Calculators›Blowdown
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Blowdown Calculator

Modified Walworth / API 521

Pressure Blowdown Calculator
Determines gas release rates and volumes during emergency depressurization events. Calculate blowdown time from orifice size, or size the orifice for a target time. Per API RP 521 depressuring guidelines.

Pipe Segment & Pressures

psig
psig
miles
inches

Vent Stack & Valve

inches

Gas Properties & Conditions

Air=1
°F
ft
psia

📚 Learn the Theory

Understand blowdown principles, calculations, and industry applications

Read Engineering Guide →

🔧 Quick Flow Rate Estimate?

Use the Gas Rate & Time Toolkit for quick blowdown flow rates, purge requirements, and gas inventory calculations

Gas Rate Calculator →

About This Calculator

Calculates pipeline depressurization (blowdown) time using modified Walworth equation with compressibility correction.
View Full Depressurizing System Design Tool →

Key Considerations:

  • API 521: 15 min to 100 psig or 50% MAWP
  • Choked Flow: Critical pressure ratio ~0.546 for natural gas
  • Z-Factor: Papay correlation for compressibility
  • Temperature: Joule-Thomson cooling effect

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