AirComParts ASME Safety Relief Valve, 1/2 in. NPT, 50 to 175 PSI

108 to 321 CFM, depending on which set pressure you pick. That range is the reason to go to a 1/2 in. valve instead of a 1/4 in. one: a quarter inch valve tops out around 275 CFM even at 325 PSI, and most of them flow far less. If your compressor outruns the valve, the valve is decorative.

Available set pressures

• 50 PSI, 108 CFM (S1300)
• 75 PSI, 150 CFM (S1301)
• 100 PSI, 190 CFM (s1306)
• 125 PSI, 235 CFM (S1302)
• 175 PSI, 321 CFM (S1304)

The 150 PSI version of this valve is listed separately as S1303, and the 200 PSI version as S1305.

Sizing it against the compressor, not the pipe

The port size is 1/2 in. male NPT, but that is a fitting dimension, not a capacity rating. What matters is whether the CFM figure above exceeds what your compressor can deliver at that pressure. Take the pump's rated output, not the motor horsepower, and not the number on the tank. A 10 HP two stage reciprocating pump at 175 PSI is doing roughly 34 CFM, so the 321 CFM valve has enormous headroom. A 50 HP rotary screw at 125 PSI is closer to 220 CFM, and now the 235 CFM valve is the right call rather than an obvious one.

Where a single valve will not cover the output, fit two. ASME allows multiple relief devices on one vessel and their capacities add.

Set pressure against the tank

Read the maximum allowable working pressure off the ASME data plate on the receiver. The valve's set pressure must be at or below that figure. Shops routinely fit a 175 PSI valve to a tank stamped 150 because the compressor is set to cut out at 165, which gets the logic exactly backwards. Replace the tank or lower the cut out, don't oversize the valve.

These are brass bodied, ASME coded, with a stainless pull ring for the manual lift test. Factory set and sealed. If one weeps at pressure, replace it rather than trying to reseat it, because a safety valve that has been apart is no longer coded.