API Standard 527, from the American Petroleum Institute, describes seat tightness testing and the maximum acceptable leakage rates for pressure relief valves. The current version of the standard was reaffirmed in 2007.


API 527 covers seat tightness testing of metal- and soft-seated pressure relief valves, including conventional, bellows, and pilot-operated designs.


API 527 specifies procedures for testing with air or nitrogen, steam, and water.



Maximum allowable leakage rate

Air or nitrogen near ambient temperature* 10% or 5 psig below than nameplate set pressure, whichever is greater
  • Metal-seated valves: Varies depending on pressure and effective orifice size
  • Soft-seated valves: No leakage for 1 minute
Saturated steam Same as above
  • No audible or visible leakage for 1 minute (both metal- and soft-seated valves)
Water near ambient temperature Same as above
  • Metal-seated valves
    • Inlet w/ nominal pipe size > 1 inch: Leakage not to exceed 10 cubic cm per hour per inch
    • Inlet w/ nominal pipe size < 1 inch: Leakage not to exceed 10 cubic cm per hour
  • Soft-seated valves: No leakage for 1 minute

* An alternative air or nitrogen testing procedure is specified for valves with open bonnets. For metal-seated valves the maximum allowable leakage rate is half that of the standard procedure, while for soft-seated valves it remains the same (i.e., no leakage for 1 minute).

A full copy of API 527, with complete testing procedures and specifications, can be purchased from the American Petroleum Institute.

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