Valve Glossary

Valve Glossary

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AARH (Arithmetic Average Roughness Height) – The measurement of the smoothness of a surface. In the case of valves refers to the flange faces and is usually given in microns or AARH. (The smaller the value the smoother the surface.)

Absolute Pressure (abs press) – Gauge pressure plus barometric pressure expressed in bar, Pascal or PSI, as measured above a perfect vacuum. Absolute pressure can be zero only in a perfect vacuum.

Absolute Viscosity (abs visc) – The product of fluid kinematic viscosity times its density. Absolute viscosity is a measure of fluid tendency to resist flow, without regard to its density. Sometimes the term dynamic viscosity is used in place of absolute viscosity. Refer to Viscosity, Absolute.

Accessory – A device attached to the actuator which provides an additional function, for example manual operation, pilot solenoid, positioner, switch, etc.

Accuracy – A measure of how close a control valve or regulator can keep downstream pressure (P2) to the set-point. Regulator accuracy is expressed as percent droop or proportional band or offset in percent of set-point or in units of pressure.

ACFH (Actual Cubic Feet per Hour) – The actual volume of fluid (gas) measured by the meter. This is not SCFH (standard cubic feet per hour).

Active/Working Regulator – A regulator that is in service performing a control function.

Actual Pressure Drop – The difference between the inlet pressure and the outlet pressure of a valve.

Actuator – A device that mechanically operates a valve by means of manual effort, air, electricity, gas, hydraulics or by a combination of these mediums. Actuators may be used when valves are remotely located (e.g., pipelines), valves are located in hazardous areas, or when manual operation would be time-consuming or difficult for the operator (e.g., with larger valves).

Actuator Stem Force – The amount of force (torque or thrust) that is required to move the actuator stem to open, close or modulate the valve.

Adjusting Screw – A screw used to change the compression setting of a loading spring.

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

AGA – The American Gas Association.

Air (Gas)-to-Close – The increase in air or gas pressure to the actuator required to cause the valve to close.

Air (Gas)-to-Open – The increase in air or gas pressure to the actuator required to cause the valve to open.

Air Filter – An accessory added to a pneumatic actuator to prevent oil, dirt, or water in the air supply from entering the pneumatic actuator.

Air Set – Air supply pressure regulator. A device used to reduce plant air supply to valve positioners and other valve control equipment.

Air Valve – Valve that is used to control the flow of air. (Typically a solenoid type valve)

Angle Valve – A globe style valve where the inlet and outlet ports are at 90° to each other.

ANSI – American National Standards Institute

ANSI Class – A strength designation for valves which show the maximum pressures at various temperatures at which a valve made of certain materials can be expected to work safely. Derives from ASME B16.34.

Anti-Cavitation Trim – A special trim used in control valves to stage the pressure drop through the valve. This will either prevent cavitation from occurring or direct bubbles formed to the center of the flow stream, away from the valve body and trim. Usually accomplished by causing the fluid to travel along a torturous path or through successively smaller orifices or a combination of both.

AOV – Air-operated valve

API – American Petroleum Institute

Appliance (Equipment) – Any device that uses gas as a fuel or raw material to produce light, heat, power, refrigeration, or air conditioning.

ASME – American Society of Mechanical Engineers

Aspirator – Any device using fluid velocity effect to produce a low-pressure zone. Used in regulator control and combustion systems.

ASTM – American Society for Testing Materials

Atmospheric Pressure – The pressure exerted by the atmosphere at a given location and time. Sea level pressure is approximately 14. 7 pounds per square inch absolute (1.0 bar absolute).

Automated Valve – A valve that is powered by a source other than a human operator.

Automatic Control System – A control system that operates without human intervention.

Automatic Cutoff – A device used on some regulators to close the main valve in the event of pressure deviation outside of a preset range. Must be reopened manually.

AWWA – American Water Works Association


Back Flow – When the normal process is reversed.

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

Back Pressure Regulator – A device that controls and responds to changes in its upstream/inlet pressure. Functions the same as a relief valve in that it opens on increasing upstream pressure.

Back Seats – In linear valves, the area of the stem that enters the valve bonnet is sealed to prevent process fluid from entering the packing box and to prevent deterioration of the sealing materials.

Ball Valve – A valve that rotates through 90° with a spherical closing element held between two seats. Ball valves are normally used as on/off valves but special designs are available for control (throttling) applications. The are four basic types, Fixed Seat, Floating, Rising Stem and Trunnion.

Barometer – An instrument for measuring atmospheric pressure, usually in inches, centimeters, or millimeters of mercury column.

Barometric Pressure – The atmospheric pressure at a specific place according to the current reading of a barometer.

Belleville Spring – A washer with a slight conical shape that gives the washer a spring characteristic. They are typically used as springs, or to apply a pre-load or flexible quality to a bolted joint. Many valves have a spring for spring-loading, to normally shift the closure element into some position by default but allow control to reposition the disc. Relief valves commonly use a spring to keep the valve closed, but allow excessive pressure to force the valve open against the spring-loading.

Bellows – A flexible, thin-walled cylinder made up of corrugations one next to the other, that can expand or contract under changing pressures. They can be used to prevent process media leaking between the valve stem and the body or bonnet.

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

Bidirectional – The ability of an isolation to allow flow in either direction, although not necessarily equally. There may be a preferred flow direction with a limited reverse flow.

Bimetallic Thermal System – A device working on the difference in coefficient of expansion between two metals to produce the power to position a valve closure device in response to temperature change.

Bleed – Removal of fluid from a higher pressure area to a lower pressure, as with draining or depressurizing a valve.

Block and Bleed – A valve configuration in which flow through a valve is blocked while the valve is bled through another small port.

Blowdown – The discharge of process fluid to reduce the pressure in a piping system. This is usually done through a pressure relief device.

Bode Diagram – A plot of log amplitude ratio and phase values on a log frequency base for a transfer function. (It is a common form of graphically presenting frequency response data.)

Body – The body of the valve is the main pressure boundary. It provides the pipe connecting ends and the fluid flow passageway. It can also support the seating surface and the valve closure member.

Bonnet – A component sitting on top of the body of a valve that contains valve stem, packing and bushings and may house a gate or other closure device. It can be screwed, flanged, or welded to the body of the valve.

Bonnetless – A valve without a bonnet relying on the stem packing or transverse seal to retain the internal process pressure. Typical of knife gate valves.

Boost – The increase in control pressure above set-point as flow is increased from low flow to maximum flow. Some regulators exhibit droop instead of boost.

Booster – A pneumatic relay that is used to reduce the time lag in pneumatic circuits by reproducing pneumatic signals with high-volume and or high-pressure output.

Bore – Passages in the valve body that allow flow in and out of the valve. The bore is obstructed or opened up by the valve closure device. Bore may be equal to or smaller than the adjacent pipe inside diameter. (See full and reduced port.)

Boronizing (or CVD) – A chemical vapour deposition process used to coat wear and sealing parts in valves, normally ball type.

Break Torque – The torque required to open or unseat a valve.

Brinell Hardness Number – A number from 111 to 745 that indicates the relative hardness of a material. As the number increases the harder the material.

British Thermal Unit (BTU) – The quantity of heat required to raise one pound of water from 59° to 60°F.

BSPT – British Standard Pipe Thread

Bubble Tight – A commonly used term to describe the ability of a valve or regulator to shut off completely against any pressure with any fluid. The term is related to a seat closure test where a small flexible tube is connected to the downstream end of a valve under test, using air or gas. The end of the tube is held under water and inspected to see if any bubbles can be seen coming from the tube over time There are many such tests based upon the requirements of API, ASME, ANSI, MSS, and FCI etc. which will include a differential pressure and a time of observation.

Butt Weld Ends – Profiles that are machined on the ends of the valves and piping, to allow the joining of components by a circumferential weld of various wall schedules.

Butterfly Valve – A valve that rotates through 90° and has a circular disc as its closing element. Butterfly valves are normally used as on/off valves but special designs are available for control (throttling) applications. There are three types: Centric (resilient Seated); Single or Double Offset (High Performance), both Position Seated and Triple Offset (TOV), Torque Seated.

Button Head – A fitting through which sealant can be injected.

Bypass Valve – A valve smaller (or equal) in size that is fitted in parallel to a larger main valve. Bypass valves may be used to reduce the differential pressure across the main valve before the main valve is opened.


Ci (Flow Coefficient) – A term used in a valve sizing equation. It is defined as the ratio of the gas sizing coefficient and the liquid sizing coefficient and provides a numerical indicator of the valve’s recovery capabilities.

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.

Capacity, Flow – The amount of a specified fluid that will flow through a valve, specific length and configuration of tubing, a manifold, fitting, or other component at a specified pressure drop in a fixed period of time. (SCFH, gpm, Nm3/h, Lpm, bph, etc.)

