Valve Terminology
A
Acme Threads:
A common thread pattern used to thread plug stems to actuator stems.
Actuation System:
A separate actuator assembly that can be installed on a manual valve and used for either on-off or throttling applications.
Actuator:
A power-driven device that provides the force to open, close, or throttle a valve.
Actuator Barrier:
A dynamic actuator part that separates the actuator chamber from the atmosphere or another actuator chamber. When the actuator barrier is acted upon by the medium in the chamber(s), the actuator moves.
Actuator Stem:
A rod used in linear valves to connect the actuator with the stem of the valve.
Air Set:
A device used to limit the air supply to an actuator, also known as a pressure regulator.
Air Spring:
A fail-safe system for actuators that employs a locked-up volume of air to drive the actuator to the failure position.
Angle Valve:
A valve-body style where the first port is perpendicular to the second port.
Attenuation:
The capability of lowering the sound pressure level radiating from a valve.
Attenuation Plate:
An anti-noise device installed downstream from a valve that uses a tortuous path, multiple holes, expanding flow area, or sudden expansions and contractions to lower the sound pressure level generated by a valve.
Automatic Control Valve:
A final control element that receives a signal from a process controller to regulate the pressure, flow, or temperature of a liquid or gas process. Also known as a control valve.
B
Belleville Disk Spring:
A cone-shaped metal washer used to produce a constant load on the packing box without retightening.
Block Valves:
Simple manual valves used to start or stop the process flow. Common block valves include gate, quarter-turn plug, ball, pressure-relief, and tank-bottom valves. Also referred to as on-off valves.
Brinell Hardness Number:
A number between 111 and 745 that indicates the relative hardness of a material. As the number increases, the material is considered harder.
Bubble-Tight:
A condition where no measurable seat leakage occurs through the closure element of a valve during a certain amount of time. The typical bubble-tight test involves air under water testing.
Butterfly Valve:
A quarter-turn rotary valve design that has a narrow body face and a circular disk closure element.
Butt Weld End Connection:
A special end connection used to prepare a valve for welding into the line. A butt weld connection has a series of angles that match up with the similar angles on the pipe. When matched up, the ends present a V-shaped gap that is filled with a weld.
Bypass Valve:
A manual valve used in a bypass line that circumvents a larger valve (usually a control valve). A bypass line usually involves one or more manual valves, which reroute the flow to the downstream side.
c
Carbon Steel:
An iron-based metal that contains 0.1 to 0.3 percent carbon. Carbon steel is a base steel from which alloys are created by mixing other metals with it.
Cavitation:
A situation in liquid services when the pressure at the vena contracta falls below the vapor pressure, followed by a pressure recovery above the vapor pressure. The pressure reduction below the vapor pressure causes vapor bubbles to form, which then collapse as the pressure recovers. This implosion of vapor bubbles can erode metal surfaces in both the valve body and downstream piping, as well as cause noise.
Charpy Impact Test:
A quality test that measures a material’s ability to resist fracture on a V-shaped notch cut into the material when an impact load is applied to the side opposite the notch. Charpy tests are normally conducted on materials that are subject to extremely cold temperatures.
Check Valve:
A valve that prevents the process flow from reversing. The closure element of a check valve normally uses gravity, fluid, and/or spring force to close the valve and does not require any type of manual operation or actuation.
Chevron Packing:
A packing style characterized by V-shaped (in cross-section) seamless rings. When axial force is applied to the ring, the radial forces cause the thin edges of the ring to press tightly against the wall of the packing box, forming a strong seal with minimal friction in moderate temperatures. Also referred to as V-ring packing.
Class:
A term used in conjunction with a pressure class, e.g., ANSI Class 2500.
Cold Flow:
The ability of a soft material to undergo plastic deformation under sustained pressure, regardless of temperature.
Concentric Butterfly Valve:
A butterfly valve design with the disk installed in the center of the body.
Control Valve:
A throttling valve equipped with an actuator or actuation system to respond to an input signal from a controller. Control valves are used to regulate the flow, temperature, or pressure of a process system. Also known as automatic control valves.
Cracking Pressure:
In check valves, the positive line pressure that allows the closure element to begin opening and permit flow through the valve.
Critical Temperature:
The temperature at which a metal’s crystal structure becomes austenite.
Cryogenic Valve:
A special valve used in services with temperatures below 50°F (-45°C). A cryogenic valve has a protective cold box as part of the body to allow a vapor barrier to form between the liquefied gas and the packing box.
Cv (Valve Coefficient):
The term used to measure flow through a valve. Cv is used to calculate the ideal valve size to pass the required flow rate while providing overall stability to the process. Cv is defined as 1 US gallon (3.8 liters) of 60°F (16°C) water flowing through a valve in 1 minute with a 1 psi (6.9 kPa) pressure drop. Also referred to as valve coefficient or flow coefficient.
