A flammable gas burns only when fuel, oxygen and an ignition source are present and the fuel-air mixture lies between the lower and upper explosive limits (LEL and UEL). Hydrogen is explosive from about 4 to 75 % in air, methane about 5 to 15 % and carbon monoxide about 12.5 to 74 %. Removing oxygen, keeping the mixture outside the limits, or removing ignition sources all prevent a fire.
- Explain the fire triangle and the explosive limits
- State approximate limits for hydrogen, methane and CO
- Compare ignition energies and why hydrogen is special
- Distinguish process analysers from gas detectors
The fire triangle
Fire needs three things: fuel, an oxidiser (usually oxygen in air) and an ignition source. Remove any one and it cannot start. Safety design uses all three: keep fuel contained, keep oxygen out or below a limit, and control ignition sources, including electrical equipment.
Explosive limits
A mixture of flammable gas and air burns only within a range of concentration. Below the lower explosive limit (LEL) there is too little fuel; above the upper explosive limit (UEL) there is too little oxygen. In between, the mixture can burn or explode.
| Gas | LEL, % in air | UEL, % in air |
|---|---|---|
| Hydrogen | about 4 | about 75 |
| Methane | about 5 | about 15 |
| Carbon monoxide | about 12.5 | about 74 |
| Propane | about 2.1 | about 9.5 |
| Ethylene | about 2.7 | about 36 |
| Hydrogen sulphide | about 4 | about 44 |
Values are approximate and depend on temperature and pressure; use the figures in your safety data sheets for design.
Ignition energy
Some gases ignite with very little energy. Hydrogen needs a tiny spark, of the order of a hundredth of a millijoule, compared with a few tenths of a millijoule for methane. That is why hydrogen service calls for the most demanding equipment group, IIC: Hazardous area classification: zones, gas groups, T-Classes.
A wide explosive range plus very low ignition energy makes hydrogen easy to ignite and hard to keep outside the limits.
Analysers versus detectors
- A gas detector warns that a gas has reached a hazardous level, usually a percentage of the LEL, in ambient air. It is a safety device for people and plant.
- A process analyser measures concentration in the process with defined accuracy, for control, protection or quality: What is online gas analysis? A plain introduction.
- An analyser can be part of a protection scheme, as when oxygen in a blanketed vessel is monitored to keep it outside the flammable region: What is limiting oxygen concentration (LOC).
Process analysers are not personal gas detectors and do not replace area gas detection.
Three ways to stay safe
- Stay below the LEL or above the UEL by dilution, ventilation or enrichment, and prove it by measurement.
- Remove oxygen by inerting with nitrogen or another inert gas: Inerting and purging: how they work and how to prove them.
- Control ignition sources by using certified equipment in classified areas: Protection concepts: flameproof, intrinsic safety, purging.
- Fire needs fuel, oxygen and ignition.
- Mixtures burn only between the LEL and the UEL.
- Hydrogen: about 4 to 75 %; methane: about 5 to 15 %; CO: about 12.5 to 74 %.
- Process analysers and gas detectors are different devices.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What does the LEL represent?
The lowest fuel concentration in air that can burn or explode; below it there is too little fuel.
Why is hydrogen especially hazardous?
It has a very wide explosive range and needs very little energy to ignite.
Name the three ways to prevent a gas explosion.
Keep outside the explosive limits, remove oxygen, and control ignition sources.
Key terms
Frequently asked questions
What is the explosive range of hydrogen?
About 4 to 75 % by volume in air.
Is a process analyser a safety device?
It can be part of a protection scheme, but it is not a personal gas detector and carries no SIL claim from us.
What does LEL stand for?
Lower explosive limit, also called lower flammable limit.
Does pressure change the explosive limits?
Yes; the range widens with pressure and temperature. Use design data for your conditions.
