Liquid gas is highly flammable, but the combustion process is more complex than simply lighting the liquid fuel. The term “liquid gas” commonly refers to two primary energy sources: Liquefied Petroleum Gas (LPG) and Liquefied Natural Gas (LNG). Both LPG (a mixture of propane and butane) and LNG (predominantly methane) are stored in a liquid state to dramatically reduce their volume for efficient transportation and storage. The liquefaction process allows these fuels to hold hundreds of times more energy in a given container than they would as a gas.
The Mechanism of Liquid Gas Flammability
Liquid gas itself does not burn; fire can only occur when the substance is in its gaseous state and mixed with air. The process of combustion requires the liquid fuel to first vaporize, transforming into a gas that then combines with oxygen. This vapor-air mixture must then encounter an ignition source, such as a spark or a hot surface.
For combustion to happen, the concentration of the gas vapor in the air must fall within a specific range known as the flammability limits. The Lower Explosive Limit (LEL) is the minimum concentration of vapor required for ignition, below which the mixture is too “lean” or diluted to burn. Conversely, the Upper Explosive Limit (UEL) is the maximum vapor concentration, above which the mixture is too “rich” in fuel and lacks enough oxygen to combust.
If a leak occurs, the escaping liquid flashes to a large volume of gas vapor, and it is this vapor cloud that poses the fire or explosion hazard. For instance, propane has an LEL of approximately 2.1% and a UEL of 9.5% by volume in air, meaning it is highly combustible only within that specific range.
Characteristics of Common Liquid Gases
The flammability characteristics of liquid gases are influenced by their chemical composition and how they are stored. Liquefied Petroleum Gas (LPG) is a mix of propane and butane, both liquefied under moderate pressure at ambient temperatures. Propane vapor is about 1.5 times heavier than air, while butane vapor is about 2.0 times heavier. If LPG leaks, the vapor cloud will sink to the ground, accumulating in low-lying areas, pits, or basements, creating a significant hazard zone.
Liquefied Natural Gas (LNG) is primarily composed of methane, which requires cryogenic cooling to approximately \(-162^{\circ}\text{C}\) to remain in its liquid state near atmospheric pressure. In contrast to LPG, methane vapor is much lighter than air, with a specific gravity of about 0.6. A methane leak will cause the vapor to rise and disperse relatively quickly in an open environment, which generally makes the hazard zone different from that of LPG.
The flammability ranges also differ. Methane has an LEL of about 5% and a UEL of 15% to 17% in air. Butane’s flammable range is tighter, starting at an LEL of approximately 1.8% and extending to a UEL of about 8.4%.
Principles of Safe Storage and Handling
Safe storage requires careful consideration of the physical properties of the specific liquid gas being used. Since LPG vapors are heavier than air, storage containers must be kept in well-ventilated areas where leaked gas cannot pool near the floor or in enclosed spaces. Storage areas should be free from drains, pits, or culverts that could trap the sinking vapor.
For containers holding LNG (methane), which produces a vapor lighter than air, ventilation must be directed upward to allow the rising gas to safely dissipate. Cylinders and tanks must always be secured and stored upright. This ensures the pressure relief valve functions correctly and prevents the liquid phase from escaping through the valve. Tanks should also be situated away from any heat sources, as excessive temperature increases the internal pressure and the risk of a rupture or leak.
Leak Detection and Monitoring
Leak detection should be performed using soapy water, which will bubble at the source of a leak, and never with an open flame. Gas detectors installed in storage or usage areas must be placed appropriately based on the gas vapor density:
- Detectors for LPG are mounted low to the ground.
- Detectors for methane are mounted high.
Even an empty tank may contain enough residual vapor to form an explosive mixture if air is allowed to enter.