Calculators › Depressurizing System
🔥

Depressurizing System Design Calculator

API RP 521 Fire Case Analysis

Emergency Depressurizing System Calculator
Calculates required blowdown area, depressuring time, and wall temperature per API RP 521 fire case methodology. Selects appropriate API 526 standard orifice and verifies material temperature limits.
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:

Vessel & Pressure Parameters

ft³
psig
psig
Target: min(50% × MAWP, 100 psig)
°F
in
ft²
Area exposed to fire, up to 25 ft above grade

Fire Case Parameters

BTU/hr
Q = 21,000 × F × A0.82 (adequate drainage)
°F
Affects MDMT and max temperature limits
—
1.0 = bare  |  0.3 = fireproofed  |  0.15 = 2" insulation  |  0.075 = 4" insulation
min
API RP 521 recommends ≤15 min for fire case

Gas & Valve Properties

—
Typical: 0.62 (orifice), 0.975 (API certified PRV)
Air=1
—
—
Typical: 1.28-1.32 (natural gas), 1.40 (air/N₂)
BTU/lb-°F
lb/ft³
°F
%
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:
Fire Heat Input: Q = 21,000 × F × A0.82 BTU/hr
Flow Coefficient: C = 520 × √(k × (2/(k+1))(k+1)/(k-1))
Critical P Ratio: Pcrit/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")

Frequently Asked Questions

What does the Depressurizing System Calculator do?

It designs emergency depressurizing systems per API RP 521 fire case methodology, calculating blowdown valve sizing, depressuring time, and wall temperature.

What is API 521 fire case depressurization?

API 521 fire case requires vessels to depressurize to a safe level within a specified time to prevent rupture during fire exposure.

Does this tool include API 526 orifice selection?

Yes, the calculator includes accurate API 526 orifice selection for blowdown valve sizing.

Related Calculators

🛡️

ESD Valve Sizing

ESD valve sizing calculator for emergency shutdown systems.
💨

Blowdown Analysis

Calculate blowdown flow rates, depressurization times, and size orifices.
⚠️

Reaction Relief

Calculate relief loads for runaway chemical reactions and tempered relief.
💨

Blowdown Reaction Force

Calculate blowdown reaction forces, thrust loads, and concrete support block.