When a container is filled with a liquid, the liquid displaces most of the air. When the liquid is drained, air replaces the liquid again, unless the tank is continuously purged with an inert gas. The ignition sources are perhaps the most variable and unpredictable. A welder's torch or the arc from an arc welder are not the only sources of ignition. Static electricity discharges pose a major threat as well as pilot lights, electrical arcs from tools, switches, lamps and appliances which are not explosion proof, and cigarette smoking.
Static electricity produces an electrical arc when it discharges.
This arc can be of sufficient energy to be an ignition source. To create a static electricity charge all that is required are two dissimilar materials to closely pass by each other. This includes flowing liquids and gases, wood, plastics and metals. When the static energy is discharged, a spark usually occurs. In situations where steam cleaning is utilized, it is important to assure proper electrical grounding of the steam nozzle and tank to minimize the possibility of static charge buildup and spark discharge.