Page 276 - Valve Selection Handbook

Sizing Pressure Relief Devices 263

in the chamber between nozzle and valve outlet. Valve
sure P' 2
manufacturers may provide equations that permit the value of P' 2 to
be calculated from the back pressure P 2 at the outlet flange of the
valve.
2. If the back pressure P' 2 cannot be determined, a coefficient of dis-
charge should be applied as determined in accordance with API
Standard 2000.

Compressibility factor Z = PjVi/RiT]. The factor Z compensates for
deviations of real gases from the ideal gas laws. Its value is evaluated at
inlet conditions. Any correction for Z however is empirical, as the entire
derivation of the equations is based on the ideal gas laws.
When critical pressure P c and critical temperature T c of the gas are
known, the reduced pressure PI/PC and the reduced temperature T^Tc
can be calculated and used to obtain the value of Z from Figures A-1
through Figure A-3 in Appendix A. Values of P c and T c for a number of
gases may be obtained from Table A-l in Appendix A. Please note that
2
the critical pressure in Table A-l is stated in terms of MPa and lb/in .
If the compressibility
factor cannot be deter-
mined, a value of 1.0 is
commonly used. This
value gives conservative
results as long as pres-
sure and temperature are
not too high.

Figure 7-2. Flow
Correction Factor F.
(Courtesy of Anderson,
Greenwood & Co.,
Source Catalog
90/9000-US.96.)