A water treatment plant used an electrolytic process to generate sodium hypochlorite (NaOCl) from sodium chloride (NaCl). The strategy of using liquid sodium hypochlorite for disinfecting water instead of gaseous chlorine (CL2) is popular because the liquid is generally safer and falls under fewer OSHA and EPA standards. The further idea of generating the liquid sodium hypochlorite on an as-needed basis and in limited quantities also has certain obvious safety advantages.
One of the disadvantages of the electrolytic process is that hydrogen gas is also created as a byproduct. The hydrogen is supposed to be vented, by design, to the atmosphere before the liquid sodium hypochlorite passes into a holding tank.
For various reasons, in this instance it is believed that the hydrogen vent line was closed, thereby forcing the hydrogen gas into the liquid holding tank where it accumulated. In order to repair a leak in the tank, plant workers had drained the tank to within a few inches and then lowered an electric pump into the tank to remove the remaining liquid. When the switch was thrown to turn on the pump, the tank exploded. One worker was killed by the blast.
- Ventilation System
- Venting System
- Process Equipment
- Process Vessels
- Motive Power Systems
The mechanisms and rates by which hydrogen gas is generated and subsequently accumulated in the holding tank need to be fully understood by vendors and employees alike. Active venting, warning signs, and local alarms designed to activate when hydrogen ventilation lines are obstructed are essential.