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)
| 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 = 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)
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)
