Basics of Valves

Basics of Valves

A valve is a mechanical device specially designed to control the fluid flow and pressure within a system or process. Valves are designed for a fluid (either liquid or gaseous) application. They are components in piping systems which, in addition to the function of ‘conducting the medium’ (directing, and changing the nominal width), also have the functions of blocking, or regulating the rate of flow and the pressure. Depending on use, different materials are normally used for a valve

Valves carry out any of the several functions which consist of (i) stopping and starting of flow of the fluid, (ii) throttling the quantity of fluid flow, (iii) controlling the fluid flow direction, (iv) regulating the downstream system or process pressure, (v) relieving the over pressure in the system, (vi) regulating the temperature of process fluid, and (vii) controlling the mixing of fluids.

Valves can be extremely simple and a low-cost item in a 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.

Since there is diversity of the types of systems, fluids, and environments in which valves are to operate, valves have several designs and types which satisfy one or more of the identified functions. By nature of their design, function, and application, valves come in a wide variety of styles, sizes, and pressure classes. A multitude of valve types and designs safely accommodate a wide variety of valve applications.  These days range of valves extends from simple water cocks to control valves equipped with micro-processors, which provide single loop control of the process.

Valves need to be designed for specific applications so that they can fulfil their desired functions correctly. When designing valves, the difference between isolation valve and control valve is an important aspect. While, as a rule, isolation valve is selected according to the nominal size and pressure rating of the pipeline, the choice of the control valve is made on the basis of the hydraulic requirements of the control task to be performed.

There are two main types of valve designs, depending on the action of the closure member: sliding-stem valves, and rotary valves. Sliding-stem valves use linear motion to move a closure member into and out of a seating surface. Rotary valves use rotational motion to turn a closure member into and out of a seating surface.

The design of isolation valves is essentially limited to determining nominal sizes and pressure ratings. As long as the flow velocity is within the range of the specifications, the isolation valve is determined in much the same way as the pipeline itself. For control valves, the hydraulic properties required for the control function is needed to be taken into account. This can mean that the different design stages have to be repeated a number of times. As regards nominal pressure, the design of the valves is based on that of the pipeline. The nominal size of the valve is determined for the maximum volume of the fluid needed. When doing this, the maximum allowable flow velocity according to the manufacturer’s specification is also to be taken into account.

The flow regulation in a valve is accomplished by the varying resistance as the valve is operated, i.e., its effective cross-sectional area is changed. As the fluid moves from the piping into the smaller diameter orifice of the valve, its velocity increases to enable mass flow through the valve. The energy needed to increase the velocity comes at the expense of the pressure, so the point of highest velocity is also the point of lowest pressure (smallest cross section). The point where the pressure is at the lowest is called ‘vena contracta’. To display the general behaviour of flow through a control valve, the valve is simplified to an orifice in a pipeline. The fluid flow behaviour through a control valve is shown in Fig 1.

Fig 1 Fluid flow behaviour through a control valve

As the liquid passes the point of highest restriction (vena contracta), its velocity reaches a maximum and its pressure falls to a minimum. Hence, the highest velocity is expected at the internal of the valve than on upstream and downstream. Beyond the vena contracta, the fluid’s velocity decreases as the diameter of piping increases. This allows for some pressure recovery as the energy which has been imparted as velocity is now partially converted back into pressure. It is important to understand how the pressure-velocity conditions change as the fluid passes through the restriction. This is best described by the continuity equation V1*A1 = V2*A2, where ‘V’ is the mean velocity and ‘A’ is the flow area. Subscript 1 refers to upstream conditions and the subscript 2 refer to downstream conditions. The equation shows that the velocity and hence the pressure can be changed by adjusting the valve opening (area).

Components of a valve

Regardless of type, all valves are 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 2 shows basic components of a valve.

Fig 2 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 several types of valves available, each having their advantages and limitations. The most common types of valves in use today are gate, globe, plug, ball, needle, butterfly, check, pressure relief, diaphragm, pinch, and safety valves etc. Valves are manufactured from a number of materials which include carbon steel, alloy steel, stainless steel, iron, brass, bronze, aluminum, plastics and a number of special alloys. Valves can have threaded end connection, flanged end connection, weld-on end connection, screwed socket connection, and puh-in-socket connection, or other type of connection like union end connections in case of some plastic 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 etc.), 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.

Though all the valves have the same function of controlling the fluid flow and have the same basic components, the method of controlling the flow can differ. Normally the following four methods are used for controlling flow through the valve. These are (i) move a disk, or plug into or against an orifice as in globe or needle type of valves, (ii) slide a flat, cylindrical, or spherical surface across an orifice as in a gate or plug valves, (iii) rotate a disk or ellipse about a shaft extending across the diameter of an orifice as in a butterfly or ball valve, and (iv) make a flexible material into the flow passage as in diaphragm and pinch 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 mor. 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.

