Valve Terminology


Absolute Pressure: The pressure expressed in bar, Pascal or PSI, as measured above a perfect vacuum.

Actuator: A device connected to the valve that allows remote movement of the valve. It can be an air, fluid or electrically powered device. Actuators may be used when valves are remotely located (e.g., on pipelines), valves are located in hazardous areas, or when manual operation would be time-consuming (e.g., with larger valves). (Learn More)

Aerodynamic Noise: Noise produced by a gas accelerating to supersonic velocity at critical or higher pressure drops through the valve trim.

Air-To-Close: An increase in air pressure to the actuator is required to cause the valve to close. This is another way of saying the valve is fail open or normally open.

Air-To-Open: An increase in air pressure to the actuator is required to cause the valve to open. This is another way of saying the valve is fail closed or normally closed.

Air Valve: Valve that is used to control air flow. This flow is generally small. (Learn More)

Angle Valve: A globe style valve where the inlet and outlet ports are at 90°.

ANSI Class: A strength designation for valves which show the maximum pressures at various temperatures at which a valve can be expected to work safely. (Learn More)

ASTM: American Society for Testing and Materials

Atmospheric Pressure: The pressure exerted by the atmosphere.


Back Pressure: The pressure exerted on the downstream side of a valve.

Ball Valve: A valve design which uses a spherical ball as the closing element. Closure is achieved by turning the ball.(Learn More)

Image Credit: Spirax

Bellows Seal Bonnet: A bonnet which uses a bellows for sealing against leakage around the valve plug stem.

Body: The body of the valve is the main pressure boundary. It provides the pipe connecting ends and the fluid flow passageway.

Bonnet: The bonnet may be used to guide the stem. It is part of the valve pressure retaining boundary and contains the packing box and stem seal. The bonnet may be integral to the valve body or bolted or screwed. The bonnet is generally the means by which the actuator is connected to the valve body.

Bubble Tight: A commonly used term to describe the ability of a control valve or regulator to shut off completely against any pressure on any fluid. Control valves are tested to ANSI B16.104 and FCI 70:2:1976 which is the American National Standard for Control Valve Seat Leakage. This standard uses six different classifications to describe a valve’s seat leakage capabilities. Although valves rarely achieve perfect shut off, valves that meet Class VI requirements (allows a number of bubbles per minute leakage, depending on the port size of the valve) have passed the most rigorous testing.

Butt Weld Ends: Profiles that are machined on the ends of the pipework components, to allow the joining of components by circumferential weld.

Butterfly Valve: A quarter-turn valve design which includes a circular body. It has a rotary motion disk closure member, which is pivotally supported by its stem, allowing the disk to rotate 90° to open and close the valve. Butterfly valves come in various styles, including eccentric and high-performance (zero-leakage) valves. Butterfly valves are high recovery valves and thus tend to induce cavitation in liquid services at much lower pressure drops and fluid temperatures than the globe style valve. (Learn More)


Cage: A hollow cylindrical trim element that is sometimes used as a guide to align the movement of a valve plug with a seat ring. It can be modified for some types of valve, to characterize the flow through the valve. The cage may also act as a noise attenuation or anti-cavitation device.

Capacity: The mass flow rate through a valve under specified conditions.

Cavitation: Cavitation is a concern for liquid services where cavities or bubbles form and then collapse. It is the two-stage process of vaporization and condensation of a liquid. Vaporization is the boiling of liquid (also known as flashing), and occurs in control valves because the pressure of the liquid is lowered, instead of the raised. As fluid passes through a valve just downstream of the orifice area, there is an increase in velocity or kinetic energy that is accompanied by a substantial decrease in pressure or potential energy. This occurs in an area called the vena contracta. If the pressure in this area falls below that of the vapor pressure of the flowing fluid, vaporization (boiling) occurs. Vapor bubbles then continue downstream where the velocity of the fluid begins to slow and the pressure in the fluid recovers. The vapor bubbles then collapse or implode. Cavitation can cause a choked flow condition to occur and can cause mechanical damage to valves and piping.

