Most off-road machinery is diesel-powered. Diesel fuel is widely regarded as safe to handle and store. In its liquid form, that is true for the most part. In its vapor form, diesel is very dangerous and can catch fire (or explode) easily in the presence of an accelerant such as fan air or oxygen.

Diesel is a stable liquid at normal outdoor temperatures. At temperatures above 100 degrees, the diesel liquid can begin to vaporize. The vaporization rate increases as the temperature increases.

Diesel vapors can ignite and explode when mixed with air. The vapors are explosive over a wide vapor-to-air mixture range. The lower explosive limit is less than 1%, and the upper explosive limit is about 10%. That means that diesel fuel vapors from a storage tank can explode with as little as 1% of the air being diesel fuel vapors.

Welding on a fuel tank is one commonly encountered hazard associated with diesel fuel. If the tank contains diesel vapors, a welding arc can easily cause an explosion.

Tank repair explosions have occurred with storage tanks, vehicle tanks, and machinery tanks. OSHA regulations specifically prohibit performing welding or cutting in the presence of explosive vapors. (See OSHA Standard 1910.252 for general safety requirements related to welding, cutting, and brazing: www.osha.gov/SLTC/weldingcuttingbrazing/)

One way to prevent an explosion of diesel vapors during weld repair of a tank is to remove all the fuel and vapor from the tank. If available, fill the tank with an inert gas such as argon or carbon dioxide (dry ice can be used, too) to eliminate the diesel fuel fumes. The liquid fuel should be completely removed and all of the vapor should be purged. Sniffers are a readily available tool and are required for testing for the presence and concentration of fuel vapors.

There are documented instances of a person using a pocket lighter as a light source to look down into a diesel fuel tank to check the fuel level. NEVER use or bring an ignition source anywhere near a fuel storage tank or fuel storage area!

The consequences of a diesel vapor explosion include severe injuries and fatalities. Treat diesel fuel in the same manner as gasoline or any other volatile fuel. Keep fuels and ignition sources separate.

Please follow equipment manufacturers’ recommendations for safe operation and maintenance procedures.