The small relief valve between the low- and high-pressure cylinders (usually set ~ 40–60 psi) opens when interstage pressure gets too high. It almost always means the high-pressure stage isn't taking the air the low-pressure stage delivers.
1
Leaking high-pressure intake valve
medReciprocating
CheckInterstage gauge (fit a test gauge in place of the relief if needed) climbs above normal; HP head runs abnormally hot.
FixReplace HP valve assembly/gaskets.
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2
Leaking high-pressure discharge valve
medReciprocating
CheckTank air leaks back into HP cylinder; interstage rises after shutdown.
FixReplace HP discharge valve.
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3
Weak or faulty relief valve
medCheckPops at a pressure below its stamped setting.
FixReplace with the same set pressure — never plug it.
SafetyThe intercooler relief valve protects the cooler tubes from rupture.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.