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 filter: An accessory used to prevent dirt, oil, or water in the air or signal supplies from reaching the actuator or other air-driven devices.
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.
Body subassembly: The portion of the valve that includes the body, flanges, and associated bolting, bonnet, packing box, cage or seat retainers, and closure element.
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.
Capacitance: The property of a capacitor device to store electric charge. The unit of measurement is the farad-coulomb per volt = ampere per second per volt.
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 rp.
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.
Diagram:
· 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 rp.
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 positioner: A positioner that uses a microprocessor to position an actuator and to monitor and record certain data.
Digital: System that handles information as numbers.
Direct-acting actuator: A term for a diaphragm actuator that allows the actuator rod to extend (closing the closure element), with air pressure placed in the chamber above the diaphragm.
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-acting positioner: A positioner that has the ability to supply and exhaust air to both sides of an actuator piston or diaphragm at the same time.
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. See also high-performance butterfly valve.
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: 1) 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. 2) 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: 1) 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. 2) Characteristic of a device or element that inherently contains or prevents an explosion.
Extension bonnet: A bonnet that is longer than normal that is used to protect the packing box and actuator from the effects of severe temperatures. Also referred to as extended bonnet.
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: 1) 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. 2) 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/in˛ 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.