Ambient conditions: The pressure and temperature of the environment surrounding a valve.
Back-flow: The flow that occurs in the opposite direction of the normal or expected fluid flow.
Back pressure: The static pressure existing at the outlet of a pressure-relief device due to pressure in the discharge system.
Block-and-bleed valve: A valve with two seating surfaces which provide simultaneous blockage of flow from both valve ends, and means for draining or venting the cavity between the seating surfaces. When the valve is closed (blocked) the drain is opened, allowing the trapped fluid between the seats to drain (bleed).
Blow-down: The difference between the set pressure and the disc-reseating pressure of a pressure-relief valve, which is expressed as a percentage of the set pressure.
Blow-down valve: A valve used to release the pressurized contents of a pressure vessel or piping.
Bonnet: A valve body closure component that contains an opening for the stem.
Bore: The diameter of the smallest opening through a valve. It is also called port.
Bubble tight: A valve is termed bubble tight when the upstream side of the valve is pressurized with air and the downstream side is filled with water and no air bubbles are detected on the downstream side with the valve in fully closed position.
Chatter: Abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc contacts the seat.
Choked flow: When the flow rate cannot be increased even if the downstream pressure is decreased. In liquid applications, it is caused by vapor bubbles, formed by cavitation or flashing, choking the flow passage. In the case of gases, choked flow is caused when the flow velocity reaches sonic proportions and a reduction in downstream pressure cannot increase the gas flow.
Closing pressure: The pressure equal to the valve of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero.
Coefficient of discharge: The ratio of the measured relieving capacity to the theoretical relieving capacity of a pressure-relief valve.
Coefficient of flow: The flow rate that passes through the fully open valve at unit pressure differential. It is measured in gal (3.8 liters) per minute of 60 F (16 C) water with 1 psi (6.9 kPa) pressure differential. It is also referred to as flow coefficient or valve coefficient.
Cold working pressure (CWP): This is the maximum flow-medium pressure at the ambient temperature to which the valve may be subjected during normal service. This is also referred to as water-oil-gas (WOG) rating.
Control valve: A valve serving as a control element in a system, providing means for varying the rate of flow of the fluid passing through the valve.
Cracking pressure: The upstream fluid pressure at which a closed check valve starts to open and allow the flow through the valve.
Cryogenic valve: A valve used in applications with fluid temperatures below 50 F ( 45 C). A cryogenic valve is provided with an extended stem contained in an extension tube so that the valve packing and operator remain at ambient temperature when the cryogenic fluid is in the valve body. The valve is oriented so that the packing is at a higher elevation than the valve body. It allows a vapor-gas pocket to form inside the extension tube, thereby making the extension tube more effective at insulating the packing from cryogenic fluid cold temperature.
Disc: The part of the valve which is positioned in the flow stream to permit or to obstruct flow, depending on closure position. In specific designs, it may also be called a wedge, plug, ball, gate, or other functionally similar expression. In international standards it is referred to as obturator.
Double-disc: A two-piece disc or two separate discs that mate with two seating surfaces. Double discs are used in some designs of gate valves.
Double-seated valve: A valve with two separate seating surfaces that come in contact with two separate seating surfaces of a disc or a double disc.
Flow characteristic: Defines the relationship between the Flow Coefficient and the valve stroke.
Flow coefficient: See Coefficient of flow.
Flow control element: The part of the valve that allows, stops, obstructs, and controls the fluid flow through the valve. See Disc.
Fugitive emission: The amount of leakage of hazardous and toxic flow medium (fluids) from a valve to the environment.
Full bore: When valve bore (port) is approximately of the same size as the inside diameter of the connecting pipe, it is called full bore (full port).
Fully stellited: A valve is termed fully stellited when seating surfaces of the valve seat(s) and the disc(s) are hard faced with wear and corrosion-resistant material (Stellite or other such material).
Half stellited: A valve is termed half stellited when only the seating surfaces of the valve seat(s) are hard faced with wear and corrosion-resistant material (Stellite or other such material).
Inside screw–non-rising stem (ISNRS): Threads on the stem are inside the valve body. The valve disc travels up and down the stem threads when the stem is rotated. Stem does not rise.
Inside screw–rising stem (ISRS): Threads on the stem are inside the valve body and exposed to the flow medium. The stem rises when it is rotated, thus opening the valve. Position of the stem indicates the position of the valve disc.
