weight. These valves are available with a soft seal ring, metal seat seal, or a metal-to-metal seal. The latter is desired for high temperature operation. The soft seal rings are replaceable, but the valve is to be removed from the line to make the replacement.
Lift check valves are normally used in piping systems in which globe valves are being used as a flow control valve. They have similar seating arrangements as globe valves. Lift check valves are suitable for installation in horizontal or vertical lines with upward flow. They are desired for use with steam, air, gas, water, and on vapour lines with high flow velocities. These valves are available in three body patterns namely (i) horizontal, (ii) angle, and (iii) vertical.
Flow to lift check valves is always required to enter below the seat. As the flow enters, the disk or ball is raised within guides from the seat by the pressure of the upward flow. When the flow stops or reverses, the disk or ball is forced onto the seat of the valve by both the back-flow and gravity. Some types of lift check valves can be installed horizontally. In this design, the ball is suspended by a system of guide ribs. This type of check valve design is normally used in plastic check valves. The seats of metallic body lift check valves are either integral with the body or contain renewable seat rings. Disk construction is similar to the disk construction of globe valves with either metal or composition disks. Metal disk and seat valves can be reground using the same techniques as is used for globe valves.
Piston check valve is essentially a lift check valve. It has a dash-pot consisting of a piston and cylinder which provides a cushioning effect during operation. Because of the similarity in design to lift check valves, the flow characteristics through a piston check valve are essentially the same as through a lift check valve. Installation is the same as for a lift check valve in that the flow is required to enter from under the seat. Construction of the seat and disk of a piston check valve is the same as for lift check valves. Piston check valves are used primarily in conjunction with globe and angle valves in piping systems experiencing very frequent changes in flow direction. Valves of this type are used on water, steam, and air systems.
Butterfly check valves have a seating arrangement similar to the seating arrangement of butterfly valves. Flow characteristics through these check valves are similar to the flow characteristics through butterfly valves. Hence, butterfly check valves are quite frequently used in systems using butterfly valves. In addition, the construction of the butterfly check valve body is such that ample space is provided for unobstructed movement of the butterfly valve disk within the check valve body without the necessity of installing spacers. The butterfly check valve design is based on a flexible sealing member against the bore of the valve body at an angle of 45-degree. The short distance the disk is to move from full open to full closed prevents the ‘slamming’ action found in some other types of check valves. Fig 11 shows the internal assembly of the butterfly check valve. Since the flow characteristics are similar to the flow characteristics of butterfly valves, applications of these valves are much the same. Also, because of their relatively quiet operation, these valves find application in heating, ventilation, and air conditioning systems. Simplicity of design also permits their construction in large diameters – up to 1,800 mm.
As with butterfly valves, the basic body design lends itself to the installation of seat liners constructed of many materials. This permits the construction of a corrosion-resistant valve at lesser expense than is encountered if it is necessary to construct the entire body of the higher alloy or more expensive metal. This is particularly true in constructions such as those of titanium. Flexible sealing members are available in Buna-N, Neoprene, Nordel, Hypalon, Viton, Tyon, Urethane, Butyl, Silicone, and TFE as standard, with other materials available on special order. The valve body essentially is a length of pipe which is fitted with flanges or has threaded, grooved, or plain ends. The interior is bored to a fine finish. The flanged end units can have liners of different metals or plastics installed depending upon the service requirements. Internals and fasteners are always of the same material as the liner. Butterfly check valves can be installed horizontally or vertically with the vertical flow either upward or downward. Care is to be taken to ensure that the valve is installed so that the entering flow comes from the hinge post end of the valve, otherwise, all flow is stopped.
Stop check valve is a combination of a lift check valve and a globe valve. It has a stem which, when closed, prevents the disk from coming off the seat and provides a tight seal (similar to a globe valve). When the stem is operated to the open position, the valve operates as a lift check. The stem is not connected to the disk and functions to close the valve tightly or to limit the travel of the valve disk in the open direction.
Relief and safety valves – Relief and safety valves prevent equipment damage by relieving accidental over-pressurization of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the set-point pressure. A relief valve (Fig 12) gradually opens as the inlet pressure increases above the set-point. It opens only to the extent as necessary to relieve the over-pressure condition.
A safety valve (Fig 12) rapidly pops fully open as soon as the pressure setting is reached. A safety valve stays fully open until the pressure drops below a reset pressure. The reset pressure is lower than the actuating pressure set-point. The difference between the actuating pressure setpoint and the pressure at which the safety valve resets is called blow-down. Blow-down is expressed as a percentage of the actuating pressure set-point.
Fig 12 Relief and safety valve
Relief valves are typically used for incompressible fluids such as water or oil. Safety valves are typically used for compressible fluids such as steam or other gases. Safety valves can be frequently distinguished by the presence of an external lever at the top of the valve body, which is used as an operational check. As indicated in Fig 12, system pressure provides a force which is attempting to push the disk of the safety valve off its seat. Spring pressure on the stem is forcing the disk onto the seat. At the pressure determined by spring compression, system pressure overcomes spring pressure and the relief valve opens. As system pressure is relieved, the valve closes when spring pressure again overcomes system pressure. Majority of the relief and safety valves open against the force of a compression spring. The pressure set-point is adjusted by turning the adjusting nuts on top of the yoke to increase or decrease the spring compression.
Pilot-operated relief valves are designed to maintain pressure through the use of a small passage to the top of a piston which is connected to the stem such that system pressure closes the main relief valve. When the small pilot valve opens, pressure is relieved from the piston, and system pressure under the disk opens the main relief valve. Such pilot valves are typically solenoid operated, with the energizing signal originating from pressure measuring systems.