Valves are available with a wide variety of valve bodies in various styles, materials, connections and sizes. Selection is primarily dependent on the service conditions, the task, and the load characteristics of the application. The basic requirements and selection of valves depend on their ability 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 (turbulence reduces head pressure), (iii) quick opening and closing mechanism (rapid response is several times needed in an emergency or for safety), (iv) tight shut off (prevents leaks against high pressure), (v) ability to allow flow in one direction only (prevents return of the fluid), (vi) opening at a pre-set pressure (this procedure control to prevent equipment damage), and (vii) ability to handle abrasive fluids (hardened material prevents rapid wear).

The most common types of valves are ball valve, plug valve, butterfly valve, globe valves, gate valve, and needle valve. Out of these types of valves, the first three types of valves are quarter turn valves while the last three types are rising stem valves as shown in Fig 3.

Fig 3 Types of quarter turn and rising stem valves

Further, there are compact fast acting on / off valves. Important among these valves are solenoid valve, coaxial valve, and angle seat valve. These valves are shown in Fig 4.

Fig 4 Types of compact fast acting valves

Further, valves can be classified in several ways. Some of the valve classification methods are described below.

By nature of their design and function in handling process fluids, valves can be classified into three categories namely (i) on-off valves which handle the function of blocking the flow or allowing it to pass, (ii) non return valves which only allow flow to travel in one direction, and (iii) throttling valves which allow for the regulation of the flow at any point between fully open to fully closed. In this type of classification of valves, it is possible that specific valve body designs such as globe, gate, plug, and butterfly etc. can fit into one, two, or all three classifications.

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.

In general, valves can be distinguished according to their (i) functional features, (ii) basic design, and (iii) type of connection. Tab 1 contains a classification of valves according to their functional features.

Tab 1 Classification of valves according to functional features
Valve designType of action on fluidExamples
Regulating valve Reducing the working pressurePressure reducing valve, throttle valve
Tapping the flow substanceSampling valve
Control deviceSeparate or combined control of pressure, temperature, and volumeControl valve, control butterfly valve, regulating cock, servo valve
Controlling a fluid levelLevel control valve
Safety valve Preventing excess pressures and subsequent shutoffOutlet valve, safety valve, safety shutoff valve
Bursting disc safety devicePreventing excess pressures without subsequent shutoffBursting disc safety device
Return flow inhibitorPreventing a reversal of flowNon-return valve, check valve

Fig 5 gives classification of valves according to basic design. The figure shows (i) working mechanism of closing device, (ii) direction of flow in the connection area, (iii) designation of basic design, (iv) type of connection, and (v) design examples.

Fig 5 Classification of valves according to basic design

Under classification as per application, valves can be classified in three categories. These are (i) general service valves which have a versatile valve design that can be used in several applications without modification, (ii) special service valves which are specifically designed for a specific application, and (iii) severe service valves which are highly engineered to avoid the side effects of difficult applications.

Under classification as mechanical motion, valves are classified in two categories. In the first category come linear motion valves which have a sliding stem design that pushes a closure element into an open or closed position. Gate, globe, pinch, diaphragm, split body, three way and angle valves fit into this classification. Linear valves are known for the simple design, easy maintenance, and versatility with more sizes, pressure class and design options. In the second category come rotary motion valves which use a closure element that rotates, through a quarter turn or 45-degree range to open or block the flow. Rotary valves are smaller in size.

Under classification as per port size, valves are classified in two categories. Valves, which are designed so that internal flow passage-ways are large enough to pass flow without a significant restriction, are called full port valves. In these valves, the internal flow is equal to the full area of the inlet port. Full port valves are used primarily with on-off services where the flow is required to be stopped or diverted. The second category valves are reduced port valves. In these valves closure elements restrict the flow. The restriction allows the valve to take a pressure drop as flow moves through the closure element, allowing a partial pressure recovery. Primary purpose of reduced port valves is to control the flow through throttling.

Valve terminology

Some of the important terms used for the valves including their physical and operating characteristics are given below.

Actuator device – It is used to operate a valve using electric, pneumatic or hydraulic means. It is frequently used for remote control or sequencing of valve operations.

Actuator lever – It is the arm attached to rotary valve shaft to convert linear actuator stem motion to rotary force (torque) to position a disk or ball of a rotary valve. The lever normally is positively connected to the rotary by close tolerance splines or other means to minimize play and lost motion.