Iron body–bronze mounted (IBBM): A valve having cast iron body and bronze trim.
Lantern ring: A spacer ring used in the lantern ring type of packing chamber to permit lubrication of the packing, purging of the shaft or stem, or a leak-off system.
Leak-off connection: A pipe or tube connected to a hole in the stuffing box at the level of lantern ring. It is used to collect leakage past a lower set of lantern ring–type of packing or to inject lubricant into the stuffing box.
Linear-flow characteristics: A flow characteristic of the valve when the flow is directly proportional to the position of the flow control element.
Liner: Lining of protective materials applied on the inside surfaces of valve body (or valve trim) to enhance resistance to corrosion, erosion, or contamination.
Non-return valve: Stop-check valve, a check valve in which the closure member can be mechanically closed.
Non-rising stem: Refer to Inside screw, nonrising stem (ISNRS).
Outside-screw-and-yoke (OS&Y): A valve design in which the threaded portion of the stem is outside the pressure boundary of the valve. The valve bonnet has a yoke, which holds a nut through which the rotating stem rises as the valve is opened. The stem part inside the valve is smooth and is sealed so that stem threads are isolated from the flow medium.
Pilot operated pressure relief valve: A pressure-relief valve in which the major relieving device is combined with and controlled by a self-actuated auxiliary pressure-relief valve.
Port: See bore.
Power actuated pressure-relief valve: A pressure-relief valve in which the major relieving device is combined and controlled by a device requiring an external source of energy.
Quarter-turn valve: A valve whose closure member rotates approximately a quarter turn (90 ) to move from full-open to full-closed position.
Reduced port: Valve port smaller than the inside diameter of the end-connecting pipe. It is approximately equal to the inside diameter of the one pipe size smaller than the end size for gate valves, and 60 percent of full bore on ball valves.
Regular port: A valve port smaller than the full bore, approximately 75 to 90 percent of full bore on ball valves and 60 to 70 percent on plug valves.
Relief valve: A pressure-relief valve actuated by inlet static pressure and having a gradual lift generally proportional to the increase in pressure over the opening pressure (set pressure).
Rotary motion valve: A valve that involves a quarter-turn motion to open or close the valve closure element.
Rupture disc: A nonclosing pressure-relief device actuated by inlet static pressure and designed to function by the bursting of a pressure-containing disc.
Safety valve: A pressure-relief valve actuated by inlet static pressure and characterized by rapid opening or pop action.
Seat: The portion of the valve against which the closure member presses to effect shutoff.
Seat ring: A separate piece inserted in the valve body to form a seat against which the valve-closure member engages to effect shut-off.
Set pressure: The inlet static pressure of the system at which a relief valve starts to open, or safety valve pops open.
Short pattern valve: A valve that has face-to-face or end-to-end dimension for a short pattern design according to standard ASME B16.10.
Steam working pressure (SWP): The maximum rated or working pressure corresponding to the steam temperature that must not be exceeded when valve is used in steam service. It is marked with S, SP, or SWP on the valve.
Stroke: The amount of travel the valve-closure member is capable of from a fully closed position to a fully open position or vice versa. In linear-motion valves it is expressed in in (mm) and in degrees, 0 to 90, for rotary motion valves.
Throttling: The process of regulating the fluid flow rate or pressure by controlling the position of the closure member between the full-open and full-closed positions.
Trim: Functional parts of a valve which are exposed to the line fluid. Usually refers to the stem, closure member and, seating surfaces. The removable or replaceable valve metal internal parts that come in contact with the flow medium are collectively known as valve trim. Valve parts such as body, bonnet, yoke, and similar items are not considered trim.
Venturi port: A valve bore or port that is substantially smaller than the full port, approximately 40 to 50 percent of full port. It is normally found in plug valves.
Wafer body: A valve body that has a short face-to-face dimension in relation to pipeline diameter and is designed to be installed between two flanges using special length studs and nuts.
Wedge: A gate valve–closure member with inclined sealing surfaces which come in contact with valve-seating surfaces that are inclined to the stem centerline. Wedge is available in solid, split, and flex designs.
Yoke: That part of the valve assembly used to position the stem nut or to mount the valve actuator.
Yoke bushing, yoke nut: Yoke nut, yoke bushing, or stem nut is the valve part that is held in a recess at the top of the yoke through which the stem passes. It converts rotary-actuating effort into thrust on the valve stem.
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