D
Design Pressure:
The pressure used to determine overall design criteria for a valve, including flange rating, bolting torques, thread wall thickness, packing box configurations, bellows seal design, etc. The design pressure includes certain allowances and safety factors to compensate for pressure surges, water hammer effects, or other unexpected phenomena in the process line. For this reason, the design pressure is always greater than the actual operating pressure.
Design Temperature:
The temperature used to determine overall design criteria for a valve, including flange rating, bolting, wall thickness, packing-box configurations, bellows seal design, etc. The design temperature includes certain allowances and safety factors to compensate for unexpected rises at or beyond the operating temperature. For this reason, the design temperature is always greater than the actual operating temperature.
Diaphragm Valve:
A manual valve related to a pinch valve that compresses an elastomeric diaphragm against the bottom of a metal body to shut off the flow.
Digital Positioner:
A positioner that uses a microprocessor to position an actuator while also monitoring and recording data.
Direct-Acting Pressure Relief Valve:
A pressure relief valve in which line pressure acts on one side of the closure element, while a predetermined spring applies a mechanical load on the other side. When the line pressure reaches its maximum limit, it overcomes the spring load, and the valve opens until the line pressure falls below the preset level.
DN:
The ISO standard abbreviation for the nominal diameter of a size, e.g., DN 100.
Double-Acting Actuator:
An actuator that can supply and exhaust air to both sides of a piston or diaphragm at the same time. Double-acting designs require the use of a positioner.
Downstream:
The portion of a process system that is located after (following) a valve.
Ductile Iron:
A special cast iron with approximately 18 percent ductility.
Dye Penetrant Test:
A quality test that uses a bright red or fluorescent dye to detect surface cracks, pits, or porosity on a nonporous surface. The test involves spraying the dye onto the part, wiping away the excess, and then examining the surface under natural or fluorescent light to highlight flaws where dye remains.
Dynamic Seal:
A seal that involves two or more elements that physically move as they act upon each other. Such seals include those associated with closure elements (e.g., between a plug and a seal, butterfly disk and a seal, ball and a seal), packing boxes (where a stem or shaft moves against the packing), or actuators (where a piston may move between two pressure chambers).
E
Eccentric Butterfly Valve:
A butterfly valve design in which the valve shaft is slightly offset from the center of the disk, allowing the disk to move in an elliptical motion as it leaves the sealing surface. This unique motion permits minimal friction and wear to the closure element.
End-to-End:
The dimension from one end connection to the opposite end connection on a valve. End-to-end is similar to face-to-face, except it is used with valves that do not have flat-faced surfaces on the end connections, such as butt-weld ends.
Equal-Percentage Characteristic:
A flow characteristic that permits a change in flow per unit of valve stroke, which is directly proportional to the flow occurring just before the change is made.
Erosion:
Material wear inside a valve or pipeline caused by the flowing action of the process fluid. Erosion deterioration is often accelerated when entrained solids are present in the flow.
Extension Bonnet:
A bonnet that is longer than normal, used to protect the packing box and actuator from the effects of severe temperatures. Also referred to as extended bonnet.
F
Fail-Safe:
An actuator system that allows a valve to move to a certain position (open or shut) or to retain the current position if the actuator power supply fails.
FEP:
Abbreviation for fluorinated ethylene propylene copolymer.
Fire-Safe:
The ability of a valve to maintain certain standards after being subjected to a fire test. A fire-safe valve is expected to minimize the amount of process fluid lost downstream or to the atmosphere during a fire. Also referred to as fire-tested.
Flangeless Body:
A rotary valve body with a short face-to-face dimension that can be sandwiched between two flanged piping end connections using long studs and nuts. Also referred to as a wafer-style body.
Flashing:
A common valve problem in which the pressure at the vena contracta falls below the vapor pressure, followed by a pressure recovery that remains below the vapor pressure. This causes vapor bubbles to form and continue downstream, accelerating velocity, leading to excessive noise and eventual erosion.
Floating Ball:
A ball or check valve design in which the ball is not fixed to the valve body.
Floating Seat:
A seat ring that is not fixed to the valve body. A floating seat can shift position to conform better to the shape of the closure element, providing better shutoff.
Flow Capacity:
The amount of flow that can pass through a valve under certain flow conditions without the valve choking. Also referred to as capacity or valve capacity.
Flow Characteristic:
The relationship between a valve’s flow coefficient (Cv) and the valve stroke (0–100%). Flow characteristics are usually classified as either inherent flow characteristic or installed flow characteristic. The three most common types are:
Equal Percentage
Linear
Quick Open
Fluoroplastic:
A polymer with a molecular structure similar to hydrocarbons, except that fluorine atoms replace some carbon atoms. Also referred to as fluoroplastic, polytetrafluoroethylene (PTFE), or Teflon.
