Flammable Gases:
Flammable gases can burn or explode under certain conditions. Flammable gases include acetylene, butane, ethylene, hydrogen, methylamine and vinyl chloride.
Gas Concentration within the Flammable Range:
The concentration of the gas in air (or in contact with an oxidizing gas) must be between its lower flammable limit (LFL) and upper flammable limit (UFL). The LFL and UFL are sometimes called the lower and upper explosive limits (LEL and UEL), respectively. The LFL of hydrogen gas in air is 4% and its UFL is 75% at atmospheric pressure and temperature. This means that hydrogen can be ignited when its concentration in the air is between 4% and 75%. A hydrogen concentration below 4% is too lean to burn whereas above 75% it is too rich to burn.
The flammable range of a gas includes all of its concentrations in air between the LFL and UFL. The flammable range of a gas is widened in the presence of oxidizing gases such as oxygen or chlorine, and by higher temperatures or pressures. For example, the flammable range of hydrogen in oxygen gas is 4% to 85% and the flammable range of hydrogen in chlorine gas is 4% to 89%.
Ignition Sources:
For a flammable gas within its flammable limits in air (or other oxidizing gas) to ignite, an ignition source must be present. There are many possible ignition sources in most workplaces including open flames, sparks and hot surfaces.
The auto-ignition or ignition temperature of a gas is the minimum temperature at which the gas self-ignites without any obvious ignition sources. Some gases have very low auto-ignition temperatures. For example, phosphine’s auto-ignition temperature of 100 °C (212 °F) is low enough that it could be ignited by a steam pipe or a lit light bulb.
Flash-back can occur with flammable gases. Many flammable compressed gases are heavier than air. If a cylinder leaks in a poorly ventilated area, these gases can settle and collect in sewers, pits, trenches, basements or other low areas. The gas trail can spread far from the cylinder. If the gas trail contacts an ignition source, the fire produced can flash back to the cylinder.
Oxidizing Gases:
Oxidizing gases include any gases containing oxygen at higher than atmospheric concentrations (above 23% to 25%), nitrogen oxides, and halogen gases such as chlorine and fluorine. These gases can react rapidly and violently with combustible materials such as,
• organic (carbon-containing) substances such as most flammable gases, flammable and combustible liquids, oils, greases, many plastics and fabrics
• finely-divided metals
• other oxidizable substances such as hydrazine, hydrogen, hydrides, sulphur or sulphur compounds, silicon and ammonia or ammonia compounds.
Fires or explosions can result.
The normal oxygen content in air is 21%. At slightly higher oxygen concentrations, for example 25%, combustible materials, including clothing fabrics, ignite more easily and burn much faster. Fires in atmospheres enriched with oxidizing gases are very hard to extinguish and can spread rapidly.
Oxygen itself is not flammable but strongly increases the combustion of flammable materials.
Hydrogen is a highly flammable gas.