API 521 Flare Purge Gas Rate (Husa) Calculator
Calculate continuous purge gas flow rates (SCFH) and velocity to prevent atmospheric oxygen ingress and internal burnback explosions per API 521 and the Husa correlation.
Flare Stack & Tip Dimensions
Purge Reduction Seal & O2 Limit
Purge Flow Rates & Operating Costs
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
Why is continuous purge gas required in flare headers?
When relief valves are closed, the flare header is stagnant. Atmospheric air naturally diffuses down through the open flare tip into the stack due to wind gusts and thermal buoyancy. If air mixes with trapped flammable gas, a flashback from the pilot burner can trigger a catastrophic internal header explosion.
What is the Husa purge gas correlation?
The Husa equation is an empirical standard adopted by API 521 to calculate the minimum continuous sweep velocity required to hold the oxygen concentration below 0.5% to 1.0% at a specified depth (typically 25 to 50 ft) below the flare tip. It accounts for tip diameter and the molecular weight ratio between purge gas and ambient air.
How does a velocity seal or fluidic seal reduce purge gas costs?
A velocity seal (cone baffle) forces entering air into an internal reverse eddy that is swept back out by rising purge gas, reducing purge requirements by 50%. A fluidic (molecular) seal uses a labyrinth flow reversal where buoyant gases form a permanent gas trap, reducing purge consumption by up to 75% to 85%.