You may have heard that there was a recent internal explosion in a Convault Aboveground Fuel Storage Tank. As is normal in our modern world, the news was quickly disseminated across the country by email. However, as has been true for centuries, news loosely communicated may begin only half-true and becomes increasingly corrupted as it travels. We thought you might want to know exactly what happened and why it happened. Following are the facts of the incident, determinations by Captain Tarkington who investigated the incident, and some recommendations to prevent such incidents in the future.
On June 28, 2010 there was an internal explosion in a 6,000 gallon aboveground fuel storage tank (AST) containing Ultra Low Sulfur Diesel (ULSD) at a US Border Patrol refueling station in Santa Teresa, New Mexico. This was the third internal tank explosion of Ultra Low Sulfur Diesel in New Mexico this summer, and the first to involve a Convault AST. There are also unconfirmed reports of internal explosions of tanks storing ULSD in other parts of the United States.
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 Border Patrol incident took place around 9:13 in the morning, when a fuel delivery truck brought a load of fuel to refill the AST. About 6 inches of diesel remained in the AST when the refill began. The tank did have a drop tube, but the tube was constructed of non-conductive fiberglass and it ended approximately 10 inches from the bottom of the tank leaving a 4” air gap. At this particular installation, the fuel is gravity fed from the delivery truck to an on-site pump, which pumps the fuel into the top of the tank. There have been some reports that temperatures at the site were over 100 degrees Fahrenheit, but this is incorrect as the incident occurred while it was still cool in the morning. The temperature of the fuel in the AST shortly before the fill began was recorded as 84 degrees F.
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.
You may wonder why the regular type of diesel is not being used, so there would be less chance of a static electricity sparked explosion. To improve air quality EPA has reduced the allowable sulfur content for diesel engines. The allowable sulfur content for ULSD in the US is now 15 ppm: much lower than the previous 500 ppm U.S. on-highway standard for low sulfur diesel (LSD). This in turn was down from regular diesel at about 3000 ppm. Reduction of sulfur in diesel fuel not only reduces emissions of sulfur compounds (a cause of acid rain), but also allows advanced emission control systems to be fitted that would otherwise be poisoned by these compounds. By December 1, 2010, all highway diesel fuel will be ULSD and by December 1, 2014 all diesel fuel including locomotive and marine diesel fuel will be ULSD.
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.
If you desire to learn more about storing ULSD, Rick Long has written an excellent article in the PEI Journal titled ULSD TANK MANAGEMENT, which you can access here. The article references the Clean Diesel Fuel Alliance's (CDFA) recently released Guidance for Underground Storage Tank Management at ULSD Dispensing Facilities. Most of the information in the article applies to shop-fabricated aboveground tanks as well.