Actuator spring – It is a spring, or a group of springs, enclosed in the yoke or actuator casing or piston cylinder which moves the actuator stem in a direction opposite to that created by loading pressure. Actuator stem – It is that part which connects the actuator to the valve stem and transmits motion (force) from the actuator to the valve.

Actuator stem extension – It is an extension of the piston actuator stem to provide a means of transmitting piston motion to the valve positioner.

Actuator stem force – It is the net force from an actuator which is available for actual positioning of the valve plug, referred to as valve travel.

All-welded construction – It pertains to a valve construction in which the valve body is completely welded and cannot be disassembled and repaired in the field.

Anchor pin – It is a pin welded to the body of a ball valve. This pin aligns the adapter plate and keeps the plate and gear operator from moving while the valve is being operated.

Angle valve – It is a variation of the globe valve in which the end connections are at right angles to each other, rather than being in-line. It is a valve design in which the inlet and outlet ports are perpendicular to each other.

Angle seat valve – It is an angle seat piston valve which is a pneumatically-controlled valve with a piston actuator providing linear actuation to lift a seal off its seat. The seat is set at an angle to provide the maximum possible flow when unseated. Angle seat piston valves are particularly suited to applications where high temperatures and large flow rates are needed. When used in reverse, some models of angle seat piston valve eliminate water hammer when operated.

Back seat – It is a shoulder on the stem of a gate or globe valve which seals against a mating surface inside the bonnet to prevent leakage of media through the bonnet stuffing box when the valve is fully opened.

Ball, full – It is the flow closure member of rotary control valves using a complete sphere with a cylindrical flow passage through it. The flow passage equals or matches the pipe diameter.

Ball, segmented – It is the flow closure member of rotary control valves using a partial sphere with a flow passage through it.

Ball, V-notch – It is the most common type of segmented ball control valve. The V-notch ball includes a polished or plated partial sphere surface which rotates against the seal ring throughout the travel range. The V-shaped notch in the ball permits wide rangeability and produces an equal-percentage flow characteristic.

Ball valve – It is a valve using a spherical closure element which is rotated through 90-degree to open and close the valve.

Bellows seal bonnet – It is a bonnet which uses a bellows for sealing against leakage around the closure member stem.

Bench set – It is the calibration procedure of an actuator spring so that it can use a pressure range to fully stroke a valve to its rated travel.

Blowdown valve – A blowdown valve is a valve or system of valves which, when activated, initiates a blowdown of a pipeline, plant, process, or platform. It is similar to an emergency shutdown valve which shuts in a pipeline, the blowdown valve opens a pipeline.

Body – It is the principal pressure containing part of a valve in which the closure element and seats are located.

Bonnet – It is the portion of the valve which contains the packing box and stem seal and can provide guiding for the valve stem. It provides the principal opening to the body cavity for assembly of internal parts or it can be an integral part of the valve body. It can also provide for the attachment of the actuator to the valve body. Typical bonnets are bolted, threaded, welded, pressure sealed, or integral with the body. This term is frequently used in referring to the bonnet and its included packing parts. More properly, this group of component parts is to be called the bonnet assembly.

Bonnet assembly (normally called bonnet, more properly bonnet assembly) – It is an assembly including the part through which a valve stem moves and a means for sealing against leakage along the stem. It normally provides a means for mounting the actuator and loading the packing assembly, and maintains proper alignment of the disk / plug to the rest of the control valve assembly.

Bottom flange – It is a part which closes a valve body opening opposite the bonnet opening. It can include a guide bushing and / or serve to allow reversal of the valve action.

Bushing – It is a device which supports and / or guides moving parts such as valve stems and disks / plugs.

Butterfly valve – It is a short face-to-face valve which has a movable vane, in the centre of the flow stream, which rotates 90-degree as the butterfly valve opens and closes.

Cage – It is a part of the valve trim which surrounds the closure member and can provide flow characterization and / or a seating surface. It also provides stability, guiding, balance, and alignment, and facilitates assembly of other parts of the valve trim. The walls of the cage contain openings which normally determine the flow characteristic of the valve.

Capacity – It is the quantity of flow through a valve, under stated conditions.

Check valve – It is a one-directional valve which is opened by the fluid flow in one direction and which closes automatically when the flow stops or reverses direction.

Clearance flow – It is the flow which occurs below the minimum controllable flow with the closure member not fully seated.

Closure member – It is the movable part of the valve which is positioned in the flow path to modulate the rate of flow through the valve.

Closure member guide – It is that portion of a closure member which aligns its movement in either a cage, seat ring (port guiding), bonnet, bottom flange, stem, or any two of these.

