Relief Valve Sizing Calculator

Relief Scenario & Flow

Relief Scenario

Service Phase

Relief Flow Rate

Required relief capacity

Wetted Surface Area

ft²

For fire case heat input (API 521 Eq. 5)

Insulation Type

Vapor Fraction

0-1 for two-phase flow (API 520 Annex C)

Pressure & Temperature

Set Pressure

psig

PRV set pressure

Allowable Overpressure

Standard: 10% (Fire: 21%, Emergency: 16%)

Back Pressure

psig

Built-up or superimposed

Relieving Temperature

°F

Temperature at relief conditions

Valve Type Affects backpressure correction (Kb/Kw)

Fluid Properties

Use Natural Gas Properties Auto-calculates MW, Z-factor using Hall-Yarborough

Gas Gravity (Air=1)

Typical: 0.55-0.65 for pipeline gas

Molecular Weight

For custom gas composition

Specific Heat Ratio (k)

k = Cp/Cv, auto-estimated if empty

Compressibility (Z)

Auto-calculated for natural gas via Hall-Yarborough

Liquid Specific Gravity

Relative to water (water = 1.0)

Viscosity

For Kv correction (API 520 Fig. 31)

Latent Heat (Fire Case)

BTU/lb

Heat of vaporization at relief conditions

📚 Learn the Theory

Understand relief valve sizing principles, calculations, and industry applications

Read Engineering Guide →

Sizing Formulas (API 520 9th Edition)

Vapor/Gas (Equation 3a):

A = (W / (C × K_d × P₁ × K_b × K_c)) × √(T × Z / M)

Where C = 520 × √[k × (2/(k+1))^(k+1)/(k-1)]

Liquid (Equation 3.1):

A = Q / (38 × K_d × K_w × K_v × K_c) / √(ΔP / SG)

Two-Phase (Annex C - Omega Method):

Ω = x√ρ_v + (1-x)√ρ_l
A = W / (C × K_d × P₁ × K_b) × √(T × Z) / (Ω × √M)

Fire Case Heat Input (API 521 Eq. 5):

Q = 21,000 × F × A^0.82 (BTU/hr)
W = Q / λ (lb/hr)

Critical Pressure Ratio:

(P₂/P₁)_critical = (2/(k+1))^k/(k-1)

Notes:
• Standard orifice areas per API 526
• K_d = 0.975 (effective discharge coefficient)
• K_b/K_w from API 520 Figures 30-32 (backpressure correction)
• K_v from API 520 Figure 31 (viscosity correction)
• K_c = 1.0 (combination correction, typically)
• Z-factor via Hall-Yarborough correlation for natural gas
• Installation factor 0.9 applied to required area

Relief Scenario & Flow

Pressure & Temperature

Fluid Properties

📚 Learn the Theory

Recommendations

Sizing Formulas (API 520 9th Edition)

Sizing Results

Performance Data

Gas Dynamics

Relief Valve Sizing Calculator

Relief Scenario & Flow

Pressure & Temperature

Fluid Properties

📚 Learn the Theory

Recommendations

Sizing Formulas (API 520 9th Edition)

Sizing Results

Performance Data

Gas Dynamics

Related Calculators

Gas Density

Pipeline Hydraulics

Line Sizing

Pipe Volume