Glossary

Absolute Pressure The pressure above absolute zero, or above a perfect vacuum (See Figure 1).

ACFH Actual Cubic Feet per Hour.  Used in measurement to refer to the actual volume measured by the meter.

Active/Working Regulator A regulator that is in service performing a control function.

Body Effect The effect of turbulence between the diaphragm and point of measurement on the downstream piping, where pressure is normally measured.

Boost The pressure increase from set pressure while flow is increased gradually from the set flow to the maximum flow (See Figure 2).

Capacity The amount of fluid, under specific conditions, that a regulator will pass while maintaining the delivery pressure within acceptable limits.  Capacity readings are limited to a droop of 1 inch of water column, or a rise of 2 inches water column, from 7 inches water column set pressure at initial flow rate (50 cfh set flow if the regulator diaphragm diameter is 8" or less).  When other outlet pressures are used, the compound should not exceed 20% and the droop 10% of the set pressure.

CFH or SCFH Cubic Feet per Hour, or Standard Cubic Feet per Hour.  This is the rate of flow.  A standard cubic foot is defined as "a volume of one cubic foot of 0.60 specific gravity gas at a temperature of 60 F and at a base pressure of 14.7 PSIA".  In contracts, this may be specified at other base conditions.

Compound or Rise See "Boost".

Control Line The external impulse pipework which connects the regulator to the point on the main line where control is required.

Creep Relief A relief valve which is installed and sized to protect against the build up of downstream pressure due to a regulator's failure to lock up.  This type of relief is not full fault protection.

Critical Flow/Capacity The maximum flow achieved for a given inlet (i.e., if the outlet set pressure is reduced, the flow will not increase).  In conventional regulators, this occurs where P₂ approximately equals ½ P₁, but this is not the case with high recovery valves.

Critical Velocity The velocity at critical flow.

Cutting Term used to describe the damage to valve seats due to either erosion or the orifice edge slicing into soft seat material.  This is normally seen on high flow rate, large pressure drop regulators.

Diaphragm Generally a composition material, whose primary purpose is to sense the outlet pressure, and in combination with the pressure adjusting spring and linkage or other loading element, to position the valve disk.  Also none as "Sensing Element".  (See Figure 3)

Diaphragm Effect The change in effective area of the diaphragm as the regulator strokes from low flow to high flow (See Figure 4)

Drifting A condition where the regulator moves away from set point then returns after a longer period of time (See Figures 5, 6).

Droop or Drop The decrease from set pressure when flow is reached, up to a maximum capacity.  Three items that contribute to this are:

Spring Extension

Flow of Gas

Loss of Inlet Pressure

Dynamic Performance The transient behavior of a regulator in controlling pressure or flow, following a deviation from set point.

External Static See "Control Line".

Full Capacity Relief A regulator that contains or has the capability of maintaining downstream pressure to within certain limits in the event of some type of failure, usually by venting the excess gas to the atmosphere.

Gauge Pressure The flowing gas pressure at line conditions as read on gauge.

Governor The term used for Low Pressure regulators (i.e., below 5 PSIG inlet).

Header A piping configuration where a number of pipes are combined at one location.

Hunting A condition where a regulator's outlet pressure fluctuates on either side of a set point, not as fast as when it is oscillating, but faster than drifting (See Figure 5).

Impulse Any piping network external to the regulator other than the main line or vent pipe.  To use this word correctly, another description is needed such as "Downstream Impulse".

Inlet Pressure Effect The increase or decrease in the outlet pressure coming from changes in the inlet pressure which results in differing degrees of force being applied to the seat disk and diaphragm (See Figure 7).

Integral Relief Valve A small, spring-loaded pressure relief valve contained within the regulator at the center of the diaphragm or attached to the external part of the regulator, usually at the outlet side of the valve body or lower casing to prevent outlet pressure from the regulator from exceeding a predetermined pressure (See Figure 8).

Load Line The line that delivers the signal from the pilot or instrument to the main regulator.

Loading Element A force placed on the diaphragm, and contained in the upper diaphragm chamber, which opposes the gas pressure exerted against the opposite side of the diaphragm.  The outlet pressure of the regulator is determined by adjustment of this force.  Types of loading elements are as follows (See Figure 9):

Spring Loading

Weight Loading

Pilot Loading

Combination Loading

Lock-Up The condition of the regulator at which no gas flows (See Figure 2).

Lock-Up Pressure The outlet pressure which must be applied to the diaphragm to stop all flow through the regulator valve orifice (See Figure 2).

Maximum WP Maximum Working Pressure or Maximum Allowable Working Pressure (MAOP).

Mesh The method of grading woven mesh, used for filter, and/or strainer elements.