Coaxial valve– It is a direct-acting solenoid valve with an unobstructed flow route for very viscous media or high-pressure applications.

Control valve – It is a valve which controls a process variable, such as pressure, flow or temperature, by modulating its opening in response to a signal from a controller.

Cryogenic valve – It is a valve which is capable of functioning at cryogenic temperatures.

Cylinder – It is the chamber of a piston actuator in which the piston moves.

Cylinder closure seal – It is the sealing element at the connection of the piston actuator cylinder to the yoke.

Diaphragm – It is a flexible, pressure responsive element which transmits force to the diaphragm plate and actuator stem.

Diaphragm actuator – It is a fluid-powered device in which the fluid, normally compressed air, acts upon a flexible component, the diaphragm, to produce a force to move the closure member.

Diaphragm case – It is a housing, consisting of top and bottom section, used for supporting a diaphragm and establishing one or two pressure chambers.

Diaphragm plate – It is a rigid plate concentric with the diaphragm for transmitting force to the actuator stem.

Diaphragm pressure span – It is the difference between the high and low values of the diaphragm loading pressure range.

Diaphragm valve – It is also called membrane valve. It consists of a valve body with two or more ports, an elastomeric diaphragm, and a ‘weir or saddle or seat upon which the diaphragm closes the valve.

Direct-acting actuator – It is an actuator, in which the actuator stem extends with increasing loading pressure.

Directional control valve – It is one of the most fundamental parts of hydraulic and pneumatic systems. It allows fluid flow into different paths from one or more sources. It normally consists of a spool inside a cylinder which is mechanically or electrically actuated. The position of the spool restricts or permits the flow and hence it controls the fluid flow.

Disk, conventional – It is the symmetrical flow closure member used in the most common varieties of butterfly rotary valves. Highly-dynamic torques normally limit conventional disks to 60-degree maximum rotation in throttling service.

Disk, dynamically-designed – It is a butterfly valve disk contoured to reduce dynamic torque at large increments of rotation, thereby making it suitable for throttling service with up to 90-degree of disk rotation.

Disk, eccentric – It is the common name for valve design in which the off-centered positioning of the valve shaft / disk connections causes the disk to take a slightly eccentric (cammed) path on opening. This allows the disk to be swung out of contact with the seal as soon as it is opened, reducing friction and wear.

Double-acting actuator – It is an actuator in which pneumatic, hydraulic, or electric power is supplied in either.

Drain plug – It is a fitting at the bottom of a valve, the removal of which permits draining and flushing the body cavity.

Drive pins – These are the pins which fit into the bottom of a ball valve stem and engage corresponding holes in the ball. As the operator turns the stem, the drive pins turn the ball.

Dynamic unbalance – It is the net force produced on the valve plug in any stated open position by the fluid process pressure acting upon it.

Effective area – In an actuator, it is the part of the diaphragm or piston area which produces a stem force. The effective area of a diaphragm can change as it is stroked. It is normally a maximum at the start and a minimum at the end of the travel range. Molded diaphragms have less change in effective area than flat sheet diaphragms. Hence, molded diaphragms are desired.

Emergency shutdown valve – It is a valve or a system of valves which, when activated, initiate a shutdown of the plant, process, or platform they are tied to.

Expanding gate valve – It is a gate valve comprised of a separate gate and segment which, as the valve operates the gate and segment, move without touching the seats, permitting the valve to be opened and closed without wear. In the closed position, the gate and segment are forced against the seats. Continued downward movement of the gate causes the gate and segment to expand against the seats. When the valve reaches its full-open position, the gate and segment seal off against the seats while the flow is isolated from the valve body.

Extension bonnet – It is a bonnet with higher dimension between the packing box and bonnet flange for direction hot or cold service.

Extension stem – It is the equipment applied to the buried valves to provide above grade accessibility to operating gear, blowdown, and seat sealant systems.

Fail-closed – It is a condition wherein the valve closure member moves to a closed position when the actuating energy source fails.

Fail-open – It is a condition wherein the valve closure member moves to an open position when the actuating energy source fails.

Fail-safe – It is a characteristic of a valve and its actuator, which upon loss of actuating energy supply, causes a valve closure member to be fully closed, fully open, or remain in the last position, whichever position is defined as necessary to protect the process and equipment. Action can involve the use of auxiliary controls connected to the actuator.

Flange – It is a formed pipe fitting consisting of a projecting radial collar with bolt holes to provide a means of attachment to piping components having a similar fitting. It is the end piece of flanged-end valves.

