At a chemical manufacturing facility a contractor was given the following task:The pressure sensor on an apparatus was to be removed and replaced by a flanged blind. Because of the corrosive medium (HCl) the plate of the pressure sensor was coated in tantalum. The flanged joint was sealed with a Teflon ring seal. After removing the pressure sensor the apparatus was to be closed using a Teflon coated, flanged blind.

The site’s workshop did not have sufficient Teflon coated, flanged blinds in stock. Therefore it was decided to issue the contractor with a standard flanged blind and a Teflon disk seal. This is an equivalent replacement.

The contractor had more than 40 years of experience and knowledge on working on this plant with these seals.

Several weeks after the task had been executed the flanged blind had been corroded through the steel plate leaving a large hole. This was discovered when there was a release of HCl gas (ca. 10 kg) which triggered alarms and an internal emergency response.

On investigation (opening the flanged joint) it was discovered that instead of the Teflon disk seal which had been issued, a Teflon ring seal had been inserted, leaving the steel blind unprotected.

As a result two measures were undertaken. Firstly Teflon disk seals (manufactured in the site’s own seal workshop) are made with a protruding tab, so that they can be readily located and that it is clear that they have been inserted and not a ring seal. Secondly, flanged joints for hazardous materials are documented when they are made-up. This includes specifications for the joint, signatures of those making the joint, photographs of bolts and seals in place, before bringing the flange faces together, so that it is clear that everything has been assembled in the correct position.

The key issue which has been identified arising from this accident is how can an operator know that the contracted work has been carried out as intended and that the contractor has not made any (undeclared) errors or alterations?

This is an issue which the safety management system needs to address through documentation or supervision procedures.