Explosion at the chlorine electrolyser of a water treatment plant

An explosion occurred at a tank of the chlorine electrolyser of a water treatment plant
The 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-product 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.

The INITIATING 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 the hydrogen venting system, or an error when operating it.

A component of the ROOT CAUSE was probably a shortcoming or wrong procedure (ill-designed or wrongly implemented). Upstream of this fact, the safety design was not able to guarantee failure-proof venting.

hydrogen vent, NaOCl storage tank, pump, switch

At the moment of the explosion maintenance works were ongoing on the NaOCl tank.

DESCRIPTION OF THE PROCESS
The water treatment plant used an electrolytic process to generate sodium hypochlorite (NaOCl) from a solution of sodium chloride (NaCl):
NaCl + H2O + ENERGY → NaOCl + H2

The on-site generation of sodium hypochlorite for water and wastewater disinfection has the advantage to provide the substance on demand, reducing the need for extensive on-site storage. Moreover, NaOCl is liquid, easier to handle than the gaseous chlorine.
Hydrogen as by-product mut be safety vented. Dilution blowers are normally used to this purpose. As explained by (Casson and Bess, On-Site Sodium Hypochlorite Generation, WEFTEC.06 Conference of the Water Environment Federation, 2006, Dallas, US), hydrogen dilution fans are not always used. Depending on the system location. Some systems are designed without fans because they are located away from ignition sources. Other systems use open-top tanks, and this allow hydrogen to separate and disperse. On-site generation systems typically have closed tanks; therefore, fans are required for normal dilution.

The main issues investigated by Chemaxx (company that carried out investigation) focused on the mechanisms and rates by which hydrogen gas is generated and subsequently accumulated in the holding tank. The need for active venting, warning signs and incidents of a similar nature were also reviewed.

The vent system of the on-site NaOCl electrolyser was a critical component of the safety design. Hydrogen had to be discharged to the atmosphere in a safe way, avoiding cumulation in the NaOCl tanks or in the venting system.
Although little is known on the reason why the system failed to vent hydrogen, it out of doubts that it was not well equipped with the required safeguards able to avoid or mitigate vent obstruction, wrong operation or hydrogen cumulation. Detection of hydrogen and out-of-spec operative conditions detection should have been in place to trigger an alarm and possibly automatic corrective actions.

The company that carried out investigation (Chemaxx) identified the need to ensure active venting hydrogen. They required also warning signs and the implementation of an alarm system able to detect failure of the venting system.