Valve Standard Reference
API 527
AmericanSeat Tightness of Pressure Relief Valves
API 527 defines the acceptable seat tightness (leakage limits) for pressure relief valves (PRVs) and safety relief valves at inlet pressure conditions below the set pressure.
Overview — API 527
API 527 defines the acceptable seat tightness (leakage limits) for pressure relief valves (PRVs) and safety relief valves at inlet pressure conditions below the set pressure. It provides maximum permissible leakage rates as a function of set pressure for both gas and liquid service. API 527 test procedures are required during manufacturing, and repeat testing is used to qualify a PRV after maintenance. A valve passing API 527 criteria is said to have 'API 527 seat tightness'.
Scope
Applies to spring-loaded pressure relief valves and pilot-operated pressure relief valves with metal-to-metal seats for gas and vapour service, and all seat types for liquid service. Covers set pressures from 15 psig (1 bar) through 6000 psig (414 bar).
Applicable Valve Types
Spring-Loaded Safety Relief Valves
Pilot-Operated Pressure Relief Valves
Applicable Pressure Classes
Key Requirements — API 527
Test pressure: 90% of set pressure for metal-seated valves
Maximum leakage at 90% of set pressure per API 527 bubble test chart
Bubble test method: discharge submerged in water, count bubbles per minute
Gas service: maximum leakage expressed in bubbles per minute per orifice area
Liquid service: maximum leakage expressed in drops per minute
Re-test after maintenance before return to service
Test conducted with the valve connected to seat tightness test stand, not in-line
Testing Requirements — API 527
Seat tightness test at 90% of set pressure
Bubble count or drop count per API 527 acceptance table
Test conducted before shipment and after maintenance
Documentation Required for API 527 Compliance
API 527 seat tightness test certificate
Set pressure and cold differential test pressure (CDTP) record
Valve nameplate data
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