Controlling corrosion in cooling tower water systems
The control of corrosion in cooling systems is essential as it can cause a number of serious problems. Corrosion of the metallic components within the system can lead to serious degradation issues, increased costs, reduced asset reliability and life-expectancy, and ultimately system failure. It can also impact heat transfer efficiencies which increases energy costs.
Cooling water related corrosion can be caused by a number of factors that typically include oxygen associated corrosion, galvanic corrosion and microbially influenced corrosion (MIC). Where stainless steel exists chloride ions can also become an issue causing pitting corrosion.
The relationship between corrosion and scaling potential in cooling waters can be predicted using the Langelier Saturation Index (LSI).
The system should be kept clean and free from microbial contamination and fouling. A specialist cooling water corrosion inhibitor, selected to account for the system metallurgy and local water conditions should also be used.
Scale control in cooling water systems
The formation of scale (sometimes called limescale) on heat exchanger surfaces and cooling tower packing can become a significant issue. If left untreated or treated incorrectly scale can build-up on surfaces and acts as an excellent insulator to reduce heat transfer efficiencies and constrict flow rates. It can also harbour dangerous pathogens including legionella bacteria, the causative organism for the potentially fatal condition Legionnaires’ disease.
The scaling potential of the water source depends on the source water hardness, and occurs when calcium and magnesium salts precipitate out on to surfaces as a result of heat and increased concentration factor. The scaling potential can be predicted using the Langelier Index (LSI) and this can be used to develop an optimum cooling water treatment programme that balances both scale and corrosion considerations.
To control scale formation in a typical cooling system the following water treatment activities should be considered.
- Pre-filtration or clarification of the water to remove suspended solids present.
- Cooling water treatment chemicals using specialist polymer dispersants to ensure suspended solids remain mobile.
- Side stream filtration of to remove suspended solids and clean cooling circuit.
- Periodic off-line cooling tower cleaning and disinfection.
Microbial control, legionella & Legionnaires’ disease
Microbial contamination of cooling systems, and in particular, cooling towers that use open evaporative cooling techniques should be considered a safety critical issue that requires careful management and control to ensure they are clean, well maintained and meet the compliance standards set in the Health & Safety Executives ACOP L8 and HSG274 Part 1 dealing with the control of Legionella bacteria and Legionnaires’ disease in evaporative cooling systems, including cooling towers.
Cooling towers are known to present a particularly high Legionella risk as their mode of operation can create an ideal mechanism for the growth, proliferation and dispersal of Legionella bacteria over wide areas, now known to be at least 6km from the source cooling tower.
Did you know that cooling towers can spread Legionella bacteria over wide areas… at least 6km in some cases
Controlling microbial contamination including legionella in cooling towers requires a robust water treatment and Legionnaires’ disease control programme. This would typically include the preparation of detailed operation and maintenance procedures for the cooling tower together with a chemical water treatment programme involving the use of specialist cooling water treatment chemicals, dosing equipment and data controllers.
Regular monitoring and testing will also be required to validate the success of the treatment programme, along with specialist laboratory tests for Legionella bacteria at least every three months. The cooling tower system will also require cleaning and disinfecting regularly.
Additionally, the Health & Safety Executive’s ACOP L8 requires that you:
