Summary
In September 2008, at a natural gas wellsite, four vacuum trucks were cleaning out two pressure vessels that had filled up with sand and fluid. During the operation, an explosion occurred within one of the vessels, which had been opened and de-pressurized for cleaning. One worker was fatally injured, and two others received severe burns.

Conclusions
Findings as to causes:

• An explosive atmosphere was allowed to develop within the tank. This was likely the result of one or both of the following: (1) the storage tank was not effectively purged, so the level of combustible substances was not at a safe limit; (2) the storage tank was not properly isolated and locked out before cleaning, allowing combustible substances to enter the tank.
• There was an uncontrolled ignition source, likely either a static discharge between the tank and the vacuum truck or a flashback from the flare stack.

• No fire and explosion hazard management plan was developed specifically for the tank-cleaning operation. As a result, a number of hazards were unrecognized and uncontrolled.
• The atmospheric testing was ineffective: the tester was insufficiently trained, the testing equipment and method were inappropriate, and the test results were not communicated to all affected workers.
• The owner, who was also the prime contractor, did not effectively coordinate the activities on this multiple-employer worksite.
• The workers were not adequately trained to recognize potential hazards, implement appropriate controls, or safely carry out the work being done at the time of the incident.