UK Health and Safety Executive notice – corrosion in trunnion supports of pipework containing hazardous substances

Published:  09 October, 2013

Problem leads one company to carry out urgent remedial work on 5% of a site’s population of trunnions.

Pipework support trunnions are common in petrochemical installations, where short lengths of tube are welded to process pipework as a means of supporting from structural steel.

A release of Liquefied Petroleum Gas (LPG) occurred on a UK refinery during processing, leading to a controlled shutdown and isolation of the associated vessel and pipework. Further investigation revealed that corrosion had occurred within the trunnion supports for the pipework, with a gradual loss of wall thickness on the LPG pipe.

The tubular support had been fitted with an end cap, forming a closed end, which in theory eliminated moisture ingress, but the trunnion also had a weep hole drilled on the underside to allow for venting. This hole was sited directly on support steelwork. As a result, the void within the trunnion gradually drew in moisture, which then condensed on the pipe wall. This in turn led to accelerated localised corrosion of the pipe. As this corrosion mechanism was not anticipated, it remained undetected until the pipe wall eventually ruptured under internal pressure.

The company have conducted further investigations on site, and have found other instances of the same corrosion. This has led to immediate replacement in some cases. In total, almost 5% of the site population of trunnions has had to have some remedial work.

Pipe support trunnions, open and closed end.

Figure 1: Pipe support trunnions, open and closed end.

The HSE is recommending that operators of plant with pipework containing hazardous substances should ensure that their inspection regime includes suitable examination of supports. In particular, where tubular trunnions are installed with an end cap that prevents visual examination, other means should be employed to ensure the integrity of the system where there is a possibility of moisture ingress.

Permanent removal of the end cap is not recommended without further design work, as the cap may be required for additional stiffness.

To read the full Notice, click here.

  • Operation Florian

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