Control Valve Glossary

Reprinted with permission of Cashco.
Linear Control Valve Design
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Actuator: An actuator is a pneumatic hydraulic, or electrically powered device which supplies force and motion to open or close a valve.

Airset: A regulator which is used to control the supply pressure to the valve actuator and its auxiliaries.

Angle valve: A valve design in which one port is collinear with the valve stem or actuator, and the other port is at a right angle to the valve stem.

Anti-cavitation trim: See “trim, anti-cavitation”. Anti-noise trim: See “trim, anti-noise”.

Bellows stem seal: A thin wall, convoluted, flexible component that makes a seal between the stem and bonnet or body and allows stem motion while maintaining a hermetic seal.

Benchset: The calibration of the actuator spring range of a control valve, to account for the in service process forces.

Body: The main pressure boundary of the valve that also provides the pipe connecting ends, the fluid flow passageway, and supports the seating surfaces and the valve closure member.

Bonnet: The portion of the valve that contains the packing box and stem seal and may guide the stem. It may also provide the principal opening to the body cavity for assembly of internal parts or be an integral part of the valve body. It may also provide for the attachment of the actuator to the valve body. Typical bonnets are bolted, threaded, welded to, pressure-sealed, or integral with the body.

Butterfly valve: A valve with a circular body and a rotary motion disk closure member, pivotally supported by its shaft.

Rotary Control Valve Design
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Cage: A part of a valve trim that surrounds the closure member and may provide flowcharacterization and/ or a seating surface. It may also provide stability, guiding, balance, and alignment, and facilitate assembly of other parts of the valve trim.

Capacity: The rate of flow through a valve under stated conditions.

Cavitation: A two-stage phenomenon of liquid flow. The first stage is the formation of vapor bubbles
within liquid system due to static pressure of fluid at vena contracta falling below the fluid vapor pressure; the second stage is the collapse or implosion of these cavities back into an all-liquid state as the fluid decelerates and static pressure is recovered.

Characteristic, flow: An indefinite term, see “characteristic, inherent flow” and “characteristic, installed flow.”

Characteristic, equal percentage: An inherent flow characteristic which, for equal increments of rated travel, will ideally give equal percentage changes of the existing flow coefficient (cv).

Characteristic, inherent: The relationship between the flow coefficient (cv) and the closure member travel as it is moved from the closed position to rated travel with constant pressure drop across the valve.

Characteristic, linear: An inherent flow characteristic that can be represented by a straight line on a rectangular plot of flow coefficient (cv) versus rated travel. Therefore, equal increments of travel provide equal increments of flow coefficient (cv).

Characteristic, quick opening: An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel.

Characterized cam: A component in a valve positioner used to relate the closure member position to the control signal.

Characterized trim: Control valve trim that provides predefined flow characteristics.

Closure member: The movable part of the valve that is positioned in the flow path to modify the rate of flow through the valve.

Closure member configurations (plug):
  • Characterized: Closure member with contoured surface, such as the “vee plug,” to provide various flow characteristics.
  • Cylindrical: A cylindrical closure member with a flow passage through it (or a partial cylinder).
  • Eccentric: Closure member face is not concentric with the stem centerline and moves into seat when closing.
  • Eccentric spherical disk: Disk is spherical segment, not concentric with the disk stem.
  • Linear: A closure member that moves in a line perpendicular to the seating plane.
  • Rotary: A closure member which is rotated into or away from a seat to modulate flow.
Coefficient, flow: A constant (cv) related to the geometry of a valve, for a given valve travel, that can be used to predict flow rate.

Control valve: A valve which controls the flow rate or flow direction in a fluid system. The final control element, through which a fluid passes, that adjusts the flow passage as directed by a signal from a cont- roller to modify the flow rate.

Dual sealing valve: A valve that uses a resilient seating material for the primary seal and a metal-to-metal seat for a secondary seal.

End connection: The configuration provided to make a joint with the pipe.
  • End connections, flanged: Valve body with end connections incorporating flanges that mate with corresponding flanges on the piping.
  • End connections, split clamp: Valve end connections of various proprietary designs using split clamps to apply gasket or mating surface loading.
  • End connections, threaded: Valve end connections incorporating threads, either male or female.
  • End connections, welded: Valve end connections which have been prepared for welding to the line pipe or other fittings. May be butt weld (bw), or socket weld (sw).
Erosion resistant trim: Valve trim, that has been designed with special surface materials or geometry to resist the erosive effects of the controlled fluid flow.

Extension bonnet: A bonnet with a packing box that is extended above the bonnet joint of the valve body so as to maintain the temperature of the packing above or below the temperature of the process fluid. The length of the extension bonnet is dependent upon the difference between the fluid temperature and the packing design temperature limit as well as upon the valve body design.

