Question regarding pre-coating of circulating water

Pre-coating with circulating water can be carried out while the equipment is in operation, or it can be done before the equipment starts to be used. In this case, we performed the pre-coating while the equipment was running after it began operation. Once the required pre-coating time has passed, the circulating water needs to be replaced and chemicals need to be added again

It’s possible to do that, but I think it’s best not to use another cooling system during the initial pre-coating stage, when rust and oil removal take place, because the water at that time is quite corrosive and dirty. It shouldn’t be a big problem if it’s done later, during the phase of extensive cleaning; in that case, the project timeline can be adjusted slightly. Personally, I believe the pre-coating process should be carried out thoroughly, as it greatly contributes to the stability of the system. By the way, for newly installed pipes, it’s better to let them experience some pressure testing, as that yields better results.

This post was last edited by Tan Zhaofeng on 2012-1-29 at 12:57. The pipes in the recirculating water system and the cooler tubes, when exposed to the air for long periods of time, develop an oxide layer and rust deposits of varying thicknesses on their surfaces. To prevent rapid corrosion or scaling of the equipment and pipes once they start operating, it is necessary to carry out system cleaning and pre-coating procedures. I. Mechanism of action for cleaning and pre-coating 1. Chemical cleaning Chemical cleaning consists of cleaning and stripping, as well as acid washing. Pickling involves using inorganic or organic acids to undergo chemical reactions with rust and calcium scale, thereby removing substances such as Fe2O3, Fe3O4, or CaCO3. Common acids include hydrochloric acid, sulfuric acid, nitric acid, citric acid, sulfamic acid, etc. 2. Pre-coating: After pickling, the pipes and cooler tubes appear in their natural metallic color, and this state makes them highly susceptible to corrosion. A pre-coating agent with corrosion-inhibiting properties should be added immediately; it readily forms complexes with divalent metal ions in water such as Ca2+, Mg2+, and Zn2+ (the reaction equations are shown below). Through an electrodeposition process, a precipitative protective film is formed that covers the metal surface evenly, thereby separating the corrosive ions in the cooling water from the metal surface and preventing corrosion. At the same time, after pre-coating, the growth of scaling can also be controlled to prevent under-scale corrosion. Common pre-coating agents fall into two main categories: inorganic phosphorus and organic phosphorus. 　　　　 2Na5P3O10 + 5Ca2+ → Ca5(P3O10)2 + 10Na+ 　　　　 Ca5(P3O10)2 + 3Na5P3O10 → 5Na3CaP3O10 II. Cleaning and pre-coating 1. Acid cleaning The acid cleaning process involves a combination of organic acids and inorganic acids, with the inorganic acids being used to help regulate the pH value; 98% industrial sulfuric acid is used, and the pH value is controlled to be between 4.5 and 5.5. During the pickling process, hanging coupons are installed in the sampling box to monitor the corrosion rate, and process parameters such as circulating water turbidity, pH value, calcium ions, and total iron are analyzed at regular intervals; the pH value is monitored continuously online. When the total iron concentration stabilizes and the calcium ion concentration shows a clear downward trend with little variation, it can be determined that the pickling process has reached its end. 2. Pre-coating: After the pickling process is completed, the system is immediately subjected to a replacement operation. Once the turbidity of the water in the circulation tank is less than 20 mg/l, a pre-coating agent is added to the system. At the same time, sulfuric acid with a concentration of 98% is used to adjust the pH value of the system to between 5.5 and 7.0, thereby facilitating film formation. After 42 hours of pre-coating, the concentrations of total iron and total phosphorus in water stabilized. Inspection of the test samples revealed no rust on their surfaces, while a distinct colored halo was present on the surface of the carbon steel, indicating that the pre-coating process was complete. 3. Through extensive water replenishment and replacement operations, the total inorganic phosphorus level in the system was reduced to around 10 mg/l, thereby meeting the conditions for normal operation. To ensure the effectiveness of the cleaning and pre-coating process for the entire circulating water system and to save costs, I recommend that this process be carried out only after all equipment and pipelines are in use, and that laboratory analyses be employed as a guide and form of oversight throughout the cleaning and pre-coating procedure.

This solution seems familiar; it can’t be GE’s solution, right, haha

Hehe, the cleaning and pre-coating steps for circulating water systems are basically the same.

