Valve Glossary

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A

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• Actuator  -  An actuator is a device used to open, close, or control valves.  The most commonly used types are electrical, hydraulic, and pneumatic.
• Actuator Stem  -  A part used to connect the valve plug with the piston actuator.
• Adjusting Screw  -  A part used to compress the actuator spring.
• All Welded Construction  -  Pertains to a valve construction in which the body is completely welded and cannot be disassembled and repaired in the field.
• Anchor Pin  -  A pin welded onto the body of ball valves.  This pin aligns the adapter plate and restrains the plate and gear operator from moving while the valve is being operated.
• Angle Valve  -  A body configuration with one port perpendicular to the other port.
• Approach Channel  -  The passage through which the fluid must pass to reach the operating parts of a pressure relief device,
• Atmospheric Pressure  -  The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth.
• Automatic Control Valve  -  Final control element used in process systems to regulate flow.

B

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• Back Pressure  -  The static pressure existing at the outlet of a pressure relief device due to pressure in the discharge system.
• Back Pressure Valve  -  A control valve that is designed to hold back pressure on a process.  This would be desirable to hold a certain level in a vessel or hold a gas pad above a tank.
• Backseat  -  A Shoulder on the stem of a valve which seals against a mating surface inside the bonnet to permit replacement, under pressure, of stem seals or packing.
• Ball  -  The closing element of a ball valve.
• Ball Check Valve  -   A check valve that is much like a swing check valve but instead of having a gate or guided disk, uses a ball to control flow.
• Ball Valve  -  A quarter turn valve used for changing the direction of a process stream (divert or shut off).
• Barstock Valve  -  Barstock valve has a body machined from a solid metal.
• Bellows Sealed Valve  -  A bellows is a sealing device which prevents line leaking between the stem and the body of a valve.  It is a flexible metal, usually made of stainless steel used to seal the stem through the shafts.
• Bevel Gear Operator  -  Device facilitating operation of a gate or globe valve by means of a set of bevel gears having the axis of the pinion gear at right angles to that of the larger ring gear.  The reduction ratio of this gearsetdetermines the multiplication of torque achieved.
• Bibb  -  A small valve with the outlet turned down.
• Bleed Valve  -  It is located at the bottom of the pipe and can be used to drain the line after hydrotest, confirmation of isolation in a double block and bleed set up, testing of a check valve.
• Blowdown  -  A blow down is the emptying or depressuring of a pipeline or vessel.  This is the difference between set pressure and re-seating pressure in a valve.
• Blowdown Pressure  -  The value of decreasing inlet static pressure at which no further discharge is detected at the outlet of a pressure relief valve after the valve has been subjected to a pressure equal to or above the popping pressure.
• Body  -  The main casing container used for the flow of fluids and to hold the moving parts and seals of the valve.
• Body Rating (actual)  -  The interaction between being safe, acceptable flowing fluid pressure and temperature and the body, excluding all internals.
• Body Rating (normal)  -  The theoretical pressure rating of the body excluding all internals.
• Bolt Tightening Procedure  -  When tightening, always use the correct sequential bolt order for the flange.
• Bolted Bonnet  -  A bonnet that is connected to the body with bolts.
• Bolted Construction  -  Describes a valve construction in which the pressure shell elements are bolted together, and thus can be taken apart and repaired in the field.
• Bonnet  -  The top part of a valve that is attached to the body that contains guids, packing, and stem.
• Bonnet bolts  -  Bolts that hold the bonnet to the body.
• Bottom Flange  -  A flange at the bottom of a valve or opposite the bonnet.
• Breaking Pin  -  A non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the breakage of a load-carrying section of a pin which supports a pressure containing member.
• Breaking Pin Housing  -  The structure which encloses the breaking pin mechanism.
• Breaking Pressure  -  The value of inlet static pressure at which a breaking pin or shear pin device functions.
• Bubble Tight  -  A commonly used term to describe the ability of a control valve or regulator to shut off completely against any pressure on any fluid.
• Built-up Back Pressure  -  Pressure existing at the outlet of a pressure relief device caused by the flow through that particular device into a discharge system.
• Burst Pressure  -  The value of inlet static pressure at which a rupture disc device functions.
• Butterfly Valve  -  A quarter turn valve (90° or less) with a circular disk as its closing element.

