Fear the Heat

Everybody with good sense is cautious with gasoline. In open air it releases highly flammable vapor that quickly spreads over a wide area. Other hydrocarbon products such as motor oil, diesel or transmission fluid are nowhere near as volatile.

Confine gasoline to a storage tank and everything changes. Think about the fire triangle – heat, fuel and an oxidizing agent such as oxygen. Because gasoline vaporizes so rapidly any space above the liquid is likely to be too rich to ignite -- too much fuel and too little oxygen.

Other hydrocarbon products that vaporize at a slower rate pose a greater threat when stored. Each withdrawal sucks outside atmosphere into the tank that is not as quickly displaced. With heat from an external fire the vaporization rate can suddenly increase. At some point the ideal ratio of air to fuel is reached.

If that external fire concentrates on a portion of the tank above the product level all three elements of the fire triangle become reality. Flames need not penetrate the tank wall. Heat alone is enough for ignition. Hopefully, when that happens, the tank is closer to full than empty. Remember, it’s the fumes that burn.

The kind of hydrocarbon in the storage tank that exploded Oct. 12 at an asphalt plant in Rogers County, Oklahoma, may be the key to what happened. With a little more knowledge, firefighters might have anticipated the result.

Ordinarily, people refer to horizontal storage tanks as “bullet tanks.” Some say this alludes to the tank’s shape. Others, mostly firefighters, say it is what the tank becomes in any incident similar to Oklahoma if not securely fastened to the ground. Gaseous hydrocarbon such as LPG evaporates at atmospheric pressure and requires a pressurized vessel. In Oklahoma, the tank that exploded was technically an atmospheric pressure storage tank, suitable for most liquid hydrocarbon.

First, let’s dismiss the idea that the Oklahoma mishap was a BLEVE (boiling liquid expanding vapor explosion), the blast when a vessel containing a pressurized liquid above its boiling point ruptures. In a BLEVE, the end caps or heads of the storage tank would have been distorted from flat to hemispheric by the building pressure. Video of the Oklahoma incident shows the pressure buildup that tore the tank apart was instantaneous.

 If not a BLEVE, then what? Remember that fire burning beneath the used oil tank and the adjacent diesel tank for some time. Foam from the crash truck played out before the last of the fire could be extinguished. Fed from a broken fuel line on the diesel tank, the resulting three-dimensional fire proved difficult to put out using a single foam line.

Remember too that heat can increase the vaporization of hydrocarbon not ordinarily considered volatile. Normally, the flammable range of hydrocarbon is eight to 15 percent vapor and the rest air. In an atmospheric tank, air is drawn inside whenever liquid product is removed. Otherwise, it would fold up like a crushed beer can. It is possible that the attempt to extinguish the fire might have drawn more air into the tank as it cooled.

Let’s say the heated contents had brought the vapor above the liquid to within a flammable range. We have enough air and fuel for ignition. Flames outside impinging on the tank above the liquid level could have provided the heat. Flames below the liquid level would have contributed to vaporization but not heated the vapor to ignition.

Structurally, the weakest point is where the end caps are welded to the cylinder. With a vertical tank, the blast blows off the roof or the bottom. Apparently the ladder on the end of the horizontal tank acted as a hinge. The end cap turned 90 degrees straight up before separating from the tank. Oklahoma could easily have been a fatality incident without this lucky break.

The easiest way to have prevented this explosion would have been a second foam line to cool the tank above the vapor space. The threat is heat on the vapor space, not the fire under the tank. Putting out a running fuel fire with foam is, at best, a difficult proposition. Using Class A foam typical of compressed air foam systems (CAFS) makes the job impossible. While it lowers the surface tension of water allowing greater saturation, using Class A foam against a Class B hydrocarbon fire is contraindicated, big time.

Finally, why did the relief valve sound only a second and a half before the explosion? The firefighters were lucky to get that much notice. Something other than the instantaneous ignition triggered it. Possibly a pocket of water in the used fuel flashed to steam, building enough pressure to open the valve a heartbeat before ignition. Otherwise, the valve would have been overwhelmed by the same thing that blew out the end of the tank.

The firefighters in Oklahoma concentrated on putting the wet stuff on the red stuff. Unfortunately, the heat, not the fire, is the real danger when it comes to storage tank firefighting.