Safety Relief Valves - Capacity vs. Pressure
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Safety valves and maximum relief free air capacity:
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| Valve Diameter (inches) | Max. Relieving Capacity of Free Air (cfm) | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Gauge Pressure (psi) | ||||||||||||||||
| 50 | 100 | 150 | 200 | 250 | 300 | 350 | 400 | 500 | 600 | 800 | 1000 | 1200 | 1600 | 2000 | 2400 | |
| 1/4 | 12 | 20 | 27 | 33 | 38 | 43 | 48 | 53 | 61 | 70 | 84 | 97 | 109 | 128 | 147 | 160 |
| 3/8 | 17 | 27 | 36 | 44 | 51 | 58 | 65 | 72 | 83 | 95 | 115 | 133 | 149 | 176 | 197 | 215 |
| 1/2 | 20 | 32 | 42 | 51 | 59 | 67 | 74 | 111 | 129 | 147 | 177 | 205 | 230 | 270 | 304 | 330 |
| 3/4 | 37 | 59 | 78 | 96 | 112 | 127 | 141 | 176 | 224 | 232 | 242 | 346 | 386 | 423 | 474 | 518 |
| 1 | 58 | 94 | 124 | 152 | 178 | 202 | 224 | 248 | 286 | 324 | 390 | 450 | 500 | 586 | ||
| 1 1/4 | 84 | 135 | 180 | 221 | 259 | 293 | 325 | 352 | 400 | 443 | 509 | |||||
| 1 1/2 | 114 | 186 | 248 | 302 | 354 | 400 | 444 | 478 | 528 | 568 | 634 | |||||
| 2 | 189 | 306 | 410 | 501 | 592 | 668 | 741 | |||||||||
| 2 1/2 | 282 | 457 | 613 | 750 | 880 | 998 | 1114 | |||||||||
| 3 | 393 | 638 | 856 | 1050 | 1230 | 1398 | 1557 |
- 1 inch = 25.4 mm
The relief capacity for a safety valve depends on
- the relief discharge area
- the compressibility factor of the actual gas
- the temperature of the gas
- the geometry of the safety valve
A safety relief valve must be capable of relieving the capacity of the connected compressor(s) at operating pressure. Note that the relief pressure can not be set higher than the maximum rated working pressure of any equipment in the system. If the operating pressure is right below the relief pressure of the valve the valve may start leaking.
A "rule of thumb" is to set the relief pressure 10% above the working pressure of the system.
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Related Topics
• Design of Control and Safety Valves
Sizing and dimensions of control valves & equipment in steam and condensate systems.
• Gases and Compressed Air
Properties of air, LNG, LPG and other common gases. Pipeline capacities and sizing of relief valves.
• Heating Systems
Design of heating systems - capacities and design of boilers, pipelines, heat exchangers, expansion systems and more.
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