Gl o s s a r y o f Te r m s
A
Absolute Pressure (abs press) - Gauge pressure plus barometric pressure. Absolute pressure can be zero only in a perfect vacuum.
Absolute Viscosity (abs visc) - The product of uid kinematic viscosity times its density. Absolute viscosity is a measure of uid tendency to
resist ow, without regard to its density. Sometimes the term dynamic viscosity is used in place of absolute viscosity. Refer to Viscosity, Absolute.
Accuracy - A measure of how close a regulator can keep downstream pressure (P2) to the setpoint. Regulator accuracy is expressed as percent
droop or proportional band or offset in percent of setpoint or in units of pressure.
ACFH - Actual Cubic Feet per Hour. The actual volume of uid measured by the meter. This is not SCFH (standard cubic feet per hour).
Active/Working Regulator - A regulator that is in service performing a control function.
Adjusting Screw - A screw used to change the compression setting of a loading spring.
AGA - The American Gas Association or Australian Gas Association.
Airsets - See Filter/Supply Regulators.
ALPGA - Australian Liqueed Petroleum Gas Association, Ltd.
ANSI - American National Standards Institute.
API - American Petroleum Institute.
Appliance (Equipment) - Any device that uses gas as a fuel or raw material to produce light, heat, power, refrigeration, or air conditioning.
ASME - American Society of Mechanical Engineers.
Aspirator - Any device using uid velocity effect to produce a low-pressure zone. Used in regulator control and combustion systems.
Atmospheric Pressure - The pressure exerted by the atmosphere at a given location and time. Sea level pressure is approximately 14.7 pounds
per square inch absolute (1.0 bar absolute).
Automatic Control System - A control system that operates without human intervention.
Automatic Cutoff - A device used on some regulators to close the main valve in the event of pressure deviation outside of a preset range.
Must be reopened manually.
B
Backpressure Regulator - This is a device that controls and responds to changes in its upstream/inlet pressure. Functions the same as a relief
valve in that it opens on increasing upstream pressure.
Barometer - An instrument for measuring atmospheric pressure, usually in inches, centimeters, or millimeters of mercury column.
Barometric Pressure - The atmospheric pressure at a specic place according to the current reading of a barometer.
Bellows - A exible, thin-walled cylinder made up of corrugations one next to the other that can expand or contract under changing pressures.
Bimetallic Thermal System - A device working on the difference in coefcient of expansion between two metals to produce the power to
position a valve plug in response to temperature change.
Bleed - Removal of uid from a higher pressure area to a lower pressure area in a regulator pilot system.
Bode Diagram - A plot of log amplitude ratio and phase values on a log frequency base for a transfer function. (It is a common form of
graphically presenting frequency response data.)
Body - Pressure retaining shell enclosing the restricting element.
Boiler - A closed vessel in which a liquid is heated or vaporized.
Bonnet - The regulator component that connects the valve body to the actuator.
713
714
Gl o s s a r y o f Te r m s
Boost - The increase in control pressure above setpoint as ow is increased from low ow to maximum ow. Some regulators exhibit droop
instead of boost.
British Thermal Unit (BTU) - The quantity of heat required to raise one pound of water from 59° to 60°F.
Build-up - In a relief valve, the pressure increase above setpoint required to produce a given ow rate.
BSPT - British Standard Pipe Thread.
C
C1 - A term used in a sizing equation. It is dened as the ratio of the gas sizing coefcient and the liquid sizing coefcient and provides a
numerical indicator of the valve’s recovery capabilities.
Cage - A hollow, cylindrical trim element that is a guide to align the movement of a valve plug with a seat ring and/or retains the seat ring in
the valve body. The walls of the cage contain openings that usually determine the ow characteristic of the control valve.
Capacity, Flow - The amount of a specied uid that will ow through a valve, specic length and conguration of tubing, a manifold, tting,
or other component at a specied pressure drop in a xed period of time. (SCFH, gpm, Nm³/h, Lpm, bph).
Capacity, Rated - The rate of ow through the regulator specied by the manufacturer for a given inlet pressure, outlet pressure,
offset, and size.
Capacity, Wide-Open - If a wide-open failure occurs, this is the amount a regulator will ow.
Cavitation - A phenomenon whereby liquid owing through a valve under reduced pressure will form gaseous bubbles that will collapse upon
pressure recovery, producing potential trim damage. This is a concern when high-pressure drops exist across the valve.
Centipoise - A unit for measurement of absolute viscosity. One centipoise is equal to one hundredth of a poise, the metric (cgs) unit of absolute
viscosity. The absolute viscosity of water at 20°C is approximately one centipoise.
