Safety Valves in Gas and Vapor Systems

Calculating relief 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

Safety Valve Coefficients for Gases
Gas	Molecular Weight	Coefficient - C -
Acetylene	26.04	342
Air	28.97	356
Ammonia	17.03	347
Argon	39.94	377
Benzene	78.11	329
N-Butane	58.12	335
Iso-Butane	58.12	336
Carbon Dioxide	44.01	346
Carbon Disulphide	76.13	338
Carbon Monoxide	28.01	356
Chlorine	70.90	352
Cyclohexane	84.16	325
Ethane	30.07	336
Ethyl Alcohol	46.07	330
Ethyl Chloride	64.52	336
Ethylene	28.03	341
Helium	4.02	377
N-Heptane	100.2	321
Hexane	86.17	322
Hydrochloric Acid	36.47	357
Hydrogen	2.02	357
Hydrogen Chloride	36.47	357
Hydrogen Sulphide	34.08	349
Methane	16.04	348
Methyl Alcohol	32.04	337
Methyl Butane	72.15	325
Methyl Chloride	50.49	337
Natural Gas	19.00	344
Nitric Oxide	30.00	356
Nitrogen	28.02	356
Nitrous Oxide	44.02	348
N-Octane	114.22	321
Oxygen	32.00	356
N-Pentane	72.15	325
Iso-Pentane	72.15	325
Propane	44.09	330
R-11	137.37	331
R-12	120.92	331
R-22	86.48	335
R-114	170.93	326
R-123	152.93	327
Sulfur Dioxide	64.04	344
Toluene	92.13	326

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)