It seems that, based on what everyone has said, it is recommended to carry out both cleaning and pre-coating at low temperatures. I’m not sure if my summary is correct

In an open-loop circulating cooling water system, the circulating cooling water is reused over extended periods of time and comes into full contact with air in the cooling tower to facilitate the transfer of matter and heat. It enables oxygen in the air to dissolve more effectively in water, until the oxygen content in the water reaches its **maximum solubility. As the circulating water passes through the heat exchange equipment, it absorbs heat and its temperature rises, which reduces the solubility of oxygen. It causes the dissolved oxygen in water to reach saturation in local areas, leading to depolarization of the dissolved oxygen and thus corrosion of the equipment. The circulating water passing through the cooling tower comes into contact with air containing low levels of carbon dioxide; as a result, the CO2 in the water is extracted by the air and released into it. This causes almost all of the bicarbonates in the water to be converted into carbonates, thereby increasing the tendency for carbonate scale to form. As the concentration ratio increases, dissolved solids and impurities in water continue to accumulate, altering parameters such as pH, turbidity, hardness, and alkalinity. This results in both corrosivity and scaling tendencies in the circulating water, with these issues becoming increasingly severe. The circulating water contains high levels of salts and impurities. The dust in the air also contains a large number of microorganisms and their spores. Moreover, the temperature of the circulating water (32–42°C), the light in the cooling towers, along with ample oxygen and nutrients, all contribute to the growth of bacteria and algae, leading to an increase in microorganisms and sludge. Due to the low rate at which this circulating water is discharged, sludge and suspended particles become highly concentrated, which can easily result in deposits on the heat exchangers and cause damage. Dirt adhering to the equipment not only causes scaling but also exacerbates corrosion, posing significant risks. Henan Longyu Coal Chemical Co., Ltd. is carrying out maintenance work on its 500,000-ton methanol production system. During such maintenance, debris such as welding slag can accumulate on the surface of the equipment, along with a large amount of sludge and impurities. Before the system can operate normally, these substances must be removed in order to create a clean and smooth metal surface suitable for coating application. I. Water balance parameters of the circulating cooling water system: The inlet temperature of this circulating water system is 32°C, the outlet temperature is 42°C, and the temperature difference between the inlet and outlet is ΔT = 100°C. Based on the principles of thermodynamics and material balance, the water balance parameters for the system are as follows: Parameter | Data | Value
---|---|---
Circulating water volume (Q) | m3/h | 42,000
Wastewater discharge volume (B) | m3/h | 500
Evaporation volume (E) | m3/h | 600
Water loss due to wind and leakage (L) | m3/h | 100
Makeup water volume (M) | m3/h | 1,200
System water storage volume (V) | m3 | 16,000
Concentration ratio (N) | — | 1.5–2.5
Circulation rate (A) | % | 96.9

II. Full analysis of the quality of the makeup water for the system is shown in the table below:
Serial No. | Analysis Item | Unit | Result
---|---|---|---
1 | pH value | — | 7.69
2 | Mg2+ | mg/L | 98.45
3 | Turbidity | NTU | 4.2
4 | Cl– | mg/L | 235.38
5 | Conductivity | us/cm | 5,640
6 | SiO2 | mg/L | 4.35
7 | Phenolphthalein alkalinity | mmol/L | 0
8 | Total dissolved solids | mg/L | 2,556
9 | Total alkalinity | mmol/L | 3.61
10 | Total iron | mg/L | 0.83
11 | Total hardness | mmol/L | 10.84
12 | Total phosphorus | mg/L | 0.04
13 | Ca2+ | mg/L | 284.62
14 | SO42+ | mg/L | 1,864.3
15 | Na+ | mg/L | 0.96
16 | HN4+•N | mg/L | 12.37
17 | K+ | mg/L | 0.45
18 | Al3+ | mg/L | 0.29