C

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• Cavitation  -  The creation and collapse of bubbles in a liquid.Chain Wheel Operated Valve  -  An overhead valve operated by a chain drive wheel instead of a handwheel.
• Chatter  -  Abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc contacts the seat.
• Check valve  -  These valves are designed to allow the process fluid to flow in only one direction to prevent backflow.
• Check Valve Standards  -
• Choke Valve  -  Valves used in severe service valves designed for oil and gas applications.  They are used for controlling the flow rates or pressure on production, steam or water injection.
• 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.  This means that higher pressure drops allow more fluid to go through the valve.
• Constant Back Pressure  -  A superimposed back pressure which is constant with time.
• Cold Box Bonnet  -  A hollow extended bonnet which allows stagnated, moderate temperature gas to form, protecting the packing from cryogenic temperatures.
• Cold Differential Test Pressure  -  The inlet static pressure at which a pressure relief valve is adjusted to open on the test stand. This test pressure includes corrections for service conditions of superimposed back pressure and/or temperature.
• Cold Rating  -  The maximum pressure that a valve or fitting is designed to withstand at room temperature.
• Clapper  -  The hinged closure of a check valve.
• Close-off Rating  -  The maximum pressure drop at which a valve can take without leaking when completely closed.
• Closing Pressure  -  The pressure at which a valve closes or reseats.
• Control Valve  -  This valve controls the flow capability by changing the flow passage size as directed by a signal from a controler.
• Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.
• Crack a Valve  -  To barely open a valve so that it leaks just a little.
• Cycle  -  A single complete operation or process returning to the starting point.  A valve, stroked from full open to full close and back to full open, has undergone one cycle.
• Cdylinder Operator  -  A power-piston valve operator using either hydraulic or pneumatic pressure. A sealed piston converts applied pressure into a linear piston rod (stem) motion.

D

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• Design Pressure  -  The maximum pressure at which a valve can operate.
• Design Temperature  -  The temperature that is used to determine allowable stresses for the purposes of design.
• Diaphragm  -  A round, thin flexible sealing device secured and sealed around its outer edge - and sometimes around a central hole in the diaphragm - with its unsupported area free to move by flexing.
• Direct Pressure  -  Pressure applied by the flow against the back of the closure and/or opposite the seat end of the valve.
• Discharge Channel  -  The passage through which the fluid must pass between the operating parts of a pressure relief device and its outlet.
• Disk  -  A valve assembly part that comes in contact with the seat to close off the flow.
• Disk Stroke  -  When the valve stem moves from a closed to open position.
• Double-acting  -  Term used to describe supplying and exhausting air to both sides of the pistonsimultaneously.
• Double Action Pneumatic Actuator  -  Require air to be used to move the piston in either direction.  A solenoid valve is used to control the air flow into and venting of the actuator.  Since there is no spring force to overcome, a smaller actuator can often be used.
• Double Block and Bleed  -  When under pressure the valve has the capability to maintain a seal across both the upstream and downstream seat rings and have the body cavity bleed down to atmospheric pressure.
• Double Top-guided  -  A packing box configuration which uses two widely spaced guides, one at each end of the packing box, to guide the plug.
• Drain Plug  -  A plug at the bottom of a valve, when removed it drains and flushes the body cavity.
• Droop  -  A drop in set (outlet) pressure of a regulator or control valve due to the travel of its valve or poppet, as the required flow increases from low to maximum.  A slight change in the control spring length due to the valve travel, will result in spring force variations, translating into a change of set (outlet) pressure.

E

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• Electric Actuator  -  It is mounted on a valve and receive a signal that moves the closing element to the desired position.
• Eccentric Ball  -  Similar to a ball valve except it usually contains a segmented ball or half ball.
• Eccentric Plug  -  Has an vertical rectangular opening in the body of a plug valve where the closure is rotated and seated against.
• Emergency Sset Seal  -  To obtain tight shut off in an emergency situation, a sealant can be injected into a specially designed groove in the seat rings.
• Equal Percentage  -  A term used to describe a type of valve flow characteristic where, for equal increments of valve plug travel, the change in flow rate with respect to travel may be expressed as a constant percent of the flow rate at the time of the change.  The change in flow rate observed with respect to travel will be relatively small when the valve plug is near its seat and relatively high when the valve plug is nearly wide open.
• Expanded Outlet  -  A body configuration with standard trim gallery and oversized ports.
• Expansion Valve  -  A valve that maintains the pressure difference between the high-pressure and low-pressure sides in a condenser/evaporator system.
• Extended Bonnet  -  A longer than normal bonnet which protects the packing and actuator from extreme heat or cold.
• Extension  -  Equipment used to provide operating gear accessibility to a below grade valve.