Face to face dimension: The dimension from the face of the inlet opening to the face of the outlet opening of a valve or fitting.

Facing, flange: The finish on the end connection that mates with gasket surfaces.

Failure mode: The position to which the valve closure member moves when the actuating energy source fails.
  • Fail-closed: A condition wherein the valve closure member moves to a closed position when the actuating energy source fails.
  • Fail-in place: A condition wherein the valve closure member stays in its last position when the actuat- ing energy source fails.
  • Fail-open: A condition wherein the valve closure member moves to an open position when the actuat- ing energy source fails.
  • Fail-safe: A characteristic of a particular valve and its actuator, which upon loss of actuating energy supply, will cause a valve closure member to fully close, fully open or remain in fixed last position. Fail-safe action may involve the use of auxiliary controls connected to the actuator.
Flangeless control valve: A valve without integral line flanges, which is installed by bolting between companion flanges, with a set of bolts, or studs, generally extending through the companion flanges.

Guides, closure component: The means by which the closure is aligned with the seat and held stable throughout its travel. The guide is held rigidly in the body, bonnet, and/or bottom plate.

Hand jack: A manual override device, using a lever, to stroke a valve or to limit its travel.

Handwheel: A mechanical manual override device, using a rotary wheel, to stroke a valve or to limit its travel.

Hard facing: A material applied to valve internals to resist fluid erosion and/or to reduce the chance of galling between moving parts, particularly at high temperatures.

Hard plating: A thin metal deposit, sometimes electroplated, used to induce surface hardening. Hard plating is many orders of magnitude thinner than hard facing.

Hysteresis: The maximum difference in output value for any single input value during a calibration cycle, excluding errors due to dead band.

Integral seat: A flow control orifice and seat that is an integral part of the body or cage.

Jacketed valves: A valve body cast with a double wall or provided with a double wall by welding material around the body so as to form a passage for a heating or cooling medium. Also refers to valves which are enclosed in split metal jackets having internal heat passageways or electric heaters. Also referred to as “steam jacketed” or “vacuum jacketed.” in a vacuum jacketed valve, a vacuum is created in the space between the body and secondary outer wall to reduce the transfer of heat by convection from the atmosphere to the internal process fluid, usually cryogenic.

Lantern ring: A rigid spacer assembled in the packing box with packing normally above and below it and designed to allow lubrication of the packing or access for a leak-off connection.

Lapping-in: A process of mating contact surfaces by grinding and/or polishing.

Leakage, class: Classifications established by ansi b16.104 to categorize seat leakage tolerances for different sizes of control valve trim.

Leakage, seat: The quantity of fluid passing through a valve when the valve is in the fully closed position with pressure differential and temperature as specified.

Leak-off gland: A packing box with packing above and below the lantern ring so as to provide a collection point for fluid leaking past the primary seal (lower packing).

Lined valve body: A valve body in which a coating or liner has been applied to internal surfaces for cor- rosion/erosion protection or for flow shut off.

Liner, slip-in: An annular shaped liner which makes a slight interference fit with the body bore and which may be readily forced into position through the body end. May be plain or reinforced. Applies to butterfly valves.

Liquid pressure recovery factor: The ratio (fl) of the valve flow coefficient (cv) based on the pressure drop at the vena contracta, to the usual valve flow coefficient (cv) which is based on the overall pressure drop across the valve in non-vaporizing liquid service. These coefficients compare with the orifice metering coefficients of discharge for vena contracta taps and pipe taps, respectively. See ansi/isa-s75.01 “control valve sizing equations.”

Lubricator isolating valve: A manually operated valve used to isolate the packing lubricator assembly from the packing box.

Lubricator packing box: A packing arrangement consisting of a lantern ring with packing rings above and below with provision to lubricate the packing.

Mechanical limit stop: A mechanical device to limit the valve stem travel.

Mounting position: The location and orientation of an actuator or auxiliary component relative to the control valve. This can apply to the control valve itself relative to the piping.

Multiple orifice: A style of valve trim where the flow passes through a multiple of orifices in parallel or in series.

Nominal size: A numerical designation of size which is common to all components in a piping system other than components designated by outside diameters or by thread size. It is a convenient round number for reference purposes and is only loosely related to manufacturing dimensions. Iso uses initials dn as an abbreviation for the term with the letters dn followed by a numerical value designating size. All equipment of the same nominal size and nominal pressure rating shall have the same mating dimensions appropriate to the type of end connections.

Packing: A sealing system consisting of deformable material contained in a packing box which usually has an adjustable compression means to obtain or maintain an effective seal.

Packing box: The chamber, in the bonnet, surrounding the stem and containing packing and other stem sealing parts.

Packing flange: A device that transfers the deforming mechanical load to the packing follower.

Packing follower: A part which transfers the deforming mechanical load to the packing from the packing flange or nut.

