Flashing Flow Sizing per CCPS DIERS / API 520 §A.3
Size a PSV for two-phase (flashing) flow per the DIERS Omega method. Single-phase API 520 sizing fails for relief scenarios where saturated liquid flashes into the relief line (runaway exotherm, fire on liquid-full vessel). Omega is a closed-form approximation to the full DIERS two-phase model and gives required orifice area directly.
Relief Conditions
psia
°F
lb/hr
psia
—
Fluid Properties (at relief)
ft³/lb
ft³/lb
Btu/lb
Btu/lb·°F
vap frac
📚 Theory
Omega derivation, choked vs subcritical, when DIERS applies, vs API 520 single-phase.