UNDERSTANDING FLOW MEASUREMENT AND RESTRICTION

A Brief Glossary of Common Terms

Accuracy: The extent to which a measurement agrees with a reference. A qualitative assessment; not the same as uncertainty (a quantitative value).

Beta Ratio (Beta): For differential producers with circular cross-sections the ratio the bore I.D.-to-pipe I.D. Typically ranging from 0.20 to 0.75, depending on meter type.

Cavitation: The implosion of vapor pockets occurring when the local pressure of a fluid rises above the vapor pressure of the line fluid.

Critical Flow (Choked Flow, Sonic Flow): The condition at which the velocity of a flowing fluid, typically a gas or vapor, reaches the velocity of sound at local conditions.

Differential Pressure: The pressure difference between high- and low-pressure taps. This is the flow signal generated by a differential pressure producing primary flow element.

Discharge Coefficient (CD): The value of the ratio of the actual flow rate to the theoretical flow rate, where the theoretical flow rate is calculated for an ideal fluid and no energy loss.

Expansibility Factor: The coefficient that accounts for the compressibility of a fluid. Y(ε) is equal to one for ideal, incompressible fluids.

Flow Profile: A graphic representation of the velocity distribution of the flow as it approaches a primary flow element. Flow through a straight, smooth, pipe of sufficient length should develop a blunt flow profile. Valves and pipe fittings introduce non-uniform velocity distributions.

Meter: Wyatt uses this term to refer to primary flow elements used for measurement, as opposed to restriction. Not to be confused with the transmitter, or secondary instrumentation.

Pressure Ratio: In gas or vapor flow, the ratio of the absolute pressure at the low-pressure tap to that at the high-pressure tap. Ideally, this value should be greater than 0.8.

Primary Element: Device (venturi, orifice, flow nozzle, etc.) that generates a signal to determine the flow rate.

Reliability: A qualitative assessment of the degree to which empirical evidence agrees with and substantiates the theoretical understanding of a phenomenon.

Transmitter: The instrument that converts the differential pressure signal to (typically) an analog or digital signal that can be recorded or processed.

Uncertainty: The estimate of an error band within which the true value falls with stated probability.