Monday, October 28, 2019

Basic Control Valve Terminology: Part 1

Control Valve
Control Valve (Cashco)

Reprinted with permission of Cashco.

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.

Control Valve
Cashco Control Valve
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.

Cage: A part of a valve trim that surrounds the closure member and may provide flow characterization 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.
Control Valve
Sanitary Control Valve
(Cashco)
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.

Basic Control Valve Terminology: Part 2 in next post.


For more information about control valves, contact Arjay Automation. Call them at (800) 761-1749 or visit their web site at https://arjaynet.com.