Flangeless valve – It is valve style common to rotary control valves. Flangeless valves are held between the flanges by long through-bolts (sometimes also called wafer-style valve bodies).

Flow characteristic – It is the relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 %. This term is always to be designated as either inherent flow characteristic or installed flow characteristic.

Flow coefficient – It is a constant related to the geometry of a valve, for a given travel, which can be used to establish flow capacity. It is defined as the flow of water in cubic meters per hour at a pressure drop of 0.1 MPa with temperature ranging from 5 deg C to 30 deg C.

Flow control valve – A flow control valve regulates the flow or pressure of a fluid. It normally responds to signals generated by independent devices such as flow meter or temperature gauge.

Gate valve – It is a straight-through pattern valve whose closure element is a wedge or parallel-sided slab, situated between two fixed seating surfaces, with means to move it in or out of the flow stream in a direction perpendicular to the pipeline axis.

Gland (or gland bushing) – It is that part of a valve which retains or compresses the stem packing in a stuffing box (where used) or retains a stem O-ring, lip seal, or stem O-ring bushing.

Gland plate – It is the plate in a valve which retains the gland, gland bushing, or stem seals, and sometimes guides the stem.

Globe valve – It is a valve with a linear motion closure member, one or more ports, and a body distinguished by a globular shaped cavity around the port region. Globe valves can be further classified as two-way single-ported, two-way double-ported angle-style, or three-way.

Handwheel operated valve – It is a valve on which the handwheel drives the stem directly to operate the valve.

High-recovery valve – It is a valve design which dissipates relatively little flow stream-energy because of streamlined internal contours and minimal flow turbulence. Hence, pressure downstream of the valve vena contracta recovers to a high percentage of its inlet value. Straight-through flow valves, such as rotary ball valves, are typically high-recovery valves.

Hydrant valve – It is also called landing valve. It provides the means to draw water for fire-fighting from the fire water piping network.

Inherent diaphragm pressure range – It is the high and low values of pressure applied to the diaphragm to produce rated valve plug travel with atmospheric pressure in the valve body. This range is frequently referred to as a bench set range since it is the range over which the valve strokes when it is set on the work bench.

Inherent flow characteristic – It is the relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel with constant pressure drop across the valve.

Inlet port – It is the end of a valve which is connected to the upstream pressure zone of a fluid system. Inner seat ring – It is the inner part of a two-piece valve seat assembly.

Inside screw, rising stem – It is the common term for any valve design in which the stem threads are exposed to the fluid below the packing and the stem rises up through the packing when the valve is opened.

Installed diaphragm pressure range – It is the high and low values of pressure applied to the diaphragm to produce rated travel with stated conditions in the valve body. It is because of the forces acting on the closure member which the inherent diaphragm pressure range can differ from the installed diaphragm pressure range.

Installed flow characteristic – It is the relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions.

Isolation valve – It is a valve in a fluid handling system which stops the flow of process media to a given location, normally for maintenance or safety purposes.

Knife gate valve – A knife gate valve works by lifting a plate with a planar or ‘knife’ edge from the path of flow which can literally cut through different impediments during closing thereby creating complete closure against a soft sealing surface. This soft sealing surface normally dictates use in rather low-pressure systems and is not designed to handle high pressure. The knife gate valve is primarily designed for isolation or on / off service rather than a throttled or partial-flow application.

Level control valve – It is an altitude control valve which automatically responds to changes in the height of a liquid in some storage system.

Loading pressure – It is the fluid, normally compressed air, applied to the diaphragm or piston in a pneumatic actuator.

Low-recovery valve – It is a valve design which dissipates a considerable quantity of flow stream-energy because of turbulence created by the contours of the flow path. Hence, pressure downstream of the valve vena contracta recovers to a lesser percentage of its inlet value than is the case with a valve having a more streamlined flow path. Although individual designs vary, conventional globe-style valves normally have low pressure recovery capability.

Modified parabolic flow characteristic – It is an inherent flow characteristic which provides equal percentage characteristic at low closure member travel and around a linear characteristic for upper portions of closure member travel.

Needle valve – It is a type of small valve, used for flow metering, having a tapered needle-point plug or closure element and a seat having a small orifice.

Non-return valve – A non-return valve is a single-way valve which allows the fluid to flow only in one direction. The main importance of non-return valves is their working of allowing flow in the downstream direction and preventing the flow in the upstream direction.

Non-rising stem – It is gate valve having its stem threaded into the gate. As the stem turns, the gate moves but the stem does not rise. Stem threads are exposed to line fluids

Normally-closed valve – It is a condition wherein the valve closure member moves to a closed position when the actuating energy source fails.