Choked Flow: Also known as critical flow, can occur in gas, steam, or liquid services. Choked flow happens when, at a fixed upstream pressure, the flow cannot be further increased by lowering the downstream pressure. Basic fluid flow equations show that flow is proportional to the square root of the pressure drop. This means that higher pressure drops allow more fluid to go through the valve. Fluids flow through a valve because of a difference in pressure between the inlet (Pl) and outlet (P2) of the valve. This pressure difference (Delta-P) or pressure drop is essential to moving the fluid. However, if the pressure drop becomes too high, the flow reaches a point where it no longer increases, this is considered choked flow.

Liquid flow in a control valve as a function of the pressure drop across the valve.
Image Credit: Valvin.

If the pressure drop is sufficiently high, the velocity in the flow stream at the vena contracta will reach the velocity of sound. Further decrease in the outlet pressure will not be felt upstream because the pressure wave can only travel at sonic velocity and the signal will never translate upstream. Choked Flow can also occur in liquids but only if the fluid is in a flashing or cavitating condition. The vapor bubbles block or choke the flow and prevent the valve from passing more flow by lowering the outlet pressure to increase the pressure drop. A good rule of thumb for gas and steam services is that if the pressure drop across the valve equals or exceeds one half the absolute inlet pressure, then there is a good chance of a choked flow condition.

Closure Member: The movable part of the valve which is positioned in the flow path to modify the rate of flow through the valve. Some of the different types of closure members are the Ball, Disk, Gate, and Plug.

Cold Rating: The maximum pressure that a valve or fitting is designed to withstand at room temperature.

Compressible Fluid: A gaseous fluid such as steam, which has a significant change in volume and density as pressure increases. (Learn More)

Control Valve: Also known as the final control element. A power-operated device used to modify the fluid flow rate in a process control system. It usually consists of a body or valve and an actuator, which responds to a signal from the controlling system and changes the position of a flow controlling element in the valve. (Learn More)

Control Valve Gain: The relationship between valve travel and the flow rate through the valve. It is described by means of a curve on a graph expressed as an installed or inherent characteristic.

Controller: A device that directs and monitors the flow of a valve. Controllers can be either pneumatic or electronic. There are pressure, temperature, pH, level, differential and flow controllers. The job of the controller is to sense one of the above variables and compare it to a set point that has been established. The controller then outputs a signal, either pneumatic or electronic, to the control valve, which then responds to bring the process variable to the desired set point. (Learn More)

Critical Flow: See the definition for choked flow.

Cryogenic Valve: A term used to describe valves designed to operate below –40°C.

CV: The valve flow coefficient is the number of US gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1psi across the valve.


Delta-P: Differential Pressure. The inlet pressure (Pl) minus the outlet pressure (P2).

Diaphragm: A flexible pressure-responsive element that transmits force to the diaphragm plate and actuator stem.

Diaphragm Actuator: Is a fluid (usually pneumatic) pressure-operated, spring-opposed diaphragm assembly which positions the valve stem in response to an input signal.

Diaphragm Valve: A bi-directional valve, operated by applying force to a diaphragm. They are often used in slurries as well as hygienic applications because they do not clog. (Learn More)

Differential Pressure: The maximum difference in pressure measured between the valve inlet and outlet, against which the valve is required to operate.

Image Credit: Spirax Sarco

Direct Acting: This term has several different meanings depending upon the device it is describing. A direct-acting actuator is one in which the actuator stem extends with an increase in diaphragm pressure. A direct-acting valve is one with a push-down-to-close plug and seat orientation. A direct-acting positioner or a direct-acting controller outputs an increase in signal in response to an increase in set point.

Direct Actuator: An actuator in which the stem extends with an increase in diaphragm pressure.

Dynamic Unbalance: The total force produced on the valve plug in any stated open position by the fluid pressure acting upon it. The particular style of valve, i.e. single-ported, double-ported, flow-to-open, flow-to-close, has an effect on the amount of dynamic unbalance.