F

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• Face-to-face  -  The distance from end to end of the valve.  These dimensions are goverened by ANSI specifications.
• Facing  -  The contact surface finish of a flange.
• Fail-safe  -  A system designed to open, close, or hold in last position should the air supply fail.
• Flange  -  A bolted connection where two pieces of pipe, equipment, fittings or valves are connected together to form a piping system.
• Flange Butterfly Valve  -  A butterfly valve with a flange on each end.
• Flange Dimension Tolerances, ASME B16.5 (in)  -
• Flange Safety Spray Shield  -  Spray shields also known as flange guard, are used on equipment (flanges in this case) where fluids are under pressure and need to be prevented from leaking or spraying on other equipment or people.
• Floating Ball  -  A closing element of a ball valve designed not to be held rigidly on a rotational axis but floats free between the seat rings.  When under fluid pressure the ball will be pushed against the downstream seat.
• Floating Ball Valve  -  Used in low pressure or small bore processes.
• Flow Coefficient  -  Can be described as the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.
• Flow Rate  -  The amount of fluid that flows in a given time past a specific point.
• Flow Rating Pressure  -  The inlet static pressure at which the relieving capacity of a pressure relief device is measured.
• Flutter  -  Abnormal, rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc does not contact the seat.
• Force  -  The push or pull of an object resulting in a change from rest or motion.
• Four-way (four-port)  -  These closure elements come with two seperate passageways in L shapes.  Each passageway connects to two different lines.
• Friction  -  The mechanical resistance to the relative movement of two surfaces.
• Full Opening  -  Describes a valve whose bore (port) is nominally equal to the bore of the connecting pipe.
• Full Port (full bore)  -  A valve port opening that is equal to the pipe size or valve opening.

G

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• Gas  -  The able to be compressed to fit a confined space and expanded when released.
• Gasket  -  A seal between two components, usually bolted.
• Gate Valve  -  One of the most frequently used valve in piping systems and is classified as either "rising-stem" or "nonrising-stem" valves.  The rising-stem gate valve has the stem attached to the gate, both gate and stem rise and lower together as the valve handwheel or actuator turns the stem.  In the nonrising-stem gate valves the stem is threaded into the wedge, rising and lowering the wedge.
• General Valve Standards  -
• Gland  -  The component that is used to compress the gland packing.
• Gland flange  -  A part used to retain and control the compression force on the packing on the bonnet.
• 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.
• Gland Plate  -  The plate in a valve which retains the gland, gland bushing or stem seals and sometimes guides the stem.
• Globe Valve  -  A type of valve used for regulating fluid flow, both on/off and throttling, it is a control valve.  The name globe valve comes from the globular shape of the valve body.
• Grease Fitting  -  A fitting through which lubricant or sealant is injected.

H

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• Handlever  -  A device attached to rotary-motion valve with preset positions to manually position, open or closed, the valve.
• Handwheel  -  A wheel operator used to turn the stem and open or close the closure elements.  Handwheels can be found on ball, gate, and globe valves.
• Hard Facing  -  A surface preparation in which an alloy is deposited on a metal surface usually by weld overlay to increase resistance to abrasion and or corrosion.
• High Performance Butterfly Valve  -  HPBV is not a butterfly valve but its function and dimensions are equivalent.
• Holiday  -  A discontinuity in painted or coated surfaces.
• Huddling Chamber  -  The annular pressure chamber located beyond the valve seat for the purpose of generating a popping characteristic.
• Hydrostatic Test  -  Pressure tests that are carried out on every valve when built to test the integrity of the pressure-containing parts.

