Jet fires

For the Transformer character see Jetfire

Jet fires are high temperature flames of burning fuel released in a particular orientation and under pressure. The material burned is a continuous stream of flammable gas, liquid or a two-phase mixture. Jet fires are a significant hazard on hydrocarbon production and processing plant where the heat flux of the jet flame can cause rapid mechanical failure thereby compromising structural integrity and leading to incident escalation.

Context

The Piper Alpha disaster in 1988 demonstrated how the accidental release of hydrocarbon can lead to the catastrophic failure of an installation with the rupture of major pipeline risers.[1] Jet fires impinged on vessels, pipework and firewalls. Under these conditions the fire proofing material was compromised within a few minutes rather than one to two hours which had been specified. Even without direct impingement, the high thermal radiation emitted by jet flames also affected plant and would have been fatal to personnel.[2]

Characteristics

Jet fires, also known as spray fires, are turbulent diffusion flames of flammable material.[3] The characteristics of jet fires depend on a number of factors. These include: fuel composition; release conditions; release rate; release geometry; direction and ambient wind conditions. Some characteristics are:

  • Sonic releases of natural gas are characterised by high velocity, low buoyancy flames that are relatively non-luminous with low radiative energy.[3]
  • Jet flames of higher hydrocarbons are lazy, buoyant, luminous, with the presence of black smoke at the tail of the flame, they are highly radiative.[3]
  • The surface emissive power (SEP) of jet flames is in the order of 200 kW/m2 to 400 kW/m2.[3] Such flames have a temperature of 1350°C. These high heat fluxes can readily compromise the integrity and can lead to mechanical failure of plant and equipment.
  • Jet fires are a particular hazard to personnel. People are able to survive and escape from exposure to heat fluxes less than 5 kW/m2, higher fluxes are assumed to be fatal.[3]

Designing for jet fires

Process plant is generally protected by a pressure relief system. But local heating of a vessel by a jet fire may compromise the integrity of the vessel before the pressure relief device operates. The measures taken for protection against jet fires are as follows:[2]

  • Prevention of leaks using effective maintenance
  • Robust external insulation
  • Depressurising systems, to reduce the inventory and pressure in the plant
  • Isolation of leaks
  • Flange orientation and elimination
  • Emergency response
  • Depressurisation of the jet fire source
  • Use recognised heat intensities:[2]
    • Jet fire, 100 kW/m2 to 400 kW/m2
    • Open pool fire, 50 kW/m2 to 150 kW/m2
    • Confined pool fire, 100 kW/m2 to 250 kW/m2.

Water deluge can reduce the heat loading of plant so that its temperature is maintained below that which failure occurs, or that temperature rise is sufficiently reduced such that shutdown and depressurisation can take place.[1]

Older plant may have been sized on an earlier version of the Pressure relieving and depressuring systems standard[2] which did not include consideration of jet fires.

There are several international standards associated with jet fire tests:[4]

  • Jet fire tests for passive fire protection materials ISO 22829-1
  • Jet fire and hydrocarbon curve test EN 13381-4 and EN 13381-8

See also

References
  1. Hankinson, G. (2007). "Jet Fires involving releases of crude oil, gas and water". Process safety and environmental Protection. 85 (3): 221.
  2. American Petroleum Institute (2014). Pressure- relieving and Depressuring Systems, API Standard 521 (6th ed.). American Petroleum Institute. pp. 53, 54, 36.
  3. Health and safety executive. "HSE offshore: Fire and Explosion Strategy - Jet Fires". Retrieved 7 September 2023.
  4. "Jet fire testing". Retrieved 7 September 2023.
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