Analysis of the Causes of Scaling in Cooling Water Heat Exchangers and Preventive Measures

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Since its commissioning, the circulating cooling water system in our plant has been operating using the same formula, with the water quality stabilizers being the corrosion and scale inhibition agents JN-1 and JN-4. During operation, chlorine disinfection is used as the primary method, with JN-2 serving as a supplementary method. The designed value of the fouling thermal resistance coefficient for the heat exchangers in our factory is only 4.78×10-5 m2·h·℃/kJ. Due to the small heat exchange area of these heat exchangers, strict requirements are placed on the quality of the cooling water. Several years of corrosion and scaling monitoring results show that although this chemical provides good corrosion and scaling inhibition effects, due to the relatively high pH level, upon opening the heat exchangers on site during maintenance and inspecting them, it was found that there was a layer of wavy, grayish-white, hard scale present on the tube walls of all the heat exchangers to varying degrees ； The temperature difference across the heat exchanger decreased from an initial value of 12°C to 8–9°C after one year, resulting in a significant reduction in its heat exchange capacity ； Especially during the hot seasons, the insufficient heat exchange capacity of the heat exchangers becomes one of the bottlenecks restricting the high-load operation of fertilizer plants. Reducing scale formation and improving the heat transfer efficiency of heat exchangers have become the primary issues to be addressed. This paper explores the causes of scale formation and proposes preventive measures. 1 Operating formulas and control parameters: To ensure good heat transfer performance in the heat exchanger and to prevent corrosion, taking into account the quality of water in our plant and through the selection of appropriate chemical formulations, JN-1 and JN-4 water stabilizers are used as per the prescribed formulas to provide corrosion inhibition and scale prevention for the cooling water. The flow diagram of the cooling water system is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0605191640133691.jpg JN-1 is mainly composed of polyol phthalate, ZnCl2, and sodium lignosulfonate, while the main components of JN-4 are acrylic acid and acrylic acid ester copolymers. Formula: pH 7.5–8.5, JN-1 40mg/L, JN-4 10mg/L. Control parameters: pH 7.5–8.5, turbidity ≤10 mg/L, organic phosphine 3.0–5.0 mg/L, residual chlorine 0.3–1.0 mg/L. 2 Scaling condition of the heat exchanger 2.1 Surface condition of the scale The surface of the scale on carbon steel heat exchangers is rough, exhibiting typical reticular patterns caused by phosphates, and is relatively hard. It contains brownish-black clay and other substances, with obvious corrosion products. The scale in stainless steel heat exchangers is grayish-white, reticulated, and hard. 2.2 Composition of scale To facilitate and improve the accuracy in analyzing and determining the composition of scale, the analysis results for scale samples from carbon steel heat exchangers and stainless steel heat exchangers are presented in Table 1 and Table 2 respectively. As can be seen from the composition of the scale samples in Tables 1 and 2, the scale samples in both carbon steel and stainless steel heat exchangers contain a considerable amount of biological sludge as well as insoluble salts composed of ions such as Ca2+, Mg2+, Zn2+, PO43—, and CO32—. Based on the ease with which they form insoluble salts, it can be seen that among these insoluble salts, CaCO3, MgCO3, Ca3(PO4)2, and Zn3(PO4)2 are the predominant ones. 3 Causes of sludge 3.1 Formation of biological sludge http://www.nmtech.com.cn/jishuwang/upload/0605191641514558.jpg Both the make-up water for cooling water and the air contain large amounts of microorganisms. In open-loop circulating cooling water systems, as water continuously evaporates and a large amount of dust from the air enters the water through the cooling tower inlets, the amount of organic and inorganic substances in the cooling water increases significantly. This creates favorable conditions for the extensive growth of microorganisms. Moreover, the water temperature and pH level in such cooling water are particularly suitable for the growth of microorganisms. During their growth and reproduction, microorganisms produce large amounts of biological waste that is somewhat sticky; this waste adheres to other microorganisms such as bacteria and algae in water, as well as metal corrosion products, various insoluble salts, sand, dust, etc., thereby forming biofouling. 3.2 Formation of insoluble salts Although water quality stabilizers are added to cooling water to prevent scaling or corrosion in heat exchangers, throughout the operation process, as water evaporates continuously in the cooling tower, makeup water containing minerals is continuously introduced, and chemicals are added and degraded, the concentrations of various cations and anions in the cooling water that can combine to form insoluble salts keep increasing, thus creating conditions for the formation of such insoluble salts. 