Glossary of Terms
ac: Alternating current or voltage that changes direction and polarity rhythmically. The most common alternating current used in industry is sinusoidal.
Accessory: A device attached to an actuator to provide a special function not normally handled by the actuator.
Accumulator tank: A pressure vessel used to store compressed gas (i.e.: air) to provide standby energy in the event of loss of system pressure.
Acme threads: A common thread pattern used to thread stems on gate and globe valves. U.S. acme threads feature 29o angles and flattened tops, with the thread height identical to the thread width. (Metric acme threads feature 30o angles.)
Actual pressure drop: The actual or measured difference in pressure between the inlet pressure and the outlet pressure of a valve.
Actuation system: An actuator assembly that can be installed on a manual valve and used for either on-off or throttling applications.
Actuator: A device that provides the force to open, close, or throttle a valve.
Actuator barrier: A dynamic actuator part that separates the actuator chamber from the atmosphere or another actuator chamber. (Such as an elastomer diaphragm.) When the actuator barrier is acted upon by the medium in the chamber(s), the actuator moves.
Actuator stem: A rod used in linear valves to connect the actuator barrier with the stem of the valve.
Actuator stem force: The amount of force (thrust or torque) that an actuator generates.
Adjusting screw: A bolt used to compress the spring in a cylinder actuator, or limit the actuator movement.
Advanced cavitation: The point where cavitation is at its maximum level.
Air consumption: The amount of air used by pneumatic equipment.
Air manifold: Enclosure containing inlet and outlet ports that route the flow of air to the active piston or diaphragm areas.
Air pressure: Available air supply pressure at the device (actuator or other pneumatic equipment); maybe less than compressor setting.
Airset: A device used to limit the air supply to an actuator. Also known as a pressure regulator.
Air spring: A fail-safe system for actuators that employs a locked-up volume of air to drive the actuator to the failure position.
Air usage: The amount of air used to stroke a pneumatic actuator.
Allowable pressure drop: The maximum pressure drop acceptable in the process. This pressure drop is used in calculating a valve size. In applications where choked flow occurs, the allowable pressure drop may be less than the actual pressure drop in some valves.
Alloy steels: A special formula of iron combined with other elements (nickel, vanadium, chromium, or manganese) to produce unique characteristics. Alloys are used to avoid corrosion or erosion by certain processes.
Ampere: Unit of measurement of electron flow or current. The amount of electricity used by the equipment at rated voltage.
Analog: Measurement system where the units and signals bear a direct linear relationship to functions.
Angle valve: A valve-body style where the first port is perpendicular to the second port.
ANSI: The abbreviation for the American National Standards Institute.
API: The abbreviation for the American Petroleum Institute.
Arithmetic average roughness height: A measurement of the smoothness of a particular surface. Usually expressed in microinches, AARH is sometimes designated as “Ra.” As AARH or Ra measurement decreases, the surface becomes smoother.
Atmospheric resistor: An attenuator trim installed outside the valve in a venting application. Also referred to as an external stack.
Attenuation: The capability or process of lowering the sound pressure level radiating from a valve.
Attenuation plate: An anti-noise device installed downstream from a valve in a gaseous service. Attenuation plates normally use staged pressure reduction to reduce sound levels.
Attenuation trim: A trim installed inside a valve that uses a tortuous path, multiple holes, expanding flow area, or sudden expansions and contractions to lower the sound pressure level generated by a valve.
Auto-ignition temperature: The surface temperature of a device that will cause a flammable material or atmosphere to ignite.
Automatic control system: Any system that can regulate a process without user involvement.
Automatic system: System that will function according to predetermined requirements without the intervention of human surveillance.
Automatic control valve: A final control element that receives a signal from a process controller to regulate the pressure, flow, or temperature of a liquid or gas process. Also known as a control valve.
Autotransformer: Static transformer having a single winding, part of which is common to both primary and secondary circuits.
Auxiliary handwheel: A special handwheel attached to an actuator to allow for manual operation of the valve.
Auxiliary switches: Auxiliary circuit switches fitted and mechanically coupled to switching devices for the purpose of indicating the position of a valve or the state of the main contacts.
Average wall thickness: The dimension of the ideal wall thickness, which includes a tolerance below and above that dimension. For example, an average wall thickness of 0.625 in with a +/-0.01-in tolerance would be between 0.615 to 0.635 in.
Backflow: Process flow direction that occurs opposite the normal or expected flow direction.
Back-seat: In linear valves, a special design that permits a portion of the stem or plug head to seat against the bottom of the bonnet to help prevent process flow from migrating into the packing box.
Ball: A spherical part in quarter-turn rotary valves that rotates against the valve seat, allowing flow to pass when the opening in the ball is exposed.
Ball, V-notch: Flow-controlling member for a popular style of throttling ball valve. The V-notch ball includes a polished or plated partial-sphere surface that rotates against the seat ring.
Ball valve: A quarter-turn valve that features a spherical closure device (ball). As the ball moves radially across the seat, the opening in the ball is exposed; this allows the flow to move through the valve.
Bearings: Cylindrical supports located around a valve shaft to provide support.
Belleville disc spring: A cone-shaped metal washer used to produce a constant load to the packing box without retightening. Also known as Belleville washers.
Bimetal strip: A strip made up of two kinds of metal with different temperature coefficients. It is enclosed by a current winding. The heat due to an overload causes the strip to deflect and to actuate a tripping bar which, in turn, acts on the auxiliary circuit switch of the release or relay. Used in overload relays and overcurrent releases.
Black box: A device that converts input signals to appropriate output values without the need for the observer to know the internal workings of the device.
Block valves: Manual valves that are used to start or stop the process flow. Common block valves include gate, quarter-turn plug, ball, butterfly, and tank-bottom valves.
Blowdown: The discharge of process fluid in a pressure-relief valve when the upstream process pressure exceeds the valve’s set pressure.
Body: The major pressure-retaining component of a valve that houses the closure device, as well as the inlet and outlet ports.
Body end connection: The part of a valve that matches and joins with the mating piping. Also referred to as end connection.
Bolt circle: The diameter of a circle where the centerlines of bolt holes interconnect-usually refers to the design of end and bonnet flanges.
Bonnet: A pressure retaining part that houses the packing box and guides, as well as seals the top-works of a valve body.
Bonnet cap: A pressure-retaining part that seals the top-works of a valve body and usually does not contain a packing box. Also known as a top cap.
Bonnet flange: The flange used to attach the bonnet to the body.
Bonnet flange bolting: Fasteners used to secure the bonnet flange and bonnet to the valve body.
Bottom flange: In reverse-acting control valves, a special cap mounted on the bottom of the valve body that allows access to the trim.
Bottom port: In angle valves, the port concentric with the rising stem.
Breakaway (unseating) torque: The turning force required to initiate the opening stroke of a valve.
Breakout torque: See Breakaway torque.
Brinell hardness number: A number between 111 and 745 that indicates the relative hardness of a material. As the number increases, the material is designated as harder.
Brinell hardness test: A standard hardness test that uses an indentor to apply a standard load to a material. The resulting indentation determines the Brinell hardness number.
Bubble-tight: A condition where no measurable seat leakage occurs through the closure element of a valve during a certain amount of time. The typical bubble-tight test involves air-under-water testing.
Bushing: See Bearing.
Butterfly valve: A quarter-turn rotary valve design that that incorporates a circular disc to provide shutoff or modulating flow through a pipe system. Butterfly valves usually have a narrow body face-to-face.
Butt weld end connection: An end connection used to prepare a valve for welding into the pipe line.
Bypass valve: A manual valve used in a bypass line that circumvents a larger valve (usually a control valve). A bypass line usually involves using one or more manual valves, which block the flow upstream of the main valve and detour the flow on the downstream side.
Cage: A cylinder contained inside the flow gallery of a globe valve body that is used to guide the plug and possibly retain the seat ring. Special designed holes in the cage can be used to provide the flow characteristic or control the pressure drop in order to manage high velocities, cavitation, flashing, or high sound pressure levels.
Calibration: The correct adjustment of a device to ensure operation within the preferred parameters.
Capacity: The amount of flow that can pass through a valve under certain conditions, without the valve choking. Also referred to as flow capacity or valve capacity.
Capacitor: Device having two conductive plates separated by a dielectric insulation substance and thus possessing the property of capacitance
Carbon steel: Iron-based metal that contains 0.1 to 0.3 percent carbon. Carbon steel is a base steel from which alloys are created by mixing other metals with it.
Cartridge: An anti-cavitation retainer or cage used inside a linear motion valve.
Cavitation: A situation in liquid services when the pressure at the vena contracta falls below the vapor pressure, followed by a pressure recovery above the vapor pressure. The pressure reduction below the vapor pressure at the vena contracta causes vapor bubbles to form, which then collapse as the pressure recovers. This implosion of the vapor bubbles can erode surfaces in both the valve body and downstream piping, as well as cause noise.
Certified dimensional drawing: A drawing that guarantees the overall critical dimensions of a valve for installation purposes.
Certified material test report: A history of a particular metal traced back to the heat number and batch number from the foundry.
Cg: Flow coefficient commonly used for gases.
Chain wheel: A handwheel design that is chain-driven for hard-to reach applications. The handwheel portion usually has a series of teeth or grooves, as well as a chain guide, to accommodate the use of a chain.
Characterizability: The ability to vary the flow characteristic of a valve by designing a special shape into a closure element, such as holes in a cage, a plug contour, the orifice shape of a ball, etc. Or the ability to vary the flow characteristic of a modulating valve by changing the positioner’s response to a control signal.
Characteristic: Relation between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent.
Charpy impact test: A quality test that measures a material’s ability to resist fracture on a V-shaped notch cut into the material when an impact load is applied to the side opposite the notch. Charpy tests are normally conducted on materials that are subject to extremely cold temperature. (In cold applications, some metals can become more brittle and apt to fracture.)
Check valve: A valve that prevents backflow. The closure element of a check valve normally uses gravity, fluid, and/or spring force to close the valve and does not require any type of manual operation or actuation.
Chevron packing: A packing style characterized by V-shaped (in cross-section) seamless rings. When axial force is applied to the ring, the radial forces cause the thin edges of the ring to press tightly against the wall of the packing box as well as the stem or shaft. Chevron packing typically provides a strong seal with minimal friction in moderate temperatures. Also referred to as V-ring packing.
Choked flow: A condition where the flow rate cannot be increased even if the downstream pressure is lowered. In liquid applications, choked flow occurs when cavitation or flashing causes vapor bubbles to form in the vena contracta, which consequently crowd the flow passage and will not allow a further increase in flow. In gas applications, choked flow occurs when the velocity reaches sonic proportions and a reduction in downstream pressure can no longer increase the gas flow.
Choked pressure drop: The point at which the pressure drop causes choked flow to occur in a valve.
Choke valve: A special angle valve used for wellhead applications.
Circuit: Continuous path for an electric current to flow through.
Circuit breaker: A switch or other device that automatically interrupts the electric current when a circuit is overloaded or extremely stressed in another way.
Class: A term used in conjunction with a pressure class, i.e., ANSI Class 2500.
Clearance flow: Any flow that occurs below the lower end of a valve’s rangeability and the actual closing of the closure element.
Closure element: A device or combination of devices used to close or open the flow passageway of a valve. Typical closure elements include a quarter-turn plug or sleeve, linear plug or seat ring, butterfly disc or seat, ball or seat, etc.
