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Safety Relief Valves - Capacities vs. Pressure

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Safety valves and maximum relief free air capacity:

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Safety Relief Valves - Capacities vs. Pressure
Valve
Diameter
(inches)		Max. Relieving Capacity of Free Air (cfm)
Gauge Pressure (psi)
5010015020025030035040050060080010001200160020002400
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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