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A Brief Analysis of the Application Research on Cathodic Protection Technology for Water Conveyance Pipelines 1. Determination of Key Technical Parameters (1) Engineering design parameters: The design life of the cathodic protection system for water conveyance pipelines should be consistent with the service life of the pipelines being protected; it is generally advisable to set this at 15–25 years. (2) Pipe protection potential: The pipe protection potential relative to the saturated copper/copper sulfate reference electrode should be at least -850 mV. (3) Minimum protection current density The minimum protection current density is an important parameter in cathodic protection design. Its value is influenced by factors such as the quality and type of the coating on the pipeline to be protected, as well as the electrical resistivity of the soil surrounding the pipeline; however, it is generally difficult to calculate theoretically. The range of protective current density values required for buried metal pipes provided in the Water Supply and Drainage Design Manual is wide, making it difficult to select appropriate values accurately. Therefore, in the design and implementation of cathodic protection, values that better reflect the actual conditions should be determined based on specific environmental factors and by referring to the operational data of similar projects. It is also possible to try using a simple forced current system on a small section of the pipeline for on-site measurement. Given the low soil resistivity in the Ningbo area, and by referring to the operating parameters of similar projects, we have determined that the minimum protective current density for the cathodic protection of the steel pipes in Ningbo’s three water diversion projects is: 0.3–0.4 mA/m2 for the impressed current method, and 0.15 mA/m2 for the sacrificial anode method. 2. Selection of sacrificial anode materials The selection of sacrificial anode materials and their specifications should be based on the measured soil resistivity along the pipeline, with reference to the petroleum industry’s \"Design Specifications for Cathodic Protection of Buried Steel Pipelines Using Sacrificial Anodes\" and \"Technical Standards for the Use of Magnesium Alloy Sacrificial Anodes\"; see Tables 5 and 6 for details. Selection of Sacrificial Anode Materials Table 5: Soil resistivity (Ω·m) and suitable anode types. >100: Striped magnesium anodes; 60–100: Magnesium (-1.7V); 40–60: Magnesium; <40: Magnesium (-1.5V); <15: Magnesium (1.5V); <5: Zinc. Selection of Magnesium Anode Specifications Table 6: Soil resistivity (Ω·m) and applicable magnesium anode specifications. >100: 2 or 4 kg; 100–50: 4 or 8 kg; 50–20: 8 or 11 kg; 20–10: 11 or 14 kg; <5: 14 or 22 kg. Based on measurements of soil resistivity and assessments of soil corrosiveness, magnesium anodes are used for the cathodic protection of water transmission pipes in Ningbo. Magnesium-aluminum-zinc alloy anodes are employed when the soil resistivity is below 40 Ω·m, while magnesium anodes are used when it is above 40 Ω·m. The technician specification for each anode is 14 kg. 3. Brief calculation for sacrificial anode design (1) The formula for calculating the output current of a single magnesium anode is: IMG = 150000 × F × Y / p Where: IMG ---- Output current of a single magnesium anode, in mA ； F------Weight correction factor Y------Potential correction factor p------Soil resistivity, Ω·m In our design, we adopt a value of 1.16 for F in accordance with the \"Water Supply and Drainage Design Manual\" ； Y is 0.93 ； p takes the average value of the locally measured soil resistivity, which is 2100 Ω·cm. It is calculated that IMG=77mA. (2) The formula for calculating the number of magnesium anodes per group is: N = b × IA / IMG. Where: b is the redundancy factor; given the low soil resistivity in the Ningbo area, a value of 2 is used ； IA----Protection current to be outputted by each group of anodes, in mA ； In our implementation, we have set the length of each group of anodic protection at 250 m, the minimum protection current density at 0.15 mA/m2, and the diameter of the pipelines to be protected at 1.6 m. Then IA=0.15×1.6×3.14×250=188.4mA. So: N=2×188.4/77= 4.89≈5 units. (3) The service life of the anode is calculated using the empirical formula from the American company HARCO for corrosion protection: T = 57.08Wη / IMG. In this formula, W represents the weight of a single anode in pounds ； We use a magnesium anode with a mass of 14 kg, and multiply it by 2.2 to convert it to pounds. η ----- is a coefficient, usually taken as 0.75. Therefore, the design life is: T=57.08×14×2.2×0.75/77≈17 years. 4. Installation of forced current protection stations The location of cathodic protection stations should take into account factors such as the availability of a reliable AC power supply, ease of maintenance and management, and protection against deliberate damage; generally, such stations are best placed within pump stations or water treatment plant complexes. In the Xiaozhen section, a cathodic protection station is installed at each of Xiaozhen and Beidu pumping stations. The XiaoZhen cathodic protection station is equipped with 2 potentiostats (1 in use and 1 as a spare), and the auxiliary anodes are 30 YJBSICr50×1200mm chromium-containing high-cast iron anodes with double-end connections, which are buried horizontally in a linear arrangement perpendicular to the oil pipeline, at a distance of 100 meters from the pipeline. The Beidu cathodic protection station is equipped with 3 potentiostats (2 in use and 1 as backup). Two sets of auxiliary anodes are buried (with the same specifications and quantity as above), with one set of anodes corresponding to each instrument. To prevent stray current-induced corrosion in the pipelines within the pump station, which lacks power protection, sacrificial anode cathodic protection was applied to these pipelines; two