Closure member: Moveable part of the valve which is positioned in the flow path to modify the rate of flow through the valve. See closure element.
Cock valve: Simple on-off valve (similar to a faucet design) used for low-pressure fluids.
Cold box: An extension used in conjunction with a bonnet (or integral to the bonnet) that is used to allow a stagnated gas to form in moderate temperatures. This extension protects the packing box from the cryogenic process flowing through the valve.
Cold flow: The ability of a soft material to undergo plastic deformation under sustained pressure regardless of the temperature.
Collar: The portion of the top-works of a quarter-turn manual valve that limits the motion of the closure element or operator.
Compressor: The part used in pinch valves to squeeze the walls of the elastomeric body together, or the plate used in a diaphragm valve to force the diaphragm against the weir or body of the valve. Or a device used to pressurize gas.
Concentric butterfly valve: A butterfly valve design with the disc installed in the center of the body.
Concentric disc: A butterfly valve’s disc that is positioned exactly at the center of the body.
Conductance: Reciprocal of resistance. Measured in siemens.
Conductor: Material in which free electrons are available to move under the influence of an electric field.
Consolidation: A reduction in a packing’s volume due to wear; cold flow; plastic deformation; or extrusion. Consolidation usually occurs with soft valve materials, such as packing and gaskets.
Contact: Condition brought about by two conductive parts (e.g., contact pieces) touching. The expression is also used to denote a contact element or contact piece.
Contactor: Remote-control switch actuated electro-magnetically and capable of switching motor loads. Also known as a starter.
Continuously connected handwheel: A handwheel design that allows for continual retraction or extension of the stem; while also acting as a low- or high-limit stop. The handwheel can also be placed in a neutral position to allow for automatic operation of the control valve.
Controller: A microprocessor dedicated to monitoring and correcting the actual pressures, temperatures, or flow levels of a process. The controller constantly monitors the set point of the condition (pressure, temperature, or flow). When the actual measurement varies significantly from the set point, the controller sends an electronic or pneumatic signal to the control valve. The valve, in turn, corrects the imbalanced service conditions until the process achieves the conditions established at the set point.
Control loop: A process feedback system that consists of a regulator or control valve, sensing element or transmitter (for flow, pressure, or temperature), and a controller. The controller receives the input from the transmitter and compares it to a set point. By comparing the actual input against the set point, the controller can correct the process by sending a signal to the control valve until the set point is reached.
Control switches: Power circuit switches for the direct control of electrical equipment, or auxiliary circuit switches for indirect control.
Control valve: A throttling valve equipped with an actuator or actuation system to respond to an input signal from a controller. Control valves are used to regulate the flow, temperature, or pressure of a process system.
Corrosion: Any deterioration of metal that is created by a chemical reaction with the metal.
Corrosion-resistant: Any material that does not react with the chemical it is exposed to. (i.e.: plastics or alloy steels).
Cracking pressure: The line pressure that allows the closure element in a check valve or relief valve to begin opening and allow flow through the valve.
Critical temperature: The temperature at which a metal’s crystal structure becomes austenite.
Cryogenic valve: A special valve used in services with temperatures below –50oF (-45oC). A cryogenic valve has a protective cold box as part of the body to allow a vapor barrier to form between the liquefied gas and the packing box.
Cs: Flow coefficient commonly used for steam.
Current: Flow of electrons through an electric circuit.
Cv: Flow coefficient commonly used for liquids. The Cv of valve is used to calculate the ideal valve size to pass the required flow rate, while providing overall stability to the process. Cv is defined as 1 U.S. gallon (3.8 liters) of 60degF (16degC) water during 1 min with a 1psi (6.9 kPa) pressure drop. Also referred to as valve coefficient or flow coefficient.
Cylinder: A pressure-retaining device used in an actuator to house the actuator barrier (a piston) as well as to contain the power (pneumatic or hydraulic) supply in a cylinder actuator.
Cylinder actuator: An actuator that uses a piston to separate pressure chambers on both sides of the piston. The piston moves by varying the power supply pressures on either side (using a solenoid valve or positioner), thereby moving the position of the valve’s closure device. May be double acting or spring-return.
dc: Direct current or voltage that has a constant value related to zero value. Can be stipulated as a positive or negative value.
DCS: Abbreviation for distributive control system:
Deadband: Range through which input can be varied without initiating observable movement of the closure element.
Delta P: The pressure difference between the upstream (inlet) and downstream (outlet) pressures. Also referred to as pressure drop, or differential pressure. Abbreviated as p.
Design pressure: The pressure used to determine overall design requirements for a valve, including materials of construction, flange rating, wall thickness, seat, seals, and packing design, etc. The design pressure is always greater than the actual operating pressure.
Design temperature: The temperature used to determine overall design requirements for a valve, including materials of construction, flange rating, wall thickness, seat, seals, and packing design, etc. The design temperature is always greater than the actual operating temperature.
Destructive test: A special test that uses mechanical or chemical methods to destroy a part in order to discover its properties.
- Block Diagrammatic representation of a circuit or system in which the various component parts of the system are represented by a rectangle or block with no detail shown for the complexities contained in the system block.
- Ladder or schematic Diagrammatic representation of a circuit or system where for the sake of clarity no relationship is made to actual physical location of components.
- Wiring Diagrammatic representation of a circuit or system where actual physical location of components with regard to each other is shown.
Diaphragm: Flexible seal area that provides for linear motion by the change of shape of the material.
Diaphragm actuator: A pneumatic actuator design that incorporates a flexible diaphragm to separate two chambers and to move the actuator rod. The actuator can be either single-acting (spring return) or double-acting.
Diaphragm case: A pressurized housing that contains the diaphragm and diaphragm plate, consisting of two sections joined together by bolting.
Diaphragm check valve: A check valve that uses a preformed elastomeric closure element, which opens upon positive flow and reverts back to its preformed closed position upon reverse flow.
Diaphragm plate: An actuator barrier that is concentric with the diaphragm and is used to transmit force from the diaphragm to the actuator stem.
Diaphragm pressure span: The range of pressure over which a diaphragm can operate. Diaphragm pressure span is noted as the maximum and minimum pressure.
Diaphragm valve: A valve that compresses an elastomeric diaphragm against the bottom of the valve body to shut off, or regulate, the flow.
Differential pressure: The pressure difference between the upstream (inlet) and downstream (outlet) pressures. Also referred to as pressure drop, or delta-P. Abbreviated as p.
Diffuser: A single-stage downstream element used to attenuate high sound pressure levels. Diffusers are designed with multiple holes, which divide the turbulent flow into smaller eddies and manage the turbulent energy so that it does not create noise.
Digital: System that handles information as numbers.
Direct-acting pressure-relief valve: A pressure-relief valve that allows line pressure to act on one side of the closure element, while a predetermined spring applies a mechanical load to the other side. When the line pressure reaches its maximum limit, the line pressure overcomes the spring load and the valve opens until the line pressure falls below the preset level.
Directivity: The reduction in noise as the source of the noise is pointed away from the hearer.
Disassembly clearance: The amount of space needed between the valve or actuator and its surroundings for removal of the valve, or to gain access to the internal structure of the valve.
Disc: Flow-controlling member used in the most common varieties of butterfly rotary valves.
Disc: The closure component in a butterfly valve used to swing through the body to open, close, or throttle the flow. Also, the closure element of a pressure-relief valve (also known as a pallet).
Disc, eccentric: Commonly referred to as a high-performance butterfly valve. See high performance butterfly valve.
Disc stop: A portion of the butterfly body that prevents the disc from over stroking.
Distributive control system: A process plant’s overall data management system that takes input from management requirements and transfers that data into process management, utilizing a number of control loops.
DN: The ISO standard abbreviation for the nominal diameter of a pipe size, i.e., DN100.
Double acting: A piston or diaphragm system in which the energizing pressure acts on both sides of the piston or diaphragm and operates the system both forward and reverse to open or close a valve.
Double-acting actuator: An actuator that requires energizing pressure to be applied to either side of the piston or diaphragm to operate.
Double-disc check valve: A check valve with two half-circle discs hinged together, that fold together upon positive flow and retract to a full circle to close against reverse flow. Also known as a split-disc check valve.
Double-ported trim: A trim with two closure elements that work in unison for reducing the unbalanced forces as the valve opens or closes.
Double-top stem guiding: A packing-box configuration that includes two guides at each end of the packing box to guide a linear motion closure device, such as a plug.
Downstream: The process portion of a system following a valve.
Downstream back-pressure device: A pressure-limiting device installed after a valve to take an additional pressure drop.
Drain: A special outlet (usually involving a threaded port with a plug) that allows process fluids to flow out of a pipe or a cavity, such as the bottom of a globe valve.
Drop-tight: A bubble-tight test that involves water-under-air testing.
Dry lubricant: A solid or powdered lubricant used to coat mating parts, such as perfluoroelastomer.
Dual springs: A heavy-duty spring actuator design that permits one small spring to fit inside another, which allows for shorter actuators with similar thrust to a longer heavy-duty spring.
Ductile iron: A special cast iron with 18 percent ductility.
Ductility: The capability of a metal to deform when placed under pressure or when acted upon by a force. Ductility is measured by the percentage increase of a stretched test specimen just before fracture.
Duty cycle: Percentage of time a device is allowed to operate over a given period of time. Expressed in percent, it equals time on divided by time off multiplied by 100.
Dye penetrant: A quality test that uses bright red or fluorescent dye to detect surface cracks, pits, or porosity on a nonporous surface. A dye-penetrant test involves spraying the special dye on the part. When the excess dye on the surface is wiped away, surface flaws are detected when natural or fluorescent light highlights the remaining dye in the crack or pit.
Dynamic seal: A seal that involves two or more elements that physically move as they act upon each other. Usually such seals include those associated with the closure element (such as between a plug and a seat, butterfly disc and a seal, ball and a seal, etc.), packing box (where a stem or a shaft move against the packing), or the actuator (where a piston may move between two pressure chambers).
Dynamic torque: The torque required to throttle a valve in midstroke.
Dynamic unbalance: The net force of the process fluid acting on the plug of a globe valve in the open position.
Eccentric butterfly valve: A butterfly-valve design in which the valve shaft is slightly offset from the center of the disc, allowing the disc to move in an elliptical motion as it leaves the sealing surface. This unique motion permits minimal friction and wear to the closure element.
Eccentric cammed disc: A butterfly valve’s disc that is offset both vertically and horizontally from the center of the valve.
Eccentric plug valve: A quarter-turn rotary valve that uses an offset plug to swing into a seat as the closure or regulating element.
Effective area: In a diaphragm actuator, the area of a diaphragm that can be acted upon by the air pressure that results in stem or shaft movement. Because the shape of the diaphragm changes as air pressure builds, the effective area can change over a given signal.
Efficiency: Ratio of useful output power to total input power.
Electromagnet: Device in which a core of ferromagnetic material is encircled by a coil of wire carrying an electric current. The core can be made magnetic only when current is passing through the coil.
Elastomer: A polymer part that is flexible and resilient, which is used to seal joints or provide a moving barrier.
Elbow: A special pipe fitting that permits an angle turn in the pipeline (typically a 45deg or 90deg turn). The standard elbow fitting has a matching end connection on both ends for mating with the upstream and downstream piping.