Effective Area: For a diaphragm actuator, the effective area is that part of the diaphragm area that is effective in producing a stem force. Usually the effective area will change as the valve is stroked – being at a maximum at the start and a minimum at the end of the travel range. Flat sheet diaphragms are most affected by this; while molded diaphragms will improve the actuator performance, and a rolling diaphragm will provide a constant stem force throughout the entire stroke of the valve.

Electric Actuator: Also known as an Electro-Mechanical Actuator uses an electrically operated motor-driven gear train or screw to position the actuator stem. The actuator may respond to either a digital or analogue electrical signal. (Learn More)

Emergency Shut Down Valve (ESD): A valve that uses energy which is stored in the actuator to close rapidly in an emergency.

End Connection: The configuration provided to make a pressure-tight joint to the pipe carrying the fluid being controlled. The most common of these connections are threaded, flanged, or welded.

Equal Percentage: A term used to describe a type of valve flow characteristic where, for equal increments of valve plug travel, the change in flow rate with respect to travel may be expressed as a constant percent of the flow rate at the time of the change. The change in flow rate observed with respect to travel will be relatively small when the valve plug is near its seat and relatively high when the valve plug is nearly wide open.


Face-To-Face: Is the distance between the face of the inlet opening and the face of the outlet opening of a valve or fitting. These dimensions are governed by ANSI/ISA specifications.
The following uniform face-to-face dimensions apply.
Specification Valve Type
ANSI/ISA S75.03 Integral Flanged Globe Style Control Valves
ANSI/ISA S75.04 Flangeless Control Valves
ANSI/ISA S75.20 Separable Flange Globe Style Control Valves

Fail-Closed: Or Normally Closed: Another way of describing an Air-To-Open actuator. Approximately 80% of all spring-return diaphragm operators in the field are of this construction.

Fail-In-Place: A term used to describe the ability of an actuator to stay at the same percent of travel it was in when it lost its air supply. On spring return actuators this is accomplished by means of a lock-up valve. On piston actuators a series of compressed air cylinders must be employed.

Fail-Open or Normally Open: Another way of describing an air-to-close actuator.

Fail-Safe: A term used to describe the desired failure position of a control valve. It could be fail-closed, fail-open, or fail-in-place.

Feedback Signal: The return signal that results from a measurement of the directly controlled variable. An example would be where a control valve is equipped with a positioner. The return signal is usually a mechanical indication of valve plug stem position which is fed back into the positioner.

Flangeless: A valve that does not have integral line flanges, sometimes referred to as a Wafer Style valve. The valve is installed by bolting it between the companion flanges with a set of bolts or studs, called line bolting. Care should be taken that strain-hardened bolts and nuts are used in lieu of all-thread, which can stretch when subjected to temperature cycling.

Flashing: Is the boiling or vaporizing of a liquid. See the definition of Cavitation. When the vapor pressure downstream of a control valve is less than the upstream vapor pressure, part of the liquid changes to a vapor. If the pressure downstream continues to decrease, the downstream pressure will eventually become less than the vapor pressure of the liquid. It remains as a vapor unless the downstream pressure recovers significantly, in which case cavitation occurs. Flashing will normally cause a choked flow condition to occur. In addition, the vapor bubbles can also cause mechanical damage to the valve and piping system in the form of smooth rivers and valves.

Velocity and pressure profile in a control valve with chocked flow and flashing.
Image Credit: Valin.

Flow Characteristic: The relationship between valve capacity and valve travel. It is usually expressed graphically in the form of a curve. Control valves have two types of characteristics: inherent and installed. The inherent characteristic is derived from testing the valve with water as the fluid and a constant pressure drop across the valve. When valves are installed into a system with pumps, pipes and fittings, the pressure dropped across the valve will vary with the travel. When the actual flow in a system is plotted against valve opening, the curve is known as the Installed flow characteristic. Valves can be characterized by shaping the plugs, orifices or cages to produce a particular curve. Valves are characterized to try to alter the valve gain. Valve gain is the flow change divided by the control signal change. This is done in an effort to compensate for nonlinearities in the control loop.

