Explosion in a chemicals plant

An explosion occurred in a plant producing organic chemistry products, in an autoclave located near a distillation column for acetone and oxygenated solvents. The explosion was followed by a fire involving the facility.
Internal and external firefighters were mobilised. Site activity was stopped and personnel evacuated. The fire was brought under control in few minutes, by means of fixed installation water hoses and water curtains to protect neighbouring facilities. The whole emergency lasted approximately 1 hour.

During a general maintenance shutdown the previous month, the operator had repaired a valve belonging to the hydrogen network and located upstream of the autoclave. It is assumed that a residual leak through the repaired valve or a handling error made during the maintenance operation caused the presence of hydrogen in the autoclave.
The activities aiming at restarting the autoclave had just begun, and during flushing of the autoclave, the hydrogen escaped through the vent and ignited upon contact with air, causing a deflagration outside the equipment. An observer noted the orange colour of the flame, which could be due to the presence of organic residues in the autoclave.

The INITIATING CAUSE was the accidental release of hydrogen from the autoclave due to the malfunctioning of a valve.

The ROOT CAUSE could be attributed to the lack of in-depth risk assessment of the autoclave operation. The procedure of restarting the autoclave was following a fixed series of step, without safety checks. Probably the action of purging the autoclave with nitrogen had been considered safe enough, and further risk analysis deemed unnecessary.
Furthermore, the absence of a pressure gauge on the autoclave made it impossible to detect the increased in pressure in the autoclave caused by the leak. The report proposes also the possibility of a human error causing the valve to release hydrogen into the autoclave, but it remains a hypothesis.

valve, autoclave

The autoclave operatios were restarting after the reair of a hydrogen feed line.

No personal injury, no property damage reported.
Probably the damage was very limited. The palnt restarte operations one week later.

Detailed timeline of the event:
7:50 – explosion at the autoclave, followed by a fire.
7:50 – 8:00 The operator brought the fire under control in few minutes the fire-fighting action using facility's fixed resources, consisting of a water curtain and water hose. The internal firefighters brigade was mobilised, and the external emergency service called.
8:00 The operator triggered the plant emergency plan and evacuated the personnel. The distillation column was inerted by injecting nitrogen. The extinguishing water was collected.
8:15 External emergency services arrived on site but did not need to intervene. Thermal, toxicological, and explosimetric camera measurements revealed no anomalies in the initial fire zone.
8:53 The emergency plan was lifted
Production restarted the following week after safety checks were implemented.

The operator planned to:
Formalise an operating procedure including a risk analysis for non-routine and complex operations.
• Add a pressure gauge to the autoclave.
• Perform leak tests of the valve in question.