Safety Valves and Relief Capacity

Maximum safety valves free air relief capacity

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The table below can be use to determine the maximum relief free air capacity of safety valves:

Maximum Relieving Capacity of Free Air (ft³/min)
Valve Diameter (inches)	Gauge Pressure (psi)
Valve Diameter (inches)	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

The relief capacity of a safety valve depends on the

relief discharge area,
compressibility factor of the actual gas,
the temperature of the gas and
the geometry of the safety valve.

A 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.

As a "rule of thumb" it is common to set the relief pressure 10% above the working pressure of the system.

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