Cavitation – Cavitation is a phenomena in liquid services where cavities or bubbles form and then collapse. It is the two-stage process of vapourization and condensing of a liquid. Vapourization is the boiling of liquid (also known as flashing) and occurs in control valves because the pressure of the liquid is lowered below the vapour pressure. 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 vapour pressure of the flowing fluid, vapourization (boiling) occurs. Vapour bubbles then continue downstream where the velocity of the fluid begins to slow and the pressure in the fluid recovers. The vapour bubbles then collapse or implode. Cavitation can cause a choked flow condition to occur and can cause mechanical damage to valves and piping.

Centipoise – A unit for measurement of absolute viscosity. One centipoise is equal to one hundredth of a poise, the metric (cgs) unit of absolute viscosity. The absolute viscosity of water at 20°C is approximately one centipoise.

Centistoke – A unit for measurement of kinematic viscosity. One centistoke is equal to one hundredth of a stroke, the metric (cgs) unit of kinematic viscosity. The kinematic viscosity in centistokes times the density equals the absolute viscosity in centipoises.

Centric Butterfly Valve – A butterfly valve with the disc installed in the center of the valve, typically resilient seated, position seated.

Certified Dimensional Drawing – A drawing that guarantees the overall dimensions of the valve that are required for installation. Sometimes called the general arrangement or GA.

Certified Material Test Report – Information on the component that covers its chemical composition and its mechanical properties.

CFH – Cubic Feet per Hour – (ft3/h). Volumetric measurement of gas flow per hour, generally at line conditions.

Cg (Flow Coefficient) – A term used in gas and steam valve sizing equations. The value of Cg is proportional to flow rate and is used to predict flow based on physical size or flow area.

CGA – Canadian Gas Association.

Chainwheel – A handwheel design that has sprockets that allow a chainwheel to be wrapped around a semicircle section of the handwheel and used as a pulley to turn the stem. This is installed on valves that are installed at an elevated position where it is not possible to erect a platform or add a ladder.

Check Valve – A valve that is designed to allow the fluid to flow in a given direction but closes to prevent back flow. Types include swing check, tilting-disc check, wafer check, center-guided check, piston check, ball check. Check valves (also called non-return valves) are usually self-acting.

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 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 vapour 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.

Class – The class is used to describe the pressure rating of the valves and piping system. For example Class 150, 300, 600, 900, 1500, 2500 or API 3000, API 5000, API 10000, etc. This relates to the maximum allowable design pressure that a flange of certain dimensions and made of a certain material can be used within a piping system, based upon a given temperature.

Closure Member – The movable part of the valve that 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, Diaphragm, Disc, Gate and Plug.

Cock – A device, such as a faucet or valve, for regulating the flow of a liquid.

Cold Rating or Cold Working Pressure (CWP) – The maximum pressure that a valve or fitting is designed to withstand at room temperature; usually used for pressures below Class 150 or PN20, e.g. 150CWP (150-psig).

Compressibility Effect – The change in density of gas or air under conditions of compression.

Compressible Fluid – A gaseous fluid such as steam, which has a significant change in volume and density as pressure increases.

Compressor – A pump or other type of machine using a turbine to compress a gas by reducing the volume.

Concentric Butterfly Valve – A butterfly valve with the disc installed in the center of the valve. See Centric Butterfly Valve.

Condensate – The liquid resulting when a vapour is cooled and/or when its pressure is increased.

Control Line – The external piping which connects the regulator actuator or pilot to the point on the main line where control is required.

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.

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.

Cooling System – An equipment system that provides water to the condensers and includes water intakes and outlets, cooling towers, ponds, pumps, valves and pipes.

Corrosion – The deterioration of a metal that is caused by a chemical reaction. This is sometimes called “weight loss.”

Chrome Carbide – A hard surface material applied typically by HVOF spray to the sealing and/or bore passages of a severe service valve.

Chrome Oxide – A hard surface material applied typically by plasma spray to the sealing and/or bore passages of a severe service valve.

Cr – Chromium, an element used in corrosion resistant alloys and hard coatings.

CrC – Chrome Carbide

CrO2 – Chrome Oxide

Critical Flow – The rate at which a fluid flows through an orifice when the stream velocity at the orifice is equal to the velocity of sound in the fluid under such conditions, the rate of flow may be increased by an increase in upstream pressure, but it will not be affected by a decrease in downstream pressure. Critical flow occurs when P2 is approximately 1/2 of P1 absolute. (See Choked Flow)

Cryogenic – Characteristic of temperature below -150°C (-238 °F).

Cryogenic Valve – Valves suited for use at temperatures below -150°C. A cryogenic valve should have a cold box as an integral part of the body to allow a vapour barrier to form between the packing box and the liquefied gas.

Cs (Flow Coefficient) – Steam valve sizing coefficient. At pressures below 1000 psig, a constant relationship exists between the gas sizing coefficient (Cg) and the steam coefficient (Cs). This relationship is expressed: Cs = Cg + 20.

CV (Flow Coefficient) – 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.

CWP – Cold Working Pressure is an indication of the pressure rating for piping, valves, and fittings at a temperature range of -20 to +100F.

Cylinder – A pressure-containing component and the part of an actuator that houses a piston that will be powered either pneumatically or hydraulically.


DCS – Distributed control system

Dead Band – The range through which an input can be varied without initiating observable response.

Delta P (DP) (Δ P) – The difference between the inlet and outlet pressures of a valve.

Demand – The rate at which fluid is delivered to or required by a system, part of a system, or a piece of equipment, usually expressed in terms of volume per unit of time.

Density – The weight of a unit volume of a substance. Also called specific weight.

Design Pressure – The pressure used during the design of a piping system, and defines the criteria for pipe wall thickness, fittings, flanges, valves, bolt torque, and threads.

Design Temperature – The temperature used during the design of a piping system, and defines the criteria for pipe wall thickness, fittings, flanges, valves, bolt torque, and threads.

Destructive Test – A test during which all or part of a component is destroyed by mechanical or chemical means to discover its properties.

Diaphragm – A flexible membrane used in a regulator or relief valve to sense changes in downstream pressure and respond to them, thus moving the restricting element or closure member to which it is attached.

Diaphragm Actuated Regulator – A regulator utilizing a diaphragm and actuator to position the valve plug.

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 Case – A housing used for supporting a diaphragm and establishing one or two pressure chambers.

Diaphragm Effect – The change in effective area of the diaphragm as the regulator strokes from low to high flow.

Diaphragm Plate – A plate used to transmit force in conjunction with a diaphragm and fluid pressure on a spring to the actuator stem or pusher post.

Diaphragm Valve – A bi-directional valve that is operated by applying an external force to a flexible element or a diaphragm (typically an elastomer). Diaphragm valves may be used for slurries (where other valve designs might clog) or in hygienic applications. Two basic forms are weir body and straight-through.

Differential Pressure Regulator – A device that maintains a constant differential pressure between a reference pressure and the pressure of the controlled fluid.

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-Operated Regulator – See Pressure Reducing Regulator.

Disc (Disk) – Closure member in a valve

Diverter Valve – A valve that can change the direction of the flow of a medium to two or more different directions.

DN – The ISO standard abbreviation for the nominal diameter of the line pipe size in metric units. For example 4″ = 100 DN, roughly 25mm is 1 inch.

Double Block and Bleed – A valve configuration in which positive shutoff is achieved at both the inlet and outlet sides. A small port is fitted to discharge fluid in the intermediate space. Fitting a gas detector to the port provides assurance of the integrity of the upstream seal. This configuration is often required to isolate high-pressure sections of a system to facilitate safe maintenance, etc.

Double Disc Check Valve – A check valve with two semicircular discs that are hinged together and that fold together when the flow is in the correct direction and swing closed when the flow is reversed. Also known as a split disc check valve or Dual Door or Dual-Flapper Check.

Double Piston Effect (DPE) – The sealing principle of a trunnion ball valve whereby the line pressure is used on both the upstream and downstream floating seats to effect a dead-tight seal simultaneously on both sides of the ball.

Double-Acting Positioner – A positioner that has the facility to supply and exhaust air on both sides of the actuator piston or diaphragm at the same time.

Downstream – Any site beyond a reference point (often a valve or regulator) in the direction of fluid flow.

Drift – A change in set point over an extended period of time.

Droop – The amount a regulator deviates below its set point as flow increases. Some regulators exhibit boost instead of droop.

Drop Tight – See Bubble Tight; but uses a liquid as the test medium. Needs to reference a known test to be of use in describing isolation ability.

Dual Seating – A valve is said to have dual seating when it uses a resilient or composition material such as TFE, Kel-F, or Buna-N, etc. as its primary seal and a metal-to-metal seat as a secondary seal. The idea is that the primary seal will provide tight shut off and if it is damaged, the secondary seal will back-up the primary seal with adequate but not necessarily equal shut-off, in an emergency situation.

Ductile – Capable of being drawn out into wire or thread.

Ductility – The characteristic of a metal to deform when placed under force. Ductility is measured by the percentage increase of a stretched test piece, just prior to fracture.

Dye Penetrant Testing – A bright red or fluorescent dye is used to detect surface cracks, pitting, or porosity. It is applied by spray and the excess dye is wiped away to expose surface flaws that can be detected by natural or fluorescent light.

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.



