This is an interesting case were water washing caused an explosion.
There was a case where a vacuum truck operator, new on the job, was tasked with washing out and vacuuming a horizontal vessel in an oil treatment plant. The vessel was about 25 feet long and situated in an open below-grade concrete vault. The plant operators had drained the vessel and opened the manways at each end. All that remained in the vessel was heavy sediments at the bottom to be removed. The vacuum truck operator began power washing the interior and bottom of the tank with a fire hose from the outside to loosen the sediments on the bottom prior to vacuuming it out. Unbeknownst to the vacuum truck operator, the sediments contained entrained oil and gas fractions – some of which were volatile. As the operator continued to hose out the interior of the vessel, his actions broke up the solids on the bottom, and they began to release volatile vapors. The vacuum truck operator was not monitoring for LEL (Lower Explosive Limit) gases. The plant operators apparently were not on scene either. Because the vacuum truck had not grounded the fire hose to the vessel, a static charge developed and the flammable gas mixture in the vessel ignited. The found the lifeless body of the operator slumped against the concrete wall of the vault opposite the manway. Evidence of a fierce flame was apparent by the charring on the wall. The contents of the vessel apparently ignited like a rocket engine and propelled the operator (with a week on the job) to his death.
As a tragic lesson learned, from then on bonding straps were then required for water washing operations. No one there had predicted that washing a vessel out with water could cause an explosion.