Jet fire of coke gas in a steel industry

The fire took place during maintenance works in one out of the three gas extractors of the coke process line. Main cause was the displacement of a flange which allowed the entrance of flammable mixture gas (methane, carbon monoxide hydrogen and benzene) which ignited probably due to electrostatic spark.
According to the ARIA report total quantity of gas released was 62 400 m³, consisting of 11,7 t CH4, 0,98 t C6H6, 5,1 t CO and 3,4 t H2.

During maintenance works, gas extractors were usually isolated and kept under continuous flow of N2. However, in this case, N2 pressure seemed to be not enough to avoid the entrance of flammable gas.

INITIATING cause was probably the inert gas pressure insufficient to prevent the entrance of the flammable mixture.

coke process line, flange

The event took place during maintenance of a gas extractor, which has a diameter of 1400 mm. During maintenance, the line was isolated from gas stream using two flanges, one before and another one after the equipment.
The accident occurred after maintenance works have finished and the gas extractor was been prepared again for operation: the procedure required as final steps the closing of the downstream flange and then the opening of the inlet one. The leak occurred immediately after the opening of the inlet flange.

Damage to property: on-site more than 2 millions of Euros, off-site more than 0.5M millions of Euros. The released susbtances were:
H2 3.4 t
CO 5.1 t
CH4 11.7 t
C6H6 0.98 t

The employers have been evacuated. The site fire brigade put the extractor under nitrogen and cooled it by water jets. The fire and the leak are stopped after approximately 1 hour.

The operator has foreseen to improve the procedure for the isolation of the section at the following maintenance and to review the risk assessment.