Document / Project Reference
Blowdown Orifice Sizing
Critical vapour flow · thick-orifice correction · critical pressure verification
Single-phase vapour blowdown screening. Final restriction-orifice design to be validated against project depressurisation study.
Company
Project
Doc. No.
Revision
Line / Tag
Date
INPUT DATA & CALCULATION BASIS
ParameterValueUnitNotes
  |  Hazem ElMancy — Blowdown Orifice
HomeCalculatorsBlowdown Orifice

RELIEF / BLOWDOWN

Blowdown Orifice Sizing

Critical-flow restriction-orifice sizing for vapour blowdown service using upstream relieving conditions, target downstream pressure, gas molecular weight, compressibility, heat-capacity ratio, discharge coefficient and thick-orifice correction.

BlowdownCritical FlowRestriction OrificePcrit Check
All Calculators

Enter parameters and click Calculate.

Calculation Basis

  • Specific gravity is calculated from molecular weight: SG = MW / 28.96.
  • Critical flow factor: Ycr = [ k/Z × (2/(k+1))^((k+1)/(k−1)) ]^0.5.
  • Initial bore estimate assumes Ftp = 1.
  • Beta ratio: β = d/D, where D is the upstream pipe internal diameter.
  • Thick-orifice correction: Ftp = 1 + [ k/2 × (2/(k+1))^((k+1)/(k−1)) ] × β^4.
  • Required bore: d = [ W / (0.1668762 × C × Ycr × Ftp × P1(kPa) × (SG/T)^0.5) ]^0.5.
  • Critical downstream pressure: Pcrit = P1 × Ftp × (2/(k+1))^(k/(k−1)).

Assumptions

  • Single-phase vapour only; no flashing liquid, two-phase flow, hydrates, solids or erosion allowance.
  • Steady isentropic critical-flow screening; final blowdown performance must be checked in a depressurisation simulation.
  • All pressures are absolute.
  • Discharge coefficient shall be confirmed against the selected orifice geometry and project/vendor standards.
  • Use a conservative downstream target pressure and validated relieving load.
  • Sub-critical result means the selected pressure ratio does not satisfy the critical-flow criterion and the equation basis is not valid for final sizing.
  • Small line ID / high beta ratio may be mechanically impractical; confirm restriction-orifice thickness, bore, plate style and piping class.
  • For formal EPC design, verify against project depressurisation philosophy, flare capacity, acoustic fatigue, reaction forces and vendor restriction-orifice datasheet.