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Relief Scenario Comparator API 521 CREDIBLE CASES · RANK BY REQUIRED ORIFICE AREA · FIND THE GOVERNING ONE
Reference values are INDICATIVE teaching figures. The API 526 orifice areas and API 520 coefficients here are indicative. They were reconstructed for illustration and are not transcribed from the standard — they may not match the published table. For any real work use your licensed copy of the standard and the vendor datasheet. Confirm orifice designations and areas against API 526 and the valve manufacturer.

PRESET PLANT CASES

EVERY CREDIBLE SCENARIO — RANKED

ScenarioRelief loadPrelArea mm²Orifice

VESSEL & PROCESS

WHAT THIS DOES NOT DO

A PSV answers overpressure only. Vacuum (needs a vacuum breaker), runaway reaction (needs a rupture disc / quench / dump), brittle fracture, corrosion and mechanical failure are all outside its job. Two scenarios are never added — they are independent events, so the largest single credible case sizes the orifice. Only a common-cause event (power failure stopping cooling and pumps at once) combines its own effects.

Educational teaching tool — simplified and illustrative. Not for engineering design use.