Fire from a hydro-desulphurisation unit of a refinery

In a refinery, a fire occurred at a new reactor for the vacuum distillate hydrodesulphurisation unit. Process gas (hydrogen, hydrocarbon, etc.) blew from a flange of upper piping in the reactor. It ignited producing 0.5-2 meters high flames. The reason for the leak was a lack of tightness of the flange.

Emergency stop of th plant was executed, thereactor was cooled and purged with nitrogen. The fire department was alarmed.

The INITIATING CAUSE was considered a loose high-pressure flange due to uneven tightening force, on uneven thermal expansion of materials at a temperature of 390 °C.

The tightening operation of hot bolting at start-up three months before was inadequate.

The ROOT CAUSE was a shortcoming in the procedure for hot-bolting, together with a lack of effective regular inspection. There was also a lack of management actions, because the difficulty of hot-bolting was known and several other plants were able to prevent occurrence of these events.

highpresure, high tmeprature flange

The reactor was new, but the operation was a standard operation.

Heat damage and some of vacuum distillate oil was lost.

Event sequence:
19:55. In a reactor with a height of 13 m, vacuum distillate containing hydrogen blew from a flange of the upper piping. It ignited and produced a 0.5-2 m high flame.
20:00. An emergency call to the fire department was made and the emergency shutdown was started. Pressure was reduced from operating pressure of 10 MPa(g).
20:08. Public fire brigades arrived. For fear of a secondary disaster with hydrogen sulphide, they stood by without extinguishing the fire with water. 20:44. The fire extinguishment was confirmed.
20:45. The pressure in the reactor lowered to 0.6 MPa(g). Nitrogen gas was introduced into the system.
21:20. Pressure lowered to 0.3 MPa(g) or less. Cooling by feeding nitrogen gas was continued.

It is well-known that to manage the tightening of flanges in high-temperature and high-pressure sections, a quantitative control of the tightening force is required. Critical as well is to determine the number of times this control should be performed, and at which moments of the pressure-temperature history of the flange.
Since not all flanges in complex high-pressure high-temperature systems are experiencing the same conditions, the hot bolting procedure should be based on the most severe conditions.

The investigation found out that:
1. There was not a sufficient margin for tightening force on hot-bolting.
2. The upper part and the lower part of the flange were different (respectively SUS321 steel and 2-1/4Cr-1Mo steel with an overlay). Therefore, their coefficients of thermal expansion differed.
3. The thermal expansion of the piping acted in the direction of opening the flange.

Findings 2 and 3 were specific to this flange only. It is possible that the special combinations of different materials, geometric configuration and harsh working conditions may have been overlooked during the design phase and/or during operation.

Based on the lessons learnt, the following was implemented:
(1) The procedures for hot-bolting were reviewed and the management system for bolt-tightening force was strengthened. To achieve an effective tightening able to withstand the harsh conditions experienced by the flange, hot-bolting shall be done every 30 °C, above the temperature of 360 °C.
(2) A dedicated check team was set up for the flange of a high-temperature and high-pressure part of newly constructed piping and vessels. This team carried out periodic inspections.