At a wellsite, two workers operating a vacuum truck were vacuuming and cleaning fluids, sludge, and sediment from a low-pressure separator vessel (LPS). Other workers opened two hatches on the LPS and inserted their vacuum hose into a debris-catch opening. Suddenly, an explosion occurred inside the LPS, resulting in a flash fire. One of the two workers was thrown against an adjoining vessel by the force of the explosion. Both workers sustained serious injuries.