A flash fire on the hydrogenation unit

The event occurred at a hydrogenation unit, when starting a new production cycle.
A wrong sequence of actions caused a residual presence of hydrogen in the reactor. When troubleshooting a leak on reactor’s manhole, the hydrogen produced an explosion when mixing with air.

DETAILED ACCIDENTAL SEQUENCE
(1) The production supervisor issued specific instructions on a new production cycle to a technician.
(2) The operator did not follow them in the right sequence as indicated in the verification instructions before staring production: to save time when pressurising the device at 9 bar of nitrogen, he simultaneously made several adjustments and carried out several checks (opening the manual H2 valves of the storage tank, the emergency shutdown valve just in front of the workshops, etc.).
(3) The reactor and safety process valves in front of the reactor were protected by a check valve that remained closed.
(4) When pressurising the reactor was with nitrogen, the operator observed N2 leaking from a manhole.
(5) He decompressed the reactor and removed the fastening bolts from the lid to change the joint. During this operation, he heard a leaking noise at the joint. Believing it to be a H2-leak, he blocked the reactor and triggered the emergency shutdown.
(5) Convinced now that the reactor was clean and the residual volume in the pipe was low, he proceeded to change the joint with assistance from a fellow technician. It followed an explosion that projected backwards the two technicians.

The INITIATING CAUSE of the explosion was the presence small quantity of hydrogen ignited by the catalyst in presence of oxygen coming from the manhole. Drying conditions could have increased the pyrophoric properties of the catalyst.

The post-incident investigation remarked that the leak test of the equipment and the pressurisation of the line were carried out simultaneously without following instructions.
This suggests a human error as ROOT CAUSE. The fact that the operator did so to gain time, hints at the possible presence of over-load and stress condition at the workplace.
Moreover, the investigation found design and procedural shortcomings as well: no leak test was foreseen for the H2 line valves, and that no specific instructions existed for opening the manhole in normal or emergency mode, nor for changing the seal. The existing H2 sampling and introduction setup increased the risk of leaks at the valves upstream of the reactor and the presence of catalyst in the reactor.

Hydrogenation unit

Few day before the accident, the hydrogenation reactor had been cleaned up and dried to eliminate all residue of the preceding synthesis.

Both the technicians were hospitalised.
The unit is closed for 48h.

Investigation is made by the gendarmerie.

A safety alert was sounded in the workshop. The Internal Operation Plan and the ETARE plan are launched. The operator decided to shut down the site for 48 hours.

The inspection expert found additional deficiencies in the procedures: no leak detection had been on the hydrogen valves and instructions were missing for opening of the manhole in normal or abnormal operation and exchange of the corresponding seal.
Moreover, the design for sampling and introducing hydrogen was favouring formation of leaks on up-stream valves in presence of catalyst in the reactor.

1) in the hydrogenation equipment: installation of pressure sensors and flame guards on regulator vents
2) in the procedures: risk analysis, verification of the status of the hydrogen line, de-commissioning the H2 pipe before opening the manhole.