Four-Way Solenoid:
A solenoid used to operate an on-off actuator, providing two-way directional control.
Full Port Valve:
A valve in which the opening of the closure element (such as a seat or a ball) has the same area as the inside diameter of the inlet and outlet ports. Also referred to as a full bore valve.
G
Galling:
Damage to two mating parts when microscopic high points interact and bond together. When each part moves against the other, this results in tearing of the surfaces. Galling typically occurs when two parts made of the same material are used together without lubrication. In the case of threaded nuts, bolts, or studs, galling can destroy both male and female threads. It is also common between guides and stems or shafts, which not only damages smooth surfaces but can also destroy packing or other soft parts.
Gland Bushing:
A part located at the top of a packing box that protects the packing from atmospheric elements and transfers force from the gland flange bolting to the packing.
H
Hardened Trim:
Valve trim that is overlaid with a special material designed to withstand the effects of cavitation, corrosion, or erosion.
High Recovery:
The inherent ability of a particular valve design to pass flow without taking a large pressure drop, allowing the downstream pressure to recover close to the upstream pressure.
Hydrostatic Test:
A valve test using water pressure to detect the presence of leaks through sealed joints, vessel walls, or the closure element. As a safety measure, a hydrostatic test typically generates 1.5 times the pressure specified by the design.
I
Impact Test:
A test used to determine the toughness of a material by measuring the energy required to fracture a test specimen.
L
Lift Check Valve:
A check valve that uses a free-floating closure element, consisting of a piston or poppet and a seat ring.
Linear Flow Characteristic:
An inherent flow characteristic that produces equal changes in flow per unit of valve stroke at a constant pressure drop.
Linear Motion Valve:
A valve with a sliding stem design that pushes the closure or regulating element into an open, closed, or throttling position. Also known as a linear valve.
Low Recovery:
The inherent ability of a particular valve design to take a large pressure drop, resulting in the downstream pressure only slightly recovering relative to the upstream pressure. Valves with low recovery factors often have highly contoured internal passageways and closure elements that remain in the flow stream when open. Examples include globe valves and butterfly valves.
Lug Body:
In butterfly valves, a body with an integral flange that has a threaded hole pattern matching the piping flanges.
M
Magnetic Particle Examination:
A method used to detect small cracks or porosity in the surface of a material that cannot be seen by visual inspection. The process involves spreading iron filings over a metal surface and passing an electric current through the metal. Flaws are revealed as the iron filings cluster around imperfections.
Maximum Allowable Operating Pressure (MAOP):
The maximum pressure a vessel, such as a valve, can safely withstand on a continuous basis. It is expressed in psi, bar, or kilopascal, and is determined by the vessel’s material, maximum temperature, and pressure class. Also referred to as Maximum Allowable Working Pressure (MAWP).
N
Nonreturn Valves:
Valves that allow the flow of process fluid in only one direction. They are designed so that any flow or pressure in the opposite direction is mechanically restricted. All check valves are nonreturn valves.
O
Operating Pressure:
The pressure at which a valve normally operates during everyday service. The operating pressure should not be confused with the design pressure, which is defined as the operating pressure plus outside factors.
Operating Temperature:
The temperature at which a valve normally operates during everyday service. The operating temperature should not be confused with the design temperature, which is defined as the operating temperature plus outside factors.
O-Ring:
An elastomer ring used as a sealing element in valves to prevent leakage between two parts.
P
Packing box:
The configuration of packing spacer (s), lantern ring, packing spring extrusion, wiper rings, guides, gland flange, live-loading, gland-flange followers etc. grouped together in a bonnet. The packing box is designed to prevent process fluid from escaping through the packing box and is used to apply axial force to the packing.
PEEK:
Common abbreviation for polyetheretheketone.
Perfluoroalkoxy:
A fluoropolymer similar to polytetrafluoroethylene (PTFE), with some different properties that makes it ideal for valve lining, seal or soft seats. Also abbreviated as PFA.
PFA:
Common abbreviation for Perfluoroalkoxy.
Pig:
A device the same shape as the inside diameter of a pipe, which is pushed down the pipe line by fluid pressure. A pig has two purposes: First, to act as a barrier between an earlier process fluid and a new fluid, and second to clean the inside walla of the pipe.
Pinch valve:
A flexible valve with an elastomeric body that can be closed by using a mechanism of fluid pressure to push the walls of the body together.
Piping schedule:
In pressure pipe, a number that describes the thickness of the pipe wall, such as
schedule 40 pipe. The larger the number, the larger the pipe wall and the more pressure the pipe
can handle.
Pneumatic actuator:
An actuation system that is powered by air pressure. Poisson effect: The assistance of process pressure to cause deformations of a soft material against a seating surface for shutoff of a closure element.
