fire from a de-sulphurisation unit

The incident occurred at the residual de-sulphurisation (RDS) unit of a refinery, during the initial start-up phase, consisting of the pressurisation by hydrogen of the unit reactors and the downstream coolers. The incident led to significant damages to the downstream coolers and some part of the reactors.
The cause of the leak was a rupture in the rectangular header box of the reactor effluent air cooler, placed downstream of the RDS reactors. The rupture led to a large hydrogen leak that spontaneously ignited with very small delay ignition time (<3s), and created a fireball with a size of around 83 m and very small overpressure. According to the investigation calculation, the hydrogen fire terminated within less than 10 minutes after ignition. However, due flame impinging on other components, a domino effect followed with the release and ignition of fuels from fuel pipes ruptures.
The fire was extinguished by shutting off the waste gas into the flare system.

DETAILED TIMELINE of the INCIDENT
The residual de-sulphurisation (RDS) unit was under routine maintenance before the incident. All hydrogen had been purged and replaced by nitrogen.
October 13, the unit was ready for pressurization and drying of catalyst by nitrogen.
October 20, the drying was completed, the temperature was reduced to 160 °C and hydrogen was fed into the system to replace nitrogen.
October 23 17:15, when the system pressure had reached 5.5 MPa and hydrogen concentration was above 88%, a small leak was found on one of the plug in head box outlet. The leak could not be sealed by tightening and thus the system was depressurised and hydrogen replaced again by nitrogen.
October 25 The plug leak was sealed by welding on and the pressurisation resumed.
October 27 19:50, the pressure reached 12.35 MPa and hydrogen concentration was above 88%. Pressurization continued and by 22:34 a large fire occurred near the REACs area.

The INITIAL CAUSE of the rupture in the air-cooled heat-exchanger was corrosion by sulphide stress cracking. The metallographic investigation found also considerable welding defects.
The ROOT CAUSE was a lack of regular and effective inspection. Moreover, the air-coolers used in this de-sulphurisation unit are characterised by specific risks related to the absence of a shell. A loss of confinement from the tube will cause a direct release into air. To mitigate the associated risks, a specific risk assessment is required, which seems having been incomplete in this case.
CONTRIBUTING CAUSES to the domino effect was the absence of sprinklers and of measures aiming at protecting from fire the individual components.

welds

The “residual de-sulphurisation “unit was shut down for routine maintenance and catalyst regeneration. The incident occurred at the second attempt of re-starting the process.

DESCRIPTION of the PROCESS
The desulphurisation of the oil residue is s a catalytic hydro-treating process aiming at reducing the sulphur content of fuel oil or other feedstock for further processing. The process converts sulphur in the residue into hydrogen sulphide at elevated pressure and temperature, by means of catalyst and hydrogen. The catalyst requires a cycling regeneration.

DESCRIPTION of the AFFECTED COMPONENT
The incident occurred at an air cooler aiming at cooling the residue effluent. It is a special type of heat exchanger that uses forced airflow to cool down the process fluid flowing in a bundle of tubes. Unlike typical shell-and-tube heat exchangers, there is no shell around the tubes and tubes are arranged and fitted to rectangular header boxes. Each tube is also finned in the outer surface to maximize the heat transfer. Each E−004 unit has a total of 138 tubes with tube length of 9.136 m and internal diameter of 22.1 mm.

One air cooler was destroyed, and the fire damaged the second air coller and the 4 residual desulphirisation units. Costs unknown.

Contrary to typical heat exchangers, reactor effluent air-coolers for RDS unit bear special risks as any crack would leak into air directly. The source (Kao et al., see references) recommends the following measures:
Preventive measures
(1) Welding and inspection of the header box should follow strictly the requirement given. Inspection of the stiffening plate welding is possible through the use of endoscope inserting into the plug holes.
(2) Existing residual effluent air coolers should be checked to prevent similar incident from occurring.
(3) In this incident, the steel used for the header box of the heat exchanger was a low grade a duplex stainless steel. The presence of ferrite and austenite makes difficult toe performance of proper welds, and this causes over-sensitivity to corrosion in presence of sulphur. A Nickel base alloy could be a good replacement.
Mitigating measures
(4) Early identification of leak location could help in minimising the impact of the leak. The exact location of the leak should be possible from the readings of the pressure and temperature data reading in the operative room. Then, correct isolation of a part of the system would be better than a complete blowdown of the units.
(5) Firefighting measures such as sprinkler system, deluge system and fireproofing insulation can mitigate the damage from fire and reduce the risk of domino effects.
On risk assessment:
(6) If the desulphurisation units contain primarily hydrogen, a fire is the most probable effect of a hydrogen leak. On the contrary, when the RDS system is operating with fuel input, a most probable accidental scenario would be a vapour cloud explosion (VCE).