API 527

Covers — methods to determine the seat tightness of metal- and soft-seated pressure-relief valves (conventional, bellows, and pilot-operated) with defined maximum acceptable leakage rates over set pressures from 15 psig to 6000 psig.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Sizing/selection of the relief device → API 520; relief loads/disposal → API 521.
• Standard orifice/dimensions → API 526.
• In-service inspection intervals and bench-test practice → API 576.

API 527 is purely the seat-leakage acceptance method; it does not size or dimension the valve.

API 527 sets minimum requirements for seat-tightness testing:

• Test Method — pressurize the valve inlet to a defined fraction of set pressure and measure leakage (bubble count for gas, drops for liquid).
• Acceptance Leakage — maximum allowable leak rates that differ for metal versus resilient seats and by orifice size.
• Applicability — conventional, balanced-bellows, and pilot-operated designs.

API 527 defines how to test and accept the seat tightness of pressure-relief valves. The valve inlet is pressurized to a set fraction of the set pressure, and leakage is measured — as a bubble rate for gas service or drop rate for liquid — against maximum acceptable limits that depend on seat type and orifice size.

It is the acceptance yardstick behind the bench tests described in API 576, and complements the sizing (API 520) and dimensional (API 526) standards. Where near-zero leakage is critical, a tighter requirement must be specified.

Engineering Note: Metal seats have a nonzero allowable leakage under API 527; if a process needs bubble-tight relief, specify a soft (resilient) seat or a supplementary tightness requirement.