Calculators›Gas Rate & Time
⏱️

Gas Blowdown, Depressurization & Purge Rate

Blowdown · Purge · Inventory · VolumeAPI 521 & ASME B31.8

Industry-Standard Gas Flow Calculator
Calculates gas flow rates, depressurization times, purge requirements, and inventory per API 521, ASME B31.8, AGA-8, and NFPA 56 standards. Suitable for preliminary engineering design.
Calculation Mode:

Blowdown Parameters

ft³
psig
psig
min

Gas Properties & Conditions

°F
psia

📚 Learn the Theory

Understand gas rate time principles, calculations, and industry applications

Read Engineering Guide →

🔧 Need Orifice Sizing?

Use the Blowdown Calculator for rigorous orifice sizing and pressure-time curves per Modified Walworth equation

Blowdown Calculator →

⚠️ Safety & Engineering Notices

  • Blowdown calculations assume isothermal conditions. Actual blowdown follows non-linear pressure decay per API 521.
  • High depressurization rates may cause hydrate formation due to Joule-Thomson cooling (≈6-7°F per 100 psi for natural gas).
  • Verify discharge meets environmental regulations (EPA 40 CFR Parts 60, 63).
  • Purge calculations assume ideal mixing. Dead legs require special attention per NFPA 56.
  • For H₂S service, follow API RP 55 safety procedures.
  • All calculations for preliminary design only. Verify with licensed professional engineer.
Z-Factor Guide (per AGA-8):
0-200 psig: Z ≈ 0.99 | 200-500 psig: Z ≈ 0.95
500-1000 psig: Z ≈ 0.90 | >1000 psig: Z ≈ 0.85
For accurate Z, use AGA Report No. 8 or Standing-Katz charts

Industry Standards Referenced

  • API 521 - Pressure-Relieving and Depressuring Systems
  • ASME B31.8 - Gas Transmission and Distribution Piping Systems
  • AGA Report No. 8 - Compressibility Factors of Natural Gas
  • GPA 2172 - Calculation of Gross Heating Value, Relative Density, and Compressibility
  • NFPA 56 - Standard for Fire and Explosion Prevention During Cleaning and Purging
  • API RP 55 - Recommended Practice for Conducting Oil and Gas Operations Involving H₂S
  • ISO 23251 - Petroleum and Natural Gas Industries - Pressure-Relieving Systems

Frequently Asked Questions

What does the Blowdown Time Calculator do?

It calculates gas blowdown time, blowdown rates, gas purge, and depressurization rates using API 521 and ASME B31.8 standards.

What standards does this gas flow rate calculator comply with?

It complies with API 521, ASME B31.8, and NFPA 56 for pipeline engineering calculations.

Can this tool calculate gas purge requirements?

Yes, the calculator handles gas purge and depressurization rates and requirements.

Related Calculators

💨

Blowdown Analysis

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

Gas Flow Through Orifice / Hole

Calculate gas flow rate through an orifice or hole.
⛽

Gas vs Diesel Cost

Compare natural gas vs diesel fuel costs for power generation and compression.
🔥

Disposal System

Size flare headers and knockout drums per API RP 521 and GPSA.