Polytheretheketone:
A rigid elstomer that is used in high temperature services. Also abbreviated as PEEK.
Polyterafluoroethylene:
A common and inexpensive gasket and packing material. Polytetrafluoroethylene is widely used in general services because it is highly elastic, generates
little friction, and requires minimal compression to achieve a strong seal . Abbreviated as PTFE.
Positioned:
A feedback device that receives a signal (pneumatic or electric) from a controller and compares that signal to the actual position of the valve .If the valve positions and the signal are not the same, the positioned sends or bleeds air pressure to or from the actuator until the correct valve positions is achieved.
Pressure relief valve:
A self-actuated valve designed to move to the wide open position when the upstream pressure reaches a preset pressure, relieving pressure from the line. When the pressure is under the preset value, a spring or puppet keeps the closure element to open. Also referred to as a relief valve for liquid applications and a safety valve for gas applications.
Psia:
Standard abbreviation of pounds per square inch, gage (gauge). The unit psia is used when pressure is expressed to standard atmospheric pressure (noted as 14.7 psia).
Q
Quak exhaust valve:
An actuator accessory that is designed to quacked vent one side of a doubleacting actuator when a full-open or full-closed position is required immediately.
Quick-open flow characteristic:
An inherent flow characteristic that produces a maximum amount of flow with minimal lift or turn of the closure element.
R
Rack-and-pinion actuator:
An actuation system used with quarter turn valves. The actuator uses a pneumatic or hydraulic force to move a flat toothed rack, turning a gear to move the closure element.
Raised-face flange:
A valve and piping end connection that includes a flange (either integral or separable). The flat portion of the valve hub extends farther than the flange. This flat portion, which is called the raised face, permits a greater load on the gasket between the valve and pipe than comparable flat-faced end connections. Raised faces are usually spiral serrated or concentric grooved for better gasket sealing. Ram valve: A valve placed at the bottom of a tank that allow for drainage. Usually the actuator or hand lever faces down with the closure element facing up into the tank. Also known as a tank bottom valve.
Relief valve:
In liquid services, a self-actuated valve designed to move to the wide open position when the upstream pressure reaches a preset pressure, thus relieving pressure from the line. When the pressure is under the preset valve, a spring or puppet keeps the closure element closed. However, when the pressure builds, the spring or puppet tension is overcome, allowing the closure element to open.
Resilient seat:
A seat in a closure element that is made wholly or partially with a flexible or semi flexible electrometer.
s
Shut off valve:
A valve that is used to block (or shutoff) the flow. Shutoff valves are usually used for emergency shutdown situations.
Sliding valve:
A manual valve that uses a flat perpendicular closure element that intersects the flow, such as a gate valve design.
Solenoid:
An actuator accessory that acts as a control device to supply air to an actuator in on-off application or to control signal pressure in throttling applications. Static seal: Any sealing design that occurs between two parts that do not move after being secured into place. Because friction is limited, static seals have a longer life and are generally more reliable than dynamic seals. Common static seals include joints between the body and bonnet, body and
the seat ring, yoke and the actuator casing etc.
Swing check valve:
A check valve designed with the closure element attached to the top of the
cap. The closure element can be pushed aside by the flow, but swings back into the closed
position if the flow reverses.
T
Threaded end connection:
A valve’s end connection that is designed with female NTP threads, which mates eight the piping end connection design with male NTP threads.
Three-way solenoid:
An actuator accessory used to interrupt a signal to a pneumatic positional or to operate a spring diaphragm valve.
Throttling:
Regulating the position of a valve between the full-open and full-closed positions so that pressure or fluid are adjusted to meet the requirements of the process system.
Thrust:
The force generated by an actuator, actuation system or a manually operated actuation device.
Trim:
A term for wetted parts and related parts of the closure element in a linear globe valve. The trim usually includes the following parts: the cage (or seat retainer or severe service cartridge or
stack), plug, and seat ring.
Trunnion-mounted ball:
A ball that is supported by both a shaft and a post opposite the shaft.
V
Vena contract:
The narrowest constriction in the fluid stream as process flow moves through a valve. The vena contract can be caused by a small orifice or a valve seat or seal. At that point, the flow velocity is at its highest rate, while pressure is at its lowest point.
w
Wafer body:
In rotary valves, a body that has a short face-to-face that can be sandwiched between two flanged piping end connections, using special length studs and nuts. Also refer to as a flangeless body.
Water-hammer effect:
In liquid services, the reaction caused when a valve is suddenly closed )or a pump is turned off), which causes a shock wave to be transmitted by the liquid throughout the piping system. Although water hammer causes noise, the primary disadvantage of allowing it to happen is the damage to piping and equipment installed in the piping system.
Weir:
In diaphragm and enclosed body pinch valves, an integral bar cast into the bottom of the metal body that is used to help compress the electrometric diaphragm or liner.