Hydrogen accidentally produced by electropolishing

A hydrogen explosion occurred in a 5-tonne stainless steel pressure vessel, while two electro-polisher workers where electropolishing internal surfaces. The reason of the explosion was possibly the build-up a hydrogen - oxygen mixture within the vessel of, generated by the electropolishing process.
The vessel laid horizontally with the only one unsealed opening: a man-way in the front-end site. There were no measure aiming at evacuating the gasses generated within vessel, and/or at avoiding sources of ignition. Significant quantities of hydrogen were likely generated due to the attempt to electropolishing the vessel using large cathode / anode area and high energy input. The vessel was approximately half full with concentrated (sulphuric / phosphoric) acid.

The explosion resulted in injuries, a fatality and in the ejection of a substantial quantities of the acid.

The INITIATING cause was the accidental production of a hydrogen-oxygen mixture while electroplating.
A CONTRIBUTING CAUSE was that this was the first such job undertaken by those injured, inexperience and failure to identify risk played a role

The ROOT CAUSE is management shortcoming. Industrial standard were available but not applied. The workers had not knowledge of the hazards involved and there was no mitigating/preventive measure.

Pressure vessel for chemical storage

Maintenance works (electropolishing) were ongoing.

The explosion resulted in one injured and one fatality and in the ejection of a substantial quantities of the acid.

The event was investigated by HSE UK.

The following comments were recorded by the inspector:
Industry standard for treatment of such vessels appears to be to:

1) Electro-polish end caps prior to assembly and then to electro-polish internal circumference with the vessel continually rotated on powered rollers and with a cathode hung from centres into a small volume of electrolyte in the base of the vessel

2) Evacuate generated gases by feeding air into vessel and / or providing extraction.

Vessel involved in accident was first such job undertaken by those injured - inexperience and failure to identify risk are considered to be the main underlying causes.

No attempt to avoid sources of ignition or to evacuate gasses generated within vessel. The vessel was laid horizontally with the only unsealed opening being a man way in the front end face.

It was the first time that the workers executed the process which brought to the incident. Inexperience and failure to identify risk are considered to be the main underlying causes. There were standard practice in place. The industry standard for treatment of such vessels appears to be to:
1) electropolishing end caps prior to assembly and then to electropolishing internal circumference with the vessel continually rotated on powered rollers and with a cathode hung from centres into a small volume of electrolyte in the base of the vessel
2) evacuate generated gases by feeding air into vessel and / or providing extraction.
None of these steps were undertaken and no preventing measure was put in place.