Holding a Flame in Place: The Balance of Burning
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Open the Flame Stability Calculator →A burner flame that sits steady at the burner tip is performing a delicate balancing act. The unburned mixture rushes outward while the flame tries to burn back inward, and only when these two speeds match does the flame stay anchored. Tip the balance either way, and the flame either races into the burner or blows out entirely.
Two Opposing Speeds
A stable flame is a truce between two velocities. The fuel-air mixture flows out of the burner at some speed; the flame tries to propagate back into that mixture at its own flame speed. When the outward flow exactly counters the inward burning, the flame front holds its position — anchored at the burner tip, neither advancing nor retreating.
Flashback: The Flame Wins
If the flow slows below the flame speed — too little gas rushing out — the flame overpowers it and travels upstream into the burner, a dangerous event called flashback. The flame burns where it should not, inside the equipment, risking damage and hazard. Flashback is the failure that occurs when the outward flow can no longer hold the flame at bay.
| Condition | Result |
|---|---|
| Flow too slow | Flashback |
| Balanced | Stable flame |
| Flow too fast | Blow-off |
Blow-Off: The Flow Wins
At the other extreme, if the mixture flows too fast, it outruns the flame's ability to burn back, and the flame detaches from the burner and is swept away — blow-off. The flame lifts off and can extinguish entirely. This is the opposite failure, where the outward flow overwhelms the flame rather than the flame overwhelming the flow.
The Operating Window
Between flashback at low flow and blow-off at high flow lies the stable operating window — the range of conditions where a flame stays put. Burner design aims to keep operation comfortably within this window across all firing rates, using flame holders and geometry to widen it. Assessing the ratio of flow speed to flame speed reveals how safely a burner sits inside that window.
Checking Stability
To assess flame stability, use the Flame Stability Calculator. Establish the flame speed with the Flame Speed Calculator, and the flashback margin with the Flashback Velocity Margin Calculator.
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