Flow characteristic curve. Image Credit: KLM Technology Group

Flow Coefficient: See the definition for CV.

Full Bore: Indicates that the internal diameter of the valve opening is that same as the pipe it is connected to.


Gain: The relationship of input to output. If the full range of the input is equal to the full range of the output, then the gain is 1. Gain is another way to describe the sensitivity of a device.

Gasket: A component softer than the parts to be sealed, which is compressed between two flanges to prevent the system fluid leaking to atmosphere.

Gate Valves: A multi-turn valve which as a gate-like disk and two seats to close the valve. This valve is used in fully open/ fully closed applications and the gate disk moves linearly, perpendicular to the direction of flow. (Learn More)

Gland: The component that is used to compress the gland packing.

Gland Nut: The gland nuts are used to exert a force on the gland.

Gland Packing: A soft conformable material fitted to a valve stuffing box to create a seal between the process fluid and the atmosphere.

Globe Valve: A valve with a linear motion, push-pull stem, whose one or more ports and body are distinguished by a globular-shaped cavity around the port region. This type of valve is characterized by a torturous flow path and is also referred to as a low recovery valve because some of the energy in the flow stream is dissipated; and the inlet pressure will not recover to the extent that it would in a more streamlined high recovery valve. (Learn More)


Handwheel: A manual override device used to stroke a valve or limit its travel. The handwheel is sometimes referred to as a hand jack. It may be top-mounted, side-mounted, in-yoke mounted or shaft-mounted and declutchable. (Learn More)

Hydrostatic Test: Pressure tests that are carried out on every valve when built to test the integrity of the pressure-containing parts.

Hysteresis: The difference between up-scale and down-scale results in instrument response when subjected to the same input approached from the opposite direction. Hysteresis can be caused by a multitude of variables, such as packing friction, loose linkage and pressure drop.


Incompressible Flow: A fluid such as water, which has no significant change in volume and density as the pressure increases.

Inherent Diaphragm Pressure: The high and low values of pressure applied to the diaphragm to produce rated valve plug travel with atmospheric pressure in the valve body.

Inherent Flow Characteristic: The relationship between valve capacity and valve travel, usually expressed graphically. It is derived from testing a valve with water as the fluid and with a constant pressure drop across the valve. The most common types of inherent flow characteristics are linear, equal percentage, modified parabolic, and quick opening.

Installed Flow Characteristic: The flow characteristic when the pressure drop across the valve varies with flow and related conditions in the system in which the valve is installed. The purpose of characterizing a control valve is to help compensate for nonlinearities in the control loop.

Integral Flange: A valve body whose flange connection is an integral or cast part of the body.

Globe valve with integral flange. Image Credit: Forged-Valve

Integral Seat: The flow control orifice and seat that is an integral part of the valve body or cage. The seat is machined directly out of the valve body and is normally not replaceable without replacing the body itself – although some can be repaired by welding and re-machining.


Leakage Classification: A term used to describe certain standardized testing procedures for control valves with a flow coefficient greater than 0. 1 (Cv). These procedures are outlined in ANSI Standard B16.104:1976, which gives specific tests and tolerances for six seat leakage classifications. It should be remembered that these tests are used to establish uniform acceptance standards for manufacturing quality and are not meant to be used to estimate leakage under actual working conditions. Nor should anyone expect these leakage rates to be maintained after a valve is placed in service.

Linear Flow Characteristic: A characteristic where flow capacity or (Cv) increases linearly with valve travel. Flow is directly proportional to valve travel. This is the preferred valve characteristic for a control valve that is being used with a distributive control system (DCS) or programmable logic controller (PLC).

Linear Valve: Another name for a globe valve. It refers to the linear or straight-line movement of the plug and stem.

Loading Pressure: The pressure used to position a pneumatic actuator. It is the pressure that is actually applied to the actuator diaphragm or piston. It can be the instrument pressure if a valve positioner is not used or is bypassed.


