Flash Fire From a Hot Combustible Liquid

Global Congress on Process Safety
April 4, 2012 - 12:00am

A truck driver loading a tanker was fatally injured in a flash fire. The accident occurred at the loading rack of a petroleum refinery. The truck driver was on top of the tank trailer loading a liquid petroleum byproduct material similar to asphalt through an open hatch on the trailer. The material was a byproduct of the petroleum refining process and had a flash point of approximately 200°F, making it a Class III combustible liquid. The truck driver, a contractor employee, had been trained in loading procedures at the refinery, and he had received from his employer specific training on handling hot asphalt and other refinery byproducts. Evidence indicated that the truck driver was trying to loosen a stuck valve by heating it with a hand-held propane torch when the flash fire occurred. The extent of thermal damage evident on the surroundings was surprising considering the tanker held a Class III combustible liquid.

The truck driver hauled a variety of commodities including carbon black oil (CBO), hot asphalt, and creosote. His formal training for the safe handling of these liquids was restricted to hot asphalt safety. The MSDS's for the other commodities were not incorporated into his training. Interviews revealed that petroleum byproduct was being delivered to a customer as fuel supplement. The trucking firm's supervisors believed the material to be equivalent to hot asphalt. In fact, in training sessions an instructor demonstrated that the liquid (at room temperature) could not be ignited with a propane torch.

Class III combustible liquids typically do not generate a sufficient amount of vapor to achieve a flammable atmosphere. Furthermore, most organic vapors are heavier than air and would not be expected to form a flammable atmosphere in the area of the tank's hatch. During the investigation it was discovered that the material was stored at the refinery at a temperature above the flashpoint. Subsequent analysis showed that at its storage temperature the CBO formed a buoyant vapor. Analysis of the physical situation indicated that air, heavier than the hot CBO vapor, could flow through the loading hatch and displace the lighter CBO vapor. This type of flow phenomenon is called an exchange flow. We analyze the flow phenomenon and the resulting fireball behavior. This paper also addresses several of the safety recommendations for the loading of heavy petroleum products that are typically considered to be a very low hazard. This accident demonstrates the importance of treating combustible liquids with care especially when they are heated to a temperature at or near their flash point.

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