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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 T1/2/ (C kd kbp P M1/2)                                 (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

kd = discharge coefficient - 0.975

kbp = 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
GasMolecular WeightCoefficient
- 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 = qs (SG T)1/2/ (1.175 C kd kbp P)                                       (2)

where

A = minimum discharge area (Square Inches)

qs = 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

kd = discharge coefficient - 0.975

kbp = back pressure coefficient - 1.0 for atmospheric discharge systems

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

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