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A Brief Guide To Combustion

Regardless of your industry, it’s good to know how combustion occurs. So here’s a brief outline:

Combustion is the exothermic chemical reaction of a fuel in the presence of oxygen which results in the release of energy in the form of heat and light. The reaction zone between burnt and unburnt products is known as a flame.

For combustion to occur and a flame to be present, the reaction requires three elements: oxygen, ignition and fuel.

When a flammable mixture is ignited the flame will propagate from the ignition source within the volume of the combustible fuel-air mix. Within pipeline systems this propagation is usually upstream (known as flashback) against the flow of gas.

The flammability of gas

The flammability of gas can be defined by its explosion limit. Each fuel gas has an explosive range defined by an upper (UEL) and lower (LEL) explosion limit in the presence of oxygen outside of which it is not combustible.

Lower Explosion Limit

The lowest concentration at which a fuel/air mixture will burn. Below this the mixture is too lean or the fuel is Insufficient. 

Upper Explosion Limit

The highest concentration at which a fuel/air mixture will burn. Above this the mixture is too rich i.e. there is not enough oxygen.

When the concentration of fuel is neither too low (lean), nor too high (rich) you get perfect, or stoichiometric conditions. In other words, complete combustion.

As well as this, some fuels have LEL of <1% (Kerosene, Jet A) in air, meaning a small mixture/leak can result in dangerous conditions. Some fuels such as hydrogen have very wide explosive ranges (4% – 75%) in air, making them particularly hazardous. Start-up/shut down conditions must be taken account of, as most accidents take place during upset conditions.

 Explosion risks

When it comes to navigating explosion risk factors, Elmac should be the first company you turn to. We’re experts at helping companies work out the risk factors involved: the likelihood that explosive atmosphere will occur, and the potential for ignition.

The types of questions we use to assess the risk include:

  • Are flammable gases/vapours present?
  • Can dispersion of the fuel in air create an explosive atmosphere?
  • Where can these explosive atmospheres occur?
  • Is the potential for creation of an explosive atmosphere reliably prevented?
  • Are the areas in which these explosive atmospheres are present controlled?
  • Is the potential ignition controlled?

In many cases it is not possible to avoid the potential for a gas/vapour explosion, so its mitigation must then be considered. And the answer is invariably: flame arresters.

Explosion characteristics

ExplosionBrief DescriptionFlame Arrester
Unconfined deflagrationCaused by ignition of a flammable vapour cloudEnd of line deflagration arrester

No physical restriction on the associated expansion of volume – flame front travels below the speed of sound due to the quick dissipation of the heat and pressure into atmosphere.

ExplosionBrief DescriptionFlame Arrester
Confined deflagrationCaused by ignition of a flammable gas mixture within a pipe.In line deflagration arrester

Gas is compressed resulting in higher pressure, accelerating the flame front – initial stages are sub sonic

ExplosionBrief DescriptionFlame Arrester
Confined detonationCaused by acceleration of a deflagration flame front and transition of pressure wave to shock wave within a pipe.Unstable detonation arrester

Deflagration to Detonation Transition (DDT) – flame speed reaches sonic velocity resulting in auto ignition of the gas at overdriven detonation stage. Flame speeds of >3000m/s and high pressures, over 100bar in some cases.

Explosion TypeDeflagration
Typical Flame Speeds ~100 m/s (subsonic) ~1000 m/s (supersonic)
Typical Peak Pressures 1 – 7 bar25 – 75 bar
Flame Arresting MechanismHeat TransferShock Attenuation

The initial conditions of the gas and the pipework layout affect the explosion characteristics. The process details needed to specify the correct type of in line flame arrester include:

  1. Gas explosion group or composition
  2. Line size (if known)
  3. Operating pressure
  4. Operating temperature
  5. Distance from source of ignition
  6. Orientation of pipework (number of bends etc.)
  7. Gas flow rate and pressure drop limitations