Explosion at a hydrocracker in a refinery

On a cold morning, the gasoil hydro-treatment unit was being brought back on line after being out of service to repair a management problem on the recycle gas compressor. Several upset trips had occurred in the previous month but these were not seen to have any direct bearing on the operation of the hydrogen heaters or the unit as a whole. Temperature at the coil outlet of both heaters was at least 100 degrees C lower than normal since the unit was in a start up mode. The reactor circuit was at full operating pressure while hydrogen was recycled via a compressor.
A violent explosion suddenly occurred within one of the heaters and a loud noise accompanied by shock was felt across the plant site. For one half hour, flames emitted from the top of the stack was felt from the top of the stack. Operators tried to isolate the heater from known fuel sources but they were unable to access the isolation valves. Fortunately, no injuries were sustained as a result of the initial explosion or the fire that followed. The fire was finally extinguished with little damage occurring outside the heater shell. In fact, the force of the explosion did not even dislodge the explosion panels on the furnace.

The INITIATING CAUSE was the local overheating of a hydrogen tube. Gas oil had backed up into at least one of the parallel hydrogen heater circuits, because the check valve had leaked permitting reverse flow. When the heater was restarted, one of the passes was flow restricted because of trapped liquid in a lower return bend. This caused the tube metal temperatures on that pass to increase. For some unexpected reason, the radiant tube did not fail initially. Instead, it expanded longitudinally upwards into the convection section. It placed stress on a tube connection, causing a crack. A small leak developed at the crack and projected a hydrogen flame onto an adjacent part of the tube. This was not immediately observed. When the tube ruptured catastrophically, a major loss occurred.
The ROOT CAUSE was indetified in an improper design and failure to conduct pre-start-up safety reviews prior to a routine start-up.

recycle gas compressor

The gasoil hydrotreater was being re-started after being out of service to repair a problem on the recycle gas compressor. Several upset trips had occurred in the previous month but these were not seen to have any direct bearing on the operation of he hydrogen heaters or the unit as a whole,

Little damage occurring outside the heater shell. In fact, the force of the explosion did not even dislodge the explosion panels on the furnace.
The furnace was taken out of service for approximately 6 weeks while half the tubes were replaced. These had been damaged during the fire following the explosion. Little refractory damage was experienced. Total cost of repairs approached $1 million (CDN) but this was a small portion of the production penalty.

It is important to prepare more effective furnace designs. However, as long as designs include hardware such as piping, there is some potential for valves and fittings to be wrongly positioned. There is no procedure that can define all the requirements for a good circuit design. On the other hand, if process designs are thoroughly reviewed under a wide range of conditions, potential failures should be picked up and addressed. New designs need to consider normal operating conditions as well as those associated with start-up, shutdown and recovery from upset. We also learnt the importance of monitoring especially during start-up periods. Data analysis after the fact showed a high stack temperature, an anomaly which should have indicated afterburning or process leakage. Start-up procedures need to include diagnostics for dealing with unusual conditions. Prior to starting up, the unit should have been thoroughly inspected to ensure that it was suitable for commissioning. Unscheduled outages can leave equipment in an intermediate state (partly filled with inventory) which does not coincide with normal start-up procedures. There may sometimes be a tendency to bypass procedures in order to get equipment back on line. Upset conditions and temporary feed outages, for whatever reason, must receive priority attention from operations and technical staff.