API RP 521 §5.15.2: For fire case, depressurize to 50% of design pressure or 690 kPa(g) [100 psig], whichever is lower, within approximately 15 minutes.

Calculation Mode:

Design (Size relief area) Verification (Check existing area)

Vessel & Pressure Parameters

Vessel Volume

ft³

Initial Pressure (P₁)

psig

Final Pressure (P₂)

psig

Target: min(50% × MAWP, 100 psig)

Gas Temperature

°F

Wall Thickness

Wetted Surface Area

ft²

Area exposed to fire, up to 25 ft above grade

Fire Case Parameters

Heat Input Rate (Q)

BTU/hr

Q = 21,000 × F × A^0.82 (adequate drainage)

Initial Wall Temperature

°F

Vessel Material

Affects MDMT and max temperature limits

Environment Factor (F)

—

1.0 = bare | 0.3 = fireproofed | 0.15 = 2" insulation | 0.075 = 4" insulation

Target Depressuring Time

min

API RP 521 recommends ≤15 min for fire case

Gas & Valve Properties

Existing Blowdown Area

in²

Discharge Coefficient (K_d)

—

Typical: 0.62 (orifice), 0.975 (API certified PRV)

Specific Gravity (SG)

Air=1

Compressibility Factor (Z)

—

Specific Heat Ratio (k = C_p/C_v)

—

Typical: 1.28-1.32 (natural gas), 1.40 (air/N₂)

Material Heat Capacity

BTU/lb-°F

Material Density

lb/ft³

Ambient Temperature

°F

Relative Humidity

Heat Transfer Coefficient

BTU/hr-ft²-°F

Typical: 5-10 for gas, 50-300 for boiling liquid

📚 Learn the Theory

Understand API 521 fire case methodology, relief vs. blowdown valves, and system integration.

Read Engineering Guide →

Quick Reference — Key Equations:

Need quick blowdown time? Use Blowdown Calc →

Fire Heat Input: Q = 21,000 × F × A^0.82 BTU/hr
Flow Coefficient: C = 520 × √(k × (2/(k+1))^(k+1)/(k-1))
Critical P Ratio: P_crit/P₁ = (2/(k+1))^k/(k-1)
J-T Cooling: T₂/T₁ = (P₂/P₁)^(k-1)/k

About This Calculator

Emergency depressurizing system design per API RP 521 (7th Ed.) fire case methodology.

API 521 Requirements:

Time: Depressure to 50% design or 100 psig within 15 min
Wall Temp: Keep below material rupture temperature
MDMT: Avoid brittle fracture from J-T cooling

Orifice Selection:

Uses API 526 standard orifice designations (D through T). Calculator includes 10% safety factor per API recommendations.

API 526 Orifice Sizes

                            D: 0.110 in² (1"×2")

                            E: 0.196 in² (1"×2")

                            F: 0.307 in² (1½"×2½")

                            G: 0.503 in² (1½"×3")

                            H: 0.785 in² (2"×3")

                            J: 1.287 in² (2½"×4")

                            K: 1.838 in² (3"×4")

                            L: 2.853 in² (3"×6")

                            M: 3.60 in² (4"×6")

                            N: 4.34 in² (4"×6")

                            P: 6.38 in² (4"×6")

                            Q: 11.05 in² (6"×8")

                            R: 16.00 in² (6"×10")

                            T: 26.00 in² (8"×10")

Depressurizing System Design

Vessel & Pressure Parameters

Fire Case Parameters

Gas & Valve Properties

📚 Learn the Theory

Design Results

About This Calculator

API 521 Requirements:

Orifice Selection:

API 526 Orifice Sizes

Depressurizing System Design

Vessel & Pressure Parameters

Fire Case Parameters

Gas & Valve Properties

📚 Learn the Theory

Design Results

About This Calculator

API 521 Requirements:

Orifice Selection:

API 526 Orifice Sizes

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