Eccentric Butterfly Valve – A butterfly valve where the shaft that carries the closure disc is slightly offset and creates an elliptical motion as it leaves the sealing surface. This effect reduces friction and wear to the closure disc. AKA the High Performance or Offset Butterfly valve, it is position seated and has a single or double offset.

Eccentric Plug – A half plug design used in both isolation and control plug valves.

Effective Area – For a diaphragm actuator, the effective area is that part of the diaphragm area 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 at a minimum at the end of the travel range. Flat sheet diaphragms are most affected by this. Molded diaphragms will improve the actuator performance and a rolling diaphragm will provide a constant stem force throughout the entire stroke of the valve.

Efficiency – Ratio of energy input to useful output.

Elastomer – A polymer that is both flexible and resilient and is used in both static and dynamic states as seals and seats.

Elastomeric – Characteristics resembling those of a polymer that is both flexible and resilient when used as a seal.

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.

Electro-Hydraulic Actuator – An actuator that supplies hydraulic power to control a valve, but has an integral electric power source used to pump the fluid.

Emergency Shut Down Valve (ESD) – A valve that uses energy which is stored in an actuator or related actuation system, to close (rapidly) in an emergency or upset situation.

End Connection – The part of the valve that joins to the piping system. This could be screwed, socket weld, flanged, butt weld, clamped, soldered, wafer.

End to End – The extremities of the valve. One connection to the other end connection. B16.10 is where many valve end to end or face to face dimensions are derived.

Energy Loss – The difference between energy input and output as a result of an energy transfer between points.

Enthalpy – Total heat content, expressed in BTU per pound, above an arbitrary set of conditions chosen as the base or zero point.

Entropy – A thermodynamic quantity that measures the fraction of the total energy of a system that is not available for doing work.

Equal Percentage – A term used to describe a type of valve flow characteristic. Equal increments of valve plug travel and the change in flow rate. 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.

Erosion – Material weight loss inside a piping system, caused by the process flow. This is not a consideration in process flows that have been adequately filtered and where entrained solids are not present.

ERV – Electronic Relief Valve

ESD – Emergency Shutdown

ESD – Electrostatic discharge

Examination – The review of a complete valve or its individual components to confirm that it complies with the user’s requirements.

Expanding Gate Valve – A gate valve that is comprised of a separate gate and segment that 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 seat. 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.

Explosion Proof – An assurance that an electrical device can be used in an area that is potentially explosive. This device must be detached from any electrical source that might arc.

Extended Bonnet – Used when the medium is at high or low temperatures, to avoid damage to the sealing elements.

Extension Stem – The equipment applied to buried valves to provide above-ground accessibility to actuator, blowdown and seat sealant injection systems.

External Pressure Regulator – A regulator with a control line. The actuator pressure is isolated from the body outlet pressure within the regulator.


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 certain specifications like B16.10.

Face-to-Face Dimension – The dimension from the face of the inlet opening to the face of the outlet opening of a valve.

Fail-Closed – A term used to describe the ability of an actuator to travel to the fully closed position in the event of power failure.

Fail-in -Place – A term used to describe the ability of an actuator to stay at the same percent of travel in the event of power failure, also known as Fail Last

Fail-Open – A term used to describe the ability of an actuator to travel to the fully open position in the event of power failure.

Fail-Safe – An actuator facility such that in the event of power failure the valve will move to a predetermined position, which could be open, closed, or an intermediate position.

Fatigue – The tendency of material to fracture under repetition of a stress Glossary 52 Pocket Guide less than the ultimate static strength. Fatigue strength, also called endurance limit, refers to the maximum stress, which can be incurred and reduced indefinitely without producing a fracture.

FCI (Flow Control Institute) – An association of manufactures of equipment for fluid (liquid or gas) control and conditioning. The institute is organized into product-specific section which address issues that are relevant to particular products and/or technologies.

FCI 70.2 most used Standard – Allowable Leakage Rates for Control Valves.

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.

Ferrous – Characteristic of relating to or containing iron.

Fire Resistant – The ability of a valve to withstand a fire and maintain the failure position. Such a valve will be equipped with devices to achieve this status.

Fire-Safe – The ability of a valve to minimize the amount of process lost downstream or to the atmosphere after or during fire.

First-Stage Regulator – A regulator used to reduce inlet pressure to a set value being fed to another regulator in series.

Fixed Factor Measurement – The measurement of gas at a controlled elevated pressure without the use of an automatic correcting device to correct the volume for variation from base or contract pressure. This is accomplished by placing an accurate regulator upstream of the meter. Also known as PPM (Pressure Factor Measurement).

Fixed Restriction – A small diameter hole in the pilot or piloting system that determines gain.

Fixed Seat Ball Valve – A severe service ball valve with a fixed integral sealing seat typically carbide coated.

Flange – End connections of valve bodies used for bolting onto another fitting or pipe element. B16.5 and B16.47 most common Standards

Flange Connection – A connection mated by means of bolts inserted through holes in the flange face.

Flange Facing – The finish on the end connection of flanged valves.

Flangeless – A valve that does not have integral line flanges. This type of valve is 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.

Flash Point – Temperature at which a liquid will give off enough flammable vapor to ignite.

Flashing – A condition when liquid changes to the vapor state caused by pressure reduction inside a valve.

Flat Face – A flange that has no raised face or a ring groove surface. These flanges are generally used in lower piping pressure classes such as ASME 125 lb or 150 lb in cast iron and carbon steel. The mating gasket will be flat and extend to the full flange circumference with holes to accommodate the flange bolting.

Flat Gasket – A circular, flat sheet with an inside and outside diameter normally used for sealing flat face flanges.

Float Valve – A valve that automatically opens or closes as the level of a liquid changes. The valve is operated mechanically by a float that rests on the top of the liquid.

Floating Ball – A ball valve where the closure ball is not attached to the body of the valve and is pushed against the sealing seat by line pressure.

Floating Seat – A seat ring that is not attached to the valve body and can move to suit the closure element and improve the shut-off.

Flow Capacity – The rated flow through a regulator under stated inlet, outlet, and droop pressures.

Flow Characteristic – Relationship between flow through the valve and percent rated travel.

Flow Coefficient – See the definition for CV.

Flow Meter – An instrument used to measure flow rate, total flow or both.

Flow Rate – The amount (mass, weight, or volume) of fluid flowing through a valve body per unit of time.

Fluid – A material that can flow; includes gases, liquids, slurries, pellets, and powders.

Forging – A part that is formed by heating followed by hammering, rolling or applying other compressive forces to create a specific shape.

FP – Full Port indicates that the internal diameter of the valve opening is equal to the pipe it is connected to.

Friction – The resistance to motion between two contacting surfaces or substances. Friction also is developed between a flowing fluid and the inner wall of the conducting pipe, resulting in a drop in pressure.

Fuel Gas – A commonly distributed gas used for fuel, such as natural gas, propane, landfill gas, etc.

Full Bore – Indicates that the internal diameter of the valve opening is equal to the pipe it is connected to.

Full Capacity Relief – A relief valve that has the capability of maintaining downstream pressure to within certain limits in the event of some type of failure, by venting the excess gas to the atmosphere.

Full(y) Closed – The position of the valve when the closure element is fully seated.

Full Lift – When a pressure relief valve is fully open upon over pressurization of the piping system.

Full Penetration Weld – The type of butt weld wherein the weld metal extends across the entire wall thickness of the joint.

Full Trim – The area of the valve’s seat that can pass the maximum flow for that particular size.


Gauge Pressure – (Psig, kPag or Barg) The difference between atmospheric pressure and the pressure being measured. Also written as gauge pressure.

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.

Galling – The damage of two mating parts when microscopic portions impact and make a temporary bond. When effort is made to separate these two surfaces, tearing of the two components can occur. This usually happens when the two materials are the same or possess several very similar mechanical characteristics.

Gas – That state of matter which expands to fill the entire container which holds it. Gas is one of the forms of matter (solid, liquid, gas, plasma).

Gas Utilization Equipment – Any device which utilizes gas as a fuel or raw material, or both.

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 Valve – A multi-turn valve that has a gate-like disc and two seats to close the valve. Generally, refers to the wedge gate type. The gate moves linearly, perpendicular to the direction of flow. This type of valve is normally used in the fully opened or fully closed position; it is not suited to throttling applications.

Gauge Pressure – Pressure reading as shown on a gauge (psig, kPag or Barg). The difference between atmospheric pressure and the pressure the gauge is measuring. Also written gage pressure.

Gearboxes – Used to ensure easier operation of manually operated valves.

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

Gland Bushing – Or the packing follower. Located at the top of the packing box, it acts as a barrier, protects the packing from the atmosphere, and transfers a force from the gland flange bolting to the packing.

Gland Flange – Part of the valve used to compress and retain the internals in the packing box.

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 multi-turn valve with a closing element that moves perpendicularly to the valve body seat and generally seals in a plane parallel to the direction of flow. This is 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 suited to both throttling and general flow control, but should not be selected for isolation duty.

Governor – An attachment to a machine for automatic control or limitation of speed. Also, an archaic term used for a low-pressure, direct­operated, pressure reducing gas regulator.

GPM – Gallons per minute.

