In studying the incident report, you will notice that the incident was not caused by the AST. The incident could have happened regardless of who manufactured the AST. The only advantage of the Convault tank over the other tank manufacturers is that the concrete encasement contained the steel tank and prevented it from causing further damage.

The use of Ultra Low Sulfur Diesel in the US is a relatively recent development having only been required for on-highway vehicles in California as of September 1, 2006, and in most of the rest of the country since September, 2007. By December 1, 2010, all highway diesel fuel will be ULSD.

The procedure for this particular fuel delivery was:

• First the hose was connected,
• next the valve between the pump and the tank was opened,
• then the pump was turned on and
• last the truck valve was opened.

The explosion happened seconds after opening the truck valve.

The delivery driver and a representative of the US Border Patrol were both standing by the tank at the time of the explosion. One received a very minor injury and the other person was not injured at all other than ringing in his ears caused by the concussion. The Fire Inspector, Captain Kellen Tarkington stated that, “The concrete really did its job because if it had been a plain steel tank they would probably both be dead.”

In the Incident Report, Captain Tarkington concluded that the “explosion was accidental in nature”. However, in his observations he made the following points, one or a combination of which could have contributed to the accident:

• The fuel tanker hooked up to the pump was using a fill hose that may not have been properly grounded and bonded. (ConVault’s note: Grounding of the AST was also not confirmed prior to removal of the tank from the site.)
• The pump connected to the AST was running prior to opening the fuel valve on the fuel tanker. The pump may have pumped air inside the AST before it started pumping fuel. This may have helped in forming an explosive mixture in the AST.
• Drop tube material was fiberglass instead of metal. Fire codes allow fiber glass drop tube for class II and III petroleum products. The tank was used for the storage of diesel fuel on this particular site, which is class II product. However, the diesel fuel was Ultra Low Sulfur Diesel which has created controversial issues and may have also contributed to the problem. No one is sure of this phenomenon, but it is a subject for discussion by many different groups.
• Drop tube ending 10 inches above the tank bottom. Fire codes require drop to be a minimum of 6 inch from the tank bottom for gasoline fuels; but it is not a requirement for the class II and III fuels such as diesel. Again the Ultra Low Sulfur Diesel contents of the fuel might have changed the circumstances.
• Spark could have resulted from static electricity generated as a result of the fuel dropping down from the fill pipe into the tank.

Taking into consideration the above observations we make the following recommendations to prevent a similar incident in the future:

• Make sure the tank is properly grounded per NFPA 780.
• Make sure there is proper grounding between the delivery truck and the tank.
• If there is a ground mounted pump, it must be properly grounded.
• Use conductive drop tube that reaches within 6” of the bottom of the tank for all Ultra Low Sulfur Diesel fuel, just as is required for gasoline tanks.
• If an external pump is used to pump fuel from a tanker into the AST, the fuel valve of the tanker should be opened first, prior to turning on the pump.

If you have any questions about this incident, please call ConVault, Inc at 800-222-7099. A copy of the official Incident Report is available from Dona Ana County Fire and Emergency Services.