Acetylene, ammonia, hydrogen, propane, propylene and methane are all flammable gases, also known as fuel gases. They burn when mixed with an oxidant and provided with a source of ignition.
The diagram below shows the flammability concentration limits for an air atmosphere. The orange bars show the percentage range within which fuel gases pose a particular danger of igniting or exploding.
Starting from low concentrations,the risk of fire increases as the percentage of fuel gas rises. Once the concentration exceeds the higher value (UEL), the air becomes “too rich” to burn, reducing the chances of ignition.
In any container or confined space, even small quantities of escaping fuel gas can form an ignitable mixture under the right conditions. Yet there is also a small risk of fuel gases reaching their lower threshold limit in open spaces or large, naturally ventilated work areas.
Fuel gas that has leaked may form an ignitable mixture with the surrounding air, resulting in fire or explosion. Some of these gases are therefore odorised so that leaks can be more easily identified by smell.