Normally closed solenoid valve – It is an electrically operated valve whose inlet orifice is closed when the solenoid coil is not energized. It energizes to open.

Normally-open valve – It is a condition wherein the valve closure member moves to an open position when the actuating energy source fails.

Normally open solenoid valve – It is an electrically operated valve whose inlet orifice is open when the solenoid coil is not energized. It energizes to close.

Offset valve – It is a valve construction having inlet and outlet line connections on different planes, but 180-degree opposite each other.

Outside screw and yoke – It is a valve in which the fluid does not come in contact with the stem threads. The stem sealing element is between the valve body and the stem threads.

Packing – It is the deformable sealing material inserted into a valve stem stuffing box, which, when compressed by a gland, provides a tight seal about the stem.

Packing box (assembly) – It is the part of the bonnet assembly used to seal against leakage around the closure member stem. Included in the complete packing box assembly are various combinations of some or all of the component parts such as packing, packing follower, packing nut, lantern ring, packing spring, packing flange, packing flange studs or bolts, packing flange nuts, packing ring, packing wiper ring, felt wiper ring, Belleville springs, and anti-extrusion ring.

Parallel slide valve or parallel gate valve – Parallel slide valve or parallel gate valve is a slide valve with a parallel-faced gate-like closure member. This closure member can consist of a single disk or twin disks with a spreading mechanism in between. Parallel slide gate valve is ideal for high temperature and pressure line to prevent thermal binding.

Pinch valve – It is a full bore or fully ported type of control valve which uses a pinching effect to obstruct fluid flow.

Piston – It is a rigid movable pressure responsive element which transmits force to the piston actuator stem.

Piston-type actuator – It is a fluid powered device in which the fluid, normally compressed air, acts upon a movable piston to provide motion of the actuator stem and provide seating force upon closure. Piston-type actuators are classified as either double-acting, so that full power can be developed in either direction, or as spring-fail so that upon loss of supply power, the actuator moves the valve in the needed direction of travel.

Plug – It is the rotating closure element of a plug valve. Also, a threaded fitting used to close off and seal an opening into a pressure-containing chamber, e.g., pipe plug.

Plug, eccentric – It is a style of rotary control valve with an eccentrically-rotating plug which cams into and out of the seat, which reduces friction and wear. This style of valve is well suited for erosive applications.

Plug valve – It is a quarter-turn valve whose closure element is normally a tapered plug having a rectangular port.

Pressure control valve – The pressure control valve enables the regulation of system pressure to adjust the force on a hydraulic piston rod or the torque on a hydraulic motor shaft. Pressure-control valves are found in practically every pneumatic and hydraulic system. They help in a variety of functions, from keeping system pressures below a desired limit to maintaining a set pressure level in part of a circuit. Different types of pressure control valves include relief, reducing, sequence, counterbalance, safety, and unloading. All of them are typically closed valves, except for reducing valves, which are normally open.

Port – It is the flow control orifice of a control valve.

Rangeability – It is the ratio of the largest flow coefficient to the smallest flow coefficient within which the deviation from the specified flow characteristic does not exceed the stated limits. A control valve which still does a good job of controlling when flow increases to 100 times the minimum controllable flow has a rangeability of 100 to 1. Rangeability can also be expressed as the ratio of the maximum to minimum controllable flow rates.

Ratchet drive – It is a shaft or valve which is operated by means of a ratchet mechanism. The ratchet delivers an intermittent stepped rotation through a gear in one direction only.

Rated flow coefficient – It is the flow coefficient of the valve at rated travel.

Rated travel – it is the distance of movement of the closure member from the closed position to the rated full-open position. The rated full-open position is the maximum opening recommended by the manufacturers.

Reducing valve – It is a regulating device which serves to reduce the fluid pressure supply to the needed delivery pressure.

Regular port valve – It is a term normally applied to plug valves. The regular port of such a valve is customarily around 40 % of the line pipe area. It corresponds to a venturi or reduced-bore valve of the same nominal pipe size. Venturi ball valves frequently are a logical alternative to plug valves with advantages in price torque and low maintenance.

Regulating valve – It is a throttling valve which exercises automatic control over some variable (normally pressure). It is not an on-off valve.

Relative flow coefficient – It is the ratio of the flow coefficient at a stated travel to the flow coefficient at rated travel.

Relief valve – It is a quick-acting, spring-loaded valve which opens (relieves) when the pressure exceeds the spring setting. It is frequently installed on the body cavity of ball and gate valves to relieve thermal overpressure in liquid services.

