SafeGuard Projects — Preliminary Sizing Tool
Relief & Vent Sizing Helper
Estimate required relief device orifice area for vapor, liquid, and fire exposure cases using simplified API 520 equations.
⚠ Professional Engineering Note
Relief device sizing for pressure vessels, fired equipment, and PSM-covered processes must be performed and stamped by a licensed Professional Engineer (PE). This tool provides preliminary estimates for feasibility and education only — it is not suitable for final design, procurement, or regulatory compliance.
Vapor case — API 520 Eq. (1) for gas/vapor relief
Liquid case — API 520 Eq. (27) for liquid relief
Fire case — API 521 wetted surface area method
Conventional and balanced-bellows PSV types
Discharge coefficient Kd / back pressure correction Kb
API nozzle designation lookup (D through T)
By proceeding, you confirm this is for preliminary estimation or educational purposes only and that final design will be performed by a licensed PE.
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Preliminary Tool — PE Required for Final Design

Relief & Vent
Sizing Helper

Estimate pressure safety valve orifice area for vapor, liquid, and fire exposure cases using simplified API 520/521 equations. Identify API nozzle designation and check sizing against standard orifice areas.

API 520 / API 521Vapor Case Liquid CaseFire Case API Nozzle TablePrint-Ready
⚠ Preliminary only. Final relief device sizing must be performed and stamped by a licensed Professional Engineer per ASME Section VIII / API 520/521.
API 520 Vapor Case (Eq. 1)
A = W / (C × Kd × P1 × Kb)
  × √(T × Z / M)
A = Required orifice area (in²)
W = Required relieving capacity (lb/hr)
C = Gas constant (function of k)
P1 = Relieving pressure (psia)
T = Relieving temperature (°R)
⚗️
Relief Case
Select the governing overpressure scenario
💨 Vapor / Gas
💧 Liquid
🔥 Fire Exposure
API 520 Vapor Formula
A = W/(C·Kd·P1·Kb) × √(T·Z/M)
P1 = Pset × 1.1 + 14.696 (psia, 10% overpressure) · T in °R (°F + 459.67) · C = f(k) · Kb from back pressure ratio
🔩
API Standard Orifice Nozzle Designations
Select next standard size above calculated area
Designation Area (in²) Typical Inlet Size Status
Sizing Results
Required Orifice Area (A)
—
in²
⚠ Preliminary Estimate Only
These results use simplified API 520/521 equations for quick screening. Final sizing requires rigorous two-phase flow analysis, detailed thermodynamics, and PE stamp. Consult a licensed Process Safety Engineer.
📚 API 520/521 Reference Data ▸ Expand
C Factor vs. Cp/Cv Ratio (k)
kCkC
1.03151.4356
1.13271.5366
1.23371.6376
1.33471.8394
Typical Latent Heat Values
FluidL (BTU/lb)MW
Water (212°F)97018
Propane18444
Butane16658
Methane (LNG)22016
Ethylene20828
Benzene16978
Toluene15692
Naphtha (avg)120100

Need a PE-Stamped Relief Study?

Preliminary sizing is just the starting point. A complete relief and vent system study — including two-phase flow, scenario selection, vent header sizing, and ASME/API documentation — must be performed by a licensed PE. SafeGuard Projects partners with experienced pressure relief engineers for PSM capital projects.

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