What is a jet fire?
A jet fire is the ignited, momentum-driven flame produced when pressurised flammable fluid escapes from a hole. Unlike a pool fire, it is directional, extremely erosive in the near field, and can reach far enough to impinge directly on adjacent equipment — which makes it the dominant escalation mechanism on most high-pressure hydrocarbon facilities.
Why jet fires dominate escalation
Flame impingement delivers heat fluxes in the region of 150 to 350 kW/m2 with strong convective contribution. Unprotected steel exposed to that can lose structural strength in minutes, and a pressurised vessel exposed to a jet flame can fail well before its relief system offers meaningful protection. Escalation from one item to another is therefore assessed mainly on impingement and on the 37.5 kW/m2 radiation contour.
What controls flame length
- Mass release rate — flame length grows with release rate, so hole size and pressure drive the geometry.
- Fuel heat of combustion and stoichiometry.
- Wind, which tilts and can lift or partly detach the flame.
- Release orientation, which sets which equipment lies in the flame path.
Modelling approach
Jet fire modelling starts with the source term: release rate from the discharge calculation. Empirical correlations then give flame length and shape, and a solid-flame model with a surface emissive power and view factor gives radiation at receptors. Two-phase and liquid jets produce longer, sootier flames with higher emissive power than a clean gas jet.
Because the flame is directional, most studies model several release directions — horizontal in multiple bearings, plus vertical — and take the worst credible orientation for each target.
Mitigation
- Emergency shutdown and blowdown to cut inventory and shorten flame duration.
- Passive fire protection sized for jet fire, not pool fire, exposure.
- Layout separation so that credible flame lengths do not reach critical equipment.
- Fire and gas detection with a confirmed executive action to isolate.
Frequently asked questions
How long can a jet flame be?
It scales with release rate. A small leak may produce a flame of a few metres, while a large high-pressure gas release can produce a flame tens of metres long. The flame length should be calculated from the specific source term rather than assumed.
Is jet fire worse than pool fire?
For escalation, usually yes — jet fires deliver higher local flux, arrive with momentum and can impinge directly. Pool fires can still be more onerous for large liquid inventories and for prolonged burning duration.
Does blowdown prevent a jet fire?
No, but it limits it. Depressuring reduces the release rate over time and shortens the flame, which is exactly why API 521 depressuring targets are framed around limiting vessel wall temperature during fire exposure.
Calculators for this topic
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Flare Radiation — Flare thermal radiation calculator
- Two-Phase Blowdown — Two-phase blowdown and depressurisation calculator