Remote control – It is the operation of a valve or other flow-control device from a point at a distance from the device being controlled. It can be accomplished by electrical, pneumatic, or hydraulic means. Resilient seat – It is a valve seat containing a soft seal, such as an O-ring, to ensure tight shutoff.

Retaining ring – It is a split ring which is used to retain a separable flange on a valve body.

Reverse-acting actuator – It is an actuator in which the actuator stem retracts with increasing loading pressure. Reverse actuators have a seal bushing installed in the upper end of the yoke to prevent leakage of the loading pressure along the actuator stem.

Reverse flow – It is the flow from the shaft / hub side over the back of the disk, ball, or plug. Some rotary control valves are capable of handling flow equally well in either direction. Other rotary designs can need modification of actuator linkage to handle reverse flow.

Rising stem – It is a valve stem which rises as the valve is opened.

Rising stem ball valve – It is a single-seated ball valve which is designed to seal by using the valve’s stem to mechanically wedge the valve’s ball into a stationary seat, affecting a bubble-tight seal. The valve’s stem operates through a guide sleeve assembly which guides the stem through a quarter turn of rotation as the stem is raised or lowered by a handwheel (or actuator). The mechanical action of the stem moves the ball away from the seat prior to the 90-degree rotation of the ball. The design provides lower operating torques and longer seat life while assuring bubble-tight shutoff

Rod end bearing – It is the connection frequently used between actuator stem and actuator lever to facilitate conversion of linear actuator thrust to rotary force (torque) with minimum of lost motion. Use of a standard reciprocating actuator on a rotary valve body normally needs linkage with two rod end bearings. However, selection of an actuator specifically designed for rotary valve service needs only one such bearing and hence reduces lost motion.

Rotary control valve – It is a valve style in which the flow closure member (full ball, partial ball, disk, or plug) is rotated in the flow stream to control the capacity of the valve.

Rubber boot – It is a protective device to prevent entrance of damaging foreign material into the piston actuator seal bushing.

Safety valve – It is a quick-opening, pop-action valve used for fast relief of excessive pressure.

Sampling valve – A sampling valve is a type of valve which allows taking a representative portion of a fluid for testing. It is a valve used for sampling. The sampling valve allows the operator to extract a sample of the product from the production line or reactor and safely store it for transportation to the laboratory where it is analyzed or to the archive room where it can be retrieved for further use.

Seal bushing – These are top and bottom bushings which provide a means of sealing the piston actuator cylinder against leakage. Synthetic rubber O-rings are used in the bushings to seal the cylinder, the actuator stem, and the actuator stem extension.

Seal, dynamic – It is a sealing element used between parts which have relative motion, e.g., stem seals, seat seal O-rings, etc.

Seal, static – It is a sealing element used as a gasket between two non-moving parts, e.g., valve bonnet O-rings, ball valve body O-rings, and flange gaskets, etc.

Seal ring – It is the portion of a rotary control valve assembly corresponding to the seat ring of a globe valve. Positioning of the disk or ball relative to the seal ring determines the flow area and capacity of the unit at that particular increment of rotational travel.

Seal weld – It is a weld which does not contribute anything to the mechanical integrity of an assembly, but is made purely to seal or prevent leakage from, for example, a threaded joint.

Seat – It is the part of a valve against which the closure element (gate, or ball) affects a tight shutoff. It is the area of contact between the closure member and its mating surface which establishes valve shutoff. In several ball valves and gate valves, it is a floating member normally containing a soft seating element.

Seat leakage – It is the quantity of fluid passing through a valve when the valve is in the fully closed position and maximum available seat load is applied with pressure differential and temperature as specified.

Seat load – It is the net contact force between the closure member and seat with stated static conditions. In practice, the selection of an actuator for a given control valve is based on how much force is needed to overcome static, stem, and dynamic unbalance with an allowance made for adequate seat load.

Seat ring – It is a part of the valve body assembly which provides a seating surface for the closure member and can provide part of the flow control orifice.

Separable flange – It is a flange which fits over a valve body flow connection. It is normally held in place by means of a retaining ring.

Shaft – It is the portion of a rotary control valve assembly corresponding to the valve stem of a globe valve. Rotation of the shaft positions the disk or ball in the flow stream and controls flow through the valve.

Short gate – It is a gate valve wherein the seat rings contact the gate only in the closed position. Such valves are not through-conduit, as the gate is completely withdrawn from the flow area in the open position.

Shut-off valve – It is a valve designed only for on / off service. It is not a throttling valve. It is sometimes referred to as a ‘block valve’.

