George,

There are two potential scenarios that you need to take into consideration in your investigation. The first is a BLEVE and the second is deflagration inside the vapor or head space in the tank.

For a BLEVE to occur in a stationary storage tank, the liquid must be heated to the right conditions to initiate the pehnomenon. Nothing in your description seems to point to a BLEVE in your case.

For a deflagration in the head space with the presence of liquid in the container, you need the vapor space to be within the flammability range at the point and time that a sufficient ignition source is available. With neat diesel fuel under these conditions, it would be too lean in the head or vapor space. At atmosphere pressure, the lower flammability limit is in the high 30 degree C range.

While I was a graduate student at WPI, I attended a thesis defense that was measuring the flashpoint of multi-component liquids. The work was performed because the large holds of ships that carried liquids were being cleaned with different solvents. It was found that the residual solvents, when mixed with some liquids, created a greater hazard than the neat liquids. The research showed that it did not take much contamination to produce a hazardous environment.

As you have indicated, you should have any residual liquids tested for composition and for at least the flashpoint. You may also want to investigate the history of the storage tank and conents as well as any tank used in the supply chain. Given the infrequency of such events with diesel storage tanks in this environment, a good question is "Why today?" Changes in pressure and temperature, at first glance, do not seem to change the lower flammability limits in your case, thus, contamination is a more likely scenario.


Also, both a BLEVE and a deflagration in the vapor space can occur when the tank is vented. Most large storage tanks where a BLEVE has occurred has been vented. The same can be true of a deflagration. It is the rate of pressure rise that is the important variable. Conservation of mass carries the day. The pressure will rise in the tank if the rate of pressure rise can overcome the rate of pressure relief offered by the vent. It is of course, dependent on the area of the vent and the density of the fluid that is being vented. At specific conditions, the vent can achieve "choked flow", this is the point where you can overcome the capabilities of the vent to relieve pressure. There are the same principles used in the design of "explosion vents" that are installed to mitigate the pressure rise in an enclosure as a result of an explosion.

Good luck and let us know the outcome of your investigation. This is an interesting case since you do not hear much about deflagrations in diesel storage tanks. You may find you had the right conditions or that you are missing some important infromation that you do nto currently have. In any event, an analysis of a deflagration potential will be benefitical.

Sincerely,

Doug Carpenter

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com