Excerpt from NACE International The National Association of Corrosion Engineers
“Hot and chilled closed loop water systems are found in many applications, ranging from space heating in commercial and industrial sites, to residential applications, as well as providing cooling at these same sites. Closed loops frequently receive little attention from the water management team, in spite of the fact that their failure is very inconvenient for correction and costly.
While most people assume that closed systems are free of oxygen, the reality is that they contain varying amounts. In a loop with low make-up, the only source is from in-leakage of air (around seals, etc.) and corrosion rates can be low. However, in systems where there is a significant amount of make-up, or poor air removal, the problem can be very severe. The presence of oxygen and temperature speeds up the corrosion processes and this can result in the formation of considerable amounts of sludge, which in turn can lead to reduced efficiency and equipment damage, as it settles out in low flow areas, such as the heat exchangers.
Traditionally, closed system treatment has involved adding corrosion inhibitor and periodically testing for the residual. However, to be effective, these treatment programs must also include cleaning (either prior to being put into service or to remove existing corrosion products) as well as monitoring. The systems being protected are generally of very high value and a comprehensive program, which assesses everything from pre-commissioning to monitoring ongoing performance, must be considered.”
