An explosion occurred in a petroleum product storage tank! at a refinery. The liquid petroleum product was a heavy oil used as an asphalt extender. There were no injuries, but the cleanup was costly. The storage tank was one of several which received the product stream from a dehydration unit. The accident occurred shortly after the refinery was brought back on-line following a shutdown for scheduled maintenance. This was the first incident of this kind to occur at this facility. Analysis of the process data and eyewitness observations indicated that the dehydration tower, which was supposed to be maintained at a minimum of 100 degrees C during the shutdown, was allowed to drift below 100 degrees C for an unknown period of time. This deviation enabled liquid water to enter the storage tank three operational factors contributed to the accident. Corrective actions were recommended to prevent a recurrence of a similar incident.