PIPEWORK CLEANING
Three examples of corrosion caused by water-quality problems. Top: Corrosion debris in half inch carbon steel pipe. Left: Crevice corrosion leading to perforation of a radiator. Right: Electron micrograph of seam weld pitting in carbon steel pipe. (Source: BSRIA/Reginald Brown)
problem. Traditional mild steel pipework is susceptible to oxygen corrosion, but thick enough to tolerate a short period of attack, during which dissolved oxygen is gradually depleted from the system water. Oxygen corrosion then more or less stops. Thin wall carbon steel pipe is less forgiving and can suffer from rapid oxygen pitting corrosion if conditions are not carefully managed. Once pitting is initiated it is likely to carry
on, even if problems with water quality are subsequently remedied. Plastic piping does not itself corrode, but in a system that is largely constructed of non-corrodible materials, oxygen levels will tend to remain higher for longer, and those materials present that are vulnerable to corrosion will be even more at risk. Steel radiators on plastic pipe
distribution systems in large buildings seem to be a recurring problem, sometimes with perforation less than 12 months after handover. The same problems rarely occur on single house systems as the period
56 CIBSE Journal November 2011
Once pitting is initiated it is likely to carry on, even if problems with water quality are subsequently remedied
between system filling and initial operation of the boiler is much shorter (hours or days rather than weeks or months) and the applied heat from early operation of the boiler will remove much of the dissolved oxygen from the system water. The main factors to reduce the risk of
corrosion in large projects are: l Ensure clean standards of construction and good quality fill water;
l Minimise the length of time when the system is unprotected, particularly the period between system filling/pressure testing and pre-commission cleaning;
l Ensure that treated water can be routinely circulated throughout the system as soon as possible in the commissioning period; and
l Regularly monitor water quality to ensure optimum concentration of inhibitors and pick up any developing problems, not just at the system pumps but also at the end of the system. This wider perspective of risk reduction
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