Omega Method PSV Calculator | API 520 Two-Phase Relief

Chemical & Process Tools

Omega Method Calculator (API 520 – Two-Phase PSV Sizing)

Implements the Omega (Ξ©) Method for PSV sizing under two-phase flashing/nonflashing flow and subcooled liquid at PSV inlet, based on API 520 Part I Annex C correlations (screening/engineering calculation). Geometry/orifice selection is per API 526 orifice areas.

Case Selection

Pressures

Thermophysical Inputs for Ξ©

Relief Rate & Factors

Actions

Results

Relieving Inlet Pressure (Po, abs): -
Omega Parameter (Ξ©): -
Critical Pressure Ratio (hc) and Critical Pressure (Pc): -
Flow Regime: -
Mass Flux (G): -
Required Area (Total / per valve): -
API 526 Orifice Selection (screening): -

Technical Notes

Variable Definitions

  • Pset: PSV set pressure (gauge).
  • Patm: atmospheric pressure (absolute).
  • Po: relieving inlet pressure (absolute), Po = PsetΒ·(1+over) + Patm.
  • Pa: outlet back pressure (absolute) at PSV discharge.
  • ρ0, ρ9: two-phase densities at Po and 0.9Β·Po (API 520 Annex C).
  • V0, V9: specific volumes, V = 1/ρ.
  • Ξ©: omega parameter describing compressibility/expansion behavior.
  • hc: critical pressure ratio; Pc = hcΒ·Po.
  • G: mass flux through nozzle (lb/ftΒ²Β·s); converted to SI for display.
  • A: required discharge area (inΒ²) total and per valve; compared against API 526 orifice areas.
  • Kd, Kb, Kc, Kv: correction factors per API 520/manufacturer.

Formulas / Logic

  • Omega parameter (two-phase at inlet): Ξ© = 9Β·(V9/V0 βˆ’ 1).
  • Critical pressure ratio: correlation from API 520 Annex C (equation form shown in β€œFormulas Used” box) or manual input from Graph C.1/C.2.
  • Flow regime check: critical if Pa ≀ Pc; else subcritical using Annex C subcritical expression.
  • Area (two-phase at inlet): Atotal = 0.04Β·W/(KdKbKcKvΒ·G), Aper=Atotal/n.
  • Subcooled inlet: uses saturated omega parameter Ξ©s=9Β·(ρlo/ρ9 βˆ’ 1) and LOW/HIGH region logic per API 520 C.2.3 (screening implementation).
  • API 526 selection: chooses the smallest standard orifice with Aorifice β‰₯ Aper (screening only).
All internal calculations are performed in US base units consistent with API 520 Annex C constants; SI values are converted for display.

Assumptions / Notes

  • This tool is intended for engineering screening / preliminary sizing. Two-phase properties (ρ0, ρ9, etc.) must come from a validated thermo/flash model and scenario definition (API 521).
  • For ρ at 0.9Β·Po, API 520 recommends an isentropic flash basis; confirm the project basis.
  • Backpressure factor Kb depends on valve type (conventional/balanced/pilot) and backpressure ratio; use API 520 figures or vendor data.
  • Final PSV sizing must consider inlet/outlet pressure losses, instability (chatter), non-equilibrium effects, and vendor-certified coefficients.

Standards / References

  • API 520 Part I (Sizing, Selection, and Installation of Pressure-Relieving Devices) β€” Annex C (Omega Method).
  • API 526 β€” Standard orifice areas (screening selection).
  • API 521 β€” Relief system design basis and scenario definition (for W, Po, two-phase conditions).
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