Page 285 - Valve Selection Handbook

272 Valve Selection Handbook

3. If the thus obtained resistance coefficient for 3% inlet pressure loss
is equal to or higher than the resistance coefficient of the proposed
inlet line, the proposed installation is adequate.

SIZING OF DISCHARGE PIPING
OF PRESSURE RELIEF VALVES 1

Prior to sizing the discharge pipe, obtain from the manufacturer the
permissible back pressure P 2 at valve outlet for the type of pressure relief
valve under consideration. In the case of bellows balanced pressure relief
valves, the permissible back pressure is the sum of built-up and superim-
posed back pressure.
Initially calculate the resistance coefficient of the proposed discharge
pipeline. Then calculate from the equations below the resistance coeffi-
cient for the permissible built-up back pressure. If the thus calculated
resistance coefficient is equal to or lower than the resistance coefficient
of the proposed discharge pipeline, the intially proposed installation is
adequate. Otherwise, redesign the discharge pipeline for a lower resis-
tance coefficient.

For liquids:

The built-up back pressure, in terms of the pressure differential across
the valve (PrP2), may be expressed by the Equation 7-19

where the pressures P are absolute pressures.
If the permissible back pressure is limited, for example, to 10% of the
set pressure, then:

1
Sizing procedure taken from Bopp & Reuter. Safety Valve Handbook for System
Planners and Operators. (Courtesy of Bopp & Reuther.)