Centistoke - A unit for measurement of kinematic viscosity. One centistoke is equal to one hundredth of a stoke, the metric (cgs) unit of
kinematic viscosity. The kinematic viscosity in centistokes times the density equals the absolute viscosity in centipoises.
CFH - Cubic Feet per Hour (ft3/h). Volumetric measurement of gas ow per hour, generally at line conditions.
Cg (Flow Coefcient) - A term used in gas and steam valve sizing equations. The value of Cg is proportional to ow rate and is used to predict
ow based on physical size or ow area.
CGA - Canadian Gas Association.
Coal/Coke Oven Gas - A gas with a high sulfur content that is produced from baking coal. It may also contain tar that can cause sticking
in moving parts of a regulator. Regulators with brass or copper parts should not be used with this gas. Often this gas requires the
use of fluoroelastomers.
Compressibility Effect - The change in density of gas or air under conditions of compression.
Compression (Spring) - The action on a spring which decreases its length relative to the force to which it is subjected.
Condensate - The liquid resulting when a vapor is cooled and/or when its pressure is increased.
Control Line - The external piping which connects the regulator actuator or pilot to the point on the main line where control is required.
Control Valve - A mechanically, electrically, or hydraulically operated valve, using an external power source to effect its operation, that
modies the uid ow characteristics in a process. It consists of a valve connected to an actuator mechanism that is capable of changing the
position of the ow controlling element or closure member in the valve in response to a signal from the controlling device.
Controller - A device that operates automatically to regulate a controlled variable.
Critical Flow - The rate at which a uid ows through an orice when the stream velocity at the orice is equal to the velocity of sound in the
uid. Under such conditions, the rate of ow may be increased by an increase in upstream pressure, but it will not be affected by a decrease
in downstream pressure. Critical ow occurs when P2 is approximately 1/2 of P1 absolute.
Critical Velocity - The velocity at critical ow. Also called sonic velocity.
715
Gl o s s a r y o f Te r m s
CSA - Canadian Standards Association.
Cs (Flow Coefcient) - Steam valve sizing coefcient. At pressures below 1000 psig, a constant relationship exists between the gas sizing
coefcient (Cg) and the steam coefcient (Cs). This relationship is expressed: Cs = Cg ÷ 20.
Cv (Flow Coefcient) - Liquid sizing coefcient. It is numerically equal to the number of U.S. Gallons of water at 60°F that will ow through
the valve in one minute when the pressure differential across the valve is one pound per square inch.
D
Dead Band - The range through which an input can be varied without initiating observable response.
Delta P (DP) (∆P) (Pressure Drop) - The difference between the inlet and outlet pressures.
Demand - The rate at which uid is delivered to or required by a system, part of a system, or a piece of equipment, usually expressed in terms
of volume per unit of time.
Density - The weight of a unit volume of a substance. Also called specic weight.
Diaphragm - A exible membrane used in a regulator or relief valve to sense changes in downstream pressure and respond to them, thus
moving the restricting element or closure member to which it is attached.
Diaphragm Actuated Regulator - A regulator utilizing a diaphragm and actuator to position the valve plug.
Diaphragm Case - A housing used for supporting a diaphragm and establishing one or two pressure chambers.
Diaphragm Effect - The change in effective area of the diaphragm as the regulator strokes from low to high ow.
Diaphragm Plate - A plate used to transmit force in conjunction with a diaphragm and uid pressure on a spring to the actuator stem or
pusher post.
Differential Pressure - The difference in pressure between two points in a system.
Differential Pressure Regulator - A device that maintains a constant differential pressure between a reference pressure and the pressure of
the controlled uid.
Digester Gas - A gas produced by sewage treatment plants. This gas is used to power burners and engines. Because of its high methane
content, stainless steel construction might be required.
Disk - A movable part that is positioned in the ow path to modify the rate of ow through the valve. It is often made of an elastomer material
to improve shutoff capability.
Downstream - Any site beyond a reference point (often a valve or regulator) in the direction of uid ow.
Drift - A change in setpoint over an extended period of time.
Droop - The amount a regulator deviates below its setpoint as ow increases. Some regulators exhibit boost instead of droop.
DVGW - Deutscher Verein des Gas - und Wasserfaches e.v. (German approval agency).
Dynamic Unbalance - The force exerted on a valve plug when uid is owing through the valve.
E
Effective Area - In a diaphragm actuator, the part of the diaphragm area that generates operating force. The effective area is less than the
total area. (The effective area of a diaphragm might change as it is stroked, usually being a maximum at the start and a minimum at the end
of the travel range. Molded diaphragms have less change in effective area than at-sheet diaphragms.)
End Connection - The style of joint used to make a pressure tight connection between the valve body and the pipeline.
Entropy - A thermodynamic quantity that measures the fraction of the total energy of a system that is not available for doing work.
Loading...