Flammable Gas Fires

 

Compartment fires generally involve combustion of natural and synthetic organic (carbon containing) materials such as wood, paper, and plastics. In order for flaming combustion to occur, fuel must be transformed into the gas phase through vaporisation or pyrolysis. Incomplete combustion of organic fuels results in production of carbon monoxide, soot, and a wide range of other products of combustion (many of which are flammable).

Pyrolysis

Pyrolysis is the chemical decomposition of a material due to the application of heat.

As fuels undergo pyrolysis, they release flammable (and toxic) gases. It is these gases that burn.

Note: Solids and liquids do not burn – they must be in the gaseous state to actually burn.

Smoke is comprised of not only the products of incomplete combustion, but also unburned pyrolysis products. As illustrated – Smoke is Fuel!

Flammable Gas Fires

There are different methods of distributing the required oxygen and fuel of combustion to a fire.

In a premixed flame, the oxygen and fuel are premixed beforehand, which results in a different type of flame.

In a diffusion flame, oxygen and fuel diffuse into each other; where they meet the flame occurs. A candle flame, for example,(a diffusion flame) operates through evaporation of the gaseous wax, which then mixes with surrounding oxygen and combusts.

Terminology

Flash Point — is the minimum temperature at which a liquid produces a sufficient concentration of vapour above it that it forms an ignitable mixture with air. At the flash point, the vapour may cease to burn when the source of ignition is removed.

The fire point, a slightly higher temperature, is defined as the temperature at which the vapour continues to burn after being ignited. Neither the flash point nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher.

Auto Ignition Temperature — of a substance is the temperature at or above which a material will spontaneously ignite (catch fire) without an external flame or spark. The flammable range is the range of a concentration of a gas or vapour that will burn if an ignition source is introduced. Below the flammable range, the mixture is too lean to burn and above the upper flammable limit, the mixture is too rich to burn. The limits are commonly called the "Lower Flammability Limit" (LFL) and the "Upper Flammability Limit" (UFL).

 Lower Flammability LimitUpper Flammability LimitFlash PointAuto Ignition Temperature
Acetylene 2.5 % 81 % −18 °C 305 °C
Ammonia 15 % 28 % 11  
Butane 1.9 % 8.4 % −60 °C 537 °C
Hydrogen 4 % 75 %   585 °C
Methane 5.3 % 17 %   537 °C
Propane, C3H8 1.7 % 10.9 %   450 °C

BLEVE — (pronunced "blevey") — Boiling Liquiid Expanding Vapour Explosion — is a phenomenon where a gas cylinder is heated due to the impact of fire. The liquid inside the cylinder boils producing vapour with a corresponding rise in pressure. Continued heating weakens the metal container. if the container fails, explosion occurs which is known as a BLEVE.

If you are not on the scene "immediately", you should prepare for evacuation of the immediate surrounds. If fire impacts a cylinder and you decide to extinguish the fire, the cylinder must be cooled. This is to lower the risk of cylinder failure and BLEVE. LPG cylinders should be cooled for a period after fire is out; this may take 30 or more minutes depending on temperature of cylinder. You will need to deploy hoses with large flow rates and establish a long term water supply.

  • Isolating a gas leak is the aim of gas fire fighting. This removes the fuel from the tetrahedron. If we extinguish the flame but do not isolate the leak we risk PUVCE.

PUVCE — (pronunced "puv–key") — Percussive Unconfined Vapour Cloud Explosion — when a cloud of gas is within its flammability range and finds an ignition source. Once ignited, the gas cloud can flash violently producing great heat and a destructive shock wave. Dispersion of a gas cloud can be done with water streams at about a 30 degree angle; this diffuses the gas into the air, keeping it below lower flammable limit; and keeps the gas cloud cool which further reduces ignition potential.