sets of 4 22 kg magnesium anodes each were buried. The forced current cathodic protection stations in the Yokoyama section are located at the Yokoyama water diversion management station and the Beilun water plant, each equipped with 2 potentiostats (one in use and one as a spare). Given that the soil resistivity at the Yokoyama cathodic protection station is relatively high (240 Ω·m), 30 YJBSICr75×1500 mm chromium-containing high-silicon cast iron anodes with double-end connections were selected as auxiliary anodes. These auxiliary anodes were buried horizontally in a bed of coke, whose width and thickness are both 600 mm, and whose length is 2 m. At the Xiao Zhen North Crossing cathodic protection station, each potentiostat covers a protection distance of 8.3 kilometers; the required protection current is I = 8300 × 1.6 × 3.14 × 0.3 = 12509 mA. The grounding resistance of the auxiliary anodes is 0.8 ohms. Select a potentiostat of 20A/24A. For the Yokoyama cathodic protection station, with an auxiliary anode grounding resistance of 10.1 ohms, a potentiostat of 30A/100V was selected. The auxiliary anode grounding resistance at the cathodic protection station of Beilun Water Plant is 0.8 ohms; a potentiostat with a capacity of 75A/35V is used. 5. Electrical insulation measures for the pipeline It is very important that the water conveyance pipelines to be protected under cathodic protection maintain electrical continuity and be insulated from the outside world; to this end, we adopt the following measures: (1) Use insulating flanges to provide insulation between different cathodic protection methods. (2) Insulating flanges shall be installed at the inlet and outlet of the pipeline at the cathodic protection station. (3) The steel pipes at all bridge ducts are insulated from the piers using rubber gaskets. (4) All other metal components directly connected to the pipeline are not allowed to be grounded directly. (5) Considering the differences in the physicochemical properties of the lake water and soil, insulating flanges are installed at both ends of the section crossing East Qianhu Lake. (6) Install a 14 kg magnesium anode at each valve chamber. The above measures have achieved good technical and economic results in the practical application of cathodic protection for water conveyance pipelines. And these measures differ from the requirements of the cathodic protection design codes for the oil industry. This specification requires that insulated flanges or insulated joints be installed at both ends of the sections where the protected pipeline passes through obstacles, as well as at the connections to the drainage wells. Ningbo is located in the water towns of the Jiangnan region, where there are many bridge pipes used to carry water across rivers. If insulating flanges had to be installed at both ends of each such bridge pipe, it would increase the difficulty of construction and raise the costs of the project. Through testing, we found that it is sufficient not to install insulating flanges at both ends of the bridge tube; instead, rubber gaskets are used to provide insulation between the water conveyance steel pipe and the support brackets of the tube bridge. Additionally, insulating flanges are not used in the pipe drainage valve chambers; instead, a set of 14 kg magnesium anodes is installed for protection. The implementation of these measures meets the requirements for cathodic protection and offers clear advantages in technical and economic comparisons. 6. Installation of test piles Cathodic protection test piles are an important aspect of project implementation, as well as a crucial element in assessing the effectiveness of pipeline protection. It is generally required to install one potential test pile per kilometer of pipeline that is under protection. The Xiaozen water diversion project is equipped with a total of 18 potential testing piles (one per kilometer), 2 current testing piles (located 5 kilometers and 12 kilometers away from the Xiaozen pump station), and 4 semi-insulation testing piles. In the Jiangdong project, sacrificial anode test piles were installed along the entire length of the project (one per kilometer), totaling 16 piles. The Yokoyama water diversion project features long pipelines, and cathodic protection work was carried out two years after the pipelines were installed; therefore, it was difficult to acquire additional land. Considering that there are 40 pipe bridges along the Yokoyama route, which can also serve as supplementary test sites, the spacing between potential measurement piles was adjusted to one every 1.8 kilometers, resulting in a total of 23 such piles being installed. In addition, 10 sacrificial anode test piles were installed. 7. Assessment of cathodic protection effectiveness: The quality of cathodic protection is determined by the protection potential. In accordance with the design standards of the petroleum industry, the protection potential of water transport pipelines under optimal cathodic protection conditions should be as follows: (1) For ordinary soil, the measured protection potential of the pipeline should be –850 mV (relative to Cu-CuSO4; the same applies hereafter) or more negative ； (2) The cathodic polarization potential between the pipe surface and the reference electrode in contact with the soil shall not be less than 100 mV; (3) The limit for the maximum protection potential should be determined based on the covering layer and the surrounding environment, taking into account the need to avoid damaging the adhesion of the covering layer, and it can generally be set at -1.5 V. During the construction of the sacrificial anodes for the Yokoyama water diversion project, based on the aforementioned criteria for evaluation and through measurements of the protection potential, it was found that in some areas the protection potential did not meet the required standards. The main reason for this was that, due to natural constraints and difficulties in acquiring