Modified Parabolic: A flow characteristic that lies somewhere between linear and equal percentage. It provides fine throttling at low flow capacity and an approximately linear characteristic at higher flow capacities


NACE: National Association of Corrosion Engineers. (Learn More)

NDE: Non-Destructive Examination. A collective term used to describe non-intrusive examination techniques such as radiographic and ultrasonic examination. (Learn More)

Needle Valve: A multi-turn device with a needle-shaped closing element. Similar in design to globe valves, but often much smaller. (Learn More)

Noise: May arise from vibration, cavitation or aerodynamic flow through the valve.

Normally Closed: See Air-To-Open

Normally Open: See Air-To-Close


P1: Used to designate Inlet Pressure.

P2: Used to designate Outlet Pressure.

Packing: A sealing system that normally consists of a deformable material such as PTFE, graphite, etc. It is usually in the form of solid or split rings contained in a packing box that are compressed so as to provide an effective pressure seal.

Packing Box: The chamber located in the bonnet that surrounds the stem and contains the packing and other stem-sealing components.

Pinch Valve: A flexible hose, pinched between two or more moving elements to stop flow. It is commonly used in slurry applications since it does not easily clog. (Learn More)

Piston Actuator: A fluid-powered, normally pneumatic device in which the fluid acts upon a movable cylindrical member, the piston, to provide linear motion to the actuator stem. These units are spring or air-opposed, and operate at higher supply pressures than a spring return actuator.

Piston Actuator. Image Credit: Spriax Sarco

Plug: See Closure Member.

Plug Valve: A multi-turn device which uses a rotating plug as the closing element. When the valve is open, the media flows through a hole in the plug, which can be cylindrical or truncated. (Learn More)

Position Switch: A switch that is normally fitted on the actuator to detect extremes of valve travel. The switch is normally electric.

Position Transmitter: A device that is mechanically connected to the valve stem and will generate and transmit either a pneumatic or electric signal that represents the valve stem position.

Positioner: A device used to position a valve with regard to a signal. The positioner compares the input signal with a mechanical feedback link from the actuator. It then produces the force necessary to move the actuator output until the mechanical output position feedback corresponds with the pneumatic signal value. Positioners can also be used to modify the action of the valve (reverse-acting positioner), alter the stroke or controller input signal (split-range positioner), increase the pressure to the valve actuator (amplifying positioner) or alter the control valve flow characteristic (characterized positioner). (Learn More)

Pressure Sealed Bonnet: A type of bonnet design where the fluid pressure is used to produce the seal between the body and bonnet.

PTFE (Polytetrafluoroethylene): a soft polymer that is compatible with almost any substance.

Push-Down-To-Close: A term used to describe a linear or globe style valve that uses a direct acting plug and stem arrangement. The plug is located above the seat ring. When the plug is pushed down, the plug contacts the seat and the valve closes. Note: Most control valves are of this type.

Push-Down-To-Open: A term used to describe a linear or globe style valve that uses a reverse acting plug and stem arrangement. The plug is located below the seat ring. When the plug is pushed down, it moves away from the seat and the valve opens.


Quarter-turn: A method of valve operation involving a 90 degree turn of the stem to move from fully open to fully closed. This describes valves such as ball, plug, and butterfly.

Quick Closing: Quick closing and quick opening refers to a valve designed to require a smaller turn to be fully closed or opened.

Quick Opening: A flow characteristic that provides maximum change in flow rate at low travels. The curve is basically linear through the first 40% of travel. It then flattens out indicating little increase in flow rate as travel approaches the wide open position. This decrease occurs when the valve plug travel equals the flow area of the port. This normally happens when the valve characteristic is used for on/off control.


Radiographic Inspection: An NDE technique that uses X-rays to detect internal flaws that are not detectable using other externally applied methods. (Learn More)

Rangeability: The range over which a control valve can operate. It is the ratio of the maximum to minimum controllable flow coefficients. Rangeability is affected by three factors: the geometry of the valve, the seat leakage and the actuator's accuracy or stiffness at near closure of the valve. Geometry is inherent due to the design of the seat and closure and excessive seat leakage can cause instability in the valve as it lifts off the seat.

