The smallest practical size relief valves available have adequate capacity for the majority of piping applications; however, valves having a ¾-inch inlet are considered the minimum for adequate mechanical strength. When relieving capacities are in doubt, relief valve size can be most conveniently selected by using slide rules or sizing charts published by valve manufacturers. Use an accumulation of 10% for determining the capacity of these relief valves when fully open.
The ¾-inch inlet, 0.74 in2 orifice Crosby relief valves referred to above will provide liquid relief for several miles of large pipe. They are the smallest, reasonably inexpensive relief valves generally acceptable for Company use. They will relieve 29 GPM when the relief pressure is 175 psig, and 38 GPM when the relief pressure is 300 psig (assumed 10% accumulation and specific gravity of stock as 0.80).
When vapor can be generated in substantial quantities as would be the case if a high vapor pressure liquid, such as LPG or NH3, were to be blocked in on the cold side of an exchanger with steam on the hot side, a much larger safety relief valve will be required.