land, the locations where the anode beds were installed could not be distributed evenly; as a result, insufficient protection was provided in those areas where the distance between the anode beds was large ； Furthermore, since cathodic protection was not applied in a timely manner after the pipelines were installed, under conditions of highly corrosive soil, the average resistance of the coating is low, resulting in significant current leakage. To ensure the effectiveness of cathodic protection and the service life of the steel pipes in this project, appropriate modifications were made to the original design. A total of 31 additional sacrificial anode sets were installed in areas where protection was insufficient, and the number of anodes per set was increased from 5 to 6 in sections with low soil resistivity. After taking the above measures, on-site measurements showed that the potential of the protected pipe section met the requirements. In the potential measurements of the forced current protection section at Yokoyama, we found that the pipeline protection potential does not decrease as the distance between the measurement point along the pipeline and the energization point increases, as shown in Table 7. Data measurement of forced current cathodic protection at the Yokoyama station of the Yokoyama water diversion project. Table 7: Measured points, energized points P1P2P3P4P5P6. Natural potential (-V): 0.60, 0.59, 0.60, 0.55, 0.585, 0.64, 0.58. Protection potential (-V): 1.226, 1.398, 0.885, 1.036, 0.904, 0.877, 0.866. This is mainly due to the large variations in soil resistivity along this protection section; the highest value reaches 400 Ω·m, while the lowest is only 38 Ω·m, a difference of one order of magnitude. These variations in soil resistivity affect the distribution of the protection potential along the pipeline. This means that although the protective potential meets the requirements in areas with low soil resistivity or far from the energized point, it does not imply that the protective potential in pipe sections with high soil resistivity or close to the energized point will also meet the requirements. The pipeline protection potential can also meet the requirements. This phenomenon should be taken into account during the measurement of pipeline protection potential to ensure that the entire pipeline meets the protection requirements. The cathodic protection for the Yokoyama water diversion project was implemented two years after the pipelines were laid. The soil resistivity in the area near the Beilun water plant is similar to that in the section between Xiaozhen and Beidu; however, theoretical calculations show that the average protection current density in the Beilun water plant area is higher than that in the Xiaozhen area. Cathodic protection in Xiaozhen was put into use simultaneously with the pipelines, which indicates that cathodic protection also plays a role in protecting the anti-corrosion coating. 8. Conclusions and Discussions (1) The promotion and application of cathodic protection technology for water conveyance pipes are highly necessary. The design and installation of cathodic protection should be carried out simultaneously with pipeline construction, which is feasible both technically and economically. (2) There are two methods for the cathodic protection of steel pipes: forced current and sacrificial anode. The appropriate method should be determined by comparing factors such as the local soil conditions and construction requirements; generally, sacrificial anode method should be used for water supply pipelines that run within urban areas. (3) The quality of the pipeline’s anti-corrosion coating and its electrical resistivity are key factors affecting both the range of protection and the uniform distribution of the protective potential; meanwhile, the soil resistivity along the pipeline also influences the distribution of the protective potential. (4) When implementing cathodic protection, it is crucial to ensure the electrical continuity of the pipeline as well as its electrical insulation from the outside world. Moreover, the form of insulation is not unique; it is crucial that insulation is not necessarily used at both ends of the bridge pipe for water transport pipes. Moreover, the form of insulation is not unique; insulated flanges need not necessarily be used at both ends of the bridge pipes for water transport pipelines, as the use of rubber insulation pads is technically feasible and offers clear economic advantages. (5) When using forced current cathodic protection, the potentiostat should be selected based on the quality of the anti-corrosion coating, the anode grounding resistance, and the results of soil resistivity tests, with a potentiostat having a different voltage-to-current ratio being chosen accordingly. As a backup, a potentiostat should be purchased after the project is put into operation, depending on the circumstances. (6) When sacrificial anode protection is used for pipelines, the selection of the anode should be determined based on the soil resistivity. In areas with low soil resistivity, the burial location of the anodes should be selected carefully to ensure that there is a sufficient distance between the anodes and the pipelines, thereby maintaining a uniform distribution of the ground potential around the pipes. (7) After the implementation and commissioning of the cathodic protection project, daily operation and management cannot be ignored. All test piles along the line must be properly protected. The measurement of the pipeline’s protection potential should be carried out regularly, and measures should be taken promptly once any issues are detected to ensure that the protection potential meets the required standards. (8) It is necessary to thoroughly summarize the practical experience in the application of cathodic protection technology for water transmission pipes, and the water supply industry should establish corresponding technical specifications.