I

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• Inhibitor  -  Can reduce the corrosion rate by presenting a protective film.
• Iinlet Port  -  That end of a valve which is connected to the upstream pressure zone of a fluid system.
• Inner Seat Ring  -  The inner part of a two-piece valve seat assembly.
• Instrumentation  -  The ability to control a process by measuring the value or the parameters set for the process.

J

K

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• Key Stop  -  A method of restricting the travel of a ball valve from fully open to fully closed. The stem key bears against the ends of an arc machined in the adaptor plate.

L

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• Leakage  -  The amount of fluid that will pass through a control valve when it is fully closed at a given pressure and temperature.
• Leak Test Pressure  -  The specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure.
• Lever  -  A device used for quarter-turn valves.
• Lift Check Valve  -  A check valve that is very much like a piston check valve where the flow lifts a disk when the valve is open.  The upstream side of the lift check is under the disk and the flow flows up through the disk.  The disk is guided to keep the motion to be perpendicular to the line flow direction.
• Lifting Device  -  A device for manually opening a pressure relief valve by the application of external force to lessen the spring loading which holds the valve closed.
• Lifting Lugs  -  Lugs provided on large ball, gate, and check valves, for lifting and positioning valves. Also called lifting eyes.
• Limit Stop  -  A device attached to the actuator used to limit or restrict the motion of the actuator.
• Limit Switch  -  A switch used to indicate a valve's open, closed, or intermediate position.
• Liquid  -  A specific volume and can hold any shape it is contained within.
• Loading Pressure  -  The pressure used to position a pneumatic actuator. It is the pressure that is actually applied to the actuator diaphragm or piston.  It can be the instrument pressure if a valve positioner is not used or is bypassed.
• Lockout/Tagout  -  Lockout/tagout procedures can vary significantly from project to project.  For this reason everyone working on a project should know the policies for equipment lockout/tagout to create a safe work place.
• Lock Closed  -  Used on valves that must be closed during normal operating conditions.
• Lock Open  -  Used on valves that must be open during normal operating conditions.
• Lock-up System  -  A system used to hold the actuator in last operating position upon air failure.
• Lubricator  -  A device that supplies lubricant to the packing box.
• Lug Butterfly Valve  -  Connects directly to the flanges by means of a lug or machine bolt.

M

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• Marked Breaking Pressure  -  The value of pressure marked on a breaking pin device or its nameplate.
• Marked Burst Pressure  -  The value of pressure marked on the rupture disc device or its nameplate or on the tag of the rupture disc and indicates the burst pressure at the coincident disc temperature.
• Marked Pressure  -  The value or values of pressure marked on a pressure relief device.
• Maximum Pressure  -  The maximum pressire limitations of the fluid that a valve can withstand.
• Maximum Temperature  -  The maximum temperature limitations of the fluid that a valve can withstand.
• Measured Relieving Capacity  -  The relieving capacity of a pressure relief device measured at the flow-rating pressure, expressed in gravimetric or volumetric units
• Metal-to-metal Seat  -  The seal produced by metal-to-metal contact between the sealing face of the seat ring and the closure element, without benefit of a synthetic seal.
• Mixing Valve  -  Has two inlet ports and one common outlet port.
• Multiport  -  These valves can either be three-way or four way valves.

N

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• Needle Valve  -  A valve with an adjustable tapered point which regulates the rate of flow.
• Non-reclosing Pressure Relief  -  A device designed to remain open after operation. A manual resetting means may be provided.
• Non-rising Stem  -  A gate valve having its stem threaded into the gate. As the stem turns, the gate moves, but the stem does not rise. Stem threads are exposed to line fluids.
• Normally Closed  -  A valve that is designated on the P&IDs as normally closed, is one that is closed during normal operating conditions.

O

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• Offset Globe Body  -  Body configuration with inlet and outlet ports parallel but offset.  They are not in-line.
• On-off Valve  -  A type of valve that only has two positions, either open or closed and can not be a throttling valve.
• Opening Pressure  -  The value of increasing inlet static pressure of a pressure relief valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling, or hearing.
• Operator  -  A device which converts manual, hydraulic, pheumatic or electrical energy into mechanical motion to open and close a valve.
• OS&Y (outside screw and yoke)  -  A valve design in which the stem threads are above the packing gland or outside the valve body and there is a yoke to support the top or outer end of the stem.
• Overpressure  -  A pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure.