Graphite – A carbon-based gasket or packing material, suitable for ambient and high temperatures.

Grease Fitting – A fitting through which lubricant or sealant is injected.

Guided Shear Gate – the fifth type of knife gate valve (KGV) known as a guided shear gate (GSG) and only one that is designed to actually cut or shear through solids. It is the highest performance style of knife gate available from 150CWP to ASME Class 1500 (PN10 to PN250).

GWe – Gigawatt Electric

GWh – Gigawatt Hour


Handwheel – A manual override device used to stroke a valve or limit its travel. It may be top-mounted, side-mounted, in-yoke mounted or shaft-mounted and declutchable.

Hard Facing – A material that is harder than the surface to which it is applied. It is normally used to resist fluid erosion or to reduce the change of galling between moving parts. Hard facing may be applied by fusion welding, diffusion, or spray coating the material.

Hardness – A property of metals that is discussed frequently when speaking of various component parts used in valve construction, particularly valve trim. There are two hardness scales which are commonly used, Rockwell & Brinell.

HART – Hybrid signal containing characteristics of both analog and digital signals.

Header – A piping configuration where a number of pipes are combined at one location.

Heat Capacity – Measured in joules per Kelvin, it is the quantity of heat needed to raise the temperature of a body one degree. Once called thermal capacity.

Heat Exchanger – Transfers heat from one fluid to another without the fluids coming into contact. Used to regulate fluid temperature or to use heat that would otherwise be wasted.

Heat Rate – Measure of generating station thermal efficiency generally expressed in BTUs (British Thermal Units) per net kilowatt-hour.

Heat Treating — The metal-producing process that involves heating and cooling to predetermined temperatures in a particular order and with specific holding times.

High Recovery Valve – A valve design that dissipates relatively little flow stream energy due to streamlined internal contours and minimal flow turbulence. Therefore, pressure down stretch of the valve vena contracta recovers to a high percentage of its inlet value. These types of valves are identifiable by their straight-through flow paths. Examples are most rotary control valves, such as the eccentric plug, butterfly and ball valve.

High-Performance Valve – A subjective marketing term for a valve that has now been replaced by the term Severe Service Valve (SSV). Is also still used to identify the Offset Butterfly as HP Butterfly.

Holiday – An imperfection or bare spot in a coating or plating.

Hot Tapping – A process by which a pipeline, under pressure, is cut into to provide
a side outlet.

HPBV – High-performance butterfly valve

HRSG – Heat Recovery Steam Generator

Hunting – A condition in which a control valve or regulator’s outlet pressure slowly fluctuates on either side of a set point.

HVAC –Heating, ventilating and air conditioning

HVOF – High Velocity Oxygen Fuel used for coating wear and sealing parts in valves, normally ball type using Chrome or Tungsten Carbides.

Hydraulic Actuator – A device fitted to the valve stem which uses hydraulic energy to open, close, or regulate the valve. The hydraulic fluid may, according to the configuration, both open and close the valve, or just open the valve. In the latter case, a spring will typically be fitted inside the actuator to return it (and the valve) to the closed position.

Hydraulic Device – A device that is moved or worked by liquid pressure.

Hydrostatic Test – Pressure tests that are carried out on every valve when built to test the integrity of the pressure-containing parts. Generally this test pressure is 1.5 times the design pressure at ambient temperature.

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.


I.D. or ID – Inside diameter

I/O – Input/Output – Electrical inputs and electrical outputs

I/P – Current to Pneumatic Transducer

Impact – A test that will determine the toughness of a particular material by measuring the force necessary to fracture the test piece.

Impulse Line – See control line.

Inch of Water – A unit of pressure measurement. The pressure required to support a column of water one inch high. Typically reported as inches w.c. (water column); 27.68-inches of water is equal to one pound per square inch (psi).

Incipient Cavitation – Is a term used to describe the early stages of cavitation. At this point the bubbles are small and the noise is more of a hiss, like the sound of frying bacon. There is normally no mechanical damage associated with incipient cavitation although it could have an effect on the corrosive properties of some fluids.

Inclusion – A foreign object or particles found in a weld, forging, or casting that will have a detrimental effect on the component and cause failure or create a leak path. Inlet. The port where the fluid enters the valve.

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 – It is the relationship between valve capacity and valve travel and is 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.

Inlet Pressure – The pressure at the inlet opening of a valve (P1).

Inlet Pressure Sensitivity – The increase or decrease in the outlet pressure caused by changes in the inlet pressure which results in differing degrees of force being applied to the seat disk and diaphragm.

Inspection – The examination of a valve or a component by the end user or an authorized third party inspector. This is to confirm that the valve or component meets the user’s requirements.

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 non-linearities in the control loop.

Instrument Data Sheet (IDS) – A table summarizing data such as service, valve size, supply pressure, etc., necessary in prescribed steps and normally in a format according to the ISA.
Instrument Pressure – The output pressure from an automatic controller used to operate a control valve. It is the input signal to the valve.

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

Integral Seat – A seat that is actually a machined part of the valve body and not one that is inserted into the valve. Aka fixed seat.

Intermediate Rated Valves – A welding or threaded valve may be assigned an intermediate pressure-temperature rating or Class, either Standard or Special, by using the interpolation method described in ASME/ANSI standard B.16.34.

Internal Relief Valve – A small, spring-loaded pressure relief valve contained within the regulator at the center of the diaphragm to prevent outlet pressure from exceeding a predetermined pressure.

Intrinsically Safe – An electrical device that is not able to produce sufficient heat to cause ignition in the atmosphere.

ISA – International Society of Automation

ISO – International Organization for Standardization

Isolation Valve – A valve that turns the flow of media on and off. All valves either isolate or control, and should never be used for both duties. Isolation valves vary in their ability to isolate and the criticality of the required isolation is an important consideration when selecting the most appropriate valve type.


Jacketed Valve – Valve designed to incorporate a so-called jacket around the valve body. Steam is introduced into the jacket to keep the fluids being controlled at the required temperature. Partial steam jackets are only attached to the body and maintain the original valve’s connection size while full steam jackets also attach to the end connections and are one pipe size oversized, i.e. a 4” Full Jacketed valve will have 6” flange connections and be identified as a 6”x 4” x 6” valve.


Kinematic Viscosity (kin visc) – The relative tendency of fluids to resist flow. The value of the kinematic viscosity includes the effect of the density of the fluid. The kinematic viscosity is equal to the absolute viscosity divided by the density. Refer to Viscosity, Kinematic.

Knife Gate Valve – A type of gate valve originally invented in 1927 and designed for pulp and paper fibre slurries. It was adapted over time to handle other slurries particularly in mineral processing. There are five distinct types: Conventional, Through-Gate, Lined, Push-Through and Guided Shear Gate. Only the latter actually cuts.

Km – Value recovery coefficient – used in liquid sizing equations to determine ΔP allowable for cavitation.


Landfill Gas – A gas produced by decaying organic matter in a garbage landfill. This gas is used to power burners and engines. This gas has a high methane content and may contain other gases including H2S; therefore, stainless steel NACE construction is usually required.

Lapped-In – A term that describes a procedure used to reduce the leakage rate on metal-to-metal seated valves and regulators. The plug and seat are lapped together with the aid of an abrasive compound in effort to establish a better seating surface than would normally be achieved by means of machining.

Leakage – Process fluid that passes through a valve to the downstream when it is fully closed.

Leakage Classification – A term used to describe certain standardized testing procedures for valves. Incorrectly, FCI 70.2 has been used to describe allowable leakage for isolation valves. Standardized seat tests found in API 598, MSS SP-61 or ISO 5208 better describe seat leakage, but new tests are underway to provide better classifications including Zero Leakage.

Leak-Off – A term used to describe a threaded connection located on the bonnet of a valve that allows for the detection of leakage of the process fluid past the packing area.

Lever operator – A manual method of operating a valve that comprises a pivot handle.

Life Cycle Costs—LCC – Total costs incurred over the life of a plant or piece of equipment, including operations, maintenance, spare parts as well as initial capital costs, total cost of ownership.

Lift and Turn – Hybrid of linear and rotary valve actions.

Lift check valve – A non-return valve that prevents back flow by having a free floating element, either a ball or a poppet. The design incorporates a piston to damp the disc during operation. Must be used in horizontal pipe and is generally less than 2” (50mm).

Limit stop – A device in an actuator that limits the linear or rotary motion of an actuator; can be adjusted.

Limit switch – An electromechanical accessory that is attached to an actuator or manual valve and used to identify the position of a valve’s closure element, typically fully Open or Closed.

Limited Class Valves – Weld- or thread-end valves in sizes 2-1/2” and smaller may be designated Limited Class Valves (if not above Class 2500 for Threaded and 4500 for socket-weld end). Note: Limited class ratings shall not be used for flanged-end valves.

Line blind – A pipeline shut-off device, whereby a flat disc is forced between two flanges. Line blinds are less expensive than valves, but require much more time to operate.

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 – A valve that has a sliding stem that pushes the closure or throttling element up and down.

