Fire from a flange of a desulphurisation unit

During startup of the hydrodesulphurisation unit, diesel and hydrogen leaked from a flange near the heat exchanger. The flammable mixture ignited spontaneously, causing a fire.
Plant personnel detected the fire. The unit was immediately shut down and depressurised, by releasing the gaseous content (hydrogen) content to the flair. Nitrogen was introduced into the unit and the diesel recovered. Ten hours later the fire was declared definitively extinguished.

A post-accident inspection determined that the source of the fire was likely near the 90° side of the flange, based on discoloration of the equipment surrounding the heat exchanger and damage to the gasket.
The reason of the leak was attributed to plastic deformation of the flange gasket due to infiltration of rainwater, which cooled and shrunk the flange bolts. The present of rainwater was due to a weather hood only temporarily fastened.

The INITIATING CAUSE was the loss of tightness of a flange due to damage of its O-ring.
Based on these findings, it is estimated that the leak occurred through the following mechanism, causing the 349°C internal fluid (diesel and hydrogen) to spontaneously ignite.

(1) The weather hood was attached to the flange, covering the upper two-thirds of its circumference. It was temporarily fastened during startup and was scheduled to be fully fastened after steady-state operation.
(2) It had continued to rain from the morning of the day before the accident until dawn on the day of the accident.
(3) Due to the temporary character of the fastening, multiple gaps opened between the weather hood and the flange, allowing rainwater to penetrate through these gaps.
(4) The infiltrating rainwater splashed onto the flange bolts, cooling them.
(5) The cooled bolts contracted, causing an imbalance in the flange's surface pressure, resulting in plastic deformation of the gasket (a 0.1 mm dent).
(6) When the rain stopped, the bolts stretched due to the temperature rise, creating a gap between the recess in the gasket and the flange, which caused the internal fluid to leak.

ROOT CAUSE
The post-accident investigation did not find corrosion or damage to the heat exchanger flange or bolts. Moreover, the bolts were tightened to the specified axial force, confirming that there were no problems with construction management or application of procedures.
Indeed, the tightening procedure aimed at preventing uneven flange surface pressure by carefully managing the axial force of the bolts.
There was, however, no procedure for managing the weather hood installation, and the weather hood was temporarily fastened at startup. This lack of a specific procedure could be interpreted as a ROOT CAUSE, worsened by harsh environmental conditions.

vlve, O-ring

The plant was re-starting.

DESCRIPTION OF THE FACILITY
The overall plant is not provided. It is probably a refinery. The data provided for the gas (hydrogen?) production facility consist of two production flows:
242,154 x 103 m3/day (Business)
7,195 x 103 m3/day (Facility)
It is unclear what these flows consist of.

This incident consisted in a loss of confinement due to unexpected thermal expansion difference acting on a flange and its O-ring.
It is important to fully understand the importance of axial force management for high-temperature flanges, which experience significant temperature increases during startups and shutdowns. It is equally important to put in place carefully all foreseen measures to avoid additional temperature-induced stresses, in particularly understanding the role of weather hoods.
It is important to specify the above points and appropriate construction methods in the construction guidelines and procedures and to educate workers accordingly.

(1) The weather hood installation management was standardised and "Heat Exchanger Weather Hood Installation Guidelines" were issued, to clarify that that weather hoods must be installed before startup (i.e. before the temperature rises) and providing examples of appropriate and inappropriate installation by means of photographs.
(2) The procedures for the construction were modified: the construction department must inspect the installation and has the responsibility to handover the procedures.
(3) The weather hood structurally modified and reshaped to prevent rainwater intrusion, including the use of screws for fastening.
(4) A verification step was included during startup to steady-state operations, foreseeing the use of a gas detector to verify absence of abnormalities inside the weather hood.
(5) The installation manuals clearly stated the importance of weather hoods and installation methods, and training was provided to subcontractors and workers.