Fire Relief Sizing

Vessel Geometry & Configuration

Vessel Orientation

Vessel Type

Head Type

Vessel Diameter

Vessel Length/Height (T-T)

Liquid Level (% of diameter)

Grade Level (from vessel bottom)

API 521 §5.15.1.1

Only wetted area within 25 ft (7.6 m) above grade/fire base is considered. Flames can exceed 130 ft, but empirical data shows this height is adequate for relief sizing.

Fire Case & Environmental Conditions

Fire Type

Drainage & Firefighting

Environmental Condition

Environment Factor (F)

—

Environment Factor (F) — API 521 Table 5

Bare vessel 1.0

Water deluge ≥0.25 gpm/ft² 0.5

Depressuring + drainage 0.3

Fireproof insulation ≤0.3

Underground storage 0.03

Fluid Properties & Relief Parameters

Fluid Type (Quick Fill)

Molecular Weight (MW)

g/mol

Latent Heat of Vaporization (λ)

BTU/lb

Specific Heat Ratio (k = Cp/Cv)

—

Compressibility Factor (Z)

—

Specific Heat Ratio (k)

Critical for C coefficient calculation. Typical values: Propane 1.13, Butane 1.09, Air 1.40, Steam 1.33. Use process simulator data at relieving conditions when available.

Relief Set Pressure

psig

Allowable Accumulation

Back Pressure

psia

Relieving Temperature

°F

📚 Learn the Theory

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⚠️ Fire Case Accumulation

Per API 520 Table 2: Max 21% for fire case (single device). This allows higher relieving pressure to maximize PSV capacity during external fire emergency.

📐 API 526 Orifice Reference

Standard Orifice Areas (in²)

D0.110

E0.196

F0.307

G0.503

H0.785

J1.287

K1.838

L2.853

M3.60

N4.34

P6.38

Q11.05

R16.00

T26.00

📚 Standards Reference

API RP 520 Part 1 — Sizing, Selection, and Installation of Pressure-relieving Devices (10th Ed.)

API Std 521 — Pressure-relieving and Depressuring Systems (6th Ed., 2014)

API Std 526 — Flanged Steel Pressure-relief Valves (5th Ed.)

Disclaimer

For preliminary sizing only. Final PSV selection requires manufacturer data and verification by qualified engineer.

📘 Calculation Methodology

// API 521 Empirical Heat Input (Pool Fire)
Q = C × F × A^0.82

// Where:
Q = Heat absorbed (BTU/hr)
C = 21,000 (adequate drainage) or 34,500 (inadequate)
F = Environment factor (0.03 to 1.0)
A = Wetted surface area (ft²)

// API 520 Relief Area (Critical Vapor Flow)
A = (W / (C × K_d × P₁ × K_b)) × √(TZ/M)

// C coefficient from k:
C = 520 × √(k × (2/(k+1))^(k+1)/(k-1))

Key Assumptions

• Liquid at saturation (no superheat correction K_sh=1.0)
• No rupture disk upstream (K_c=1.0)
• Preliminary discharge coefficient K_d=0.975
• Critical flow conditions (back pressure < critical)

Vessel Geometry & Configuration

Fire Case & Environmental Conditions

Fluid Properties & Relief Parameters

📚 Learn the Theory

⚠️ Fire Case Accumulation

📐 API 526 Orifice Reference

📚 Standards Reference

📘 Calculation Methodology

Results

Related Calculators

Gas Density

Pipeline Hydraulics

Line Sizing

Pipe Volume