Liquefied Petroleum Gas (LPG) – Butane, propane, or a mixture of the two, obtained from oil or gas wells, or as a by-product from the refining of gasoline. It is sold in metal bottles under pressure as a liquid; hence, sometimes called bottled gas.

Liquid Expansion Thermal System – A closed system containing liquid whose expansion and contraction in response to temperature changes provides the power to position a valve member.

Liquid Penetrant Testing – See Dye Penetrant Testing

LNG – Liquified natural gas, mostly Methane CH4 with some Ethane C2H6

Loading Element – In a regulator, the means for placing a measured amount of force against the regulator’s diaphragm. The loading element is commonly a spring.

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 may be the instrument pressure if a valve positioner is not used or is by passed.

Locking device – A device that can be attached to a valve or an actuator and that will enable it to be locked closed or locked open. Prevents accidental operation as only authorized personnel can operate the valve.

Lockup Pressure – Increase over setpoint when the regulator is at no-flow condition.

Lock-Up Valve – A special type of regulator installed between the valve positioner and valve actuator is used to sense the supply air pressure. If that pressure fails below a certain level, it locks or traps the air loaded into the actuator causing the valve to fail-in-place.

Low Recovery Valve – A valve design that dissipates a considerable amount of flow stream energy due to turbulence created by the contours of the flow path. Consequently, pressure downstream of the valve vena contracta recovers to a lesser percentage of its inlet value than a valve with a more streamlined flow path. The conventional globe style control valve is in this category.

Lubricated Plug Valve – A quarter-turn valve 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. The valve is designed to have lubricant or sealant injected between the plug and the body to help allow the valve to be turned and create a seal.

Lug body – A body of a flangeless wafer butterfly valve that requires bolts to pass through the body to flanges on either side of the valve. These holes can be tapped to allow the line to be dismantled without “dropping” the valve. Tapped lugged valves are sometimes called “end of line” valves.


Magnetic Particle Inspection (MP) – Material testing where iron filings are spread over the area under examination. On passing an electric current through the examination piece, the filings will collect where there are imperfections.

Main Fuel Valve – The valve controlling fuel input to a burner.

Main Stop Valve – The valve connected to the boiler allowing steam to exit the boiler.

Manual Handwheel – A handwheel to open, close, or position a closing element, which does not require an actuator to make it function.

Manual Valve – A valve that is worked by manual operation, such as a handwheel or a lever. These valves are generally used for on-off service.

Manufacturers Standardization Society – A non-profit technical association organized for the development and improvement of industry, national and international codes and standards for valves and fittings. Currently co-producing Standard Practices with ANSI.

Maximum Allowable Operating Pressure – The maximum pressure that can be safely held in a piping system, expressed in bar (kilopascal) or psi. Determined by the material of construction, the maximum operating temperature, and the piping class. Also called the Maximum allowable working pressure (MAWP)

Maximum Operating Pressure – The maximum pressure existing in a piping system during normal operation.

Measuring Element – A diaphragm that senses (measures) changes in downstream pressure and causes the regulator restricting element to move toward the open or closed position.

Media – The gases, liquids or loose materials that flow through an aperture, such as a pipe.

Megawatt – (MW) – One million watts. Mwe = one million watts of electric capacity / MWh = one million watt hours.

Metal Seat – A seat design where the fixed mating surface with movable closure component is made of metal.

Metal Seat Ball Valve (MSBV) – a true metal seat ball valves designed for severe service (not a soft seated valve with the soft seats exchanged for metal ones). Typically MSBVs feature carbide coated balls and seats and are capable of the tightest shut-off at temperature beyond where soft seats can operate.

Metal-to-Metal Seal – The seal produced by metal-to-metal contact between the closure device and the seat without the benefit of a synthetic seal.

Meter Run – The section of pipeline in which a meter is installed to measure the volume of fluid passing through the line.

Mill Test Report – Report of the chemical testing and physical testing performed on a base material. This documentation is normally produced by the manufacturer and is often requested by the purchaser to confirm compliance to the specification.

Minimum Controllable Flow – The lowest flow at which a steady regulated condition of the controlled variable can be maintained.

MMCF – The abbreviation for “million cubic feet” used to designate gas volume and gas flow rates in pipelines. (MMcf/d or MMcf/hr)

Modbus – Protocol used for communications between electronic devices developed by Gould Modicon.

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

MOV – Motor Operated Valve

MSS – Manufacturers Standardization Society

Multiported – Multiported valves include additional inlet/outlet ports, to allow fluids to be directed. The ball and plug valve types are ideally suited to multiport designs.

Multi-Turn – Category of valves (such as gate, globe, needle), which require multiple turns of the stem to move the valve from the fully open to the fully closed position. Also known as linear valves.

MW – Megawatt


NACE – National Association of Corrosion Engineers

National Pipe Thread – A tapered thread that is used for pressure connections for piping.

Natural Gas – A hydrocarbon gas consisting mainly of methane.

NDE – Non-Destructive Examination. A collective term used to describe non-intrusive examination techniques such as radiographic and ultrasonic examination.

Needle Valve – Multi-turn valve that derives its name from the needle shaped closing element. The design resembles that of the globe valve. Typically available in smaller sizes, they are often used on secondary systems for on/off applications, sampling, etc.

NEMA – National Electrical Manufacturers Association

Net Available Capacity – The gross available capacity less than the unit capability used for that unit’s station service or auxiliary loads.

Net Heating Value – The amount of heat generated by combustion of hydrocarbons including water vapor.

Nitriding – A surface treatment process used to protect wear and sealing parts in valves, normally ball type.

Nm3/h – meters cubed per hour (normal); measurement of volume rate of a gas at atmospheric pressure and 0°F. Also refer to Sm3/h.

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

No-Load Loss – Power and energy lost by an electric system when not operating under demand.

Non-Destructive Examination – Non-Destructive Examination. A collective term used to describe non-intrusive examination techniques such as radiographic and ultrasonic examination.

Non-Return Valve – A valve that allows the flow of a process fluid in only one direction and will not allow flow reversal, aka Check Valve.

Non-Rising Stem – A valve where the stem is threaded and the turning of a stationary operator will result in the closure element rising to open and lowering to close.

Normally Closed – A valve that is normally closed during operation. In many cases these valves are locked closed by using a mechanical device.

NPT – National Pipe Thread is a tapered thread that is used for pressure connections
for piping.

NRS – Non-rising stem


O.D. or OD – Outside diameter

Offset Design – A design in which a valve controller is installed off geometric center of
the valve.

Offset – The deviation from setpoint for a given flow. Negative offset is equivalent to droop. On-Off Valve. Basic operation for a manual valve used to start or stop the flow of a process fluid.

Open-Ended Valve or Line – Any valve, except safety relief valves, having one side of the valve seat in contact with process fluid and one side open to the atmosphere, either directly or through open piping.

Operating Availability Factor – The percentage of time a unit is available for service, whether
operated or not.

Operating Medium – The power supply used to operate an actuator: can be pneumatic, hydraulic or electric.

Operating Pressure – The pressure at which a valve usually operates under normal conditions. This is lower than the design pressure.

Operating Temperature – The temperature at which a valve usually operates under normal conditions. This is lower than the design temperature.

Operator – A person, device, handwheel, lever, or wrench used to open, close, or position the closing element of a valve.

Orbit® Valve – A rising stem ball valve where the ball is lifted by an internal mechanism before the 90 degree rotation of the ball. (Orbit is a registered name of the Cameron Co.)

Orifice – A fixed opening, normally the inside diameter of a seat ring, through which fluid passes. The term can also refer to the inlet or outlet of a regulator or pilot valve. Also called a port.

O-Ring – An elastomer ring that forms a sealing material for the internals of a valve.

OS&Y – Outside screw and yoke, typical of gate and globe valves.

Outage – The period of time when a generating unit, transmission line, or other facility is out of service.

Outlet Pressure (Reduced Pressure) – The pressure leaving the outlet opening of a valve (P2).

Output – The amount of power or energy produced by a generating unit, station or system.

Over-Pressure Cut-Off Device – A mechanical device incorporated in a gas pipework system to shutoff the supply of gas when the pressure at the sensing point rises to a predetermined value.

Overpressure – The pressure increase over the set pressure of a pressure relief valve. Also, the amount of pressure in a media flow system that is over the set pressure of the pressure relief valve.


P1 – Inlet or upstream pressure of a valve.

P2 – Outlet or downstream pressure of a valve.

P&ID – Process and Instrument Diagram, a map of the process with the elements identified by standard symbols.

Packing – A soft sealing material that is used to prevent leakage of process fluid from around the stem of a valve. It is located in the packing box.

Packing Box – The chamber located in the bonnet that surrounds the stem and contains the packing.

Packing Follower – A part that transfers a mechanical load to the packing from the packing flange or nut.

Parallel Slide Gate Valve – A gate valve that has a flat disc gate that slides between two parallel free floating seats. Most designs are position seated.

Particulate – A fine grained particle(s) small enough to be suspended in a gas or liquid but large enough to be filtered out.

Peak Demand – The maximum load during a specified period of time.

PEEK – The abbreviation for polyether ether ketone. A robust higher temperature soft seating material.

