H2 explosion at a chlorine production plant

An explosion happened at a chlorine production plant (Seveso-classified)
The plant had been shut down for two weeks, and maintenance works were ongoing. Two subcontractor employees were cutting a purge pipe on a hydrogen manifold in the chlorine electrolysis room while a plant worker was purging the brine circuit (brine is used as a raw material in chlorine production).
A through-cut of the pipe caused an explosion inside the hydrogen manifold that blew apart the rubber sleeves connecting the manifold to the electrolysis cells. Reason of the explosion was ignition during the cutting operation of an air-hydrogen mixture. The air was present because of the electrolysis cells were opened. The presence of hydrogen was probably due to insufficient nitrogen purging and/or several weeks of hydrogen desorption from the steel of the manifold.

The facility was placed in a safe condition and the fire-fighters were alerted.

The INITIATING CAUSE was cutting a hydrogen-containing pipe, followed by its ignition due to sparks from the cutting tools.

According to the ARIA report, the work authorisation issued to the subcontractor had not identified the risk of hydrogen in the manifold, which had been purged with nitrogen when the electrolysis room was shut down in December. On top of that, the work authorisation did not call for closing the valve which isolated the manifold from the hydrogen condensate pipe. This was due to a misunderstanding about the exact location of the cut.

Contributing to a ROOT CAUSE were (i) an inadequate purging of the hydrogen system and (ii) the inadequate risk assessment underlying an ill-written working permit for the subcontractors.

hydrogen manifold, electrolyser cells

Maintenance work were ongoing.

The operator was knocked down by the explosion and sustained injuries from flying pieces of sleeve and plastic, brought to the hospital and released the same evening. The two subcontractor employees, shaken by the blast, were treated in the site’s infirmary.

According to the ARIA report (see reference), the lesson learnt by the plant operator was the implementation of regular audits of the risk assessments conducted when work authorisations are issued.