Electro-hydraulic actuator: A hydraulic actuator with a self-contained hydraulic source that is a physical part of the actuator and is electrically driven.
Electromotive force: Force that causes the movement of electric charges. Measured in volts.
Electronics: That part of electrical engineering that specializes in the conductance properties of semiconductor materials.
Enclosed-body pinch valve: A pinch valve in which the elastomeric body is protected by a solid metal housing.
End connection: The part of a valve that matches and joins with the mating piping. Also referred to as a body end connection.
End-to-end: The dimension from one end connection to the opposite end connection on a valve. End-to-end is similar to face-to-face, except that it is used with valves that do no have flat-faced surfaces on the end connection, such as butt weld ends.
Equal-percentage characteristic: A flow characteristic that permits a change in flow per unit of valve stroke, which is directly proportional to the flow occurring just before the change is made.
Erosion: Material wear inside a valve or pipeline caused by the flowing action of the process fluid. Erosion deterioration is often hastened when entrained solids are present in the flow.
Erosion: Damaging effects of gritty or dirty fluids on control valve components. Erosion is forestalled with valve designs which separate flow stream from critical valve components and with hardened materials.
Examination: The physical review of a part or finished valve to ensure that it is in full compliance with user requirements. Examinations are typically conducted by the user, while inspections are conducted by the user or a designated third-party agent.
Expanded outlet valve: A body configuration that has oversized inlet and outlet ports for a given valve size, for example, a 2-in valve with 4-in ports.
Expanding-teeth trim: An anti-noise trim that uses a stack of grooved discs to provide a series of sudden expansions and contractions of the fluid.
Explosion-proof: The assurance that an electrical device can be placed in a potentially explosive atmosphere. An explosion-proof device must separate from any electrical device that may arc from the atmosphere.
Explosion-proof: Characteristic of a device or element that inherently contains or prevents an explosion.
External stack: An attenuator trim installed outside the valve in a venting application. Also referred to as an atmospheric resistor.
Extrusion: A condition that occurs when force is applied to a soft material, causing it to deform and eventually fill empty spaces or migrate through openings.
Face: In the closed position, the side of a butterfly disc that faces the seat.
Face-to-face: The body dimension between the face of one end connection and the opposite face.
Fail-closed: A planned design of a valve and/ or actuator that allows the valve to move to the full-closed position upon loss of power to the actuator.
Fail-lock-in-place: An actuation system that uses lock-up valves to allow the valve to remain in its last position upon loss of power to the actuator.
Fail-open: A planned design of a valve and/ or actuator that allows the valve to move to the full-open position upon loss of power to the actuator.
Fail-safe: An actuator system that allows a valve to move to a certain position (open or shut) or to retain the current position, should the actuator power supply fail.
Fail safe: Characteristic of a system that provides that a unit will be operated into a safe condition upon the failure of electric power, power source, etc.
Failure mode: Upon electric power failure or air pressure loss to the actuator, the valve is operated to a predetermined position (fail open, fail closed, fail in last position).
Far field: The noise that is generated beyond 10 ft (3 m) from the source.
Farad: Unit of capacitance.
Feedback: Signal indicating the actual position or the electrical or pneumatic condition of an element in a control system.
Feedback system: In control valves using a positioner, the system in which a return signal from the stem position is mechanically fed to a positioner, allowing for verification and/ or correction of the valve’s position.
FEP: Abbreviation for fluorinated ethylene propylene copolymer.
Fieldbus: A standardized digital communications language that allows field devices to communicate directly with a controller or DCS as well as other field devices.
Field-reversible: The ability for actuators and positioners to be modified from air-to-open mode to air-to-close mode (or vice versa) with no additional parts or special procedures requiring removal from the line.
Filter: A device used to screen the actuator’s power supply medium of impurities.
Final control element: High-performance process equipment that provides power and accuracy to control the flowing medium to the desired service conditions.
Fire-resistant: The ability of a valve to withstand and survive afire in terms of reaching and maintaining the failure position of the closure element. A valve developed for fire-resistant service can be designed to allow the flow direction to assist with the failure action of the valve. It can also be equipped with backup metal seats to soft seats, external fire resistant insulation, external fire-resistant enclosures, and other design features that intrinsically resist fire damage.
Fire-safe: The ability of a valve to maintain certain standards after being subjected to a fire test. A fire-safe valve is expected to minimize the amount of process fluid lost downstream or to the atmosphere during a fire. Also referred to as fire-tested.
Flange: A flat round portion of the valve’s end connection that has a greater outside diameter than the valve hub. The flange has a number of bolt holes for connecting a valve to a similar end connection on the end of a length of pipe. A flange that is a solid part of the body is called an integral flange. A flange that slides over the body hub and is held in place with half-rings is called a separable flange.
Flangeless body: A body of a rotary valve with a short face-to-face that can be sandwiched between two flanged piping end connections, using long studs and nuts. Also referred to as a wafer-style body.
Flangeless body: Body style common to rotary-shaft control valves. Flangeless bodies are held between ANSI class flanges by through bolts. (Sometimes also called wafer-style valve bodies.)
Flashing: Phenomenon observed in liquid service when the pressure of the medium falls below its vapor pressure and does not recover to a higher pressure.
Flashing: A common valve problem in which the pressure at the vena contracta falls below the vapor pressure, followed by a pressure recovery that remains below the vapor pressure. This pressure reduction below the vapor pressure at the vena contracta causes vapor bubbles to form and to continue downstream. This liquid-gas mixture downstream causes the overall velocity to accelerate, which can lead to excessive noise and eventual erosion.
Flat face: An integral flanged end connection that has no raised face or serrations for the gasket to adhere to. The face is continually smooth from the outside diameter of the inlet or outlet port to the outside diameter of the integral flange. The gasket used with flat-face end connections extends to the outside diameter of the entire flange.
Flat gasket: A gasket produced with simple inside and outside diameters with even, flat surfaces.
Flexible gate: A closure element in a gate valve that uses a solid gate and a flexible seat. Also known as a split wedge gate.
Flexible valve: A manual valve with an elastomeric closure element, such as a pinch or diaphragm valve design.
Floating ball: A ball- or check-valve design that features a ball that is not fixed to the valve body.
Floating seat: A seat ring that is not fixed to the valve body. Floating seats can shift position to conform better to the shape of the closure element, providing better shutoff.
Flow booster: Actuator accessories used to provide a quick stroking action when large input signal changes are made.
Flow capacity: The amount of flow that can pass through a valve under certain flow conditions, without the valve choking. Also, referred to as capacity or valve capacity.
Flow characteristic: A valve’s relationship between the flow coefficient (Cv) and the valve stroke, from 0 to 100 percent. A flow characteristic is usually differentiated as either an inherent flow characteristic or an installed flow characteristic. The three most common types of flow characteristics are equal percentage, linear, and quick open.
Flow characteristic: Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This is a special term. It should always be designated as either inherent flow characteristic or installed flow characteristic. Common flow characteristics are linear, equal percentage, and quick opening.
Flow coefficient Cv: The number of U.S. gallons per minute of 60oF water that will flow through a valve with a 1 lb/in² pressure drop.
Flow coefficient: The measurement of flow that is commonly applied to valves. The flow coefficient is used to determine the best valve size to pass the required flow rate while providing overall stability to the process. The flow coefficient is expressed by the term Cv, which is defined as 1 U.S. gallon (3.8 liters) of 60oF (16oC) water during 1 min with a 1 psi (6.9 kPa) pressure drop. Also referred to as valve coefficient.
Fluid: Any material that can flow given a particular set of circumstances. Fluids include gases, liquids, powders, pellets, and slurries.
Fluorinated ethylene propylene copolymer: A common fluoropolymer used to provide linings for bodies as well as seals for soft seats and soft seals. Also abbreviated as FEP.
Fluoroplastic: A polymer with a molecular structure similar to hydrocarbons, except that fluorine atoms take the place of carbon atoms. Also referred to as fluoropolymer, polytetrafluoroethylene, or PTFE.
Fluoropolymer: A polymer with a molecular structure similar to hydrocarbons, except that fluorine atoms take the place of carbon atoms. Also referred to as fluoroplastic, polytetrafluoroethylene, or PTFE.
Four-way pilot valve: Pilot valve used with double-acting actuators.
Four-way positioner: A positioner that sends and exhausts air to both sides of an actuator.
Four-way solenoid: A solenoid used to operate on-off actuators, providing two-way direction.
Fracture toughness: A measurement of a metal’s ability to resist fracture using a Charpy test. Fracture toughness is a major concern with metals placed in services in which cryogenic temperatures can exist.
Free air: The flow or volume rate of air at standard atmospheric temperature (70oF or 21oC) and pressure 04.7 psia or 1 bar).
Frequency response: The measurement of how a system or actuation device responds to a constant-amplitude sinusoidal input signal. In other words, frequency response determines the overall speed of the system by measuring how well the system keeps up with changing input signal. With frequency response, the phase shift and the output amplitude are measured at different frequencies and are plotted as amplitude ratio and phase shift versus frequency.
Full-bore valve: Any valve where the opening of the closure element (such as the seat) has the same area as the inside diameter of the inlet and outlet ports. Also referred to as full-port.
Full-closed: The valve’s position when the valve’s closure element is fully seated.
Full lift: A pressure-relief valve design in which the valve opens to the full-open position immediately upon overpressurization.
Full-open: The valve’s position when the valve’s closure element allows for maximum flow through the valve.
Full-port valve: Any valve where the opening of the closure element (such as a seat or a full ball) has the same area as the inside diameter of the inlet and outlet ports. Also referred to as a full-bore valve.
Full trim: The area of a valve’s seat that can pass the maximum amount of flow for that particular size of valve.
Fusible element: A specially shaped copper conductor (silver is used in some cases) in the ceramic body of the fuse element. It melts on overload or short circuit. Its special features are the low-melting-point insert and the so-called necks. The insert melts on overload, and the necks will interrupt a short circuit.
Gage: A device that measures a particular function and reports the results through a display. Gages can display information through a dial or through a digital display. Also spelled gauge outside North America.
Gain: The ratio of actuator pressure unbalance to instrument pressure change when the stem is locked in place.
Galling: Damage to two mating parts when microscopic portions interact and bond together, which, when each part moves against the other, results in tearing of the two surfaces. Galling typically happens when two parts of the same material are used together without lubrication. In the case of threaded nuts, bolts, or studs, galling can destroy both male and female threads. Galling is also common between guides and stems or shafts, which not only damages smooth surfaces, but can also destroy packing or other soft parts.
Gasket: A hard or soft material used to seal a joint from the process fluid. Gaskets are used with flanged end connections. They are also used between the bonnet and the body, and the body and seat ring.
Gate: A flat and broad closure element that intersects the flow steam.
Gate valve: A linear-motion valve in which the closure element (a gate) is flat and broad. The gate slides up and down seating surfaces found on both sides of the body. Gate valves are normally used for on-off service.
Gear operator: A manual operator that uses gearing to produce high output thrust. General-service valve: A versatile valve design that can be used in several applications without modification to the design.
Gland bushing: Apart found at the top of a packing box that protects the packing box from atmospheric elements and transfers force from the gland-flange bolting to the packing. Also referred to as a packing follower.