Penstock valve – A type of simple gate valve, used to contain fluids in open channels. Often found in waste water treatment plants.

PFM (Pressure Factor Measurement) – The measurement of gas at a controlled elevated pressure without the use of an automatic correcting device to correct the volume for variation from base or contract pressure. This is accomplished by placing an accurate regulator upstream of the meter. Also known as Fixed Factor Measurement.

PID – Proportional/Integral/Derivative device. Usually used as a controller.

Pig (Pipeline Inspection Gauge) – A device, closely conforming to the pipe bore, which is forced through a pipeline to clean the pipe of all foreign materials and debris.
(See also Smart Pig)

Pilot (Amplifier) – A relatively small controlling regulator that operates the main regulator. They are used to increase accuracy.

Pilot Valve – Small valve requiring little power that is used to operate a larger valve. See also Solenoid valve.

Pilot-Operated Regulator – Two regulators connected so that one increases the effect of downstream pressure changes on the other. This arrangement is used to provide increased accuracy and flow capacity compared to direct-operated regulators.

Pilot-Operated Relief Valve – Two relief valves connected so that one increases the effect of inlet pressure changes on the other. This arrangement is used to provide increased capacity and reduced buildup compared to other relief valve types.

Pinch valve – A valve in which a flexible hose is pinched between one or two moving external elements to stop the flow. This valve is often used in slurry and mining applications, as its operation is not affected by solid matter in the medium. It is also used with certain gases, as the absence of possible leak paths to the atmosphere ensures good emission control.

Piping (Pipe) Schedule – A method of noting the wall thickness of a pipe, for example Sch 40, Sch 80, Sch 160. The larger the number the thicker the wall thickness of the pipe at a given nominal diameter.

Piping and Instrument Diagram – A schematic drawing that indicates the process system, and includes items of equipment, valves, and associated instrumentation.

Piston Actuated Regulator – A regulator utilizing a piston rather than a diaphragm actuator.

Piston Actuator – A fluid powered normally pneumatic device in which fluid acts upon a movable cylindrical member, the piston, to provide linear motion to the actuator stem.

Pitot Tube – A hollow tube that connects the area beneath the regulator diaphragm with the vena contracta area of gas flow. The pitot tube causes the diaphragm to sense a pressure lower than that which exists downstream of the regulator, and thus allows the regulator to open more for any given change in downstream pressure. The result is increased regulator accuracy.

Pitting Corrosion – Surface corrosion that appears as small holes or cavities. Over time these cavities will increase in size and join to create larger cavities.

PL – Loading pressure of fluid on the main diaphragm that is controlled by a pilot regulator.

Planned Outage – The removal of a unit from service to perform work on specific components. The removal is scheduled well in advance and is of a predetermined duration.

Plasma Spray – A process used to ceramic coat wear and sealing parts in valves, normally ball type, especially Chrome Oxide, CrO2

PLC – Programmable Logic Controller

Plug – Valve component that moves against an orifice or seat to increase and decrease flow.

Plug Valve – A quarter turn valve 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 round or truncated. Can be lubricated or sleeve-lined.

Pneumatic Actuator – A device fitted to the valve stem which uses pneumatic energy to open/close or regulate the valve. The compressed air may, according to the configuration, both open and close the valve, or just open the valve. In the latter case, a spring will typically be fitted inside the actuator to return the valve to the closed or open position. Available in Rack & Pinion, Scotch Yoke, and Vane styles of Rotary actuators and Piston Cylinder for linear actuators.

Pneumatic device – A device that is moved or worked by air or gas pressure.

Poise – A metric unit for measuring absolute viscosity. One poise equals one dynesecond per square centimeter, or one gram per centimeter second.

Polyethylene – A flexible thermoplastic that is used for valve seats.

Polypropylene – A thermoplastic that is not as flexible as polyethylene.

Pop Type Relief Valve – A spring-loaded poppet type relief valve.

Poppet – A closure element in a check valve that is held in place by a spring. Used frequently in hydraulic actuator control systems.

Porosity – Small air bubbles that were created in the casting when the metal was molten. When the metal has cooled, these trapped bubbles weaken the structure and can cause failure in the component.

Port – See Bore.

Position Seated – A type of valve where the method of actuation positions the closure device and seal; and under or over travel reduces the sealing ability and may even damage the valve.

Position Switch – A switch that is fitted on an actuator to detect extremities of valve travel or limit the travel to a certain position.

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

Positioner – A device that receives a signal—pneumatic or electric—from a controller and compares it to the actual position of the valve. If the signal is not correct then the positioner actuates the valve so that the correct position is achieved.

Positive Material Identification – A testing process that identifies the chemical composition of a material.

Post Weld Heat Treatment (PWHT) – Heating a piece of equipment to a sufficient temperature to relieve the residual stresses resulting from mechanical treatment and welding.

Powder Paint Coating – A paint process that uses dry powder with no solvents for surface finish. Dry powder can be reused, thereby reducing waste and pollutants. The powder coating over a clean surface provides better corrosion resistance than liquid coat.

Pressure – Force per unit area.

Pressure Buildup – In a relief valve, the pressure increase above set point required to produce a given flow rate.

Pressure Differential – The difference in pressure between two points in a system.

Pressure Drop – The difference between the upstream pressure and the downstream pressure of a valve.

Pressure Gauge – An instrument that measures the pressure of a fluid.

Pressure Reducing Regulator – A valve that satisfies downstream demand while maintaining a constant reduced pressure. As the pressure decreases, the valve opens to increase flow.

Pressure Reducing Valve – A self-operating valve used to reduce any excess pressure in a system. Also known as a PRV. The valve opens if the internal pressure exceeds that holding the closing element onto the seat.

Pressure Relief Valve – A valve which relieves pressure beyond a specified limit and recloses upon return to normal operating conditions.

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

Process Flow Diagram – A schematic that outlines the process in a plant and shows all major in-line equipment and essential instrumentation.

Propane – An easily liquefiable hydrocarbon gas. Propane is one of the components of raw natural gas, and it is also derived from petroleum refining processes. Its chemical formula is C3H8.

Proportional Band (Amount of Deviation) – The amount a regulator deviates from setpoint as the flow increases from minimum to maximum. Also referred to as droop or offset.

Proximity Switch – A limit switch that indicates the valve position without making mechanical contact. The switch will use a magnetic or an electronic sensor to determine the valve position.

PRV – Pressure Relief Valve.

PSG – Parallel Slide Gate

psi (PSI) – Pounds per square inch.

psia (PSIA) – Pounds per square inch absolute is the pressure above a perfect vacuum, calculated from the sum of the pressure gauge reading and the (local or ambient) atmospheric pressure (approximately 14.7 psia).

psid (PSID) – Pounds per square inch, differential.

psig (PSIG) – Pounds per square inch gauge is used when the pressure is expressed to standard atmospheric pressure (approximately 14.7 psia).

PT – The abbreviation for dye penetrant test. (See Dye Penetrant Testing)

PTFE – The abbreviation used for polytetrafluoroethylene (Teflon).

PTFE (Polytetrafluoroethylene) – A soft polymer that is compatible with almost any substance.

Pump – A mechanical device used to increase fluid pressure or move fluids.

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.

Push-Through Knife Gate – The fourth version of the knife gate valve that uses a gate to separate two sleeves that isolate the pressure retaining body from the process media. This knife gate was designed for neutral pH mineral slurries. It discharges a volume of process media with every stroke.


Q – Required media flow, expressed in gallons per minute.

QA – Quality Assurance

QC – Quality Control

Quarter-turn – The 90° angle through which a valve’s closing element must move from the fully open position to the fully closed position. Examples are ball, plug, and butterfly valves.

Quick Closing – Quick closing and quick opening refers to a valve designed to require a smaller turn to be fully closed or fully 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 characteristics is used for on/off control.


Rack and Pinion Actuator – An actuator used in conjunction with quarter-turn valves. This actuator will supply either a pneumatic or a hydraulic force to move a flat-toothed rack that turns a gear to open and close the closure element. The torque curve of this type of actuator is a flat line.

Radiographic Inspection – An NDE technique that uses X-rays to detect internal flaws that are not detectable using other externally applied methods.

Raised Face Flange – A flange face that has a raised section on the mating surface. This raised section can come with various types of serrated finish. This allows greater loadings to be applied to the gasket and creates a more efficient seal than a flat face flange.

Range – The region between the limits within which a quantity is measured, received, or transmitted, expressed by stating the lower and upper range values (Example: 3 to 15 psi; -40° to 212°F [-40° to 100°C]).

Rangeability – The ratio of maximum rated capacity to the minimum controllable flow within the specified accuracy band.

Rate of Flow – The volume of material passing a given point in a system per unit of time.

Rated Capacity – The rate of flow through a control valve or regulator specified by the manufacturer for a given inlet pressure, outlet pressure, offset, and size.

Rated Working Pressure – The maximum allowable pressure specified by the manufacturer.

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

Reduced Bore or Port (Standard Bore or Port) – Indicates that the internal diameter of the valve is lower than that of the piping to which the valve is fitted.

