Fire in a chemical plant

Hydrogen and butyl acetate released and caught fire on the vent of a hydrogenation reactor, part of a dyes manufacturing plant.
The pressure inside the reactor increased due to excess hydrogen resulting from a faulty pressure measurement, till a safety valve opened and released the gas: the event took place on the roof, the roof vent was equipped with a flame arrestor. The wrong pressure reading was caused by the clogging of the connection used for the pressure measure.
The reactor’s hydrogen supply and the injection of nitrogen in the installation were stopped. The Classified Installations Inspectorate performed an investigation.

The INITIATING cause was due to the fact that the pressure gauge inside the reactor was not working well due to clogging of a connector.
The investigation revealed the following ROOT or CONTRIBUTING CAUSES:
1. A lack of an explosive zone in the area around the vent, despite the presence of electrical equipment in the immediate vicinity (lighting, ventilation, air conditioners).
2. Some of this equipment was explosion-proof, but it was not clear is designed for the class of gases to which hydrogen belongs.
3. The opening of the valve and the release of gases and flammable vapours were detected belatedly.

pressure gauge,
PRD,
vent stack,
hydrogenation reactor

The pressure in the reactor was higher than normal, due to a wrong pressure reading.

All the equipment located in the surrounding of vents, susceptible to free flammable gas, will be checked for ATEX conformity. ATEX zoning will be made around each vent. Safety systems and monitoring will be doubled and regularly checked.

The Classified Installations Inspectorate performed an investigation.

The report found:
1. The lack of an explosive zone in the area around the vent despite the presence of electrical equipment in the immediate vicinity (lighting, ventilation, air conditioners).
2. Some of this equipment was explosion-proof, but it was not clear is designed for the class of gases to which hydrogen belongs.
3. The opening of the valve and the release of gases and flammable vapours were detected belatedly.

Feeding of hydrogen and nitrogen was stopped.

The safety valve worked as designed, releasing the hydrogen to the roof without any further consequence.

The operator was required to implement a safety improvement program (not directly related to the cause of specific event, but aiming at improving consequences):
1. technical and organisational measures to prevent such an event from happening in the future,
2. designation of explosive zones around all vents in the building likely to release flammable gases or vapours into the atmosphere and to search for such zones in the site’s other installations,
3. verification of the compatibility of the explosion-proof equipment near the vent involved with the hydrogen.
Several technical provisions were undertaken before the workshop was restarted: doubling up of safety devices to disconnect the supply of hydrogen in the event of overpressure, improvement of the pressure tapping and implementation of preventive maintenance for this device.