Shuttle valve – A shuttle valve, also known as a double-check valve, allows pressure in a line to be obtained from alternative sources. It is primarily a pneumatic device and is rarely found in hydraulic circuits. Construction is very simple and consists of a ball inside a cylinder.

Sliding seal – It is the lower cylinder seal in a pneumatic piston-style actuator designed for rotary valve service. This seal permits the actuator stem to move both vertically and laterally without leakage of lower cylinder loading pressure, allowing for a single rod end bearing.

Sluice valve – It is a gate valve or a sluice gate valve and is described as a valve which uses a gate or wedge shape disk which slides perpendicular to the flow of the fluid into or out of the pipeline.

Solenoid valve – It is a small electrically operated valve used in the control piping and powered by hydraulic or pneumatic cylinder operations.

Spring adjuster – It is a fitting, normally threaded on the actuator stem or into the yoke, to adjust the spring compression.

Spring rate – It is the force change per unit change in length of a spring. In diaphragm actuators, the spring rate is normally stated in newtons per millimeter of compression.

Spring seat – It is a plate to hold the spring in position and to provide a flat surface for the spring adjuster to contact.

Standard flow – For those rotary control valves having a separate seal ring or flow ring, it is the flow direction in which fluid enters the valve body through the pipeline adjacent to the seal ring and exits from the side opposite the seal ring. It is sometimes called forward flow or flow into the face of the closure member.

Static unbalance – It is the net force produced on the valve stem by the process fluid pressure acting on the closure member and stem with the fluid at rest and with stated pressure conditions.

Stem connector – It is the device which connects the actuator stem to the valve stem.

Stopcock – A stopcock is a form of valve used to control the flow of a liquid or gas. The term is not precise and is applied to many different types of valves.

Swing check valve – It is a check valve in which the closure element is a hinged clapper which swings or rotates about a supporting shaft.

Throttle valve – It is a valve designed to regulate the supply of a fluid. It is operated by a handwheel, a lever, or automatically by a governor.

Trim – It is the internal components of a valve which modulate the flow of the controlled fluid. In a globe valve body, trim typically includes closure member, seat ring, cage, stem, and stem pin.

Trim, soft-seated – It is the valve trim with an elastomeric, plastic, or other readily deformable material used either in the closure component or seat ring to provide tight shutoff with minimal actuator forces.

Trunnion – It is the part of a ball valve which holds the ball on a fixed vertical axis and about which the ball turns. The torque requirements of a trunnion mounted ball valve are considerably less than for a floating ball design.

Trunnion mounting – It is a style of mounting the disk or ball on the valve shaft or stub shaft with two diametrically opposed bearings.

Valve – It is a device which controls the flow of a liquid or gas in a conduit or pipeline.

Valve body – It is the main pressure boundary of the valve which also provides the pipe connecting ends, the fluid flow passageway, and supports the seating surfaces and the valve closure member. Among the most common valve body constructions are (i) single-ported valve bodies having one port and one valve plug, (ii) double-ported valve bodies having two ports and one valve plug, (iii) two-way valve bodies having two flow connections, one inlet and one outlet, and (iv) three-way valve bodies having three flow connections, two of which can be inlets with one outlet (for converging or mixing flows), or one inlet and two outlets (for diverging or diverting flows). The term ‘valve body’, or even just ‘body’, is frequently used in referring to the valve body together with its bonnet assembly and included trim parts. More properly, this group of components is required to be called the valve body assembly.

Valve body assembly (normally valve body or valve, more properly valve body assembly) – It is an assembly of a valve body, bonnet assembly, bottom flange (if used), and trim elements. The trim includes the closure member, which opens, closes, or partially obstructs one or more ports.

Valve plug (plug) – It is a term frequently used to reference the valve closure member in a sliding-stem valve.

Valve stem – In a linear motion valve, it is the part which connects the actuator stem with the closure member.

Venturi valve – It is a reduced-bore valve. A valve having a bore smaller in diameter than the inlet or outlet. For example, a 200 mm x 150 mm x 200 mm ball valve has 200 mm inlet and outlet connections, while the ball and seats are 150 mm. The flow through a venturi valve is reduced because of the smaller port. Venturi valves frequently can be economically substituted for plug valves.

Vena contracta – It is the portion of a flow stream where fluid velocity is at its maximum and fluid static pressure and the cross-sectional area are at their minimum. In a control valve, the vena contracta normally occurs just downstream of the actual physical restriction.

Weld end – It is the end connection of a valve which is to be installed by welding into the line.

Yoke – It is the structure which rigidly connects the actuator power unit to the valve.

Yoke nut – A yoke nut is an internally threaded nut and is placed in the top of a yoke by which the stem passes.


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