A globally confined toxic leak
In a chemical plant, an isocyanate (HDI) production facility was restarted after a shutdown period involving dismantling/cleaning as a result of fouling. At 2:55 p.m., a leak fuelled by phosgene, a toxic gas used as a raw material, occurred in the pressurised equipment’s containment structure. The phosgene sensors, installed in the enclosure, triggered an alarm. At 3 a.m., the installations were shut down and the ventilation flaps were closed to isolate the enclosure (via an automatic safety procedure). At around 3:20 a.m., two external sensors detected a slight leak on the containment’s large door (between 0.5 and 1 ppm). The phosgene, spreading throughout the containment, was drawn into the soda scrubbing columns which neutralized it until the evening. Between 200 and 350 kg of phosgene was released into the containment, and less than 1 kg outside. No personnel were affected. The operator issued a press release and informed the administration. The workshop was started up again the next day once the leaks had been identified and repaired. The damage and production losses were evaluated at a few tens of thousands of euros.
Faulty reassembly of a valve was determined to be the cause of the leak
The leak occurred on a connection flange of an expansion valve located upstream from the phosgenation reactor. The seal on this valve had been incorrectly installed after a cleaning procedure. Before restarting, a nitrogen leak test did not detect anything. An evaluation of the accident showed the absence of the graphite track over 30 mm of the circumference on the upstream joint. The seal’s outer centring ring, caught in the threaded rod, was found slightly warped. This deterioration resulted from damage to the seal during the reassembly phase. An inspection fault was noted. At the same time, the airlock seal on the large door was leaking due to its age and frequent use, particularly for passing equipment as no dedicated equipment airlocks were available.
The valve was replaced and requalified. The bolts on the main door (20 bolts out of 460 were loose) and the dry or corroded inflatable seals on the two ancillary airlocks were replaced. All containment openings were checked.
The operator has improved the formal testing procedure of the equipment after cleaning: systematic acceptance testing of work performed by subcontractors, and systematic helium testing of the valve.
The graphite seal technology was changed (from B29 to spiral). Raising awareness/training of subcontractors was initiated (identification and location of seals). Pressure testing of the containment is foreseen each time the large containment door is opened. Its bolts were replaced with non-deformable bolts.
