When should the cooling tower be put into use for circulating water cleaning of the pre-coating process?

After the pre-coating process begins, cooling towers and side filtration tanks are put into use to prevent impurities in the water from clogging these systems

Please explain why cleaning does not affect the operation of the cooling tower and the bypass filter tank. Thank you!

I think it should be put into use after pre-coating the film. During the cleaning of the pre-coating, the presence of a large amount of suspended solids can clog the fillers in the cooling tower. The oil removed during cleaning may contaminate the filter media of the side filter, so it is put into use after pre-film completion.

The treatment process for circulating cooling water consists of: A. Manual cleaning, B. System washing, C. Chemical cleaning of the system, D. Pre-coating of the system, and E. Normal operation. Before a factory begins operations, it is necessary to carry out various cleaning tasks in order to remove oils, weld residues, rust deposits, sediment, and other debris from the pipes and heat exchangers that come into contact with the cooling water. After a factory has been in operation for a certain period of time, the cooling water, under the influence of physical or chemical conditions during operation, or due to inadequate treatment of its quality, often develops biological sludge, salt deposits, and corrosion products. These pollutants directly affect the heat exchange efficiency of the heat exchangers as well as the flow area of the pipes; therefore, systematic water washing and chemical cleaning are necessary. Cleaning at these two different stages is an effective measure to maintain the proper operation of the process and ensure optimal cooling efficiency of the heat exchanger; it is also a necessary condition for the pre-film treatment of the cooling water system. 1. Manual cleaning: Before filling the circulating cooling water system with water, it is necessary to manually clean the collection tank and remove completely all kinds of debris such as sediment from within the tank; this step is very important. 2 System washing 2.1 Fill the collection tank with water; it is not necessary to fill it completely – only enough water to allow the circulation pump to draw from it is required ； 2.2 Start the circulation pump and increase the circulation rate to maximum ； 2.3 During the water washing process, analyze the turbidity every 20 minutes and plot the variation curve ； 2.4 As the cycle time increases, the turbidity of the circulating water rises. When it reaches its highest level, the washing process can be stopped and the water drained; this step takes about 24 hours ； 2.5 If the turbidity after washing does not increase significantly, proceed directly to the next step ； 2.6, Explanation: a. If the turbidity increases significantly during the washing process, it is best to drain the water after washing is complete and clean the collection tank again, thereby creating favorable conditions for the subsequent film-forming step and normal operation ； b. If the turbidity of the circulating water is below 20 mg/L, chemical cleaning can be carried out directly. 3 Chemical Cleaning of the System 3.1 Purpose of Chemical Cleaning The purpose of chemical cleaning of the entire system is to remove oil, corrosion products, dirt, and other impurities from all the heat exchangers in the system, thereby improving the system’s heat exchange efficiency and activating its surfaces to lay a good foundation for film formation. 3.2 Steps of chemical cleaning: Sludge removal → Scale and corrosion product removal → Displacement → Pre-coating → Displacement → Normal operation. 4.3.3 Chemicals used for cleaning and their quantities: Name of chemical, Quantity (Kg): Z-33E cleaning agent – 800; Z-34 defoamer – 600. 3.4 Items and frequency of cleaning inspections: 3.4.1 Sludge removal; 3.4.2 Scale and corrosion product removal. 3.5 Cleaning procedures: 3.5.1 Preparation work: (1) Form an on-site cleaning team to address any issues that arise during the cleaning process; regularly monitor those areas that require special attention, and take immediate action in case of any emergencies. (2) Check each component of the circulating water system, including the chemical dosing equipment (other than manual dosing), drain valves, make-up water valves, pH meters, as well as flow meters for make-up and drain water, to ensure that they can meet the requirements for chemical dosing, drainage, and make-up water during cleaning. If they cannot meet these requirements, temporary measures should be taken. (3) Transport the chemicals required for chemical cleaning and pre-coating to the site, adjust the dosing equipment (except for manual dosing), make all necessary preparations for analysis as required, and be in standby mode the day before cleaning. 3.5.2 Chemical cleaning: (1) First, check the liquid level in the collection tank; adjust the liquid level in the circulation tank to a safe level, and set the flow rate of the circulating cooling water to its maximum value ； (2) Analyze pH, turbidity, alkalinity, total iron, and hardness in the circulating cooling water ； (3) During the cleaning process, the valves of all water inlet heat exchange equipment must be opened to full extent to ensure an effective cleaning result ； (4) Add 220 kg of Z-33E to the system for the first time based on its water storage capacity. After one hour of circulation, analyze and measure the pH value of the circulating water system ； (5) If the pH of the circulating water is higher than the specified control value, more cleaning agent should be added; the cleaning agent Z-33E should be added gradually, and the pH of the circulating water should be monitored continuously to keep it above 5.0 ； (6) If the cleaning agent Z-33E is exhausted and the pH of the circulating water is above the specified range, industrial-grade sulfuric acid shall be used to adjust the pH of the circulating cooling water to within the controlled range ； (7) If the system is quite dirty, then around 6 hours into the cleaning process, when there is a large amount of waste material produced and the water becomes highly turbid, it is necessary to drain the system, refill it with water, and then add more cleaning agent. The purpose of doing this is to prevent excessive concentrations of the various components that are washed away from occurring, which could lead to secondary deposition. Another purpose is to extend the cleaning time in order to achieve a thorough cleaning of the system; the specific cleaning time should be determined based on changes in the total iron content. Cleaning can be stopped once the indicator stops rising; generally, the cleaning period is around 24 hours, depending on the specific cleaning conditions ； (8) A small amount of foam will be generated during cleaning; at this time, Z-34 antifoam agent can be added in an appropriate amount to eliminate the foam. The typical concentration for addition is around 100 ppm, but it is not necessary to completely remove all the foam ； (9) During cleaning, try to avoid discharging waste water or adding water, so as not to reduce the concentration of the cleaning agent. If the concentration decreases, cleaning agent should be added promptly ； (10) The chemical cleaning time should be determined based on changes in system turbidity and total iron content; generally, the cleaning time is 24 hours. (11) After cleaning is complete, drain as much water as possible, then add water again for displacement until the total phosphorus level in the system is below 30 ppm and the turbidity is below 15 ppm; at that point, the displacement process can be stopped and proceeding to the next step can begin. Otherwise, the pH of the circulating water should be maintained between 7 and 8 (by using sulfuric acid) to carry out the displacement process. (13) Note: a. The analysis frequency can be adjusted as appropriate as needed. b. During the chemical cleaning process, if the pH of the circulating water drops below 5.0 due to an excessive amount of cleaning agent being used, an appropriate amount of liquid alkali should be added to the system or the circulating water system should be properly replaced to ensure that the pH of the circulating water remains within the specified range ； 3.5.4 During the cleaning process, the following parameters are monitored and analyzed: Analysis Parameter, Analysis Frequency, Control Limits. PH: once every 1 hour; range: 5.0–6.5. Turbidity: once every 2 hours, to determine whether the curve shows stability. Total phosphorus: once every 4 hours; limit: >60 ppm. Orthophosphorus: once every 4 hours, to determine whether the curve shows stability. Total iron: once every 4 hours, to determine whether the curve shows stability. Ca or total hardness: once every 4 hours, to determine whether the curve shows stability. 3.5.4.1 During the chemical cleaning process, water quality analysis is carried out in collaboration between Party A and Party B, with Party A taking the lead role. 3.5.4.2 During the chemical cleaning process, the addition of chemicals to the system is carried out by both Party A and Party B, with Party B taking the lead. 4.4 System Pre-coating 4.4.1 Purpose of Pre-coating Pre-coating is intended to protect the metal surface from corrosion. It involves using certain components in chemical agents to form complexes with divalent metal ions in the cooling water (such as Ca2+, Zn2+, Fe2+, etc.), thereby creating a very thin yet dense protective layer on the metal surface. This layer adheres firmly to the metal surface, helping to prevent corrosion caused by the cooling water; it also helps to prevent corrosion due to microorganisms. Such a layer is often referred to as a protective layer or anti-corrosion layer. Practice has shown that the cleaning and pre-coating of cooling water systems are among the effective measures for reducing corrosion, improving heat exchange efficiency, extending the service life of equipment, and ensuring continuous operation over the long term; therefore, cleaning and pre-coating are essential components in the current treatment of cooling water systems. 4.4.2 Requirements for the quality of the pre-coated film: The surface of the carbon steel test pieces coated with this film must be free of rust, and these test pieces should exhibit a distinct color halo when viewed under sunlight.

