Flammability range is the concentration range in which gas or vapor of combustibles or fuels to normal air, on specific pressure and temperature, can produce flame propagation in the presence of an ignition source. The flammability range has a lower and upper limit which are presented in volume percent.
Lower Flammability Limit (LFL) dan Upper Flammability Limit (UFL)
LFL and UFL are the lower and lower limit of the flammability range, respectively. Table 1 shows the LFL and UFL of various fuel gases at atmospheric pressure and 20oC temperature.
Table 1. List of LFL and UFL of various fuel gases at atmospheric pressure and 25oC temperature*.
To help us understand what LFL and UFL are, let us take a look at fires due to gas leakage, particularly, liquified petroleum gas (LPG).
According to the Directorate General of Oil and Gas of Indonesia, LPG is classified into three types. The types are based on LPG’s main components. Those types are LPG propane, LPG butane, and LPG mix (a combination of propane and butane). Let us use LPG propane as an example.
When there is no leakage, the kitchen is full of air. Once the gas leaks, propane starts to fill the kitchen. At some point, the volume of propane will reach 2.1% of the air volume. As long as the concentration of the mixture between propane and air in the atmosphere lies within the flammability range, the atmosphere in the kitchen is considered flammable. A small ignition source (e.g., turning the stove on) can lead to flame propagation and causes a fire (or even an explosion). Thus, it is important to have good ventilation in a room where gases are stored or used.
Effect of Temperature and Pressure on Flammability Range
As stated above, the LFL and UFL in Table 1 are for gases under atmospheric pressure and 20oC temperature. Why is this information important? In fact, with different pressure and temperature, the LFL and UFL will be different. Increasing temperature and pressure will widen the gap between LFL and UFL (LFL shifts to lower concentrations and UFL to higher concentrations). Hence, the gas mixture will be easier to catch fire or, even worse, explode.