Fire at a hydrogenation unit of a petrochemical plant

The event occurred at a plant for the production of ethylene, and affected the unit for the hydrogenation of pyrolysis gasoline. During a shutdown operation of the hydrogenation plant, the recycled gas leaked and ignited at the top flange of the hydrogenation reactor.
The normal operation pressure of the reactor was about 2.8 MPa and the temperature about 350 °C. During the transient triggered by the shutdown, the gas started leaking when it reached 150 °C and a pressure of 1.3 MPa.
The reason of the leak was an insufficient tightening torque applied at the start-up. Due to this insufficient torque, the flange started to leak due to the decrease of both falling temperature and pressure at shutdown.

The INITIATING/direct cause was the recycled hydrogen gas leak at a reactor flange.
The loss of confinement was due to the fact that the flange bolt tightening torque applied at the start-up had been insufficient, and that the temperature reduction rate during shutdown was too quick, worsening the lack of tightness due differential dilatation of the joint materials.
A contributing cause was that a quantitative control of tightening torque was not carried out, hinting at a human error or a shortcoming of the maintenance procedures.

flange, bolt

The incident occurred during turnaround shutdown.

DESCRIPTION OF THE PROCESSES
Pyrolysis gasoline is a naphtha-range product with high aromatics content. It is a by-product of high temperature naphtha cracking during ethylene and propylene production. Also, it is a high octane number mixture that contains aromatics, olefins, and paraffin (Wikipedia, https://en.wikipedia.org/wiki/Pyrolysis_gasoline ). The main purposes of hydrogenation are desulphurization and saturation of olefin such as styrene.

The fire the highway triggered a major fire service emergency operation. Around 50 firefighters were busy extinguishing the fire for two hours. To reduce the risk of explosion, the fire brigade cooled the gas bottles with extinguishing water at the scene of the fire. As there was not enough water at the scene of the accident, water hoses were laid over several hundred meters.
The motorway was closed for around eight hours.

The tightening force required to guarantee tightness during operation is determined during the design phase taking into account the operative steady-state conditions. However, during transients (for example at start-up or shut-down) other conditions may occur, falling outside the range used in the design phase. These cases can lead to loss of confinement and must be as well considered in the risk assessment.