Rating: An alpha numeric classification used to define the pressure capability of a pipework system.

Reduced Trim: Is an undersized orifice. A reduced or restricted capacity trim is used for several reasons including, adjusting a large valve to handle smaller flow requirements, reduced inlet and outlet fluid velocities, and correct errors in over sizing.

Reverse-Acting: This term has several deferent meanings depending upon the device it is describing. A reverse-acting actuator is one in which the actuator stem retracts with an increase in diaphragm pressure. A reverse-acting valve is one with a push-down-to-open plug and seat orientation. A reverse-acting positioner or a reverse-acting controller outputs a decrease in signal in response to an increase in set point.

Reverse Flow: Flow of fluid in the opposite direction from that normally considered the standard direction. Some rotary valve are considered to be bi-directional although working pressure drop capabilities may be lower and leakage rates may be higher in reverse flow.

Rotary Valve: A valve style in which the flow closure member is rotated in the flow stream to modify the amount of fluid passing through the valve (i.e. Ball Valve).


Seat Load: The contact force between the seat and the valve plug. When an actuator is selected for a given control valve, it must be able to generate enough force to overcome static, stem and dynamic unbalance with an allowance made for seat load.

Seat Ring: A part of the flow passageway that is used in conjunction with the closure member to modify the rate of flow through the valve.

Socket Weld: A connection made by entering a pipe into a matching socket in the end of a valve fitting, and welding the two together.

Sonic Velocity: The local speed of sound in the fluid. See Flashing

Special Class: A term applied to a Class designated threaded or weld end valve, where the body and cover have been subjected to non-destructive examination (NDE) and any defects removed. This allows the valve to have a higher pressure capacity than a standard class valve.

Split Body: A valve whose body is split. This design allows for easy plug and seat removal. Split-bodied valves are made in both the straight-through and angle versions.

Static Unbalance: The net force produced on the valve stem by the fluid pressure acting on the closure member and stem within the pressure retaining boundary. The closure member is at a stated opening with a stated flow condition. This is one of the forces that an actuator must overcome.

Stem: The valve plug stem is a rod extending through the bonnet assembly to permit positioning of the plug or closure member. The actuator stem is a rod or shaft which connects to the valve stem and transmits motion or force from the actuator to the valve.

Image Credit: Process Operation Technology

Stem Guide: A guide bushing closely fitted to the valve stem and aligned with the seat. Good stem guiding is essential to minimize packing leakage.

Stroke: See Travel.

Supply Pressure: The pressure at the supply port of a device such as a controller, positioner or transducer. Common values of control valves supply pressure are 20 psig. for a 3-15 psig. output and 35 psig. for a 6-30 psig. output.


Throttling: Modulating control as opposed to on/off control. (Learn More)

Transducer: An element or device that receives information in the form of one quantity and coverts it to information in the form of the same or another quantity.

Travel: The distance the plug or stem moves to go from a fully closed to a fully open position. Also called Stroke.

Trim: Includes all the parts that are in flowing contact with the process fluid except the body, bonnet and body flanges and gaskets. The plug, seats, stem, guides, bushings and cage are some of the parts included in the term trim.

Turndown: A term used to describe the ratio between the minimum and maximum flow conditions seen in a particular system. For example, if the minimum flow were 10 G.P.M. and the maximum flow were 100 G.P.M. the turndown would be 10:1. See Rangeability.


Valve: A device that dispenses, dissipates or distributes energy in a system. (Learn More)

Valve Flow Coefficient: See CV.

Valve Plug: See Closure Member.

Vapor Pressure: A pressure at which, for a given temperature, vapor bubbles form in the liquid.

Vena Contracta: The location where the cross-sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level, and where fluid pressure is at its lowest level. The vena contracta normally occurs just downstream of the actual physical restriction in a control valve.


Water Hammer: Shock waves generated in a pipework system caused by a valve closing too quickly.

Video Credit: GefaAnWendungen


Glossary Terms gathered from Kentintrol - Glossary of Terms and Valve Word – Glossary

Additional Resources

Liquid Flow in Control Valves