Hydrocarbons and hydrogen fire in a refinery

During a routine inspection, a flame was observed at the bottom flange of a reactor in a refinery. Few moments later, sensors near the leak detected a rise in temperature.
The facility was shut down in accordance with emergency procedures. The sprinkler system was triggered to protect the equipment. The rapid decompression of the plant towards the safety flare reduced the leak flow rapidly. One hour later, the fire was extinguished.
15,7 t of hydrocarbons were sent to the flare for burning on the day of the accident.
No injuries were reported on site and no inconvenience was felt outside. The leak involved naphtha, hydrocarbon gases and hydrogen. Due to the thermal effects of the ignited leak, cables, instruments and insulation materials were damaged.
[ARIA event 58577]

The INITIATING CAUSE was the leak of a process gases due to loss of sealing of a reactor flange.

The leak was due to a loss of tightness at a flange, caused by an incompatibility of materials with operating conditions.

The ROOT CAUSE lies in the decision of the operator to allow the replacement of a defective component with another one with different material properties. A possible additional factor could have been the absence of strict guidelines for the replacement of consumables.

flanges, bolts

Six months earlier, a first leak had already occurred on the same flange seal. The flange rods had to be replaced but those specified consumable parts were not available in stock, therefore, rods with a higher chromium alloy were mounted.

The source does not specify the unit/reactor affected by the leak and fire. From the composition of the process gases reelased it is is plausible that it was a desuphurisation and/or isomerisation unit of the refinery.

No injuries were reported on site and no inconvenience was felt outside.
15,7 t of hydrocarbons were sent to the flare for burning on the day of the accident
Due to the thermal effects of the ignited leak, cables, instruments ,insulation materials and some pipes were damaged. After checking the hardness of some pipes, 3 sections were replaced and two units were shut down for 2 months.
The fire caused an estimated operating loss of 2.5 M€ and material damage of around 500 k€.

A similar leak on the same flange seal had already occurred before. To improve the sealing at high temperature, the flange rods had to be replaced. As the specified replacement part was not available in stock, the supervisor and the modification coordinator allowed the installation of rods with a higher chromium alloy. Thie metal composition was indeed compatible with the temperatures, pressures and substances of the process. However, they did not consider the difference of expansion coefficient between the flange and the rods, due to the different chemical composition. Moreover, during the modification, they did not open the flange and therefore did not notice that the lower metal ring seal was incompatible with the flange.

These findings highlight the importance of three factors:
(1) The procedure for consumable replacements must be clear, fit-for-use and strict. They must be informed with the (safety) design principles at the base of the material and component choices, so that the operator can take decision based on pre-existing know-how.
(2) procedures and guidelines for preventive and corrective maintenance must also be detailed and fit-for-the-purpose. In this event, after the first leak the flange was not dismantled, so that operator could not discover one of the reasons for the leak, the incompatibility of the gasket.
(3) To careful asses of materials compatibility when taking a decision deviating from the procedure. The type of competences required to be able to o so could be provided by (1) and (2) and completed with proper training.
(4) To secure the supply chain of spare materials.