Page 40 - Valve Selection Handbook

Fundamentals 27

FLOW THROUGH VALVES

Valves may be regarded as analogous to control orifices in which the
area of opening is readily adjustable. As such, the friction loss across the
valve varies with flow, as expressed by the general relationship.

where
v = flow velocity
Ah = headless
Ap = pressure loss

For any valve position, numerous relationships between flow and flow
resistance have been established, using experimentally determined resis-
tance or flow parameters. Common parameters so determined are the
resistance coefficient £ and, dependent on the system of units, the flow
coefficients C v, K v, and A v. It is standard practice to base these parame-
17 18
ters on the nominal valve size. '
Resistance Coefficient £
The resistance coefficient £ defines the friction loss attributable to a
valve in a pipeline in terms of velocity head or velocity pressure, as
expressed by the equations