Gland flange: Valve part used to retain and compress the guides, packing, anti extrusion rings, and packing spacers in the packing box. The gland flange is usually attached to the bonnet in linear and quarter-turn valves and the body in rotary valves.
Globe valve: A valve with a rounded valve body made from a casting with in-line ports. Normally the inlet and outlet passages feature constant areas and are streamlined to ensure smooth flow through the valve. The closure element is usually a round member called the plug, which fits into a seat, and is perpendicular to the inlet and outlet passages.
Globe valve: Valve construction style with a linear motion flow-controlling member with one or more ports, normally distinguished by a globe-shaped cavity around the port region. Two categories are commonly recognized depending on the method of plug guiding: cage-guided and stem-or plug-guided.
Graphite: A carbon-based packing or gasket used in high-temperature applications. Graphite is normally produced as die-formed rings or braided rings.
Grease fitting: A small check mechanism that allows grease to be injected into an area containing a bearing.
Ground: Main body of the earth which is considered to be at a zero electric potential. The ground itself and any conductor connected to it are termed at zero potential.
Guide: A cylindrical part that aligns the valve stem or shaft with the closure element. In most cases, the guide or guides are found in the packing box or close to the closure element. In some designs, the upper guide is also used to transmit axial force from the gland-flange bolting to the packing. Also referred to as guide bushing.
Guide bushing: Another term for a guide.
Guiding: Mechanism used to maintain the correct position of the stem or shaft of the closure device. In the case of linear valves, the guiding ensures that the plug aligns with the seat ring. With rotary valves, the guiding ensures that the disc or ball aligns with the body seal.
Half-ring: A ring divided in half that is used to retain a separable flange in place on a valve body.
Handle: A simple lever used to manually open, close, or position a quarter-turn, butterfly, or ball valve, which mayor may not include a latching device. Also referred to as a handlever or actuator lever.
Handlever: A manual operator with two spring-loaded levers used to operate a quarter-turn rotary valve. In a static position, it is locked in place. To operate the valve, the two levers must be squeezed together to disengage the handlever.
Handwheel: A manual operator with a wheel that turns clockwise or counterclockwise to manually operate a valve.
Hard facing: Process of applying a material harder than the surface to which it is applied. This technique is used to resist fluid erosion and/or to reduce the chance of galling between moving parts, particularly at high temperature.
Hardened trim: Valve trim that is overlaid with a special material designed to withstand the effects of cavitation, corrosion, or erosion.
Hardfacing: The welding of a harder alloy over a softer base material (such as steel) to produce additional resistance to the effects of severe services, such as cavitation and flashing.
Hardness: A material’s ability to resist indentation.
Hardness Rockwell test: Measurement of a material’s hardness depending on the size of an indentation. Higher numbers indicate greater hardness of the material. Also referred to Rockwell hardness, or specifically as HRB or HRC (depending on the scale used).
Hazardous environment: Any combination of combustible or potentially explosive mixtures existing in the surrounding atmosphere.
Hazardous location: Any location where an explosion or fire may result because of flammable vapors or materials in the atmosphere.
Header: A large process line that is designed to feed several smaller process lines.
Heat treating: The process of heating and cooling a metal in order to create or enhance the properties of the metal. In some cases, a record of the temperature cycle can be generated by temperature recorders, which are called heat treat charts.
Hertz Unit of frequency: Number of times per second that alternating current flows in one direction, reverses, and flows in the other direction. Power companies in the United States bold this figure constant at 60 cycles per second (hertz).
High-performance butterfly valve (HPBV): Common name for valve design in which the positioning of the valve shaft-disc connections causes the disc to take a slightly eccentric path on opening. (This allows the disc to be swung out of contact with the seal as soon as it is opened, thereby reducing friction and wear.)
High-performance valve: A valve designed specifically for exceptional throttling performance.
High recovery: The inherent ability of a particular valve design to pass flow without taking a large pressure drop, allowing the downstream pressure to recover close to the upstream pressure. Valves with high-recovery factors have similar design characteristics, such as straight-through or streamlined internal passageways, and closure elements that, when open, are outside the flow stream. Gate, quarter-turn plug, and ball valves are good examples of valves with high recovery factors.
Horsepower: Unit of power equal to 746 W, or 33,000 ft lb of work per minute. The relationship between horsepower and torque is horsepower equals torque times revolutions per minute divided by 5250.
HRB: Abbreviation for B scale of the Rockwell hardness test.
HRC: Abbreviation for C scale of the Rockwell hardness test.
Hydraulic actuator: A highly accurate and fast actuator that uses a plant’s hydraulic fluid (or other external hydraulic supply) to provide the thrust to open, close, or position a valve.
Hydrodynamic torque: Turning force exerted on a valve stem due to fluid flow through the valve and its effect on the closure element.
Hydrostatic test: A valve test using water pressure to detect the presence of leaks through sealed joints, vessel walls, or the closure element. As a safety measure, a hyrdotest typically generates 1.5 times the pressure called for by the design.
Hysteresis: In actuation devices, the amount of position error caused when an identical input signal is approached from opposite directions.
Inductance: Property of a circuit to generate an electromagnetic force by reason of a change in the magnetic flux through the circuit.
Inherent flow characteristic: Flow characteristic when constant pressure drop is maintained across the valve.
Inlet: Body opening through which fluid enters the valve.
In-rush current: 1.)Current input of a motor on starting. 2.)Current input of a solenoid or contactor.
Installed flow characteristic: Flow characteristic when pressure drop across the valve varies as dictated by flow and related conditions in the system in which the valve is installed.
Insulation: Nonconductive material, e.g., vacuum, glass, nylon, rubber.
Integrated circuit: Semiconductor device containing circuit elements which are manufactured in a single piece of material and which are individually connected to perform a function.
Impact test: A test used to determine the toughness of a material by measuring the force required to fracture a test specimen.
Incipient cavitation: The point when vapor bubbles begin to form in cavitating services.
Inclusion: A foreign object or particle found in a casting, forging, or weld that may weaken the material or create a potential leak path.
Increaser: A special pipe fitting designed to expand one pipe size to another pipe size. If the increaser has universal end connections, such as flanged or butt weld, it may be used to either reduce or expand the pipeline. If the expander has one-way connections (male-to-female connection, for example), it may only be used to expand.
Independent linearity: The amount of deviation of an actuator stem from a true straight line.
Indicator: A pointing mechanism (usually attached to the stem or shaft) that provides visual verification of valve position.
Indicator plate: A plate with increments of travel that shows the entire length of the valve’s stroke. The valve’s indicator moves along the indicator plate to show current position.
Inherent flow characteristic: A flow characteristic of a valve that operates with a constant pressure drop. A valve’s inherent flow characteristic does not account for the effects of piping or other process equipment, such as pumps.
Inherent rangeability: The ratio of maximum to minimum flow that can be recognized and acted upon by a throttling valve after receiving a signal from a controller, taking into account any significant deviation from the inherent flow characteristic.
Inlet: The port where the process fluid enters the valve.
Inspection: An examination of a valve, part, or system by the user or by an authorized inspector (not associated with the manufacturer) to ensure that it was manufactured to the specifications approved by the user.
Installed flow characteristic: A flow characteristic of a valve that changes from the inherent flow characteristic as it takes into account the system effects of valves, pumps, piping configurations, etc.
Installed set: In throttling valves, the high- and low-pressure values applied to an actuator that indicates the range that the valve will stroke-after installation in a process system.
Instrument pressure: The output pressure from a controller to an actuator (usually without a positioner).
Instrument signal: A signal (electric or pneumatic) between a controller and a valve that communicates the desired valve position.
Integral flange: An end connection that features a solid flange either cast or fabricated as part of the valve body.
Integral seat: A seating surface machined into the body itself. Integral seats are usually used with oversized closure elements to provide greater flow than is normally provided in a given valve size.
Intelligent system: A microprocessor-based controller installed on a control valve that provides local process control, diagnostics, and safety management functions.
Intrinsically safe: An electrical device that is not capable of producing enough heat to cause an ignition of a flammable material or atmosphere.
Isolating valve: A valve placed between the packing box and the packing lubricator used to control or stop the flow of lubricant to the packing box.
Lapping: A process in which an abrasive compound is placed on the seating surfaces of a plug and a seat; the plug is turned in the seat until a full-contact seal is created.
Leakage: The measured amount of process fluid that continues to flow through the closure element of a valve (at a certain temperature, pressure, and pressure drop) when in the full-closed position.
Leakage: Quantity of fluid passing through an assembled valve when the valve is in the fully closed position under stated closure forces, with pressure differential and pressure as specified. Leakage is usually expressed as a percentage of the valve capacity at full rated travel.
Lever actuator: Another name for a lever operator.
Lever operator: Manual operator with a pivot handle used to open or close, or adjust the position of the closure device of a valve. Lever operators may also be used to position other flow devices, such as louvers or dampers. Also referred to as a lever actuator.
Lift check valve: A check valve that uses a free-floating closure element, consisting of a piston or poppet and a seat ring.
Limit-stop: A device in an actuator that restricts or limits the linear or rotary motion of an actuator. Limit-stops may be adjustable or fixed.
Limit switch: An electromechanical accessory attached to an actuator used to indicate or verify the valve’s position (open, closed, or an intermediate position). Usually the signal (or lack of a signal) generates some action in the process, such as operating a motor, pump, or other device.
Limit switch: Set of electric contacts that are activated mechanically at a preselected position.
Line: A long container used to move fluid or pressure from one point to another in a process plant. Also referred to as piping.
Linear flow characteristic: The inherent flow characteristic that produces equal changes in flow per unit of valve stroke at a constant pressure drop.
Linear flow characteristic: Inherent flow characteristic which can he represented ideally by a straight line on a rectangular plot of flow coefficient versus percent rated travel. (Equal increments of travel yield equal increments of flow coefficient change at a constant pressure drop.)
Linear-motion valve: A valve with a sliding stem design that pushes the closure or regulating element into an open, closed, or throttling position. Also known as a linear valve.
Linear valve: Another name for a linear-motion valve.
Line voltage: Voltage existing between the two lines of an electric supply.
Liquid-penetrant examination: A test that is used on nonporous surfaces to detect small flaws, such as porosity or cracks. Liquid penetrant is usually a red or fluorescent dye that is applied to the surface and all excess removed, revealing the flaws in the surface.
Loading pressure: In positioners installed on actuators, the output pressure from the positioner to position an actuator.
Locking device: A mechanism attached to a valve or an actuator that prevents accidental operation or use by unauthorized individuals.
Lock-up system: System used with an actuator to hold the actuator in position if the power supply fails.
Lower valve body: In split-body valves, the half of the valve body that houses the flow area under the seat ring.
Low recovery: The inherent ability of a particular valve design to take a large pressure drop, resulting in the downstream pressure only slightly recovering in relationship to the upstream pressure. Valves with low recovery factors have similar design characteristics, such as highly contoured internal passageways and closure elements that remain in the flow stream when open. Globe and butterfly valves are good examples of valves with low recovery factors.
Lubricator: A mechanical device that feeds lubricant to a packing box to minimize friction between the packing and the stem or shaft. Also referred to as the packing lubricator.
Lug body: In butterfly valves, a body with an integral flange that has a threaded hole pattern identical to the hole pattern of the piping flanges.
