Explosion of at the Chlorine Electrolyzer of a Water Treatment Plant
The incident occurred at a tank of the chlorine electrolyzer of a water treatment plantThe plant products were sodium hypochlorite as a disinfectant for water treatments. The production process used was electrolysis of NaCl brines, which has hydrogen as by-products and has to be vented. Perhaps, the cause of this event was the erroneous closure of the hydrogen vent line, forcing the hydrogen gas into the liquid holding tank where it accumulated. During a maintenance stop, plant workers had drained the tank to within a few inches, to be able to repair a leak. They lowered then an electric pump into the tank to remove the remaining liquid. When the pump was switched on, the tank exploded.
Event Date
December 31, 1969
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
The immediate cause was the ignition of the hydrogen present in the NaCl tank. According to the investigation of the company CHEMAXX, the presence of hydrogen in the tank was probably due to the failure of or wrongly operated hydrogen venting system. The root cause is then probably a wrong procedure (ill-designed, or wrongly implemented).
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
water treatment plant, vent, NaOCl storage tank, pump, switch
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
The water treatment plant used an electrolytic process to generate sodium hypochlorite (NaOCl) from sodium chloride (NaCl). NaCl + H2O + ENERGY NaOCl + H2At the moment of the explosion maintenance works were ongoing on the NaOCl tank.
Currency
Lessons Learned
Lessons Learned
The use of liquid sodium hypochlorite (NaOCl) instead of the more classic gaseous chlorine (CL2) has the advantage to have to handle a liquid, which is generally safer. The disadvantage is the production of hydrogen as a by-product of electrolysis. The hydrogen has to be safely handled, for example by venting it to the atmosphere before the storage of the sodium hypochlorite in a tank.The company that carried out investigation (Chemaxx) identified the need to guarantee active venting hydrogen. They required also warning signs and the implementation of an alarm system able to detect failure of the venting system.
Event Nature
Emergency Action
Unknown
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
originally a CHEMAXX investigation, not available anymore.
This event is also in H2TOOLS,
https://h2tools.org/lessons/hydrogen-explosion-water-treatment-facility
For an overview of the NaOCl process, incuding safety handling:
Casson and Bess,
On-Site Sodium Hypochlorite Generation,
WEFTEC.06 Conference of the Water Environment Federation - Oct. 21-25, 2006, Dallas, US