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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

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

Frequently Asked Questions

What does the Blowdown Calculator calculate?

It calculates blowdown flow rates, depressurization times, and sizes orifices for emergency depressurization per API 521.

Does this tool generate pressure-time curves?

Yes, the calculator includes pressure-time curves and Joule-Thomson cooling analysis for blowdown scenarios.

What standard is used for blowdown orifice sizing?

The calculator follows API 521 for sizing orifices and calculating emergency depressurization parameters.