Page 273 - Valve Selection Handbook

260 Valve Selection Handbook

SIZING OF PRESSURE RELIEF VALVES
GAS, VAPOR, STEAM

The equations for the sizing of pressure relief valves for gas or vapor
are based on the ideal gas laws in which it is assumed that flow through
the valve is isentropic. Deviations from the ideal gas laws are corrected
by the compressibility factor Z.
When the ratio of the absolute downstream to the upstream pressure
across the nozzle is higher than about 0.5, flow through the valve nozzle
is subsonic. At a pressure ratio of about 0.5 and lower, flow through the
valve nozzle is sonic. The actual pressure ratio at which flow through the
valve nozzle turns sonic is a function of the isentropic coefficient as
expressed by the equation

P CF is referred to as the critical nozzle flow pressure and the pressure
ratio as the critical pressure ratio. Flow pressure at a pressure ratio below
the critical pressure ratio is referred to as subcritical.
Values of the critical pressure ratio for isentropic coefficients 1.01
through 2.00 may be taken from Table 7-2.

Table 7-2
Values of Critical Pressure Ratios and Coefficients C

PCF/P. C k Pc F/P,
1.01 0.60 317 1.45 0.52 360
1.02 0.60 318 1.50 0.51 365
1.04 0.60 320 1.55 0.50 369
1.06 0.59 322 1.60 0.50 373
1.08 0.59 325 1.65 0.49 376
1.10 0.58 327 1.70 0.48 380
1.15 0.57 332 1.75 0.48 384
1.20 0.56 337 1.80 0.47 387
1.25 0.55 342 1.85 0.46 391
1.30 0.55 347 1.90 0.46 394
1.35 0.54 352 1.95 0.45 397
1.40 0.53 356 2.00 0.44 400