At 9:30 a.m., an explosion and fire occurred in a hydrocracker unit of a Reffinery producing 40 000 t/d. Difficulties had been experienced since 8:50 (drop in oxygen value, smoke…) and an experienced technician had been sent to assess the situation and make manual adjustments. He called for assistance as he could not find the solution. When the engineer and team leader left the control room to respond the problem, a violent explosion occurred in the east cell of the furnace, killing the technician on the spot. The 2 men, slightly burned, returned to the control room and initiated emergency shutdown and fire-fighting procedures. The fire department battled the blaze which produced intense heat and a thick black smoke visible over 30 km. The fire was extinguished within 2 hours. Material damage was extensive and the unit had to be rebuilt.

The heater was a “cabin” style furnace, consisting of 2 individual ground level cells (called respectively east /west radiation cells). Each cell was made from steel plate and lines with monolithic refractory insulation. An interior framework of structural steel, resting on concrete foundation piers 1 foot off the ground supports the box-like structures. Each cell had single centrally located longitudinal bank of tubes (20 cm diameter made of 347 stainless steel) stacked 8 high. The explosion occurred following the sudden rupture of a tube in the east cell of the unit that caused a flammable cloud (process fluid mixed with air). Tube 2 had a stress rupture resulting from extensive local overheating or hot spots.

A committee issued a number of recommendations including :

1- the implementation of a formal, regulatory scheduled preventive maintenance program for burners to prevent flame impingement

2- the introduction of a mechanism to monitor more accurately tube skin temperature to prevent hot spots

3- the implementation of complete procedures to deal with hot spots once detected and reported

4- the introduction of a more accurate method to determine the impact on tube life once a hot spot is detected.

5- Documentation and personnel training to recognise and respond to the changes in metallurgical properties.