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Hydrocarbon Fire: Its Nature and the Dangers

Feb 10, 2020

Ultimately a Hydrocarbon fire is a type of combustible liquid fire and is considered one of the most dangerous classes of fire today. It comes in two types: pool fire and jet fire, both known It comes in two types: pool fire and jet fire, both known to spread rapidly and endanger people as well as damage equipment instantaneously when left unattended.  

This article aims to shed light on the nature, behaviour, as well as quickly touching base with regards to the fire protection measures required for the prevention and mitigation of hydrocarbon fire.

What is a hydrocarbon fire? 

Hydrocarbon fire is a liquid fire that usually occurs in industrial spaces and in the oil and gas sector. Hydrocarbons – referring to the molecular structure of something – made of hydrogen and carbon often generate significant vapour emissions, enough to create a flammable concentration which when combined with oxygen form a volatile compound.

There are two types of hydrocarbon fire: pool fire and jet fire. The first, pool fire, is characterised by leakage of the combustible liquid from a vessel or pipeline. The leaked fuel forms a pool or a fluid reservoir that lies dormant until the moment a nearby spark sets it off, exploding into flame. 

Jet fire, on the other hand, starts when a pressurised vessel or gas line ruptures, causing the flammable liquid to escape into the air in sprays or mists. Again, all it needs is a small spark and this becomes a flame thrower of catastrophic proportion.

For purposes of fire protection, hydrocarbon fires are given a Class B rating, which means that they can only be put out by dry fire-retardant chemicals.  

What triggers hydrocarbon fire? 

Hydrocarbon fires are in no way similar to cellulosic fires. Where the latter originates from light combustible materials like paper and wood, the sources for hydrocarbon fires are much more complex and much less common. You may want to read a blog we wrote on The Differences Between Cellulosic and Hydrocarbon Fires to get your head fully around all the differences.

But back to the fuel sources of a hydrocarbon fire. The first ones fall under the light hydrocarbon group which includes petrol, heptane, terpenes, and cyclohexane. The second category is called heavy hydrocarbons and include kerosene, gasoline, diesel, and the like. The last source of hydrocarbon fire falls in the aromatic hydrocarbon group like benzene and toluene. 

But to generalise broadly, most prevalent cases of hydrocarbon fires are triggered either by gasoline, liquid gases, solvents, or crude oil. 

Burning Behaviour of Hydrocarbon Fire

Hydrocarbon fires behave differently from cellulosic fires as they can reach 1,100 degrees Celsius in as fast as five minutes after the initial combustion (compared to a peak temperature of around 1000 degrees for cellulosic) and with heat fluxes of around 150kW/sqm. They are considered highly dangerous compared to fires ignited by simple combustibles because the elements involved in hydrocarbon fires not only have the capacity to burn on a massive scale but also potentially trigger an explosion if the fluid’s release cannot be controlled nor contained. 

One type of Hydrocarbon fire called a Pool fire interestingly, spreads through convection and radiation. Once a pool of flammable liquid is set alight, the escaping gas is heated by radiation which creates a feedback loop, making the gas more vapourised, thus increasing the surface fire. Heat is also transferred to the escaping gas because of the scorching flame, thereby spreading the flaming zone to a much bigger area. With convection and radiation contributing to heat release, pool fires are capable of heating up the surrounding spaces with a range of 30 to 50 kW/sqm. 

Jet fires, on the other hand, ignite within two minutes after the combustible gas or atomised liquid is unleashed from a pressurised pipe or vessel. As a result of the unanticipated release of liquid and an accompanying spark, a high-velocity jet flame occurs, which only stabilises when the leakage is stopped. 
Jet fires are more dangerous than pool fires because while they are localised at 1,000 degrees Celsius, they have a much bigger heat flux, double that of a pool fire’s, estimated at around 300 to 320kW/sqm. They can spread heat as much as 40 per cent of the flame length. 

How does hydrocarbon fire react with water? 

The human instinct is to put out a fire with water, however, when it comes to hydrocarbon fires, the scenario is different. Water and oil clearly don’t mix together so when you try to use water to stop a hydrocarbon fire, the result is chaos. 

When you try to stop the fire with water, the water will just sink into the ground or evaporate, carrying fuel particles with it, thus allowing the flammable liquid to disperse. These chemicals are heated up by the surrounding heat and burst back into flame worsening the situation. This happens because the burning fuel is way hotter than the boiling point of water. Instead of putting out the fire, water — the moment it touches the flame — will just create steam that will splatter and further spread the fire. 

Take note, however, that you can cool down exposed surfaces — those hot but not yet burning area s— with water. The cooling effect will delay the spread of fire, if not contain it in the flaming space alone. 

Hydrocarbon Fire Protection

The right way to extinguish a hydrocarbon fire is to use an agent that will effectively isolate the fuel source or to stop the flow of the combustible liquid. It must be able to inhibit the release of the flammable vapours as well as delay the combustion process because this class of fire re-ignites easily. Killing a hydrocarbon fire is only possible with a Class B fire extinguisher and agents made of dry, powdered chemicals.  
At Permax we do not actively sell any intumescents designed to protect steel from the heat of Hydrocarbon flame, though there are a number out there. If you want any advice on who to call, please get in touch and we will do our best to put you in touch with someone.

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