Calculators
Emergency Depressuring
Isothermal vessel blowdown through a choked restriction orifice. Estimates time to depressure from operating pressure to target pressure and screens against the API 521 15-minute guideline.
Method: Isothermal blowdown of ideal gas through a choked orifice. Mass flow W = Cd · A · P · √[k/(Z·R·T) · MW · (2/(k+1))(k+1)/(k−1)] (P/P0) decays as mass leaves. Integration is a 1-minute explicit Euler step from t = 0 until pressure reaches target or 120 min elapses. Gas temperature is held constant (conservative — real depressuring is faster due to Joule–Thomson cooling reducing downstream density). Source: API 521, 7th Edition §5.20 (Pressure-Relieving and Depressuring Systems). The 15-minute criterion targets reaching 50% of design pressure or 100 psig, whichever is lower, for a fire-exposed vessel.
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