Magnetic particle examination: A method used to discover small cracks or porosity in the surface of a material that cannot be detected by visual examination. Magnetic particle examination involves spreading iron filings over a metal surface and passing an electric current through the metal. Flaws are then revealed as the iron filings cluster around imperfections in the metal.
Maintenance repair operations: The function of servicing a valve, either through valve failure or periodic servicing. Also referred to as MRO.
Manual operator: Any device that requires the presence of a human being to provide the energy to operate the valve.
Manual override: Capability provided to actuators that allows the manual positioning of the actuator.
Manual valve: Any valve that operates through a manual operator, such as a handwheel or handlever. Most manual valves are used for on-off service, although some can be used for throttling service.
Maximum allowable operating pressure: The maximum pressure a pressure-retaining vessel, such as a valve, can safely hold on a continuous basis. The maximum allowable operating pressure is expressed in psi or bar (or kilopascal) and is determined by the vessel’s material, maximum temperature, and given pressure class. Also referred to as maximum allowable working pressure.
Maximum flow capacity: The volume of air that can flow into an actuator during a particular time period.
Mechanical preload effect: A butterfly seat design that allows the inside diameter of the seat to interfere slightly with the outside diameter of the disc.
Mechanical tubing: Hollow piping that is used for non-pressure retaining purposes, such as structural supports or to hold wiring, etc. Typically, the piping is round, but other shapes are possible.
Metal bellows seal: An accordion-shaped, flexible device that is welded to a stem or a shaft to provide a solid barrier between the process fluid and the atmosphere.
Metal seat: A seat design in which the mating surfaces in the closure element are both made from metal, lacking an elastomer surface. Metal to-metal seats have greater leakage rates than soft seats but can handle higher temperatures and pressures.
Milliampere: The electrical current measurement used in process instrumentation. Also designated as mA.
Mill test report: A document that indicates the results of chemical testing and physical testing performed on a base material. Mill test reports are normally provided by the producer of the material to the valve manufacturer. Some users require access to mill test reports to ensure compliance to user specifications and requirements.
Minimum wall thickness: Specifying a wall thickness so that the wall measurement never falls below a particular dimension. Tolerance is usually added to the minimum wall thickness; for example, a minimum wall thickness of 0.625 in would have a 0.01-in tolerance (0.625 to 0.635).
Modulate: Function of a controller which causes a valve to respond to an infinite number of positions between the closed and full-open positions.
Modulating lift: In pressure-relief valves, a design that permits the valve to open only enough to relieve the overpressurization and not the entire line pressure.
Moment arm: Linear distance from the center of rotation upon which a force may act to produce torque.
Multi-hole trim: An anti-noise trim that uses a number of cylinders with drilled or punched holes.
Multiple-spring design: Unique design where two or more springs may be used in parallel to provide a variable spring rate within a fixed space.
National pipe thread: A slightly tapered thread used with pressure connections in piping. As the male and female ends are threaded together, the tapered threads have a tendency to seal the connection (especially when a tape thread or sealant is used in conjunction with connection).
National straight thread: A straight thread used with piping that requires a gasket to seal the joint.
Near field: The noise that is measured or heard within 3 to 10 ft (1 to 3 m) of the source.
Needle-valve trim: A low-flow trim set in which the plug head is designed to be very narrow and sharp, with the general appearance of a needle. Needle-valve trims are used for extremely low Cv’s and are normally required for research applications.
NEMA classification: Code established for the construction of electrical components by the National Electrical Manufacturers Association.
Noise, control valve: Generally refers to aerodynamic noise associated with flow-stream turbulence in compressible fluids. Noise levels can be reduced to safe levels defined by OSHA and the EPA with noise-abatement trim (source treatment) and with silencers and diffusers (path treatment).
Normally closed control valve: A valve which closes when the actuator pressure is reduced to atmospheric.
Nominal diameter: A dimensionless numerical designation for a valve or a pipe. Also designated as DN with a number, such as DN 50.
Nominal pipe size: A reference to the size of a pipe or valve. Nominal pipe size is expressed in inches measured across the inside diameter of the pipe or the inlet or outlet of the valve.
Nominal pressure: The pressure rating used with ISO standards. Also, designated as PN with a number, such as PN 100.
Nondestructive examination: Any test that determines the characteristics of a material without requiring destruction of the material.
Nondestructive test: Any test that determines the function, performance, or reliability of a part, assembly, or entire valve without destroying it or damaging its operation.
Nonreturn valves: Valves that allow the flow of the process fluid in only one direction. Nonreturn valves are designed so that any flow or pressure in the opposite direction is mechanically restricted. All check valves are nonreturn valves.
Nonrising stem: In manual handwheels, in which the closure element is threaded and the turning of a stationary operator causes the closure element to lift or close.
Normally closed: A valve design in which the valve remains closed unless it receives a control signal or is manually operated.
Normally open: A valve design in which the valve remains open unless it receives a control signal or is manually operated.
Nozzle: In pressure-relief valves, the seating portion of the closure element that the disc interacts with.
Off-balance area: The difference in the surface areas between the two sides of a closure element, usually the upstream side and the downstream side. The greater the off-balance area, the greater the process forces act against or with the closure element.
Offset globe body: A special body configuration that features an inlet port and an outlet port that are parallel but not in-line.
Ohm: Measure of resistance. Even in a good conductor, such as copper wire, there is some resistance to current flow. Three factors determine this resistance: the conductor’s substance, its diameter, and its length. Large-diameter wiring offers less resistance than smaller-diameter wiring, and the farther the current has to flow from the power source to the equipment, the greater the resistance and the greater the voltage drop.
Ohm’s law: Explains the relationship of amperes, volts, and ohms. One ampere of electric current will flow when pushed by 1V against 1 of resistance. Ampere=volts/ohms.
On-off valve: Any of a number of valve types used for either full-open or shutoff service. Terminology indicates an automated assembly.
On-off valves: Valves used to start or stop the flow of the process fluid.
Open-body pinch valve: A pinch valve with no metal casing, surrounding the elastomeric body, which is supported by a metal skeleton structure.
Open bonnet: In safety valves, a bonnet that exposes the spring to the atmosphere and is used in high-temperature applications in which an enclosed spring would normally lose some strength.
Opening pressure: In full-lift pressure-relief valves, the overpressurization required before the valve moves to the full-open position.
Open-Ioop gain: The ratio of unbalance that occurs when an instrument signal change is made when the actuator is locked in position.
Operating medium: The power source used to operate an actuation system or actuator, which is generally pressurized air, electricity, or hydraulics.
Operating pressure: The pressure that a valve normally operates under during everyday service. The operating pressure should not be confused with the design pressure, which is defined as the operating pressure plus outside factors.
Operating temperature: The temperature that a valve normally operates under during everyday service. The operating temperature should not be confused with the design temperature, which is defined as the operating temperature plus outside factors.
Operative limits: The range that a device can operate under without damage to the device or impaired operation.
Operator: Any device that is used to provide force, leverage, mechanical action, or torque to open, close, or regulate the closure element.
O-ring: An elastomer ring that is used to seal a joint or to seal two dynamic pressure chambers, such as is the case with a piston actuator.
Outlet: The port where the fluid exits the valve.
Outlet: Body opening through which fluid exits the valve.
Output torque characteristics: Equation or graph that shows torque versus position, usually 0 through 90o operation. Overload Conditions in a healthy electric circuit that result in an overload of current flow. A sustained overload may cause the circuit to suffer damage.
Outside diameter: The measurement of a round structure from one side to its opposite side.
Overload relay: Relay which operates as a function of the current. Used for the protection of electrical equipment (e.g., motors) against overload. Its mode of operation is to actuate a built-in auxiliary switch by thermal or magnetic means. The auxiliary switch causes the switching device to be tripped remotely. Overload relays must be protected by fuses to prevent them from being destroyed by a short circuit.
Oversized actuator: The selection of a larger actuator than is normally required for a given service in order to provide more thrust.
Oxidation: A chemical reaction between iron-based metals and oxygen, resulting in rust or scale.
Packing: A soft material used to prevent the leakage of process fluid from around a valve’s stem or shaft and the bore of the bonnet.
Packing box: The configuration of packing, packing spacer(s), lantern ring, packing spring, extrusion rings, wiper rings, guides, gland flange, live-loading, gland-flange followers, etc. grouped together in a bonnet. The packing box is designed to prevent process fluid from escaping through the stem or shaft. The gland flange is located above the packing box and is used to apply axial force to the packing.
Packing box (assembly): Cavity in the valve body or the part of the bonnet assembly used to seal against leakage around the valve plug stem or shaft.
Packing follower: Similar in appearance to an upper guide, a part in which the basic purpose is to transfer the axial load from the gland flange to the packing, as well as protecting the packing from the outside environment. Also referred to as a gland bushing.
Packing lubricator: Another term for lubricator.
Pallet: The closure element of a pressure-relief valve. Also known as a disc.
Parallel: Two or more circuits which are said to be connected in parallel when the total current flow is divided between them.
Parallel gate valve: A gate valve that uses a flat disc gate, which fits between two parallel free-floating seats.
PEEK: Common abbreviation for polyetheretherketone.
Perfluoroalkoxy: A fluoropolymer similar to polytetrafluoroethylene (PTFE), with some different properties that make it ideal for valve linings, seals, or soft seats. Also abbreviated as PFA.
Permanent pressure drop: The difference between the upstream and downstream pressures, which is caused by frictional losses as the fluid moves through the valve.
PFA: Common abbreviation for perfluoroalkoxy.
PID control: A loop-tuning process where the proportional, integral, and derivative settings are adjusted continuously.
Pig: A device the same shape as the inside diameter of a pipe, which is pushed down the pipeline by fluid pressure. A pig has two purposes: first, to act as a barrier between an earlier process fluid and a new fluid, and second, to clean the inside walls of the pipe.
Pilot actuation: The process in which a pilot valve mechanism inside a pressure-relief valve monitors the system pressure and triggers the opening of the main valve when the line pressure exceeds the limit.
Pilot valve: Device acting between the source of air pressure and the actuator that directs air flow to the required actuator air inlet ports.
Pinch valve: A flexible valve with an elastomeric body that can be closed by using a mechanism or fluid pressure to push the walls of the body together.
Pinion: A small gear that works in conjunction with a larger gear or a flat-tooth rack. Pinion gears are usually used in actuation systems.
Piping: A hollow cylinder of a particular length and diameter used to retain pressure and fluid. Standard pipe lengths have uniform wall thickness and outside diameter. Also referred to as a line.
Piping and instrument diagram: A schematic indicating the design of a process system, including the piping, valves, pumps, controllers, sensors, and related instrumentation. The piping and instrument diagram is not to scale and only shows the general order of the equipment and does not address equipment orientation or specific location.
Piping schedule: In pressure pipe, a number that describes the thickness of the pipe wall, such as schedule 40 pipe. The larger the number, the larger the pipe wall and the more pressure the pipe can handle.
Piping tee: A common pipe fitting that has three end connections and has the general appearance of the letter T. In piping configurations, tees are generally used for joining or separating flows.
Piston: A round, flat actuator barrier that is used inside a piston actuator to separate and seal between two air chambers. The piston remains stationary when the air pressures on both sides are equal. When the air pressures change, the piston moves toward the air chamber with less pressure, until both sides are equal. In check valves, a free-floating closure element that seats against a fixed seat ring.
Piston area: Net cross-sectional area upon which air pressure may act to provide an output force.
