Fire at a hydrocracker of a refinery

A leakage and fire occurred during normal operation at a medium-pressure hydrocracker of a refinery. This unit perform a desulphurisation and a decomposition of heavy gas oil.

High-temperature and high-pressure gas leaked through a flange with a special shape located at the reactor outlet, during a change of the operating conditions of the hydrocracker implying a reduction of the temperature of the reactor. The following fire damaged the pipe network in the area.

The assumed cause of the leakage was differential thermal contraction off the flange when the operation temperature was lowered. The flange spacer shrank as temperature fell, but the bolt did not decreasing the tightening torque. Probably also the axial alignment of the flange component was not sufficient, or worsened during the differential contraction, forming a gap through which the mixture could escape.

Sequence
October 27th - The feed rate was decreased, and the reactor operating temperature was lowered to 320 °C from 370 °C.
22:30- High-temperature and high-pressure gas leaked from a spacer flange at the reactor outlet and ignited. First, a small fire occurred from a small leakage. By further opening of the gap, more oil and gas was released.
Day after, 5:03, the fire was declared extinguished.

The INITIATING cause was the loss of gas confinement induced by a temperature reduction and different thermal expansion of a flange components.
Contributing cause was the complex construction of the flange, with a spacer and one gaskets at each end of it, which made difficult the correct tightening and caused a complex differential expansion behaviour of components.
Therefore, root cause was probably a design one, due to the choice of the component for the flange, and a lack of correct maintenance procedures, considering also that the plant was very old, and had been previously reconverted. Possibly an inadequate management of changes plaid a role as well.

hydrocracker, flange, pipeline

DESCRIPTION OF THE FLANGE
The flange with a spacer is a special flange that has a flange space enlarged by inserting a piping component between the flanges. The gasket is put at both ends of the spacer, and two gaskets and the spacer are tightened together. Tightening becomes difficult, and the inside spacer approximates to the process temperature because of direct contact with the process fluid. However, as the tightening bolts are located away from the spacer external surface, the thermal expansion is affected less by the process temperature change.

HISTORY OF THE PLANT
1962. The unit was designed and built as an indirect heavy oil desulphurization unit, and started System design error.
In 1983. An additional reactor was built, and the unit was re-modelled into a hydrodesulphurization and decomposition unit for heavy gas oil.
2002. This fire occurred.

The unit was damaged, as well las the hydrogen and steam piping, and the fuel oil piping on the rack.

The plant was shut down and depressurised. A nitrogen purging was carried out in all components. The other plants at the refinery were shut down.
Company and public fire brigades were called Thirty employees took refuge in the explosion-resistant control room.

1. The ageing of plant must be taken into account when reviewing risk assessment and safety provisions. Prolonged operation may induce lie time degradation at unexpected places. In particularly, modification of a plant by re-using existing components should be matter of concern.
2. Complex flanges with a spacer use longer bolts than the simpler ones. Their tightening management is crucial, because their components tend to expand/contract differently at temperature changes.

The following measures were adopted to improve the technical performance, operation control and knowledge:
1. Removal of the unnecessary spacers and flanges.
2. Flanges axial alignment corrected for thermal gradients.
3. Changes of the material of the bolt.
4. Inspections of the operations with large changes of operating conditions and strengthening inspection scope.
5. Adoption of a facility and risk management specifically for age degradation.
6. Strengthening risk management and management of changes.