PIPES AND DUCTWORK
Talking about pipe life
Corrosion under insulation (CUI) can severely affect the performance of a HVAC system, impacting negatively on efficiency and causing operational issues. Tom Merton, Technical Specialist at Armacell, looks at how pipe insulation can prove an effective weapon in the fight against CUI.
P
ipe insulation may not necessarily be at the very top of the list when it comes to HVAC systems, but it’s critical not to underestimate its importance when it comes to long-term performance, as well as energy efficiency. CUI is an all too common issue in HVAC systems that don’t benefit from the right pipe insulation material. It’s caused by moisture penetrating the pipework, which can be as a result of either no thermal insulation protection or by using the wrong material to insulate. It is most severe when the system includes carbon and low alloy steel with brass or copper metals because it results in galvanic corrosion. Bi-metallic or galvanic corrosion occurs when dissimilar metals – like steel and copper or brass – are in close contact and in the presence of condensation. When this occurs in a HVAC system it results in the more anodic of the two metals undergoing accelerated corrosion, whilst the other metal (called the ‘noble’ metal) remains unaffected.
For example, copper pipework and brass
fittings (ranked higher on the Galvanic series – more noble) with steel hanger brackets (ranked lower) will result in the steel undergoing accelerated corrosion. It’s worth bearing in mind that the two metals don’t need to be in contact for galvanic corrosion to occur. When a more ‘noble’ metal corrodes slightly
and dissolves in moisture from condensation and subsequently drips over a less noble metal
36 April 2020 What is closed cell insulation?
Closed cell insulation As the name suggests, is made up from millions of tightly packed, completely closed cells. Each cell is bonded to those around it, creating an impenetrable barrier for both air and moisture. Closed cell foams retain their thermal performance for the life of the material without thermal drift, principally because the cells are filled with air. Flexible closed cell insulation tends to be a better insulator than some open cell materials.
Open cell insulation Is less dense than closed cell, which means it is easily compressed. Because of that it not as good an insulator and requires a thicker layer to achieve the same thermal performance. Open cell insulation allows condensation to form on pipework which leads to corrosion because moisture-laden air passes through it and condenses on cooler surfaces.
it will result in corrosion of the latter. Galvanic corrosion is therefore an issue that needs to be considered when specifying insulation.
Pipework supports A potential trouble spot can be pipe support brackets because they are susceptible to
galvanic corrosion (e.g. steel bracket connected to copper pipework). In addition, direct fixing of cold services pipework to their mounting brackets without insulation acts as a thermal bridge from surrounding warmer ambient air, resulting in rapid energy losses. As moisture is continually
drawn to the cold brackets it condenses on the metal parts. If this condensation spreads because it is insulated using open cell material, thermal efficiency is lost along the pipework and corrosion and consequential secondary damage will result.
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