Piston cylinder actuator: A pneumatic actuator that features a piston inside a cylinder. Because piston actuators are double-acting, positioners are usually required for operation. They typically provide higher thrust than other types of pneumatic actuators. Also referred to as a cylinder actuator.
Pitting corrosion: A specific form of corrosion with the appearance of small holes or cavities. Over time, pitting corrosion will cause the cavities to combine and create larger cavities.
Plug: In linear globe valves, the part that extends into the seat as a closure device and, in some cases, provides the flow characteristic. In quarter-turn plug valves, a cone-shaped part with a flow passage that turns to open, close, or regulate the flow.
Plug, eccentric: Flow-controlling member of the eccentric rotary plug valve. Because of its eccentric action, it clears its seat soon after opening.
Plug valve: Device that uses a truncated V-shaped element to provide tight shutoff or flow through a pipe system. One of several types of 90o operation valves.
Plug head: In linear globe valves, the portion of the plug that fits into a seat ring.
Plug stem: In linear globe valves, the portion of the plug that connects the plug head with the operator.
Plug valve: A manually operated quarter-turn valve featuring a cylinder-shaped or cone-shaped closure element with an internal flow passage. Plug valves can be turned to allow flow through valve or turned 900 to block the flow.
Pneumatic actuator: An actuation system that is powered by air pressure.
Poisson effect: The assistance of process pressure to cause deformation of a soft material against a seating surface for shutoff of a closure element.
Polyetheretherketone: A rigid elastomer that is used in high-temperature services. Also abbreviated as PEEK.
Polyethylene: A common thermoplastic used for piping and some valve parts, such as the seat. Polyethylene is known for its flexibility.
Polypropylene: A common thermoplastic used for piping as well as pipe liners. Polypropylene is not as flexible as polyethylene.
Polytetrafluoroethylene: A common and inexpensive gasket and packing material. Polytetrafluoroethylene is widely used in general services because it is highly elastic, generates little friction, and requires minimal compression to achieve a strong seal. Abbreviated as PTFE.
Poppet: A closure element in a check valve that uses a spring to keep the element closed unless positive line pressure is applied.
Porosity: Small air bubbles in the molten metal during the casting process. After the metal has cooled, these bubbles leave small pits or holes in the casting. Porosity can cause a failure in a pressure-retaining vessel.
Port: Fixed-area holes in the valve body where the process fluid enters and exits after passing through the closure element.
Port: Fixed opening, normally the inside diameter of a seat ring, through which fluid passes.
Positioner: A feedback device that receives a signal (pneumatic or electric) from a controller and compares that signal to the actual position of the valve. If the valve position and the signal are not the same, the positioner sends or bleeds air pressure to or from the actuator until the correct valve position is achieved.
Positive material identification: An independent process in which the identity of a material is verified. A nuclear analyzer is typically used for verification.
Power supply: There are two types of power supply: direct current and alternating current. The latter is the most widely used power supply.
Pressure: The force that fluids exerts on the containment walls.
Pressure-assisted pinch valve: A pinch valve that uses an outside pressure source rather than a mechanical device to close the valve.
Pressure-balanced trim: A special trim modification that allows the upstream pressure to act on both sides of the plug, significantly reducing the off-balance forces and requiring less thrust to close the valve. Pressure-balanced trim is often used when high pressure drops exist or when smaller or lower-thrust actuators are used.
Pressure class: The amount of pressure that a valve’s wall thickness and connections are designed to handle without failure.
Pressure drop: The difference between the upstream and downstream pressures of a valve.
Pressure drop: Difference between upstream pressure and downstream pressure that represents the amount of flow-stream energy that the control valve must be able to withstand.
Pressure drop, maximum allowable: Maximum flowing or shutoff pressure drop that a control valve can withstand. While maximum inlet pressure is commonly dictated by the valve body, maximum allowable pressure drop is generally limited by the internal controlling components (plug, stem, disc, shaft, bearings, seals). Maximum allowable pressure drop may apply to the pressure drop while flowing process fluids or at shutoff.
Pressure recovery: The difference between the pressure at the vena contracta and the downstream pressure.
Pressure regulator: A small valve used to limit the air supply to an actuator.
Pressure relief valve: A self-actuated valve designed to move to the wide-open position when the upstream pressure reaches a preset pressure, relieving pressure from the line. When the pressure is under the preset value, a spring or poppet keeps the closure element closed. When the pressure builds, the spring or poppet tension is overcome, allowing the closure element to open. Also referred to as a relief valve for liquid applications and a safety valve for gas applications.
Pressurized bonnet: In relief valves, bonnets that totally enclose the spring and do not allow leakage of the process to atmosphere.
Process control: Act of maintaining or correcting a process within preset limits using a measurement of the actual process conditions in comparison to an established, desired condition.
Process flow diagram: A schematic that outlines the overall processes in a plant, including the major control instrumentation and equipment. The process flow diagram generally does not include specific piping sizes or detailed instrumentation.
Proportional controller: Device that provides an output signal based on a valve(s) or a measured process parameter(s) and an input set-point command.
Proximity switch: A special limit switch that indicates valve position without using a mechanical contact. Rather, a proximity switch relies on a magnetic or electronic sensor to determine valve position.
psi: Standard abbreviation of pounds per square inch.
psia: Standard abbreviation of pounds per square inch, absolute. The unit psia is used when pressure is expressed without considering ambient pressures.
psig: Standard abbreviation of pounds per square inch, gage (gauge). The unit psig is used when pressure is expressed to standard atmospheric pressure (noted as 14.7 psia).
PT: Abbreviation for penetrant test.
PTFE: Standard abbreviation for polytetrafluoroethylene.
Push-only handwheel: A handwheel design part of an actuator that can extend the actuator stem by pushing against the actuator stem. It can also act as a high-limit stop.
Quarter-turn motion: The movement of a valve’s closure element from 0o (full-closed) to 90o (full-open).
Quick-open flow characteristic: An inherent flow characteristic that produces a maximum amount of flow with minimal turn of the closure element.
Quick-opening flow characteristic: Inherent flow characteristic in which there is maximum flow with minimum travel.
Rack: In rack-and-pinion operators, a flat plate with a series of linear teeth that move linearly with the piston and rotates the pinion.
Rack: Linear gear that operates in conjunction with a pinion gear to provide rotary motion.
Radiography: An examination method that uses x-rays to produce an internal image of a material. Radiography is used to reveal porosity, cracks, and other inclusions under the surface of the material.
Raised-face flange: A valve and piping end connection that includes a flange (either integral or separable). The flat portion of the valve hub extends farther than the flange. This flat portion, which is called the raised face, permits a greater load on the gasket between the valve and pipe than comparable flat-faced end connections. Raised faces are usually spiral serrated or concentric grooved for better gasket sealing.
Ram valve: A valve placed at the bottom of a tank that allows for drainage. Usually the actuator or handlever faces down with the closure element facing up into the tank. Also known as a tank bottom valve.
Range: The region between the lower and upper limits. The range in valves usually applies to temperatures (such as -50 to 100o C) or instrument signals (such as 3 to 15 psi).
Rangeability: The ratio of maximum to minimum flow that can be recognized and acted upon by a throttling control valve after receiving a signal from a controller.
Rangeability: Ratio of maximum to minimum flow within which the deviation from the specified inherent flow characteristic does not exceed some stated limit. Rangeability is expressed as the ratio of the maximum flow co-efficient to the minimum usable flow coefficient of a control valve.
Range spring: In single-acting diaphragm actuators, the internal spring that opposes the air chamber. In positioners, the internal spring that opposes the incoming pneumatic signal.
Rated Cv: The flow coefficient Cv of a valve when the valve is in the full-open position.
Rated Cv: Value of Cv at the rated full-open position.
Rated travel: The measured linear movement of the valve closure element from the full-closed position to the full-open position.
Rating: Specified operating conditions or performance of a piece of equipment by way of its input-output values.
Recovery: Relative term used to describe how much flow-stream pressure is reduced because of the design of the control valve; the ratio of maximum (valve fully open) downstream pressure to upstream pressure.
High-recovery valve: A valve design that dissipates relatively little flow-stream energy due to streamlined internal contours and minimal flow turbulence. Therefore, pressure downstream of the valve vena contracta recovers to a pressure significantly higher than the pressure at the vena contracta. (Straight-through flow valves, such as rotary-shaft ball and butterfly valves, are typically high-recovery valves.)
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, downstream pressure recovery is reduced when compared to high-recovery valves. (Although individual designs vary, conventional globe-style valves generally have low-pressure recovery capability.)
Reduced-port valve: A valve inlet or outlet which is smaller than the pipe size.
Reduced trim: In throttling valves, a smaller diameter seat that is expected to pass a smaller amount of flow than that valve’s rated flow capacity.
Reducer: A special pipe fitting designed to decrease (or increase) one pipe size to another pipe size. If the reducer has universal end connections, such as flanged or buttweld, it may be used to reduce or expand the pipeline. If the reducer has one-way connections (male to female connection, for example), it may only be used to reduce.
Regulating element: A closure element that can be used for throttling control of the valve. Also known as a throttling element.
Regulator valve: A valve designed to ensure process output is constant, regardless of pressure fluctuations.
Relays: May be integral parts of, or used as accessory items for, individual switching devices. They operate as a function of the current or voltage or of time to control other circuit elements connected through auxiliary circuits.
Reliability: A valve’s ability to stay in service between routine maintenance without any unplanned failures.
Relief valve: In liquid services, a self-actuated valve designed to move to the wide-open position when the upstream pressure reaches a preset pressure, thus relieving pressure from the line. When the pressure is under the preset value, a spring or poppet keeps the closure element closed. However, when the pressure builds, the spring or poppet tension is overcome, allowing the closure element to open.
Repeatability: The maximum amount of change in the input signal that requires a change in valve stem or shaft position when approached from the same direction.
Reseating pressure: In pressure-relief valves, the point at which the line depressurization allows the valve to return to the closed position.
Resilient seat: A seat in a closure element that is made wholly or partially with a flexible or semiflexible elastomer.
Resistance: Opposition to the flow of electric current exhibited by matter.
Resolution: The minimum amount of change in valve stem or shaft position when an input signal is received.
Response level: The maximum amount of input signal required by the valve in order for the valve stem or shaft position to move in one direction.
Reverse flow: When a valve is capable of being installed backwards, with the upstream flow passing into the outlet port and the inlet port discharging flow to downstream. Only some valves are capable of reverse flow (for example, some globe and butterfly valves), although the flow coefficient and leakage limits may vary. In addition, some actuators may not be able to provide the correction failure mode with reverse flow (especially if the process flow direction assists with the failure mode).
Ring-type joint: Flanged end connections with a special groove on each face, where a soft piece of metal is placed before the flange is tightened. Also abbreviated RTJ.
Rising stem: A valve design in which the stem of the manual operator lifts with the opening of the closure element.
rms: Abbreviation for root mean squared.
Rockwell hardness: Specific hardness of a material measured by comparing the size of an indentation against a standard scale. Also. abbreviated as HRB for the Rockwell B scale and HRC for the Rockwell C scale. Also referred to as a hardness Rockwell test.
Root mean squared: A measurement in microinches of how rough a surface is. Although it is not exactly equivalent to the arithmetic average roughness height, the two values for the same measurement are very close. Also abbreviated as rms.
