Corrosion in Heating and Cooling Systems.
Strategy for Managing Corrosion in Circulating Heating and Cooling Systems.
The major strategy for reducing corrosion and leaking in heating systems covers the whole lifetime of the installation.
At the design stage it is important to produce a design that does not enable air to enter the system and enables any oxygen in the water to escape. This can be through AAV (automatic air vents) in closed systems or via a header tank in open systems.
Entrained oxygen or air can cause general corrosion problems, galvanic corrosion and can also result in cavitation. Small amounts of air in completely enclosed systems can result in oxygen pitting and water line corrosion.
Cavitation in heating and cooling systems can be made worse by the presence of debris and result in erosion problems. A typical example would be the seats of
the three way valves often associated with FCU’s (fan coil units).
Refrigeration units, boilers, evaporator units, heat exchangers, chiller units, pumps and general pipe work can also be affected by similar corrosion problems. Erosion corrosion in pipework can also be reduced at the design stage by the inclusion of strainers into the system.
Before a heating or cooling system enters service it is important to carry out a very thorough pre-commission clean to ensure correct functioning and corrosion free operation.
From a corrosion point of view, it is important to remove mill scale from the inside of the steel pipework and to remove residual carbon films from copper pipe work. If residual organic matter is left in the system, corrosion under compacted deposits can occur as the corrosion inhibitor is unable to reach the corrosion sites.
Residual organic matter that is left in a system can act as a food source for bacterial organisms and this can allow MIC (microbially induced corrosion) to occur. The corrosion problem with MIC is enhanced if the inhibitor system contains sodium nitrite, which can also act as a food source for microbes.
Corrosion in heating and cooling systems is therefore reduced by a thorough pre-commission cleaning or flushing. BSRIA publish application guides AG1/2001 and AG 8/91 which should be carefully followed.
After the pre-commissioning cleaning the final part of the strategy is to introduce water treatment chemicals into the system. These water treatments are commonly formulated with corrosion inhibitors, anti-scaling agents and a mixture of other chemicals such as oxygen scavengers. Sodium sulphite is the most common chemical used to prevent corrosion by scavenging the oxygen. Control of oxygen down to 20 parts per billion is possible.
Inhibitors for preventing galvanic corrosion caused by mixed metals such as copper, steel and aluminium should also be present. Sodium benzoate is a common galvanic corrosion inhibitor.
The most common general corrosion inhibitor is sodium nitrite as it is both cheap (inexpensive) and very effective. However, it is regarded as a “dangerous inhibitor” as its performance depends on the concentration. If the nitrite concentration drops too low it can actually increase the corrosion rate. Often sodium nitrite is used with sodium molybdate with which it has synergistic properties.
In systems with MIC (microbial corrosion) problems, nitrite is not used and molybdate may be the main corrosion inhibitor in the inhibitor package. The mixture for heating and cooling systems can also contain tannins, phosphonates and film forming inhibitors such as amines.
Maintenance Of Heating & Cooling Systems - Corrosion Issues.
After the heating and cooling system is commissioned and enters service it is common to award the maintenance of such a system to a water treatment contractor. This contractor will monitor the system for inhibitor levels and top them up via either a dosing point manually or using automatic inhibitor dosing pumps. When automatic pumps are used either the corrosion rate is monitored or the water constituent variables such as iron or inhibitor level are measured and monitored.
Routine inspections should be carried out with greater frequency immediately after commissioning as inhibitor levels may drop quickly due to them being used up to control the initial corrosion or by being absorbed onto internal surfaces or debris.
Once the system has been stabilised then the conditioned water and good system design, together with a well carried out pre-commission clean and flush, should result in the system reaching its design life (typically of 25 years) without major problems.
Initially, a few small leaks or drips from joints, flanges, FCU’s and valves may be expected due to poor fitting or minor equipment faults. Increasing failure rates after the initial period should be treated as a major concern.
If red rust appears at sites such as the above it can be taken that air has entered the system and that the inhibition provide by the water treatment chemicals is insufficient. Failures in fan coil units like those shown in the header photo are often the first indication of system problems.
Corrosion Failures in Heating Systems.
Corrosion failures taking place more than six months after the commissioning stage should be viewed with concern. Rapid initial corrosion failures in environmental control systems should be thoroughly investigated or substantial system corrosion and degradation can occur.
Indications of corrosion in the system are rapid inhibitor consumption, the need for excessive inhibitor dosing, the presence of debris at the lowest point, the presence of slimes and biomass from microbial corrosion and changes in the colour of any water drawn from the sampling point.
Corrosion that leads to leaking radiators, pipe work or fan coil units should be reported. Increasing numbers of leaking components associated with poor water quality should trigger a review of the whole system by either a corrosion engineer or an M&E engineer in combination with a corrosion engineer.
Unusual rapid corrosion causing leaks or leakages in larger components such as risers, distribution pipework or boilers may be due to other types of corrosion such as deposition corrosion or stray current corrosion due to electromagnetic interference. Stray current corrosion often shows as very rapid corrosion on the faces of flanges or other gasket components such as rapid corrosion of valves.
Rapid corrosion due to air leaks in closed systems is a common problem. Water leaks can often result in inhibitor depletion and loss of protection.
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