Micron A unit of measuring particle size, used to rate filter performance (25 micron = .001").

Minimum WP Minimum Working Pressure.

Monitor Regulator A regulator installed in series with an active/working regulator that will operate automatically should the pressure rise to its set point.

Negative Shock The momentary minimum outlet pressure caused by a sudden start of the downstream flow.

Normal WP Normal Working Pressure.

One Way Drift A condition where the outlet pressure moves away from set point in one direction, above or below set point (See Figure 6).

OPSO Overpressure Shut Off.

Operating Pressure Normal Operating Pressure.

Overpressure Shut Off A device designed to shut off the flow of gas when the outlet pressure increases to a predetermined point.  Overpressure Shut Off devices normally require to be reset manually if tripped.

Override A safety pilot which is set to take control if the working pilot malfunctions and allows pressure of flow to go outside preset limits.

P₁ Inlet Pressure.

P_1e Maximum inlet pressure for abnormal or emergency service, without damage to internal parts.

P₂ Outlet Pressure.

P_2e Maximum outlet pressure for abnormal service, without damage to internal parts.

P_2f Maximum outlet pressure which can be contained by pressure carrying components without leakage to atmosphere.

Partial Relief See "Creep Relief".

Pilot Control A subsidiary regulating circuit which provides a control signal to the main regulator, which in turn controls the main flow (See Figure 10).

Pinching Used in connection with a diaphragm to describe the clamping action produced by bolting up the flanged periphery of a diaphragm casting.

Positive Shock The momentary maximum increase in outlet pressure obtained with a sudden decrease of the downstream flow.

Position Indicator A travel indicator incorporated in a regulator to show valve movement from the closed to open position.

PSI Pounds per square inch – generally accepted as gauge pressure.

PSIA Pounds per square inch absolute pressure.

PSIG Pounds per square inch gauge pressure.

Pressure Drop The difference between inlet and outlet pressure of any control device.

Pressure Elevation The proportionate change in outlet pressure from set pressure while inlet pressures are increased, the flow remaining the same.

Pressure Loss The pressure loss through a length of pipe.

Proportional Band The change in input signal required to produce a full range change in output (stroke).

Pulsation or Chattering A rapid oscillation of the regulator diaphragm, linkage or valve plunger which may cause noise or fluctuating outlet pressure (See Figure 5).

Rangeability The ratio between the maximum and minimum capacity of the regulator at which it will continue to operate satisfactorily.

Restricting Element See "Valve Orifice".

SCFH Standard Cubic Feet per Hour.

Sensing Element See "Diaphragm".

Set Flow The flow rate used for the initial setting of the regulator at a specified outlet pressure (See Figure 2).

Slam Shut Valve A valve arranged to close in the event of an abnormal pressure being at a "selected point" and then reset manually.  See "OPSO".

Specific Gravity The ratio of the density of a substance to the density of a reference substance, both at specified physical conditions.  As applied to gas, air is the reference substance.  Two specific gravity definitions are recognized in gas measurement:

Real Specific Gravity – The density ratio between a gas and air determined by measurement at the same temperature and pressure.

Ideal Specific Gravity – The ratio of the molecular weight of a gas to the molecular weight of air. 

(Molecular weight of air = 28.96440)

Spring Effect The change in spring force applied to the diaphragm as the regulator strokes from low flow to high flow.  As the spring is extended, it will exert less force against the diaphragm plate (See Figure 11).

Standby Regulator A regulator that is installed in such a manner that should the active/working regulator fail to supply the gas required, as indicated by the reduction of pressure, the standby regulator will automatically come into service.

Stroke Indicator See "Position Indicator".

Underpressure Shut Off A device designed to shut off the flow of gas when the outlet pressure drops to a predetermined point.  Underpressure Shut Off (UPSO) devices normally must be reset manually.

Valve Disk or Seat Disk A composition disk or similar device which when positioned with respect to the regulator valve orifice controls the flow of fluid through the regulator, and which when pressed against the valve orifice by the valve linkage, shuts off the flow of fluid through the regulator (See Figure 12).

Valve Orifice The part of the regulator where flow of fluid is controlled.  A seating surface is machined on one or both sides.  It is the position of the valve disk with respect to the seating surface which determines the amount of fluid flowing through the regulator (See Figure 13).

Vent The opening to the atmosphere side of the regulator diaphragm through which the regulator breathes (See Figure 12).

Vent Damper A restrictor or flapper design installed in a vent to stabilize the regulator and prevent the diaphragm from fluctuating.

Water Column (W.C.) A unit of pressure measurement where 27.7 inches water column equals 1 PSIG.

Working/Active Regulator See "Active/Working Regulator