Packing lubricator assembly: A device for injection of lubricant/sealer into a lubricator packing box.

Pinch or clamp valve: A valve consisting of a flexible elastomeric tubular member connected to two rigid flow path ends whereby modulation and/or shut off of flow is accomplished by squeezing the flexible member into eventual tight sealing contact.

Plug: A term frequently used to refer to the closure member.

Plug valve: A rotary motion valve with a closure member that may be cylindrical or conical. Port: The flow control orifice of a control valve.

Port guiding: A valve closure member with wings or a skirt fitting into the seat ring bore.

Positioner: A position controller, which is mechanically connected to a moving part of a final control element or its actuator, and automatically adjusts its output pressure to the actuator in order to maintain a desired position that bears a predetermined relationship to the input signal. The positioner can be used to modify the action of the valve (reversing positioner), extend the stroke/controller.

Positioner, double acting: A positioner with two outputs, suited to a double acting actuator.

Positioner, single acting: A positioner with one output, suited to a spring opposed actuator.

Position switch: A position switch is a pneumatic, hydraulic or electrical device which is linked to the valve stem to detect a single, preset valve stem position.

Position transmitter: The position transmitter is a device that is mechanically connected to the valve stem or shaft and generates and transmits a pneumatic or electrical signal representing the valve position.

Post guiding: A design using guide bushing or bushings fitted into the bonnet or body to guide the plug’s post.

Pressure energized seal: A seal energized by differential pressure.

Rangeability inherent: The ratio of the largest flow coefficient (cv) to the smallest flow coefficient (cv) within which the deviation from the specified inherent flow characteristic does not exceed the stated limits.

Rated travel: The amount of movement of the valve closure member from the closed position to the rated full open position.

Seat: The area of contact between the closure component and its mating surface which establishes valve shut-off.

Seat ring: A part of the valve body assembly that provides a seating surface for the closure member and may provide part of the flow control orifice.

Shaft: The mechanical member used to support a rotary closure member.

Spring rate: The force change per unit change in length of a spring.

Stem connector: The device which connects the actuator stem to the valve stem. Stem guide: A guide bushing closely fitted to the valve stem and aligned with the seat.

Three-way valve: A valve with three end connections, used for mixing or diverting flow.

Throttling: The action of a control valve to regulate fluid flow by varying the position of the closure member. This service generates a variable pressure drop.

Transducer: A device that is actuated by power from one system and supplies power in another form to a second system.

Travel: The movement of the closure member from the closed position to an intermediate or rated full open position.

Travel indicator: A pointer and scale used to externally show the position of the closure member; typically in terms of units of opening percent of travel or degrees of rotation.

Trim: The internal components of a valve which modulate the flow of the controlled fluid.
  • Trim, anti-cavitation: A combination of control valve trim that by its geometry reduces the tendency of the controlled liquid to cavitate.
  • Trim, anti-noise: A combination of control valve trim that by its geometry reduces the noise generated by fluid flowing through the valve.
  • Trim, balanced: Control valve trim designed to minimize the net static and dynamic fluid flow forces acting on the trim.
  • Trim, reduced: Control valve trim which has a flow area smaller than the full flow area for that valve. Trim, soft seated: Valve trim with an elastomeric, plastic or other readily deformable material used
  • either in the closure component or seat ring to provide tight shutoff with minimal actuator forces.
Unbalance, dynamic: The net force/torque produced on the valve stem/shaft by fluid pressure acting on the closure member and stem/shaft at stated travel and flowing conditions.

Unbalance, static: The net force produced on the valve stem by the fluid pressure acting on the closure member and stem with the fluid at rest and with stated pressure conditions.

Valve: A device used for the control of fluid flow, consisting of a fluid retaining assembly, one or more ports between end openings and a movable closure member which opens, restricts or closes the port(s).
  • Balve, ball: A valve with a rotary motion closure member consisting of a full ball or a segmented ball.
  • Valve, diaphragm type: A valve with a flexible linear motion closure member which is moved into the
  • fluid flow passageway of the body to modify the rate of flow through the valve by the actuator.
  • Valve, floating ball: A valve with a full ball positioned within the valve that contacts either of two seat rings and is free to move toward the seat ring opposite the pressure source when in the closed position to effect tight shutoff.
  • Valve, globe: A valve with a linear motion closure member, one or more ports and a body distinguished by a globular shaped cavity around the port region.
Vena contracta: The location in a flow stream where fluid velocity is at its maximum and fluid static pressure and the cross-sectional area are at their minimum. In a control valve, the vena contracta normally occurs just downstream of the actual physical restriction.

Yoke: The structure which rigidly connects the actuator power unit to the valve.

Glossary courtesy of Cashco, Inc. For more information about Cashco products, contact Miller Energy, Inc. Call them at 800-631-5454 or visit their web site at