Page 280 - Valve Selection Handbook

Sizing Pressure Relief Devices 267

Sizing Equations for Liquid Flow

Customary Mixed Imperial Units
Volumetric flow—US gpm

Customary Mixed SI Units
Weight flow—kg/h

Note:

K = Coefficient of discharge:

1. For pressure relief valves that open fully on liquid within an over-
pressure of 10%, obtain the certified value of K from manufacturer.
2. If the pressure relief valve is of the type that requires an overpres-
sure of 25% to open fully, a coefficient of discharge of 0.62 for an
overpressure of 25% is commonly assumed. At lower overpressures,
a capacity correction factor KP to be obtained from Figure 7-5
must be applied. At an overpressure of less than 10%, however, the
valve will display a heightened tendency to chatter. For overpres-
sures above 25%, the valve capacity is affected only by the change
in overpressure.

K w = Capacity Correction Factor Due to Back Pressure

The correction factor applies to balanced bellows-type pressure relief
valves. Typical values may be obtained from Figure 7-6. For product
applicable values, consult valve manufacturer.
Conventional pressure relief valves are affected only by built-up back
pressure, though correction for capacity is not commonly allowed for.