Reduced Pressure – The pressure leaving the outlet opening of a valve (P2). More commonly called outlet pressure.

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.

Regulating Valve – Valve type used to regulate flows to provide a constant pressure output.

Reliability – A measure of how well equipment will operate without failure.

Relief Valve – A valve which opens at a designated pressure and bleeds a system in order to prevent a buildup of excessive pressure.

Relieving Pressure – The sum of the set pressure and overpressure pressure relief valve.

Repeatability – The closeness of agreement of a regulated value when returned to the same steady-state conditions after upset(s).

Reseat Point – In a relief/backpressure valve which is opened by an increase in inlet pressure, the point where the valve closes.

Resilient – Capable of withstanding shock without permanent deformation or rupture; tending to recover from or adjust easily to misfortune or change. As applied to the seats in some valves.

Restricting Element – The element that restricts and controls fluid flow in a system. In a regulator this element is typically a disk and orifice combination, or plug and cage assembly.

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.

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.

RID – Resistance Temperature Detector. A resistance device used to measure temperature.

Ring Type Joint (RTJ) – A flanged end connection with a circular groove on the mating face, where a softer metal ring is placed before mating up to a similar flange face and bolting up. The softer ring, usually oval or hexagonal, will deform when the flanges are bolted up and create a tight seal. Ring type joint connections are used on higher-pressure piping systems in ASME Class 900 and above.

Rising Stem – A linear valve where the stem rises upon opening past the handwheel and/or yokes. Typical of gate valves of all types.

Risk Based Inspection (RBI)– The process of developing a scheme of inspection based on knowledge of the risk of failure. The essential process is a risk analysis. This is the combination of an assessment of the likelihood (probability) of failure due to flaws damage, deterioration, or degradation with an assessment of the consequences of such failure.

Rockwell Hardness Test – Method of testing and registering a material’s hardness based on the depth of indentation. The higher the number the greater the hardness. This hardness is identified as HRB or HRC depending on the scale used.

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).

RS – Rising stem

RTU – Remote Terminal Unit or Remote Telemetry Unit


Safety Relief Valve – A pressure relief valve for liquid or vapor/gas service.

Safety Shut-off Valve – A valve that is automatically closed by the safety control system or by an emergency device to completely shut off the fuel supply to the burner.

Safety Valve – A pressure relief valve for liquid or vapor/gas service.

Sampling Valve – A valve that is fitted to a vessel or pipeline to allow small samples of a fluid to be withdrawn for further testing.

SCFH – Standard cubic feet per hour is the volumetric gas measurement of flow per hour at standard or at base conditions.

Screwed Bonnet – A valve bonnet with male threads to join a valve body with female threads.

Screwed End Connections – End connections that have female national pipe thread (NPT), which mates with male NPT on a pipe.

Seal Load – For linear valves, the force that must be generated by an actuator on a stem to overcome the various forces acting on the shaft during the opening, closing, or positioning of the closure element.

Seal Weld – The fillet type weld applied to socket weld fittings to prevent leakage.

Seat Leakage – Flow of fluid past a seat or seal when in the closed position.

Seat Load – The contact force between the seat and the valve closure device. When an actuator is selected for a valve, it must be able to generate enough force to overcome static, stem and all hydrodynamics with an allowance made for seat load.

Seat Pressure Differential – The difference between the operating pressure and the set pressure for the system; the set pressure is the higher.

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.

Seat – The portion of the seat ring or valve body which a closure member contacts for shutoff.

Self-Contained Regulator – A valve with a positioning actuator using a self-generated power signal for moving the closure member relative to the valve port or ports in response and in proportion to the changes in energy of the controlled variable.

Set Pressure Range – The range of pressures, specified by the manufacturer, within which the device can be adjusted.

Set Pressure – The inlet gauge pressure at which a pressure relief valve is set to open

Severe Service – A valve application in which the valve will be exposed to excessive conditions such as temperature, vibration or corrosion.

Severe Service Valve – A valve that is designed to survive excessive conditions such as temperature, vibration or corrosion over a prolonged interval. Severe Service Valves (SSVs) have been defined recently by MSS and are one of the three criticalities of valve application severity.

SG – Specific gravity

Shaft – The rod that connects the closure element and the closure operator (handwheel or actuator).

Shutdown Valve – An automatic valve used to isolate a component in a system.

Shut-Off – When the valve is in a closed position and flow ceases.

Shut-Off Valve – The valve to achieve shut-off.

Single Piston Effect (SPE) – The standard sealing principle of a trunnion ball valve whereby the line pressure is used on either the upstream or downstream floating seats to effect a dead-tight seal on either side of the ball.

Single-Acting Actuator – An actuator in which air is applied to one chamber. This air pressure acts against and pushes a plate.

Sliding Gate Valve – A gate valve that has a flat rectangular plate as a closure element. Sometimes called a sluice valve and used for large bore irrigation and waterworks systems.

Slurry – A process fluid that contains undissolved suspended or trailing solids.

Sm3/h – Meters cubed per hour (standard); measurement of volume rate of a gas at atmospheric pressure and 60°F. Also refer to Nm3/h.

Smart Pig (Intelligent Pig) – A device, closely conforming to the pipe bore, which is forced through a pipeline to internally inspect the pipe and measure wall thickness.

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.

Soft Seat – An elastomeric, plastic, or other readily deformable material used either in the valve plug or seat ring to provide tight shutoff with minimal force.

Soft Seat Plug – An elastomer that is placed within a metal ring at the seating area of a globe valve to provide tight shut-off.

Soft Seated – A term used to describe valve trim with an elastomeric or plastic material used either in the valve plug or seat ring to provide tight shutoff with a minimal amount of actuator force. A soft seated valve will usually provide zero seat leakage capability.

Solenoid – An accessory to an actuator that acts as a control device. It can regulate the air supply to an actuator for on-off or throttling of the valve.

Solenoid Valve – Valve, typically of the needle globe type, that is operated by an electrical solenoid. Such valves are often deployed as pilot valves, which is, fitted to actuators that in turn control larger valves.

Sonic Velocity – The speed of sound for a particular gas at a given inlet pressure and temperature.

Sour Gas – Gaseous fuel that contains a relatively large proportion of sulfur or sulfur compounds.

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.

Specific Gravity (SG) – The ratio of weight of a given volume of fluid to the weight of an equal volume of liquid/gas at stated temperature.

Specific Weight – The weight per unit volume of a substance. The same as density.

Speed of Response (Stroking Speed) – The amount of time it takes the valve closure device to travel from completely closed to completely open (0 to 100%).

Spiral Wound Gasket – A gasket that contains hard and soft elements to create a seal. A stainless-steel strip is coiled to create a circular disc with small spaces that are then filled with graphite or another soft non-metallic material. A spiral wound gasket is held between two flanges and bolted up.

Split Body – A valve where the body is split to allow for easy assembly and disassembly.

Spray and Fuse – A type of coating that is applied to the wear and sealing parts of valves, normally ball type.

Spring – In diaphragm actuators, this is the component that applies the force to act against the piston in the chamber. It provides the force necessary to move the closure element to the correct failure position.

Spring Adjustment Screw – A screw used to compress the spring to establish the regulator set point.

Spring Rate (K) – Spring rate is defined by the amount of force required to compress a spring a given distance. Spring rate is given in force/ length (for example, lbf/m).

SSCV – Severe Service Control Valve.

SSCKV – Severe Service Check Valve.

SSIV – Severe Service Isolation Valve.

SSKGV – Severe Service Knife Gate Valve.

SSV – Severe Service Valve.

Stability – The ability to hold a steady controlled variable within the limits of stated accuracy of regulation.

Standard Atmosphere – The accepted normal atmospheric pressure at sea level, equal to 14.696 pounds per square inch.

Standard Barometer – The reading of a barometer for standard atmospheric pressure; equal to 29.92 inches of mercury column.

Standard Class Valves – All valves following the ASME standard B16.34 (except section 8) are designated as standard class.

Standard Gravity – Standard accepted value for the force of gravity. It is equal to the force which will produce an acceleration of 32.17 feet per second per second.

Standard Pressure – The same as standard atmosphere; equal to a pressure of 14.696 pounds per square inch.

Start-Up – The procedure used in starting a plant’s prime equipment and supporting auxiliaries.

Static Line – See Control Line.

Static Pressure – The pressure in a fluid at rest.

Static Unbalance – The force exerted on a valve closure device due to fluid pressure in the non-flowing condition.

Stellite™ – An alloy material used in valve trim known for its hardness, wear and corrosion resistance. Stellite is a trade name of the Deloro Stellite Company.

Stem – The rod that connects the closure element and the closure operator (handwheel or actuator).

Stem Guide – A guide bushing closely fitted to the valve stem and aligned with the seat.

Stoke – The cgs unit of kinematic viscosity. One stoke equals one centimeter squared per second.

Stress Relief – The heating of a substance to a specific temperature to relieve any residual stress.

Stroke – The full travel of a valve from one end of its travel to the other like Open to Closed. The addition of the return stroke equals one cycle – Open to Closed to Open or vice versa.

