Hydrogen explosion at a chlorine chemical production plant

Hydrogen explosion at a chlorine chemical 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 manufacturing operator was purging the brine circuit (brine is used as a raw material in chlorine production).
A through-cut caused an explosion inside the hydrogen manifold that blew apart the rubber sleeves connecting the manifold to the electrolysis cells.

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

According to the ARIA report (see reference), 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.
Air brought in while the electrolysis cells were opened and hydrogen created an explosive mixture that ignited during the cutting operation. The presence of hydrogen is supposed to come from insufficient nitrogen purging and/or several weeks of hydrogen desorption from the steel of the manifold.

For the ARIA report the following can be deduced:
INITIATING cause was a cut in a hydrogen-containing system.
A contributing cause was the inadequate procedure for the purging of the hydrogen system.

Intermediate or root cause was mistakes in the working permit for a subcontractor, which did not identified correctly risks and intervention areas.

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.