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NUON Energy part of Vattenfall company, had in their in Utrecht site moisture and condensation problems with some district heating lines. Although the pipe, and cladding were  in good condition, inspection showed heavy water accumulation. The pipes were insulated with mineral wool and finished with aluminium cladding. And according common practice a 10mm drain hole was drilled at the lowest point.

The above shown unforeseen situation led to discussion about the design. Is only a drain hole sufficient to prevent moisture accumulations within permeable insulation materials?

Also 2 conclusions were drawn :

1. Unwanted loss on thermal capacity.
2. Risk of Corrosion Under Insulation CUI.

Various options were considered which led to only one viable solution and PMU drainplugs were installed. For a test period  between November 2011 and March 2012 the lines were monitored by thermography inspection. And  results after inspection in March showed that there was no accumulation anymore and the condition of insulation material was well within clients margins.

This successful test resulted in a new NUON standard , which means more of these district heating lines will be executed with PMU drainplugs in the near future.