3.2.1 Formation of CaCO3 (1) Decomposition of bicarbonates During the circulation of cooling water, the bicarbonates dissolved in it decompose when exposed to heat as they pass over the heat transfer surfaces of the heat exchanger, resulting in the formation of CaCO3 precipitates. During the cooling process in the cooling tower, CO2 dissolved in the cooling water also escapes, thereby promoting the formation of CaCO3 precipitates. Meanwhile, the release of CO2 increased the pH of the cooling water, further promoting the decomposition of bicarbonates under alkaline conditions. (2) Calcium carbonate water treatment index: In the actual operation of cooling water, important criteria for determining whether CaCO3 precipitation will occur, as well as whether scaling or corrosion will take place, include the Ryznar Stability Index R.S.I., the saturated pH value for calcium carbonate (pHs), the practical operation index Fpract, and the critical pH scaling index pHc (where pH = 2pHs – R.S.I., pHpract = pHs + 1, and pHc = pHs + 1.7). A plot combining all the indices is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/0605191643084142.jpg According to this index chart, at different saturation pH values (pHs), the pH of the cooling water must be kept within the corresponding ranges within the intersection areas ABCD indicated by the indices in order to prevent scaling and corrosion. The operating parameters of the cooling water in our plant are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload/0605191644386893.jpg Note: In the table, except for pH, all other values are expressed in mg/L. For this water quality, pHs = (9.3 + A + B) – (C + D); where A, B, C, and D represent the total dissolved solids coefficient, temperature coefficient, calcium hardness coefficient, and M-alkalinity coefficient, respectively. Using the conversion table for the calcium carbonate saturation index coefficients (with a temperature of 50 °C): A=0.2, B=1.50, C=2.17, D=2.43, the pHs value is 6.40. At these pH values, Table 2 indicates that the pH of the cooling water should be controlled between 7.40 and 8.10. The average pH value during actual operation was 8.29, indicating that the cooling water in our plant has been operating in the scaling zone for a long time. Although water quality stabilizers were added to the cooling water, and the scale-inhibiting agents in these stabilizers were used to distort the crystal lattice of CaCO3 as well as to adsorb and disperse it, **this reduced the crystallization rate of CaCO3 and effectively prevented the excessive formation of CaCO3 deposits. However, due to operation in a scaling-prone environment for an extended period, some CaCO3 still crystallizes on the surface of the heat exchange tubes, forming CaCO3 scale that impairs heat transfer. 3.2.2 Formation of calcium phosphate: Since the water quality stabilizers added to cooling water contain polyol phosphonates, these can decompose to produce orthophosphate ions, which readily combine with the high concentrations of calcium ions in the water to form insoluble calcium phosphate precipitates: 3Ca2+ + 2PO43− → Ca3(PO4)2↓ In phosphorus-based water treatment formulations, the saturated pH value for calcium phosphate (pHs) is determined in order to predict whether calcium phosphate scale will form. The PHS value is affected by the cooling water temperature, calcium ion concentration, and phosphate ion concentration. When the actual pH value of the cooling water minus pHs is greater than 1.5, it indicates a possibility of calcium phosphate precipitation. In the actual operation of the cooling water in our plant, the pH value is on average 8.29, while the pHs value is 6.33; thus, the actual pH-pHs difference is greater than 1.5, which inevitably leads to the formation of Ca3(PO4)2 scale. In accordance with the requirement of pH-phs=1.5, the pH should be controlled at 7.83 during production. 3.2.3 Formation of Zn(PO4)2 JN-1 water-stabilizer contains not only polyol phosphonates but also zinc salts (ZnCl2). In cooled water, Zn2+ can provide protection for metals, thereby achieving anti-corrosion effects; however, it also tends to form the insoluble compound Zn3(PO4)2 with orthophosphate ions produced by the hydrolysis of polyol phosphonates under alkaline conditions. Furthermore, the pH value has a significant impact on the stability of zinc ions in cooling water. The results of studies on the stability of zinc ions in JN-1 water stabilizer, as shown in Figure 3, indicate that zinc ions are relatively stable when the pH is between 7.5 and 8.0 ； When the pH increases from 8.0 to 8.5, the zinc ion concentration in the cooling water decreases by 33.3%; zinc ions precipitate out of the water and form the insoluble salt Zn3(PO4)2, which deposits on the heat transfer surfaces of the heat exchanger. Since our plant’s cooling water operates within this pH range for an extended period, large amounts of zinc salts are present in the scale samples. http://www.nmtech.com.cn/jishuwang/upload/0605191645456372.jpg 4 Measures to prevent scaling of cooling water in heat exchangers (1) Strengthen the control of the pH value of the cooling water. As discussed earlier regarding the formation of insoluble salts, the pH value of the cooling water plays a very important role in suppressing the deposition of CaCO3, Ca3(PO4)2, and Zn3(PO4)2 in heat exchangers, as well as in preventing corrosion of carbon steel heat exchangers by the cooling water. By taking into account the calcium carbonate water treatment index, the calcium phosphate saturation index, and the pH requirements for the stability of zinc ions in cooling water, sulfuric acid is used to adjust the pH of the cooling water to 7.5–8.0, thereby achieving multiple benefits including corrosion prevention and the inhibition of the formation of insoluble salts. (2) Continuously and uniformly add water quality stabilizers to maintain a stable concentration of the chemicals, thereby preventing a decline in the cooling water’s corrosion inhibition and scale prevention capabilities due to reduced chemical concentrations. (3) Strengthen the sterilization and stripping treatment. The cooling water system is disinfected and cleaned by combining shock chlorination with periodic addition of non-oxidizing biocides, thereby strictly controlling microbial growth, reducing the deposition of biological sludge in heat exchangers, and preventing under-scale corrosion. By taking the above measures to keep the pH value of the cooling water and the concentration of organic phosphines at 7.5–8.0 and 3.0–5.0 mg/L respectively, and by applying a non-oxidizing bactericide for sterilization and removal once a month, the operational conditions of the system have been significantly improved. After 2 years of monitoring, the scaling problem has been effectively resolved. This post was last edited by eddy-8280103 on 2009-2-20 16:26.]