Page 272 - Valve Selection Handbook

Sizing Pressure Relief Devices 259

Table 7-1. (continued)
Nomenclature
Mixed SI /Mixed US
Units 1Jnits
P'2 Pressure in space between valve seat
2
and valve outlet bara lb/in. a
Atmospheric pressure (abs.) bara lb/in. 2 a
P 3
PC Critical vapor pressure, the pressure
of saturated vapor at the critical
temperature T c
PE Pressure at valve entry during
2
pressure relief bara lb/in. a
PN Pressure at valve nozzle face during
2
pressure relief bara lb/in. a
PR Reduced pressure, P R = PI/PC
PT Pressure at face of discharge pipe 2
outlet during pressure relief bara lb/in. a
RO General gas constant 8314.3 1545 ft
J/kmol°K lb f/lbmol°R
RI= Individual gas constant 8314.3 1545 ft
RO/M J/kg°K lb f/lb°R
Q Rate of flow of liquid US gal/min.
T, Temperature absolute of gas or
vapor at relieving pressure K = °C + 273 °R = °F + 460
Critical temperature absolute, the
T c
maximum temperature at which the
vapor and liquid phases exist in
equilibrium
TR Reduced temperature, T R = Tj / T c
V Volumetric flow rate of gas or vapor
3
at standard pressure and temperature Nm /h scfm
3
3
Vl Specific volume at inlet conditions m /kg ft /lb
W Mass rate of flow kg/h Ib/h
z Compressibility factor at actual
relieving pressure and temperature,
correction for deviations from the
ideal gas laws
u Viscosity Saybolt universal seconds SU
n Dynamic viscosity centipoise mPajS
p Density of liquids kg/m 3 lb/ft 3