During restart of the ammonia (NH3) shop at a chemical plant, the operations teams noticed around 8:50 am a jet flame lasting just a short time on the air supply pipe of the post-combustion reactor.

The NH3 production shop included several secondary units; the secondary reforming unit (post-combustion) was equipped with a reactor to complete methane cracking. During restart of the workshop’s post-combustion section, pressurised inflammable gas containing methane released from the primary reforming unit came into contact with hot air (~500°C) at the level of the air supply pipe of the post-combustion reactor.

Since self-ignition conditions of the air-gas mix had been met, the mix ignited, causing an explosion inside the pipe, which was deformed over a 1-m length, and an 80-mm diameter opening was created on the lower part near a circular weld. System decompression and ignition of the air-gas mix led to the jet flame detected by technicians.

The installations were placed in secure mode and the workshop was shut down around 7 pm. Bleeding the pipes along with vapour capacities were responsible for generating a loud noise.

Subsequent to this incident, the operator planned to restart the installations after replacing the damaged pipe and verifying (with the potential for replacement) the check valve installed on this pipe section. During its investigation however, the classified facilities inspectorate observed that out of several hundreds of operations or verifications included in the “check-list” of the “OPERGUID” start-up manual of installations (whose objective was to facilitate process controls and installation operations under optimized safety conditions), only 3 of them had not been recorded and several tens of others had given rise to handwritten modifications. Moreover, the stable state verification document complementing this manual had only been formally validated by a single person, namely the foreman.

The elements presented by the operator to the classified facilities inspection exposed the inappropriate implementation of the shop’s start-up phase, initiated on 5th April, in addition to the inadequacy of measures adopted to remedy cases of guideline noncompliance, as evidenced by the internal audits conducted to ensure compliance with procedures established as part of the safety management system. The classified facilities inspectorate noted these facts, and a formal notification was issued by the Prefect on 30th April, 2010. The NH3 production installation start-up procedures were revised, with the workshop being placed back into service at the end of May 2010 under stronger operating supervision.