Safety Valves in Gas and Vapor Systems

Flow (lb/hr)

The minimum discharge area of a relief safety valve in a gas or vapor system can be calculated as

A = m T^1/2 / (C k_d k_bp P M^1/2)                                 (1)

 where

A = minimum discharge area (Square Inches)

m = relieving capacity (Lbs per Hour)

T = absolute temperature (^oR = ^oF + 460)

C = coefficient determined from ratio of specific heats - depends on the gas - see below

k_d = discharge coefficient - 0.975

k_bp = back pressure coefficient - 1.0 for atmospheric discharge systems

P = relieving pressure (psia) - set pressure (psig) + over pressure (psig) + atmospheric pressure (14.7 psia)

M = molecular weight of gas

Flow (SCFM)

The minimum discharge area of a relief safety valves in a gas or vapor system can alternatively be calculated with the equation

A = q_s (SG T)^1/2 / (1.175 C k_d k_bp P)                                       (2)

where

A = minimum discharge area (Square Inches)

q_s = relieving capacity (Standard cubic feet per minute)

SG = specific gravity of gas

T = absolute temperature (^oR = ^oF + 460)

C = coefficient determined from ratio of specific heats - depends on the gas - see below

k_d = discharge coefficient - 0.975

k_bp = back pressure coefficient - 1.0 for atmospheric discharge systems

P = relieving pressure (psia) - set pressure (psig) + over pressure (psig) + atmospheric pressure (14.7 psia)