Ten kilos (22 pounds) of hydrogen chloride gas was released from a flange that had been fitted by a contractor just a few weeks earlier, triggering alarms and resulting in an internal emergency intervention. The incident had no significant consequences. A contractor performing maintenance was instructed to remove the pressure sensor from a device and replace it with a blind flange. Due to the corrosive environment (HCl), the plate on the pressure sensor was coated with tantalum. After being removed, the pressure sensor was supposed to be closed off with a Teflon®-coated full flange. As none were in stock in the site’s workshop, the contractor was given a standard flange and a Teflon® sealing disc. This type of substitution is routine.
After the incident (which occurred several weeks after the maintenance operation), the flange was found to be corroded and a large hole could be seen extending through the steel plate. It is this degradation that caused the hydrogen chloride release. When the flanged seal was opened, the operator saw that instead of the Teflon ® sealing disc, there
was a Teflon® O-ring. The result was that the steel flange was left unprotected.
The contractor had 40 years of job experience and regularly used this type of seal at the plant. The main issue raised by this incident is the supervision and checking of maintenance work performed by contractors. The operator’s safety management system should be revised to include supervision and post-maintenance checks. In addition, two technical measures were immediately put in place:
- the Teflon® sealing discs (made in the site’s workshop) have a protruding tab that makes it easy to locate and identify them (impossible to mistake them for O-rings);
- documentation is compiled when flanged seals in contact with hazardous materials are fitted: seal specifications, manufacturer certificates, photos of the fitted bolts and seals prior to tightening.
SOURCE: European Union Network for the Implementation and Enforcement of Environmental Law (IMPEL)