Explosion of a flammable mixture in a petrochemical plant

The incident occurred in a catalytic reforming unit. At 13:41, a leak occurred at the 90° elbow of a pressure pipe containing a mixture of hydrogen and naphtha. This pipe was located between the pre-hydrogenation heat exchanger and the pre-hydrogenation product tower heat exchanger. The flammable material leaking from the pipe burst into flames, and at 13:51 and 14:21 there were two more explosive combustions.
Some of the nearby towers, pipelines and other equipment and facilities affected by the following high temperature flames continued to burn, with varying degrees of damage or rupture and leakage of combustible materials. The intensified combustions and fire triggered two subsequent deflagrations, 10 minutes and 40 minutes later respectively, after the first explosions.

The INITIATING CAUSE of the pipe rupture was internal corrosion. The wall thickness of the pressure pipe 90 ° elbow had become thinner and ruptured under an internal pressure of 0.2 MPa with an opening of about 950mm × 620mm. The hydrogen gas mixture ejected to form an explosive mixture. The friction between the ejected gas and the pipeline generated electrostatic sparks, which triggered a fire.
Regarding the root technical causes, the affected pipeline was continuously in an acidic environment. The company was not monitoring and analysing the acidic water in the pre-hydrogenation tank. The continued recycling of this acidic water led to increasing concentrations of H2S, HCl and NH3 and intensified the corrosion of the pipeline.
A contributing cause was the temperature of the pipeline. The operating temperature of the affected pipe was supposed to be 150°C, and the design temperature was 170°C. However, the actual operating temperature of the pipeline at the time of the accident was about 180°C, and this accelerated the corrosion process.
The ROOT CAUSE could be tentatively attributed to incapacity to control process parameter to avoid operating beyond the design values. The investigation found also organisation causes in the failing of implementing preventive safety measures (see lessons learnt).

process gas pipe,
heat exchangers,
naphta reforming unit

The accident caused different degrees of damage to buildings, equipment and facilities in the pre-hydrogenation unit, reforming unit and product refining and separation unit of the catalytic reformer with no casualties.

The municipality safety department intervened with 49 fire trucks and 121 firefighters. They acted according to the strategy: “first control, then elimination”, and “"cooling and suppressing explosions, focusing on protection, and preventing spread", using equipment such as fire-fighting robots and mobile fire-fighting water cannons. The public security department dispatched 628 police officers to implement traffic control and maintain order at the scene. The health department dispatched 5 ambulances to stand by on site. Approximately 5 hours later, the fires were completely extinguished.

The investigation revealed sever shortcoming in the definition and execution of safety measures for the site:
(1) The safety management system of special equipment was not implemented, in particular the part related to corrosion monitoring and prevention. The operator did not execute the annual thickness measurement procedures. Also, they did not realise that measurements performed previously by an external inspection were demanding corrective actions.
(2) The procedures for the analysis of acid water quality were not well designed and not implemented.
(3) The pressure pipeline were not respecting the requirements of the more recent regulations, and therefore, de facto, illegal.
(4) The operator did not perform the regular (annual) inspection of all the special equipment on site.