P

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• Packing  -  Material uesd to seal the valve from leaking around the plug stem.
• Packing Box  -  Internal bore of bonnet which contains guiding and packing.
• Pig Valve  -  These valves are suited for the handling of slurries, liquids with large amounts of suspended solids, and systems that convey solids pneumatically.  They consists of a sleeve molded of rubber or other synthetic material and a pinching mechanism.  The sleve can be external or internal inside a body.
• Piping and Instrumentation Diagram  -  In the design process, a piping and instrumentation diagram, abbreviated as P&ID, is created following the Block Flow Diagram and Process Flow Diagram.
• Piping and Instrumentation Diagram/how to  -  It is important that the Piping and Instrumentation Diagram contains all the information for the process because this is the basis for the Piping Design and HazOP.
• Piping Geometry Factor  -  The pressure and velocity changes caused by fittings such as bends, expanders, reducers, tees, and Y's if directly conected to the valve.
• Piston  -  Part used to seperate two air chambers of piston actuator.
• Piston Actuator  -  Double-acting actuator design with piston inside actuator.
• Piston Effect  -  The sealing principle involved in utilizing line pressure to effect a seal across the floating seats of some valves.
• Plug Valve  -  A plug valve is a quarter turn valve that has a conical or cylindrical plug that is rotated within the valve body to stop flow when needed.  Plug valves function similar to a ball valve where the plug has an opening in the plug that allows fluid to flow through the valve when it is open.
• Poppet valve (mushroom valve)  -  An oval, round, tapered plug, or disk shaped end with a shaft, used to control the closing and opening for gas and vapor.
• Popping Pressure  -  The value of increasing inlet static pressure at which the disc moves in the opening direction at a faster rate as compared with corresponding movement at higher or lower pressures. It applies only to safety or safety relief valves on compressible-fluid service.
• Port  -  A passageway that allow fluid to flow through the closure element / valve.
• Positioner  -  A device that attaches to a valve actuator that receives an electronic or pneumatic signal and moves the valve to a certain position.
• Position Indicator  -  Any external device which helps you get a clear visually view of whether a valve is in the closed and open position.
• Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.
• Pressure Containing Member  -  A part which is in actual contact with the pressure media in the protected vessel.
• Pressure Differential  -  The maximum difference in pressure measured between the valve inlet and outlet, against which the valve is required to operate.
• Pressure Loss  -  The difference in upstream and downstream pressure.
• Pressure Relief Valve  -   Relief valves removes excessive pressure from a system by limiting its pressure level to a safe level.  The valve opens gradually during normal operation to maintain an optimal pressure level inside the vessel.
• Pressure Retaining Member  -  A part which is stressed due to its function in holding one or more pressure containing members in position.
• Pressure Safety Valve  -  Also known as a pressure relief valve and is used to protect vessels and tanks from overpressure.
• Pressure Valve  -  Both the relief and safety valves work to release excess pressure, but each go about it is a little different.
• Primary Pressure  -  The pressure at the inlet in a safety, safety relief, or relief valve.

Q

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• Quarter Turn  -  A shut-off valve that turns one-quarter on the center vertical axis to close or open the valve.
• Quick Opening  -  A two-way valve that creates the maximum flow capacity as soon as the stem raises.

R

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• Rachet Drive  -  A shaft or valve that is operated by means of a ratchet mechanism. The ratchet delivers an intermittent stepped rotation through a gear in one direction only.
• Rated Lift  -  The design lift at which a valve attains its rated relieving capacity.
• Rated Relieving Capacity  -  That portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relief device.
• Reduced Port (reduced bore)  -  A valve port opening that is smaller than the pipe size or valve opening.
• Reduced Port Ball Valve (standard or regular)  -  A reduced bore valve has a smaller bore diameter in the ball than the internal diameter of the pipe.
• Regulating Valve  -  A valve that controls the flow rate or pressure in a process.
• Relieving Pressure  -  Set pressure plus overpressure.
• Resilient Seat  -  A valve seat containing a soft seal, such as an o-ring, to assure tight shut-off.
• Reverse Flow  -  When the flow moves in the oposite direction of the norman system operation.
• Reverse Pressure  -  The pressure applied against the face of a valve closure.
• Reynolds Number  -  Measures the ratio of inertial forces (forces that remain at rest or in uniform motion) to viscosity forces (the resistance to flow).
• Rim Pull  -  The force required at the edge of the handwheel to generate the required torque at the center of the handwheel.
• Rising Stem  -  A stem that rises as the valve is opened.
• Rupture Disc  -  A non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc.
• Rupture Disc Holder  -  The structure which encloses and clamps the rupture disc in position.