I would like to ask regarding the 4th floor: the required turbidity level during pre-coating is ≤10. If bypass filters and cooling towers are not used, then the pipes associated with these devices have not undergone pre-coating; will corrosion be more severe in such cases?

During the cleaning of the pre-coating, it is best not to cool the water tower, in order to avoid damage to components such as the cooling tower fill; the water supply valve on the cooling tower’s vertical pipe can be closed to allow the cooling water to circulate within the tank.

In my opinion, after pre-coating, a large amount of dirt is generated during cleaning, and starting up the system can cause damage to the cooling tower and the side filtration tanks. If pre-coating is carried out first, then by replacing the water in the system, the corrosion rate is significantly reduced; as a result, there is less sediment in the water, and scaling does not occur. Therefore, that is the best time to start up the system.

Thank you all for your help. I understand now that there is a lot of rust and impurities during cleaning, and these will contaminate the packing when it goes into the tower； The side filtration tank cannot be used during pre-film formation either; the automatic backwashing of this tank would result in the loss of the pre-film agents. Therefore, it is more appropriate to use the cooling tower and the side filtration tank after pre-film formation.

My experience in handling this issue: During chemical cleaning, the cooling water is not sent to the cooling tower nor to the side filter. There are three main reasons for this: ① The pH level during the cleaning process is low, usually between 4.0 and 5.0; sending this water to the cooling tower could damage some of the fill materials in the tower as well as those in the side filter；       ②Chemical cleaning may produce a large amount of foam (depending on the ingredients of the cleaner), and foam can easily escape into the tower ；       ③During the cleaning process, the iron and turbidity removed can easily contaminate the packing and the side filter.           Cleaning and pre-coating procedure: 1. Shut down the side filter tank before cleaning; if there is an air separation unit, it is best to close the valve leading to that unit ； Close the water supply valve on the cooling tower ； The entire process operates in a closed-loop manner, with zero waste discharge, to maintain the liquid level in the cooling tank and keep the system volume constant.      2. Before pre-film formation, reduce the turbidity of the circulating water to 15 mg/L (it can be reduced to 10 mg/L if the discharge requirements permit), and ensure that the total iron content is ≤1 mg/L. Once these conditions are met, open the valve leading to the upper tower; the circulating water is then directed toward the cooling tower to initiate the pre-film formation process.      3. Once pre-coating is complete and normal operation (or cold operation) resumes, the side filter can be put into use (it should be shut down after hydraulic flushing but before chemical cleaning).           The above is based on years of experience in handling such matters; I hope friends will offer their insights and suggestions to address any shortcomings. This post was last edited by davoo on 2009-3-4 18:26.]

It is best to send it to the cooling tower after cleaning is complete