A flash fire on the hydrogenation unit

The event occurred while starting a new production cycle at a hydrogenation unit. The production supervisor issued specific instruction on a new production cycle to a technician. The technician 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.). The reactor and safety process valves in front of the reactor were protected by a check valve that remained closed. The reactor was pressurised using nitrogen. The operator observed N2 leaking from a manhole. 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. He was convinced that the reactor was clean and the residual volume in the pipe was low and proceeded to change the joint with assistance from a fellow technician. It followed an explosion that projected backwards the two technicians.

The probable INITIATING CAUSE of the explosion was identified by the inspection in 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 root cause was the operator not following the chronological order of checking as specified by the instruction.

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.

The fire was left extinguish by itself. The manufacturing site was evacuate, as well as a neighbourhood.

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.