Rotary actuator: Device that provides torque or rotating movement to a rotary valve.
Rotary-motion valve: Any valve design that involves a quarter-turn rotation to open or close the valve’s closure element. Also known as a rotary valve.
Rotary-shaft control valve: Valve style in which the flow closure member (full ball, partial ball, or disc) is rotated in the flow stream to modify the amount of fluid passing through the valve.
Rotary valve: Another term for a rotary-motion valve.
RTJ: Common abbreviation for ring-type joint.
Safety barrier: An electrical restriction device located outside a hazardous location that is designed to restrict the voltage and current being sent to a device inside a hazardous location.
Safety valve: A pressure-relief valve designed to reduce overpressurization in gas or steam services. Also, a valve used with volume tanks to prevent overpressurization of the tank.
Sanitary trim: Special self-draining, stainless steel trim used for the food and beverage industry.
Scraper: A device in the shape of the inside diameter of a pipeline and operated by line pressure that is used to clean the inside walls of a pipeline.
Screwed bonnet: A bonnet designed with male threads at the joint with the valve body (which is threaded with female threads).
Screwed end connection: Body end connection that features a female National Pipe Thread (NPT), which mates with a male NPT on a pipe.
Scotch yoke: Device that translates linear motion to rotary motion. The linear motion is defined by a pin or roller element trapped within a channel in the rotary element.
Seal: The static or fixed portion of a closure element in rotary valves. A seal is normally associated with quarter-turn manual plug or ball valves.
Seal weld: Following assembly of two parts, the welding of a joint to prevent any leakage to atmosphere.
Seal-welded bonnet: A special bonnet that can be welded to the valve body after assembly.
Seal-welded cover: A special cap that can be welded to the valve body following assembly.
Seat: The static or fixed portion of a closure element in a valve. A seat is normally associated with ball, globe, gate, butterfly, and eccentric plug valves.
Seat: That portion of the seat ring or valve body which a valve closure member contacts for closure.
Seating torque: Value depicting the turning force required to set a valve into its closed position.
Seating pressure differential: In pressure-relief valves, the difference between the operating and set pressures.
Seating torque: The amount of torque that must be produced by a rotary actuator to close or open the valve.
Seat retainer: In linear globe valves, a trim component that transmits force from the tightened bonnet to the seat ring, when a clamped-in seatring design is provided. Unlike cages, seat retainers are not used to guide the plug or to provide a flow characteristic.
Seat ring: In linear globe valves, a round orifice in which the plug fits to achieve shutoff. Seat rings may be either solid metal or include a soft seal material.
Segmented ball: A style of ball in which only a portion of the sphere is used instead of the entire sphere.
Sensitivity: The specific change in the flow area opening produced by a given change in a regulating element when compared to the previous position.
Separable Range: A flange that is not integrally connected to the body, but rather is held in place with half-rings placed within a groove in the body hub. Because the separable flange is not part of the body, it can be made from less costly materials, such as carbon or stainless steel, when alloy bodies are specified.
Series: Method of connecting the elements of an electric circuit so that the same current passes through them.
Set point: The input value that determines the best desired position of a controllable device.
Set pressure: In pressure-relief valves, the point at which the pressure of the system overcomes the spring force holding the disc to the nozzle and the valve begins to open.
Severe-service valve: A valve that is highly engineered to handle difficult applications, such as those with cavitation, high noise levels, flashing, a high pressure drop, etc.
Shaft: In rotary valves, the component attached to both the closure device and the actuation device or handlever.
Shaft: Portion of a rotary-shaft control valve assembly corresponding to the valve stem of a globe valve. Rotation of the shaft positions the disc or ball in the flow stream and thereby controls the amount of fluid which can pass through the valve.
Shim: In ball valves, extremely thin gaskets used in a series to adjust the deflection of the ball to the seal.
Short-circuit current: Prospective current which will flow under the conditions of a terminal short circuit
Shutoff: See leakage.
Shutoff: The point at which the valve’s closure element is in the closed position and flow ceases through the valve.
Shutoff valve: A valve that is used to block (or shut off) the flow. Shutoff valves are usually used for emergency shutdown situations.
Side-mounted handwheel: An auxiliary handwheel mounted on the side of the actuator.
Side port: In angle valves, the port perpendicular to the rising stem.
Sigma (0″): The cavitation index that describes the ratio of the potential for preventing cavitation to the potential for causing cavitation.
Signal: An electronic or pneumatic piece of information sent from one controlling device to another.
Silencer: A downstream antinoise device that offers large noise reduction with a series of compartments of acoustic material to absorb noise.
Single-acting: A term used to describe the action of diaplliagm actuators in which air pressure is applied to one air chamber. This pressure pushes a plate, which is opposed by a range spring on the opposite side of the plate.
Single-acting positioner: A positioner that can only send air to one side of a piston or diaphragm.
Single-Ioop control: The process of an input sensor sending information to a controller, which sends a correcting signal to a control valve until the correct process condition is achieved.
Single-seated trim: A closure or regulating element with a sole point of closure.
Sleeve: In globe valves, the part that provides a sliding seal surface for a pressure-balanced plug. In quarter-turn plug valves, an elastomeric cylinder used to seal and retain the plug.
Sliding gate valve: A gate valve that uses a flat, rectangular gate for irrigation or waterway services. Also known as a sluice valve.
Sliding seal: In actuators used with rotary valves, a special seal located where the actuator stem exits the lower pressure chamber of the actuator. Because of the rotary shaft connection (usually a rod end bearing), the motion of the actuator stem must move in a nonlinear fashion, thereby needing a moving seal.
Sliding valve: A manual valve that uses a flat perpendicular closure element that intersects the flow, such as a gate-valve design.
Slotted trim: A single-stage cage or seat retainer that uses long, narrow slots to reduce noise.
Sluice valve: A gate valve that uses a flat, rectangular gate for irrigation or waterway services. Also known as a sliding gate valve.
Slurry: A process fluid characterized by a mixture of undissolved solids and liquids.
Smart valve: A control valve equipped with an onboard microprocessor or a digital positioner.
Socketweld end connection: An end connection that allows the inside diameter bore of the body hub to mate with the outside diameter of the pipe. The body and the pipe are then connected using a weld between the face of the body hub and pipe’s outside diameter.
Soft-seat plug: In globe valves, a linear plug in which an elastomer insert is placed in the seating area of the plug and is used with a metal seat ring. This design usually requires the plug to be disassembled to install or replace the elastomer. Soft seat plugs are used to provide bubble-tight shutoff.
Soft-seat ring: A seat ring made up of two metal pieces, with an elastomer insert sandwiched between the two, that is used with a metal plug. Soft seats are usually required to achieve bubble-tight shutoff. Also referred to as a resilient seat.
Solenoid valve: Electrically powered device with a movable element that switches flow as a method of controlling actuator position.
Sound pressure level: The standard measurement of noise in a valve. One sound pressure level is equal to 90 dB and doubles every 6 dB.
Span: The difference between the high and low limits of a range. For example, a 3- to 15-psi (0.2- to 1.0-bar) signal has a span of 12 psi (0.8 bar),
Special-service valve: A valve designed only for a specific application.
Specification: Any requirement that a valve must conform to in order to meet the user’s expectations. Specifications can vary significantly, outlining requirements for performance, materials, design tolerances, quality, delivery, and method of shipment.
Speed of response: The rate of travel provided by a particular actuator, given certain conditions. Also referred to as stroking speed.
Spiral-wound gasket: A gasket consisting of alternate layers of metallic and nonmetallic materials wound together.
Spline: A series of equal-sized grooves cut into a shaft clamped to a matching coupling.
Split body: A body that is separated into parts with the closure or regulating element found at the joint.
Split-disc check valve: A check valve with two half-circle discs hinged together, which fold together upon positive flow and retract to a full circle to close against reverse flow. Also known as a double-disc check valve.
Split range: In positioners, the partial use of an available signal range.
Split wedge gate: A closure element with agate valve that uses a solid gate and a flexible seat. Also known as a flexible gate.
Spring: In diaphragm actuators, the part that provides force to act against the opposing air chamber. In piston cylinder actuators, the part that provides force for the actuator to move to the correct failure position.
Spring button: A flat, round part found in actuators that is responsible for the holding the spring in place.
Spring compression force: Characteristic of a spring expressed as the restoring force available when the linear spring dimension is at minimum length.
Spring end torque: Published value depicting the potential turning force of a spring-return actuator at the end of travel, when air pressure is zero.
Spring rate: The amount of force generated by a spring when the spring is compressed to a certain measurement.
Spring-return actuator: Device that contains a spring element having the capability of returning a valve-actuator system to its normal position in the absence of air pressure, electric power, etc.
Spring-start torque: Published value depicting the initial turning force of a spring-return actuator when air pressure is zero.
Staged pressure reduction: A measure to reduce effects of a high pressure drop by taking a series of small pressure drops, rather than one large pressure drop.
Stainless steel: An iron alloy that may be mixed with different elements to produce a metal that resists oxidation in the open atmosphere. The capabilities of stainless steels vary depending on the percentage of chrome, nickel, and other elements added to the iron.
Standard: Any procedure or requirement that is part of a written document.
Standard flow: In butterfly and ball valves, where the flow enters into the port of the body that is closest to the seat and exits from the port that is farthest from the seat.
Starter: Switch and/or resistance unit for running motors up to speed either steplessly or in steps.
Static seal: Any sealing design that occurs between two parts that do not move after being secured into place. Because friction is limited, static seals have a longer life and are generally more reliable than dynamic seals. Common static seals include joints between the body and bonnet, body and the seat ring, yoke and the actuator casing, etc.
Static unbalance: The difference between the forces of the process fluid pressure that act on both sides of the closure element when in the closed position.
Steady-state air consumption: In double-acting actuators and four-way positioners, the air consumption required to maintain a required position.
Steam jacket: A metal covering that is welded or attached to a valve body to provide space for a separate fluid. A steam jacket is used to provide external cooling or heating of the body or the process.
Stem: The rod portion of a closure or regulating element that is attached to the actuator or a hand-operated device. The stem is sealed by the packing box and transmits the force applied by the actuator to the closure or regulating element.
Stem clamp: A component used to secure or clamp the actuator stem to the plug stem of a linear valve. A stem clamp also has secondary purposes, such as providing feedback to a positioner, indicating the position of the valve, preventing plug rotation, and tripping limit switches.
Stop: A mechanical device attached to a rotary-valve shaft or closure element that prevents further motion. In particular, rotary valves have stops to prevent excessive actuator force from driving through the seat and destroying it.
Stopper valve: A manual valve that uses a linear-motion, round closure element perpendicular to the centerline of the piping, such as a globe- or piston-valve design.
Stroke: The amount of travel a valve is capable of. In linear valves, the stroke is usually linear measurement. In quarter-turn rotary valves, the stroke is usually a measurement of an angle between 0 and 90o.
Stroking: The act of moving the position of a closure element.
Stroking speed: The rate of travel provided by a particular actuator given certain conditions. Also referred to as speed of response.
Stud: A threaded rod in which one end is threaded into a drilled and tapped hole and the other end is secured with a nut.
Stuffing box: Another term for packing box, commonly used in Great Britain.
Supply pressure: In positioners and actuators, the air pressure that drives the actuation system.
