Fire at the desulphurisation unit of a refinery

The incident occurred at the direct desulfurization unit of heavy oil. A 130×90 mm oval opening occurred in the bypass pipe of the hydrogen sulphide absorption tower for the circulating gas, at a height of about 20 m from ground. The process fluid, consisting mainly of hydrogen, leaked and ignited, causing a fire.
The fire broke out in combination with an explosive noise. It burned a part of the system installed in the site in a range of the radius of about 30 m, and was extinguished after about 10 hours.
The heavy oil supply was stopped, waiting for the complete burning-off of the hydrogen and the heavy oil of about 1,000 kl left in the system.

The fire station dispatched 27 fire engines for chemical fire, fighting the fire by pouring nitrogen in the piping system. The fire was extinguished 10 hours later.

The bypass piping was made ofSTS48-S steel, had a nominal diameter of 10 inches, and a wall thickness of 28.6 mm. The failure occurred at a dead-end branch of the pipe, positioned vertically, which upon inspection resulted extremely corroded and thinned. The cause was supposed to be the consequence of rapid corrosion due to accidental formation of ammonium (hydro)-sulphide from nitrogen, hydrogen and hydrogen sulphide, in presence of condensed water. The operative condition of that part of the pipe were almost stagnation due to absence of flow, temperature variation and humidity variation with consequent cycling condensation/evaporation.

The INITIATING cause was the corrosion of the pipe due to accidental formation of corrosive substances from hydrogen, hydrogen sulphide and nitrogen impurities.
The ROOT CAUSE was the lack of effective inspection and of operative feedback which should have contribute to improve the design.

pipe,

According to the plant, a plurality of gas detectors sensed abnormality in the area including the high-temperature and high-pressure separation tank to separate heavy oil from gas in a part of the system before the explosion. These were probably casued by the intiation of the leak. The fire broke out while the staff started to check the system.

An area of about 30 meters in diameter around the air-cooled cooler of the desulfurization system burned. The damage was preliminarily assessed at about 7 billion to 8 billion yen (ca 45000 Euros).

Despite the fact that safety measures and safeguards are part of the original plant design, it is important to review their effectiveness during operation. This probably implies a thorough inspections programme.

Removal of the bypass pipe.
Inspection of similar pipes of the plant.