III. Organizational responsibilities:
1. Water supply and drainage department:
(1) In accordance with the requirements of the cleaning plan, it is necessary to verify that all components of the circulating water system, such as chemical dosing devices, acid dosing devices, wastewater discharge valves, makeup water valves, online pH meters, and flow meters, can meet the needs related to chemical addition, acid addition, wastewater discharge, and makeup water supply during cleaning. If these components do not meet the requirements, temporary measures must be taken, such as adding additional makeup water and wastewater discharge pipelines, as well as more chemical dosing and acid dosing equipment ； (2) Coordinate with the technical staff from the cleaning and pre-coating manufacturers to resolve various issues that arise during the cleaning and pre-coating processes ； (3) Adjusting the amount of water required for circulation and storage during cleaning and pre-coating processes, carrying out water supply and drainage tasks, as well as operating the chemical dosing and acid addition devices in the circulating water system ； (4) Carry out proper safety supervision and inspections. 2. Production Technology Department: The scheduler is responsible for coordinating the various factories, instructing them to put all the equipment and pipelines in the circulating water systems into operation, and ensuring that all parameters are controlled as specified in the plan. Meanwhile, the laboratory carries out analyses and tests in accordance with the requirements of the plan. 3. Supply and Sales Department: Responsible for notifying the manufacturers of cleaning and pre-coating services to provide an adequate amount of chemicals as required by the plan, and arranging technicians to be on site to guide the cleaning and pre-coating process. 4. Manufacturer responsible for cleaning and pre-coating: (1) The manufacturer’s technical staff should arrive at the site in advance, familiarize themselves with the conditions there, and ensure that the pumps used for adding chemicals and acid are sufficient to meet the requirements for cleaning and pre-coating. Make specific arrangements for the chemical addition ports, acid addition ports, and storage areas for chemicals, and determine the locations for monitoring probes and sampling points together with your company’s technicians ； (2) A chemical cleaning and pre-coating analysis room should be set up in a room equipped with water, electricity, and other facilities that meet the requirements for water quality chemical analysis; standard reagents should be prepared, and the analysis instruments should be calibrated. In accordance with the project and frequency requirements outlined in the plan, assign monitoring and analysis personnel as appropriate, and install and test equipment for adding chemicals and acids ； (3) The on-site project supervisor shall arrange for proper safety measures during cleaning operations. All personnel involved must comply with the safety regulations during construction as well as various rules and procedures set by Henan Longyu Coal Chemical Industry; they must also receive safety training and be familiar with the relevant safety protection measures. III. Cleaning Procedure 1. Chemicals to be used: WT-301 cleaning agent, WT-309 defoamer. 2. Concentration of chemicals: 1000 ppm for WT-301 cleaning agent. 3. Method of addition: Shock addition. 4. Operation time: 10 hours. 5. Amount of chemicals required: (V + M × 10h) × 1000 ppm. Amount of WT-301 cleaning agent required (kg) = —————————— / 1000 = (12000 + 800 × 10) × 1000 / 1000 = —————————— = 20000 kg = 20 tons. In the formula: V – Volume of water in the system, in m3 ； (The minimum safe water level is approximately 12,000 m3) M – System water replenishment rate, in m3/h. (The water replenishment rate is approximately 800 m3/h). The consumption of WT-309 antifoam agent is 30 ppm; its addition should be adjusted according to the operating conditions of the system, with an estimated consumption of 400 kg. In the formula: V —— amount of water stored in the system, m3 ； (The minimum safe water level is approximately 12,000 m3) M – System water replenishment rate, in m3/h. (The water replenishment rate is approximately 800 m3/h.) 6. Operating method and end-point determination: Reduce the circulating water level to a safe low level; adjust the pH value of the circulating water using sulfuric acid to between 5.5 and 6.5. After about 1 hour for uniform distribution, gradually add the WT-301 cleaning agent. During operation, pay attention to the changes in various water quality indicators; test the pH value and turbidity of the circulating water every 2 hours. When the turbidity in the circulating water stops increasing, it indicates that the cleaning is complete. Throughout the cleaning process, attention should be paid to whether foam forms in the circulating water; if there is an excessive amount of foam, WT-309 antifoaming agent should be added promptly. After cleaning is complete, increase the water supply and drainage to change the water. The entire cleaning process is expected to take about 10 hours. 