S

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• Seat  -  The inner surface of the valve body which creates a tight shut-off.
• Seat Load  -  The contact force between the seat and the valve disk.
• Seat Retainer  -  Trim component that clamps the seat ring in place.
• Seat Ring  -  Creates a tight shut-off surface inside the body for the disk.
• Secondary Pressure  -  The pressure existing in the passage between the actual discharge area and the valve outlet in a safety, safety relief, or relief valve.
• Service Classification  -  A service classification or service class is a one to three letter designation that shows what service the line is in.
• Set Pressure  -  The set pressure is the point where a pressure relief valve (PRV) is set to start opening.
• Shaft  -  Component in a rotary valve that connects the closure member to the actuator.
• Shut-off Pressure  -  The differential pressure against the closure when closed.
• Shut-off Valve  -  A block valve that only has two positions, either open or closed and can not be a throttling valve.  Sometimes used for emergeny shutdown of the system.
• Slam Retarder  -  A device designed to prevent the clapper of a check valve from slamming as it closes upon flow reversal.  Hydraulic damping cylinders, rotary vanes, and torsional springs are all used for this purpose.
• Specified Burst Pressure (of a rupture disc device)  -  The value of increasing inlet static pressure, at a specified temperature, at which a rupture disc is designed to function.
• Spring Button  -  A part that prevents movement of the actuator spring and permits the adjustment screw to compress the spring.
• Spring Return Pneumatic Actuator  -  Use compressed air to move the piston in one direction, and a spring to push it back in the other when the air is stopped and allowed to vent.  This requires a larger actuator since it has to overcome the force of the spring in addition to providing enough torque to operate the valve.
• Spur Gear  -  The simplest of gears.  In a gear set, the input spur gear and output spur gear are aligned on parallel shafts.  An idler gear may be used to the direction of rotation on the two shafts is in the same direction.
• Start-to-leak Pressure  -  The value of increasing inlet static pressure at which the first bubble occurs when a pressure relief valve is tested by means of air under a specified water seal on the outlet.
• Stem (shaft)  -  A shaft that is connected to the handwheel or gear operator and then to the closing element of the valve.
• Stem Extension  -  An extension of the valve stem used to provide operating gear accessibility to a below grade valve.
• Stem Guide  -  A guide bushing closely fitted to the valve stem and aligned with the seat. Good stem guiding is essential to minimize packing leakage.
• Stem Nut  -  A threaded nut that surrounds the valve stem threads and causes the stem to move when rotated.
• Stop Check Valve  -  It is essentially, two valves built into one.  It can act as a globe valve to isolate or control the flow rate.  It also acts as a check valve by preventing reverse flow.  This is critical to prevent boiler flooding by preventing steam and condensate from back flowing into the generato
• Stop Collar  -  The collar on a ball valve which restricts the ball to 90° of rotation from the fully open to the fully closed position.
• Stop Valve  -  Used to shut off or partially shut off the flow of liquids.
• Stress  -  The force per unit area of cross-section.
• Stroke  -  The distance the valve piston, plug, or stem, moves from a fully closed to fully open position or vice versa.
• Stuffing Box  -  The chamber surrounding the stem where it emerges from a pressurized space used to contain the packing.
• Superimposed Backpressure  -  The pressure that is on the discharge side of a pressure relief valve (PRV) when the device is required to operate.
• Supply Pressure  -  The pressure at the supply port of a device such as a controller, positioner or transducer. 
• Swing Check Valve  -  It is inexpensive and simple to operate and repair.  A check valve that has a disk that pivots about a hinge pin.

