Glossary of Valve Terms

The commonly used terms and definitions related to valves is given below;

Actual pressure drop: The difference between the inlet pressure and the outlet pressure of a valve.

Actuator: A device used to open/close or control the valve. Key types include electrical, hydraulic, and pneumatic. Movement may be quarter-turn or multi-turn.

Actuator stem: A rod used in linear designed valves connecting the actuator with the stem of the valve.

Back seat: 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 quarter-turn valve with a spherical closing element held between two seats. Characteristics include quick opening and good shutoff.


Bellows: A sealing device that prevents line media from leaking between the stem and the body.

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

Breakout torque: The torque required to open or unseat a rotary valve.

Bubble-tight: When there is no measurable seat leakage over a certain period of time during test conditions.

Butterfly valve: A quarter-turn valve, which has a circular disc as its closing element. The standard design has the valve stem running through the center of the disc, giving a symmetrical appearance. Later more complex designs offset the stem so that the disc “cams” into the valve seat.

Bypass valve: A valve smaller in diameter that is fitted in parallel to a larger main valve. Bypass valves are used to reduce the differential pressure across the main valve before this main valve is opened (as otherwise this larger, more expensive valve, may suffer damage to internal components).

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, and wafer check, non-slam (piston type).

Class: The class is used to describe the pressure rating of the piping system. For example, Class 150 lb, 300 lb, 600 lb, 900 lb, or API 3000, API 5000. 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.

Concentric butterfly valve: A butterfly valve with the disc installed in the center of the valve.

Control valve: A valve that regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges. Control valves are normally fitted with actuators and positioners. Pneumatically actuated globe valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball and butterfly valves may also be used.

Cryogenic valves: Valves suited for use at temperatures below 45 degrees Celsius. A cryogenic valve should have a cold box as an integral part of the body to allow a vapor barrier to form between the packing box and the liquified gas.

Cv: The Cv of a valve is defined as 1 U.S. gallon of 60°F water for 1 minute with a 1 psi pressure drop. Also known as the valve coefficient or the flow coefficient.

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.

Direct-acting actuator: A diaphragm actuator that allows the actuator stem to extend.

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.

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 assures 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 semi-circular 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.

Drop tight: A bubble-tight test that involves a water-under-air test.

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.

Elastomer: A polymer that is both flexible and resilient when used as a seal.

End to end: The extremities of the valve. One connection to the other end connection.

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

Fail closed: An actuator facility such that in the event of power failure the valve will move to a fully closed position.

Fail open: An actuator facility such that in the event of power failure the valve will move to the fully open position.

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.

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 a fire test.

Flat face:  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 circumference with holes to accommodate the flange bolting.

Flat gasket: A circular, flat sheet with an inside and outside diameter.

Floating ball: A ball valve where the closure ball is not attached to the body of the valve.

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.

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

Full-bore valve: Any valve where the closure element has the same inside diameter as the inlet and outlet of the valve. Also called a full-port valve and has a lower pressure drop than a reduced bore.

Full closed: The position of the valve when the closure element is fully seated.

Full open: The position of the valve when the closure element is fully open allowing maximum flow through the valve.

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

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.

Gate valve: A multi-turn valve that has a gate-like disc and two seats to close the valve. 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. Gate valves provide robust sealing and are used extensively in the petrochemical industries. This class of valve also includes knife gate valves, conduit gate valves, and wedge gate valves.

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.

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 type of valve is suited to both throttling and general flow control.

High-performance valves: A valve specifically designed for accurate throttling applications.

Hydrostatic test: A test using water under pressure to detect any leaks through the body, sealing joints, or closure element. Generally, this test pressure is 1.5 times the design pressure at ambient temperature.

Integral flange: A flanged connection that is either fabricated or cast to the body of the valve.

Integral seat: A seat that is actually a machined part of the valve body and not one that is inserted into the valve.

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.

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.

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 and used to identify the position of a valve’s closure element.

Linear valve: A valve that has a sliding stem that pushes the throttling element up and down.

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.

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.

Multi-ported:  These 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.

National pipe thread: A tapered thread that is used for pressure connections for piping.

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.

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.

Normally open: A valve that is normally open during operation. In many cases, these valves are locked open by using a mechanical device.

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

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

Packing box: A chamber through which the stem passes. This chamber houses the packing material, packing spacers, lantern rings, guides, and other seal accessories necessary to prevent leakage of process fluid.

Parallel gate valve: A gate valve that has a flat disc gate that slides between two parallel free floating seats.

PEEK: The abbreviation for polyether ether ketone. A robust soft seating material.

Pilot valve: Small valve requiring little power that is used to operate a larger valve.

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.

Plug valve: This multi-turn valve derives its name from the rotating plug that forms the closing element. The plug may be cylindrical or truncated. In the open position, the fluid flows through a hole in the plug. Lubricated plug valves rely on a sealing compound injected between the plug and the valve body, whilst sleeved plug valves are fitted with a “soft” insert between the plug and the body.

Poppet: A closure element in a check valve that is held in place by a spring.

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 sends pressure to, or bleeds pressure from, the valve so that the correct position is achieved.

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.

Psi: The abbreviation for pounds per square inch.

psia: The abbreviation for pounds per square inch absolute. The psia unit is used when the pressure is expressed without taking into account ambient pressure.

Psig: The abbreviation for pounds per square inch gauge. The psig unit is used when the pressure is expressed to standard atmospheric pressure (noted 14.7 psia).

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.

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.

Reduced-port valve: A valve that has a smaller internal bore than those of the inlet and the outlet. A reduced-port (bore) ball valve will have a greater pressure drop than a full-port (bore) ball valve.

Ring type joint: 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, ASME 900 lb and above. The abbreviation is RTJ.

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.

Seating Torque: The torque value produced by a rotary actuator to open or close the valve.

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

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.

Soft seat plug: An elastomer that is placed within a metal ring at the seating area of a globe valve. This will provide a bubble-tight shut-off.

Speed of response: The speed provided by an actuator to operate a valve. Sometimes called the stroking speed.

Split body: Usually refers to a ball valve that comprises more than one piece and houses the closure element.

Stroke: The travel required by a valve, either linear type or rotary type, generally from fully open to fully closed.

Sub-sea valve: A valve that is designed for use in seawater. For example, installed in a pipeline on the seabed.

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 the closed position.

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

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: The regulation of the process fluid by positioning the closure element of the valve between open and closed to create the desire flow regime.

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.

Trim: The trim of the valve is the parts of the closure element that are exposed to the process flow, sometimes called the wetted parts.

Trunnion mounted ball valve: A robust ball valve, where the closure element ball is supported at the base by a shaft. This design is more common on larger valves and higher ratings, because of the weight of the ball.

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 the pipeline. The benefit is lower bolting requirements. Typically used with certain butterfly and check valves.

Water hammer effect: The reaction when a valve is suddenly closed and a shock wave is transmitted through the piping system. This is generally caused by the 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.

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

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


To read about the Butterfly Valves, Please click here

To read about the Ball Valve, Please click here

To read about the Plug Valves, Please click here

To read about the Gate Valves, Please click here

To read about the Globe Valves, Please click here

To read about the Safety Valves, Please click here

To read about the Valve Basics, Please click here


This article is written by

Mr. Goutham Rathinam

(BE-Mechanical & CSWIP 3.1-TWI, UK)             

Email ID:

Sandeep Anand

I am a Mechanical Engineer with more than ten years of work experience in the field of welding and NDT.

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