7. Monitoring: The reference standard is the \"Standard of China Association for Engineering Standardization\", with standard number CECS103:99. Specifically, the corrosion rate for carbon steel should be ≤3 g/m2•h, while the corrosion rate for copper and stainless steel should be ≤0.3 g/m2•h. IV. Pre-coating (I) Preparation work: 1. After chemical cleaning, pre-coating treatment should be carried out as soon as possible to prevent the heat exchange surfaces and water flow pipes from being corroded by dissolved oxygen in the water ； 2. Pre-film conditions: (1) The turbidity of the circulating cooling water should be <20 mg/L, and the total iron content should be <1 mg/L. High turbidity in the water or a high level of iron ions can affect the quality of the membrane. (2) Ca2+ ≥ 50 mg/L. Generally speaking, an important factor in the pre-coating process is the presence of sufficient calcium; when the Ca2+ level in the water is between 100 and 200 mg/L, the pre-coating effect is good. When the Ca2+ level is below 50 mg/L, CaCl2 should be added. (3) Sufficient pre-coating agent; (4) Appropriate amount of H2SO4. Controlling the pH value during the pre-coating process is one of the most important factors. The optimal pH value for film formation of WT-302 is 5.5–6.5 ； The film-forming effect declines when PH > 7.5 ； When PH > 8, the tendency for calcium phosphate deposition increases, affecting the density of the membrane as well as the binding force between the membrane and the metal surface ； When PH < 5.5, the polyphosphorus complex film will be solvated and destroyed. (5) At temperatures >15°C, the WT-302 pre-coating agent facilitates film formation when the cooling water temperature is high; in cases where it is difficult to increase the cooling water temperature, it can operate at room temperature. (6) Flow rate: During the pre-coating process, a slightly higher flow rate is appropriate, around 1.0~1.5 m/s. A high flow rate facilitates the diffusion of the pre-coating agent and dissolved oxygen in water; simultaneously, it promotes the electrodeposition process, allowing for an increased film formation rate and the creation of a uniform and dense film. However, if the flow rate is too high (greater than 2.5 m/s), it may cause the pre-coating solution to erode the metal through scouring. (7) Residence time: Due to the hydrolysis or degradation of polyphosphates in the system, a longer pre-coating time is not necessarily better; an excessive residence time can lead to the formation of calcium phosphate scale deposits. Normally, 24 hours after pre-coating, the pH value returns to its natural level; it is necessary to add a dispersant at a certain concentration to the system in order to prevent the deposition of calcium phosphate scale and zinc salts. (8) Monitoring: Pre-treated test pieces are attached to the hanging device in the tower tank for monitoring. The evaluation of the pre-coating effect can be carried out using the following methods: a) Visual inspection method: In the case of an excellent pre-coating effect, there will be a uniform colored film on the test pieces, or they will emit a blue light and exhibit a metallic luster, with no pitting. Depending on the pre-coating agent used, the color of the film formed on the test pieces varies. b. Copper sulfate solution method: Weigh 15 g of CaCl2 and 5 g of CuSO4, dissolve them in 100 ml of distilled water, and drop this solution onto the pre-coated test pieces; simultaneously, determine the time it takes for red spots to appear on the test pieces. It should be greater than 10S. (II) Start of pre-coating: Pre-coating involves using chemical methods to create a dense protective film on the surface of the equipment, thereby preventing direct contact between the heat exchange surface and the circulating water. This enhances the equipment’s corrosion resistance and ensures effective water treatment. Therefore, the equipment must be pre-coated. 1. Chemical to be used: WT—302 pre-coating agent 2. Dosage concentration: 300 ppm 3. Method of application: Shock dosing 4. Operation time: 30–36 hours 5. Amount of chemical required: (V + M × 36h) × 300 ppm. Amount of WT—302 pre-coating agent required (kg) = —————————— 1000 / (12000 + 800 × 36) × 300 = —————————— = 12240 kg = 12.24 tons. In the formula: V – Volume of water in the system, in m3 ； (The minimum safe water level is approximately 12,000 m3) M – System water replenishment rate, in m3/h. (The water addition rate is approximately 800 m3/h.) 6. Operating method and end-point determination: Operate the circulating water at a safe low level; adjust the pH value of the circulating water to between 5.5 and 7.5 using sulfuric acid, and gradually add the WT-302 pre-coating agent to the water intake tank. During operation, the pH value of the circulating water and the concentration of the WT-302 pre-coating agent are tested every 2 hours, and test pieces are used to monitor the effect of pre-coating; when a stable multicolored pattern appears on the surface of the test pieces, it indicates that the pre-coating process is complete. At this point, an appropriate amount of WT-304E scale and corrosion inhibitor should be added for dispersion purposes, in order to prevent the formation of calcium phosphate precipitates in the system; meanwhile, the volume of water added and removed should be increased, and the water should be replaced. 7. The high-concentration pre-coating solution in the system should be removed from the circulating cooling water system as quickly as possible; once the water quality meets the requirements of the operating process, the addition and removal of water should be stopped, and the system will then operate normally.

Pre-coating is essential; there are no major issues whether in cold or hot conditions

Hot-state pre-coating? Could you explain it roughly?

The hot state refers to the pre-film formation during the operation of the heat exchanger