HSE Safety AlertLaura Nelson
Catastrophic Rupture of Dead-Leg Pipework
The HSE have circulated a document warning Process Plant Operators of the increased risks of primary containment loss associated with dead-legs that are present in process pipe-work systems.
“A recent failure on a UK refinery involved an 8” diameter vertical relief line, approximately 5m in length, which catastrophically failed during normal operation part way along its length, releasing approximately 75 tonnes of extremely flammable material at elevated temperature and pressure. Connected to an insulated process header and vessels, the failure involved complete separation of the relief line (which was not insulated)”.
“Subsequent examination of the line revealed excessive internal thinning which was very local to the failure point, but relatively normal wall thickness elsewhere along its length. The inspection regime included regular thickness testing at four locations, but not at the area local to the failure. Notably, an adjacent relief line was also found to have very similar localised and severe internal corrosion, which had also gone undetected.”
“Advice relating to dead-leg inspection can be found within Section 6.6.2 of API 570 and Annex I.6 of the Energy Institute guidance, and includes comment on the level of rigour that needs to be applied; over and above regular pipework examination. The API guidance includes the following points pertinent to the cases outlined:
- Operators should consider the removal of non-essential dead-legs;
- Dead-legs can require special attention due to increased corrosion rates;
- Risk assessment should be employed for lines at risk of accelerated corrosion, or which may be difficult to isolate. Any thermal gradient, liquid interface or other feature that could exacerbate corrosion should be included in the risk assessment as this may lead to highly localised deterioration.
- Various non-destructive testing methods are available depending on the size and duty of the pipework, but for high risk dead-legs, a reliance on spot UT is not considered adequate, and operators should ensure that a significant proportion of the entire dead-leg is tested for remaining wall thickness to help identify local corrosion.”
*extracted from HSE original document
What the HSE are effectively saying is that “dead legs should be inspected in their entirety using techniques that will find very localised areas of metal loss”.
Most of the dead legs we encounter, or are most aware of, are the small-bore vents and drains originally fitted to the pipe work for use during the pressure testing of the lines after fabrication. Many problems with these short length vents and drains can be resolved by ensuring the Inspection company carrying out the radiography use large films or cassettes ensuring 100% of the dead legs are captured.
There’s no doubt this will be a wake-up call for many to check for the less obvious dead legs. For example, the long lengths of pipe work previously supplying an in-service unit but which are now blinded off at one end after the unit was shut down or mothballed. In some instances, the dead leg may be 10, 20 30 or 50 metres long. Potentially a very big problem and expensive to have to scan regularly.
So, What is the Solution?
As the HSE outline, remove any dead legs wherever possible, carry out detailed inspections (if you are not doing this already), investigate any problems and identify the causes which will help you understand and predict future problems and share your findings with the wider community – you may be able to pool knowledge with others and find solutions you may not have been aware of.
Finally, continuing with or adopting a robust Asset Integrity Management regime is an effective tool to manage and reduce the risks of failure in your piping systems, including any dead legs.