Hydrogen fire in a hydrogen production plant

An ignited hydrogen leak occurred at a hydrogen production plant. The hydrogen conditioning unit consisted in a gas holder supplying hydrogen to 4 compressors. The compressors were delivering hydrogen at 5.4 bar to a PSA (pressure swing absorption) unit,

The facility had been shut down for maintenance and was restarting by performing a series of pressurisation and performance tests. One of these tests revealed a leak from a pressure gauge line of one of the compressors.
The section of the leaking compressor was stopped and purged with nitrogen. When the compressor was inspected, additional repair needs became evident due to local external corrosion. When the repair works started, an undetected hydrogen escaping from the compressor drain valves were ignited by a spark generated by a grinder.
The fire was extinguished with a portable fire extinguisher, and the drain valve was closed to stop the leak.

The leak locations were two blow out lines which had the function to discharge hydrogen that may escape from the compressor’s shaft areas, routing it safely to a vent pipe. Since the compressor was of a two-stage type, two lines were required to guarantee safe hydrogen venting. During normal operation, the lines were connected to the vent pipe by flexible hoses under a constant flow of nitrogen. When the lines were isolated for inspection, the nitrogen flow was stopped after the internal nitrogen purge was completed, and the flexible hoses and the vent pipe valves were removed.
These vent lines were also connected to the PSA unit. On the day of the incident, by mistake not all flexibles were removed. Moreover, the startup preparatory works on the PSA was continuing. Pressurised hydrogen flow back from the PSA to the vent line, into the left flexible and eventually was released through the compressor vent line which gad been removed.

The INITIATING CAUSE was the accidental release of hydrogen from a dismantled venting line. Despite the correct purging of the compressor unit to be repaired, the hydrogen from the PSA unit accessed the compressor unit due to a combination of several shortcomings of different types.

(1) Lack of communication regarding risk assessment and safety of the repair works: maintenance team did not involve and inform the operation supervisor. Moreover, they gave only a verbal explanation to the manufacturing manager and obtained approval. The consequence was that two teams were active simultaneously, one busy with renewing hydrogen in the PSA, the other with purging and repairing the compressors section.
(2) Lack of system knowledge. The compressor maintenance team knew that there were six flexible hose connections to be removed but did only for 4 of them. They had also not understood well the function of the vent lines, considering them part of a nitrogen system.
(3) lack of proper supervision and checks on the proper execution of the operation steps. No final check was performed at location by supervisors, to ensure that all required lines and valves had been removed. Nobody realised that two teams working in parallel without action coordination was creating additional risks.
(4) Lack of adequate safety preventing measures. The compressor team used combustible gas detectors but did it at a moment when the PSA was not yet filled with hydrogen. One negative measure at one time created the wrong conviction that the compressor section was hydrogen-safe.

1. When different work streams are executed at the same time, by two different teams, the internal communication must be thorough and involve all the involved members of both teams and their supervisors.
2. Any equipment modification must be preceded by a comprehensive risk assessment covering operations, safety, and engineering.
3. Unplanned works, such as repair, must be covered by a dedicated work instructions, communicated in written, and supervision of their executions strictly ensured (for example by checks on location).

Technical corrective measures
1. The hydrogen line from the gas holder to the PSA unit was modified so that no back pressure could be applied to the compressor side. The PSA vent line became a dedicated single line,
2. The vent line exits from the hydrogen compressors were extended to a higher, thus safer position. A hydraulic seal device was installed at the entrance of the vent line, to further improve safety.

Management-related corrective measures
3. Review of SOP (safety operative procedures). After the accident, the importance of these documents was re emphasised, and regular revisions were planned to ensure their optimisation.
4. Revision of work procedure regulations. The manual for ad hoc work was revised to eliminate the distinction between ad hoc and regular work, allowing the same safety management framework to be applied on both situations. The overall manual was overhauled, with clearer definition of hazardous zones, better defined procedure steps and a pre work safety assessment in addition to the already foreseen safety meeting.