Supercompressibility – Many gases are more compressible under high pressure at ordinary temperatures than indicated by Boyle’s Law. These gases, measured at the high pressures, will occupy a greater volume when the pressure is reduced to near atmospheric pressure.

Supply Pressure – The pressure at the supply port of a device such as a controller, positioner or transducer.

Swing Check Valve – A check valve with a single plate pivoted at the top and secured to the body of the valve. The flow of the process fluid pushes the plate open and in the event of flow reversal the plate swings to close through gravity and reduction of flow velocity.


Tank Valve – A valve arranged for fitting at the bottom of a tank or process vessel.

Tensile Strength – The maximum amount of force that can be applied to a piece of material, before failure occurs. Also called the ultimate tensile strength, UTS.

Therm – 100,000 BTU.

Thermal Expansion – The increase in volume of a fluid or length of a solid as a result of a change in temperature.

Thermoplastic – A common term for plastic used for piping that loses strength as the temperature rises. Such plastic is used for utilities and fluids of a corrosive nature, usually operating at ambient temperatures.

Thermostat – A device that automatically maintains a predetermined temperature in an appliance or component.

Three Piece – A type of valve that has 3 pieces that make up the body and end caps of the valve, all joined by bolted connections.

Three-Way Valve – A diverter type valve that has three ports and allows the flow path of the process fluid to be switched, or two different flow paths to be combined.

Throttling – Modulating control as opposed to on/off control.

Through-Conduit Gate Valve – A full-bore gate valve that has a very low pressure drop and allows for the passage of pipeline pigs or scrapers for cleaning, de-watering, batching, etc.

Thrust – The force generated by any type of actuator to open, close, or position the closure element of a valve, measured in Pounds or Newtons.

Top Entry – A type of valve where the unit can be serviced or repaired by leaving the body in the line and accessing the internals by removing the top portion of the valve.

Top Mounted Handwheel – An accessory handwheel that is mounted on top of the actuator and used if there is a power failure.

Top Works – Any number of parts that are located above the bonnet of the valve. They could be the yoke, the handwheel, the positioner, the actuator.

Torque – The rotational force (in in/lbs, ft/lbs, Nm) applied to the stem or shaft of a valve.

Torque Seated – A type of valve that required extra effort from the method of actuation, to force the closure device into the seal.

Toughness – A material’s ability to remain undamaged when a force is applied. A tough material will deform first, before failure occurs.

TOV – Triple Offset Butterfly Valve. The TOV is torque seated.

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 amount of linear movement of the valve closure device from the closed position to the rated full-open position.

Travel Indicator – An external, visible device used to indicate the travel of the valve plug.

Trim – The trim of the valve is the parts of the closure element that are exposed to the process flow, also known as the wetted parts.

Triple Eccentric – A type of butterfly valve where the stem is located behind the disc and below the centerline of the disc with the cone axis offset from the centerline of the disc. This type of butterfly is Torque seated and is know as TOV.

Trunnion – A pin or pivot on which something can be rotated or tilted.
Trunnion Mounted Ball Valve – A ball valve, where the closure element ball is supported at the base by a shaft and the seats and seals are energized by the process fluid.

Trunnion Mounting – A style of mounting the disc or ball on the valve shaft or stub shaft with two bushings diametrically opposed.

TT – Total torque

Tubing – Small bore piping used to supply air or hydraulic fluid to an actuator.

Tungsten Carbide – A very hard material applied to the sealing and/or bores of valves, usually by HVOF. Chemical formula WC. It has an as applied hardness of Rc 70-72.

Turbine – A machine/motor that consists of a rotating shaft with blades driven by a fluid or steam used to generate rotary mechanical power.

Turbulence – A flow characteristic that is created when higher velocities and obstructions are experienced in a valve or a process system.

Turndown – A term used to describe the ratio between the minimum and maximum flow conditions seen in a particular system.

Two Piece – A type of valve that has 2 pieces that make up the body of the valve joined by a flanged connection.


Ultrasonic Testing (UT) – A testing method that requires the material to be bombarded with high frequencies to detect inclusions, pits, and cracks within the material. These reflected sound waves will find the depth at which the flaws occur.

Under-Pressure Cut-Off Device – A mechanical device incorporated in a gas pipe work system to shutoff the supply of gas when the pressure at the sensing point falls to a predetermined figure.

Unplanned Outage Hours – Sum of all unplanned outages, start-up failures, maintenance outages and the scheduled outage extensions for maintenance.

Upstream – The process fluid before it reaches the valve.


Vacuum Breaker – A valve used to limit an increase in vacuum. An increase in vacuum (decrease in absolute pressure) beyond a certain value registers on the diaphragm. The valve closure device will open permitting atmospheric, positive pressure, or an upstream vacuum that has a higher absolute pressure than the downstream vacuum, to enter the system and restore to set point.

Vacuum Regulator – A device that maintains a vacuum at a set point. A decrease in this vacuum (increase in absolute pressure) beyond this value registers underneath the diaphragm and opens the valve. This permits the downstream vacuum of lower absolute pressure than the upstream vacuum to restore the upstream vacuum to its original pressure setting.

Valve Body – A pressure retaining housing for internal parts having inlet and outlet flow connections.

Valve Closure Member (Device) – The movable part which is positioned in the flow path to modify the rate of flow through the valve, often made of an elastomer material to improve shutoff.

Valve Data Sheet (VDS) – A data sheet defining the minimum level of a valve, including the materials of construction, design requirements, testing, inspection and certification requirements.

Valve – A mechanical or electromechanical device by which the flow of a gas, liquid, slurry, or loose dry material can be started, stopped, diverted, and/or regulated. All valves only perform two basic functions: Isolation or Control.

Valve Flow Coefficient – See CV.

Valve Linkage – A lever or levers connecting the diaphragm to the valve plug or valve plug stem.

Valve Plug – A movable part which provides a variable restriction in a port.

Vapour Pressure – A pressure at which, for a given temperature, vapour bubbles form in the liquid.

VDC – Volts direct current

Velocity – The speed at which the process fluid passes through the valve.

Vena Contracta – The location where cross-sectional area of the flow stream is at its minimum size.

Vent – An opening in a piping system that can be exposed to the atmosphere and allows fluid to be released.

Venting – Discharge into the atmosphere, allowing excessive or unwanted media to escape as planned.

Viscosity – The resistance of a process fluid to flow. The “thickness” or “thinness.” Highly viscous fluids (thicker) require more energy to move through a piping system.

Visual Examination – Surface examination of a specimen that is carried out with the human eye without any supplementary test.

VOC – Volatile organic compounds

Volume Corrected – The volume metered times metering pressure plus atmospheric pressure/base pressure equals volume corrected.

V-Ring Packing – A stem packing that is V shaped in cross section. Radial forces that are applied will force out the packing radially and create a tight seal against the wall of the packing box and the stem/shaft.


Wafer Design – The construction of wafer design valves allows them to be “sandwiched” between flanged sections of pipeline. The benefit is lower bolting requirements. Commonly used with certain butterfly and check valves and some ball valves..

Wall Thickness – The thickness of the pressure-retaining shell of a valve. It must be designed to satisfy all the necessary tests that the valve will be subjected to during examination.

Water Column – A unit of measurement. The pressure required to support a column of water one inch high. Typically reported as inches w.c. (water column); 27.68-inches of water is equal to one pound per square inch (psi).

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

Water Hammer Effect (Transient) – The reaction when a valve is suddenly closed and a shock wave is transmitted through the piping system. This is generally caused by under sizing of the piping system. It is not only noise, but it can also cause mechanical damage to the piping system and associated equipment.

Wedge Gate Valve – A valve that uses a wedge as the closure device and the surfaces are inclined to the direction of closing thrust so that mechanical force on the stem produces tight contact with the inclined seat rings. This is the most common form of the Gate Valve.

Weir – An obstruction in a diaphragm valve, against which the elastomer liner is compressed to isolate the flow of the process fluid.

Welded Body – A type of valve where the entire body is welded closed such that no process media can leak to atmosphere. Normally used for butt weld valves.

Wellhead Valve – Used to isolate the flow of oil or gas at the takeoff from an oil or gas well. The design is usually a plug or gate valve.

Wide-Open Capacity – If a wide-open failure occurs, this is the amount a control valve or regulator will flow.

WOG (Water, Oil, Gas) – An older standard that did not apply a maximum temperature requirement. Higher temperatures can significantly degrade pressure handling in piping systems thus, CWP is the preferred standard for communicating a fitting or valve’s rated capacity.

WOG – Water, oil, gas

Worm – A self-locking type of gear.


Yield Strength – The force at which a material will begin to deform or stretch.

Yoke – A structure by which the diaphragm case or cylinder assembly is supported rigidly on the bonnet assembly.


Zero Leakage – A term to describe the result of a valve test using API 598, MSS SP-61 or ISO 5208’s resilient seat test or the ultimate isolation state. The latter is the desired ability for most severe service isolation valves. The in-situ performance of this isolation state can more easily be made with resilient seated (soft seat) isolation valves than with metal seated isolation valves, which are all allowed to leak under the above tests.

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