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Cooling towers are typically made of concrete and galvanized steel and transfer waste resulting from processing plants to the atmosphere. These are the beaker-shaped towers commonly seen at energy-processing plants. Passivation is a process whereby materials are made corrosion-resistant. or passive, to corrosion. Passivation of cooling towers protects them against the harmful effects of white rust.
New cooling towers contain dirt, different kinds of oils, grease films and rust as a result of the manufacturing process. Traditional cleaning processes and HVAC (heating, ventilation and air conditioning) water-treatment processes use water with high alkaline and high PH levels. This can cause white rust to form on galvanized surfaces, including galvanized steel. This can be avoided by combining the cleaning process with a prior passivation process. Such a process results in the formation of a zinc corrosion-inhibition layer on the inner surface of the tower. This is a dense, adherent film that protects against corrosion.
Before the cleaning and passivation process is carried out, it is first recommended that cooling towers undergo hydrostatic testing. First, all debris inside the tower must be inspected and removed. Second, the tower must be filled with treated water. The treated water should include a mixture of orthophosphate or hexametaphosphate at between 40 and 60 parts per million, as well as tolyltriazole at between 10 and 20 parts per million. This mixture should be thoroughly circulated through the system.
Cleaning and Passivation
If the tower passes the hydrostatic test, the pre-operational cleaning and passivation procedure should be carried out. First, flush and drain and remaining debris left behind by the hydrostatic testing procedure. Second, add an amount of cleaning formula appropriate for the volume to be cleaned and circulate the formula through the system for at least 24 hours. Third, flush and drain the system so it is free of cleaning solution. Ensure that all strainers and filter have been cleaned. Finally, circulate the system with makeup water for 30 minutes.
The standard responsible for governing the cleaning and passivation of cooling towers is UFC 3-240-13FN. This standard was developed and published by the National Institute of Building Sciences. The full standard may be viewed on the Web site of the Whole Building Design Guide, which is program of the National Institute of Building Sciences.