Supply-pressure effect: The effect on a valve’s position when the actuator’s air supply is changed by 10 psi (0.7 bar).
Swing check valve: A check valve designed with the closure element attached to the top of the cap. The closure element can be pushed aside by the flow, but swings back into the closed position if the flow reverses.
System actuation: In pressure relief valves, the process in which line pressure acts on one side of the closure element and a predetermined spring applies a mechanical load to the other side. When the line pressure reaches its maximum limit, the line pressure overcomes the spring load and the valve opens until the line pressure falls below the preset level.
Tack weld: A small weld designed to hold two parts together during major welding or to allow handling without misalignment of the parts.
Tank-bottom valve: A valve placed at the bottom of a tank to allow for drainage. The actuator or handlever usually faces down (underneath the tank) with the closure element faces up into the tank. Also known as a ram valve.
T-body: In linear-motion valves, a body style designed to allow the valve to be installed in a straight piping configuration with the rising stem action perpendicular to the piping.
TEFZEL: Thermoplastic [25 percent ethylene, 75 percent tetrafluoroethylene (TFE)]. Used as lining and seat material in valves.
Tensile strength: The maximum amount of force that can be applied to a part before failure occurs. Also referred to as ultimate strength.
Test certificate: Documentation that outlines the results of a physical or chemical test.
Thermoplastic: A common plastic material used in plastic piping and valve parts. Thermoplastics are sensitive to temperature variances, losing their strength as the temperature increases, while gaining strength as the temperature decreases.
Threaded end connection: A valve’s end connection that is designed with female NPT threads, which mates with the piping end connection designed with male NPT threads.
Three-way adapter: An upper-body extension that adds a third port to a conventional T-style globe or angle body.
Three-way pilot valve: Pilot valve used with single-acting (spring-return) actuators.
Three-way positioner: A positioner that sends and exhausts air to only one side of an actuator.
Three-way valve: A valve-body configuration that has three ports, allowing for the flow to be diverted to one of two paths or to combine two separate flows.
Throttling: Action of a control valve in motion as it modulates flow.
Throttling: Regulating the position of a valve between the full-open and full-closed positions so that pressure or fluid are adjusted to meet the requirements of the process system.
Throttling element: A closure element that can be used for throttling control of the valve. Also known as a regulating element.
Throttling valve: A high-performance valve that is used to regulate the flow, temperature, or pressure of a process. Although a throttling valve can fully open or close, its main purpose is to provide an exact measurement somewhere between those two points.
Through-bolt connection: A piping-to-valve connection that involves a narrow face-to-face, flangeless valve body installed between two piping flanges with longer bolting.
Through-conduit gate valve: A full-area gate valve that has a body the same shape as the pipe. Not only does a through conduit provide for continuous flow without interruption, but it also allows the unrestricted passage of pigs or scrapers.
Thrust: The force generated by an actuator, actuation system or a manually operated actuation device.
Tilting disc check valve: A check valve with a round closure element with two pivot points located on each side of the element.
Top cap: A pressure-retaining part that seals the top-works of a valve bodyand does not contain a packing box. Also known as a bonnet cap.
Top-mounted handwheel: An auxiliary hand wheel mounted above the actuator.
Top-works: Any equipment or part mounted on a valve that is located above the bonnet. Top-works can include any number of parts depending on the design, but typically the term refers to the operator, yoke, positioner, position indicator, etc.
Torque: Product of a force acting upon a moment arm to produce rotary motion.
Torque: Rotational force applied to a shaft.
Torque switch: A switch that opens or closes when a certain amount of rotational force is applied to a shaft.
Tortuous-path trim: A trim designed with a series of holes, channels, teeth, etc., to increase flow resistance, lower velocities, and reduce the pressure drop.
Toughness: A material’s ability to remain intact when force is applied. A tough material will deform under the force, rather than fracture.
Transducer: A positioner accessory used to convert an electrical signal to a pneumatic signal.
Transducer: Device that senses a process parameter, such as temperature and pressure, and converts this information to an electric or pneumatic signal for use in control systems.
Transfer case: A mechanism used in rotary valves that generates rotary shaft motion from linear actuator motion.
Transflow valve: A valve with three or more ports, in which the flow from the inlet port is always flowing through one of the remaining outlet ports. In a transflow valve, if flow is switched from one outlet to another, the increasing flow in one outlet is inversely related to the decreasing flow in the other outlet. Therefore, flow is continual, although being channeled to another line.
Transformer: Basic transformer consists of two inductors (termed primary and secondary windings) wound onto a ferromagnetic core. If an alternating current or voltage is passed through the primary winding, a corresponding output will appear on the secondary windings in a direct relationship to the windings ratios.
Trim: Internal parts of a valve which are in contact with the process fluid.
True installed cost over time: Term used to describe all the costs related to the valve from the point of purchase to final replacement.
Trunnion-mounted ball: A ball that is supported by both a shaft and a post opposite the shaft.
Tubing: Any length of metal pipe that is purposely manufactured according to nonstandard piping measurements and materials.
Turbulence: Flow that is characterized by high velocities and a series of eddies created by obstructions or odd angles in the flow stream, such as the closure element on a valve.
Turndown ratio: A ratio of the range ability that can be used by a valve. If a valve has a range ability of 100:1 and the maximum controlled flow is equivalent to 75 percent of the valve’s capacity, the turndown ratio is 75:1.
Turning force: Characteristic of a rotary valve resisting its change of position, or the output of a rotary actuator. Also known as torque.
UHMWPE: Ultrahigh-molecular-weight polyethylene. Seat material offers high abrasion resistance superior to TFE. Temperature limit is 200oF (93oC).
Ultimate strength: The maximum amount of force that can be applied to a part before failure occurs. Also referred to as tensile strength.
Ultrahigh-molecular-weight polyethylene: A higher grade of polyethylene used in closure seals of ball and butterfly valves.
Ultrasonic testing: A testing process that involves bombarding a surface of a material with high frequencies to find hidden inclusions, cracks, pits, etc. As the sound waves make contact with a flaw, the reflected signal can be evaluated to determine the depth of the flaw.
Union end connection: A piping end connection found on some smaller valves that involves an external nut is held in place on one end connection and the mating end is threaded. The end with the external nut is then engaged with the threaded end and tightened.
Unseating torque: See breakaway torque.
Upper guide: The guide that is furthest from the closure element, usually at the entrance to the packing box. The upper guide not only provides correct alignment of the stem or shaft with the closure element, but it is also used to transmit axial force from the gland-flange bolting to the packing and to protect the packing from the outside environment.
Upper valve body: In split-body valves, the half of the valve body that houses the flow area over the seat ring.
Upstream: The process preceding a valve, including the fluid, pressure, temperature, piping, tank, process equipment, etc.
Valve: A mechanical device designed to divert, start, stop, mix, or regulate a process fluid, which may be either a liquid or a gas. Specifically, a valve is used to handle the flow, pressure, or temperature of a process fluid.
Valve capacity: The amount of flow that can pass through a valve U11der certain circumstances, without the valve choking. Also referred to as capacity or flow capacity.
Valve coefficient: The measurement of flow that is commonly applied to valves. The valve coefficient is used to determine the best valve size to pass the required flow rate, while providing overall stability to the process. The flow coefficient is expressed by the term Cv which is defined as 1 U.S. gallon (3.8 liters) of 60øF (16øC) water during 1 min with a 1 psi (6.9 kPa) pressure drop. Also referred to as flow coefficient.
Vapor pressure: Pressure at which a given liquid begins to vaporize.
Variable speed: Characteristic of the actuator providing for revised (slower) operating time over the range of operation.
Velocity: The speed at which a fluid moves through a valve.
Vena contracta: The narrowest constriction in the fluid stream as process flow moves through a valve. The vena contracta can be caused by a small orifice or a valve seat or seal. At that point, the flow velocity is at its highest rate, while pressure is at its lowest point.
Vena contracta: Location where cross-sectional area of the flow stream is at its minimum. (The vena contracta normally occurs just downstream of the actual physical restriction in a control valve.)
Vent: A portion of a pipeline where process fluid (usuallya gas, such as air) is allowed to escape to atmosphere.
Vent element: Similar to a severe-service trim stack, except that it is mounted downstream from the outlet of the valve as an antinoise measure when venting to atmosphere. Vent elements allow for the use of larger stacks than physically possible inside the valve, thus providing greater staged pressure reduction and reduced velocity control.
Venturi seat ring: A special extended seat ring used in an angle valve to protect the outlet portion of the valve body from damage caused by gas-born particulates, cavitation, flashing, process erosion, etc. In some cases, the Venturi seat ring can be extended through the entire length of the downstream portion of the valve and even into the piping itself.
Viscosity: A measurement of how thick or thin a fluid is. Highly viscous fluids require more energy in the form of pressure or heat to move through the process system.
Visual examination: A sight review of the quality of a casting or manufacturer part by someone trained to perceive flaws.
V-notch ball: In rotary ball valves, a ball-valve design that features a half-sphere with a V-shaped notch flow passage. In general, V-notch balls provide better rangeability than full-ball designs.
Volt: Unit of potential difference or electromotive force. The final velocity of an electron which starts from rest and traverses a space across which a potential difference of 1 V exists is 593 km/s.
Voltage: Potential force available to move electrons (current). When you start equipment which uses a great deal of electricity, the voltage may drop. However, if your power service and wiring are properly sized, the voltage drop will not he enough to affect the motor.
Volume booster: An actuator accessory used to increase the stroking speed when large input signal changes are received, decreasing stroking times by up to 90 percent (depending on actuator size and packing friction). Volume boosters also permit normal air flow from the positioner, with minor changes in the positioner input signal. Also referred to as flow booster.
Volume tank: A large tank attached to an actuator that is used to supply additional air to stroke the valve upon air failure.
V-ring packing: A packing style characterized by V-shaped (in crosssection) seamless rings. When axial force is applied to the ring, the radial forces cause the thin edges of the ring to press tightly against the wall of the packing box, as well as the stem or shaft. V-ring packing typically provides a strong seal with minimal friction in moderate temperatures. Also referred to as chevron packing.
Wafer body: In rotary valves, a body that has a short face-to-face that can be sandwiched between two flanged piping end connections, using special length studs and nuts. Also referred to as a flangeless body.
Wafer-style valve body: Flangeless type of butterfly or gate, short face-to- face valve body. Also called a flangeless valve body; it is clamped between pipeline flanges.
Wall thickness: The thickness of the pressure-retaining outside shell of a pipe or a valve body.
Water-hammer effect: In liquid services, the reaction caused when a valve is suddenly closed (or a pump is turned off), which causes a shock wave to be transmitted by the liquid throughout the piping system. Although water hammer causes noise, the primary disadvantage of allowing it to happen is the damage to piping and equipment installed in the piping system.
Weir: In diaphragm and enclosed-body pinch valves, an integral bar cast into the bottom of the metal body that is used to help compress the elastomeric diaphragm or liner.
Welded end connection: Another term for a buttweld end connection.
Wrench: Similar to a handle, a wrench is not attached to the valve stem and can be moved from valve to valve or removed altogether for security reasons.
Y body: A globe or check valve body where the linear action is inclined 45 to 60ø from the axis of the inlet and outlet ports.
Yield strength: The force needed before a material begins to stretch, deform, or fracture.
Yoke: The part of an actuator that supports and secures the actuator to the top-works of a linear valve.