T

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• Temperature  -  Normally described as the amount of heat or cold, but it is neither heat or cold.
• Tension Strength  -  The capacity of a material to resist a force tending to stretch it.
• Theoretical Relieving Capacity  -  The computed capacity expressed in gravimetric or volumetric units of a theoretically perfect nozzle having a minimum cross-sectional flow area equal to the actual discharge area of a pressure relief valve or relief area of a non-reclosing pressure relief device.
• Three-way (three-port)  -  These closure elements come in T or L shaped passageways.  The passageway connects to two or three different lines.
• Throttling  -  The intentional restriction of flow by partially closing or opening a valve.  A wide range of throttling is accomplished automatically in regulators and control valves.
• Tilting Disc Check Valve  -  A check valve where the disc has a pivot point at the center of the disc.  It is also known as a tilt check valve or a tilting-disk check valve.  When flow is moving forward, the valve easily opens and when there is flow reversal, the valve immediately closes.
• Torque  -  The rotational force used to move a shart.
• Transducer  -  A device used to transfer an electrical signal to a pneumatic signal.
• Transfer Case  -  Component in a ball and butterfly valve that converts linear actuator motion to rotary shaft motion. 
• Trim  -  Valve trim is the inner components of the valve that are exposed to the process fluid.  Because they are exposed to the process, they are more likely to wear out than other components like the valve body or the bonnett.  Because of this, valve trim is usually designed to be removed and replaced when necessary.
• Trunnion  -  That part of a ball valve which holds the ball on a fixed vertical axis and about which the ball turns. The torque requirement of a trunnion mounted ball valve is significantly less than that for a floatingball design.
• Trunnion Ball  -  You can tell the difference between the floating and trunnion ball valve by the fixed shaft on the bottom of the body and grease injection fitting.
• Trunnion Ball Valve  -  Trunnion design is simply the use of upper and lower supports to retain the ball under pressure.
• Turns to Operate  -  The number of complete revolutions of a handwheel or the pinion shaft of a gear operator required to stroke a valve from fully open to fully closed or vice versa.
• Two-way (two-port)  -  These closure elements come in straight through passageways.  The passageway connects to one line.

U

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• Union Bonnet  -  A type of valve construction in which the bonnet is held on by a union nut with threads on the body.

V

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• Vacuum  -  A contained space having little or no matter or a volume having a pressure lower than the outside atmospheric pressure.
• Vacuum Support  -  An auxiliary element of a rupture disc device designed to prevent rupture or deformation of the disc due to vacuum or back pressure.
• Valve Flow Coefficient Cv / Kv  -
• Valve Installation Guide  -  The valve should be inspected to make sure the opening is free from dirt and foreign matter.
• Valve Selection Guide  -
• Valve Service Guide  -
• Valve Sizing for Gas and Steal  -
• Valve Sizing for Liquid  -
• Vapor  -  The gas state of a liquid or solid.  Both vapor and gas are basically the same thing.
• Variable back Pressure  -  A superimposed back pressure that will vary with time.
• V Port  -  The ball closure element has a V shape opening turned on its side.  This allows the closure to be opened and closed so to control the flow much better.
• Velocity  -  The rate of change or displacement with time. 
• Vena Contracta  -  The point immediately downstream from an orifice or valve seat which is the point of lowest pressure and highest velocity.
• Venturi Valve  -  A reduced bore valve.  A valve having a bore smaller in diameter than the inlet or outlet.
• Viscosity  -  The measure of the internal friction/resistance to the flow of a liquid. 
• Volume Booster  -  A device used to increase the actuator stroking speed.
• Volumetric Flow Rate  -  The amount of fluid that flows in a given time past a specific point.
• Volume Tank  -  Air tank used to supply additional air to stroke the valve upon air failure.

W

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• Wafer Butterfly Valve  -  A valve that is sandwiched between two flanges.
• Water Hammer  -  Occurs when a valve is suddenly opened or closed.
• Wedge  -  The closure element of a gate valve.  A wedge is opened and closed by the stem.
• Working Pressure  -  Working pressure, abbreviated as WPR, is the normal pressure that a system operates at.

X

Y

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• Yield Strength  -  The minimum stress that leads to permanent deformation of the material.
• Yoke  -  A bracket to support the top or outer end of the stem and stem bearing used in gate and globe valves.

Z