ABSOLUTE PRESSURE: Gauge pressure plus barometric pressure. Absolute pressure can be zero only in a perfect vacuum.
BLEED: Removal of fluid from a higher-pressure area to a lower-pressure area in a regulator pilot system.
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.
FAIL OPEN: In the event of a regulator failure, a condition wherein the valve port remains open. All regulators can fail open or closed.
GAUGE PRESSURE: Pressure reading as shown on a gauge (PSIG or bar g). The difference between atmospheric pressure and the pressure the gauge is measuring. Also written gage pressure.
HEADER: A piping configuration where a number of pipes are combined at one location.
MEASURING ELEMENT: A diaphragm that senses (measures) changes in downstream pressure and causes the regulator restricting element to move toward the open or closed position.
NPT: “National Pipe Thread,” a standard for tapered thread used on pipes and pipe fittings.
ORIFICE: A fixed opening, normally the inside diameter of a seat ring, through which fluid passes. The term can also refer to the inlet or outlet of a regulator or pilot valve. Also called a port.
PITOT TUBE: A hollow tube that connects the area beneath the regulator diaphragm with the vena contracta area of gas flow. The pitot tube causes the diaphragm to sense a pressure lower than that which exists downstream of the regulator, and thus allows the regulator to open more for any given change in downstream pressure. The result is increased regulator accuracy.
RANGEABILITY: The ratio of maximum-rated capacity to the minimum controllable flow within the specified accuracy band.
VENA CONTRACTA: The location where cross-sectional area of the flow stream is at its minimum size, where fluid velocity is at its highest level, and fluid pressure is at its lowest level. (The vena contracta normally occurs just downstream of the actual physical restriction in a regulator.)
This glossary of terms and definitions was contributed by Emerson Process Management Regulator Technologies (www.fisherregulators.com), a provider of regulators, control valves, instrumentation and performance services to the process control industries.