Hydrogen and chlorine concentrations at a certain plant are measured once each shift. On the morning of the explosion, the hydrogen concentration in the chlorine header leaving the cell bank was 0.47 percent. After passing through the chlorine coolers and liquid/gas separators, the hydrogen concentration of the gas streams increased to 2.5-3.2 percent H2, i.e., 63-80 percent of the lower flammability limit.

About 5 hours after the measurements were made, the DC power to the electrolysis cell bank was shut down because of intermittent power supply problems. At that time, a low-order explosion was heard from the chlorine dryer area of the plant. Thirty seconds later, chlorine gas began escaping from the chlorine header pumps, and another explosion occurred in the electrolysis cell room. The first explosion blew off the steel plate cover on the dryer by shearing off the 3/8-inch bolts on the cover. The second explosion blew off the heads of two electrolysis cells and broke 108 glass connections between the chlorine header and the cell bank. There was considerable damage to other chlorine processing equipment.

A loss investigation indicated that the hydrogen concentration in the chlorine header probably increased when DC power to the cells was cut off without completely shutting off the chlorine vacuum pumps. Thus, the vacuum pumps probably drew additional hydrogen into the chlorine header, and increased the hydrogen concentration further downstream where ignition occurred.

Although most of the incidents seemed to occur on the chlorine side of the cell bank, at least one analogous incident occurred on the hydrogen side of the cells, at a plant where poorly designed safety interlocks allowed the hydrogen compressor to operate after DC power to the cells was automatically shut down. This created a sufficient vacuum in the hydrogen header to draw residual chlorine in from the cell bank. It also broke the water seal on the hydrogen compressor, thus allowing air to enter. The hydrogen-chlorine-air mixture exploded in the hydrogen header, causing considerable damage to the header and the cell connections.

Incident Date
Dec 31, 1969
  • Motive Power Systems
  • Compressor
  • Electrical Equipment
  • Safety Systems
  • Electrical Safety Interlocks
Damage and Injuries
Contributing Factors
When Incident Discovered
Lessons Learned

This incident highlights the need to properly design safety interlocks. These safety interlocks need to be carefully incorporated into the initial building/plant designs and should consider all of the unexpected occurrences, such as the electrolysis cell bank losing power in this case, and the potential ramifications of such occurrences.