Corrosion prevention of buried pipelines

:lol 1 Cathodic protection design 1.1 Determination of cathodic protection types Cathodic protection is a type of electrochemical protection; it is a corrosion prevention technique that uses an external current to change the corrosion potential of metals, thereby reducing their corrosion rate. Cathodic protection for buried steel pipelines is divided into forced current cathodic protection and sacrificial anode cathodic protection. Forced current cathodic protection is mainly suitable for main gas pipelines in areas with single underground pipeline networks, such as suburbs, or for urban gas distribution networks. Its advantages are a high and adjustable output current, no limitation by soil resistivity, and a large protection radius ； The system has a long operational life and excellent protection performance ； Changes in the output current of the protection system can reflect changes in the performance of the pipeline coating. Its drawback is that it requires dedicated personnel for maintenance and management, needs a continuous external power supply, and is prone to shielding and interference, especially in areas with complex underground metal structures. Sacrificial anode cathodic protection is mainly suitable for gas pipelines of various pressure classes in densely populated areas and towns. Its advantages are that it requires no external power supply, is easy to install, does not need regular specialized maintenance, does not cause shielding effects, does not interfere with other structures, and features a uniform distribution of protective current along with high efficiency. Its disadvantage is a low output current and a limited protection range ； It needs to be replaced regularly; it cannot monitor changes in the output current in real time, nor can it indicate the condition of the pipeline coating. Based on past experience and our practical findings, sacrificial anode cathodic protection is recommended for buried steel pipelines in towns to mitigate the electrochemical corrosion of the pipelines caused by the soil. 1.2 Determination of cathodic protection current To ensure adequate protection for buried gas pipelines, it is necessary to have sufficient current to prevent the pipelines from corroding. The minimum protective current for steel pipelines is one of the most important parameters in cathodic protection design. Its calculation formula is as follows: I = AIP (1) Where I represents the minimum protective current required for the pipeline, in mA; A is the total surface area of the pipeline, in m2; and IP is the minimum protective current density, in mA/m2. The minimum protective current density Ip depends on the type and quality of the pipeline’s anti-corrosion coating. For newly installed pipelines with asphalt-glass cloth coatings, Ip is approximately 0.1 mA/m2, while for newly installed triple-PE pipelines, it is about 0.001 mA/m2. For old pipelines, Ip is set at 0.3 mA/m2. 1.3 Selection of the sacrificial anode ① Soil resistivity: Soil resistivity reflects the electrical conductivity of the soil medium. Generally, soils with low resistivity are more corrosive, while those with high resistivity are less corrosive; an appropriate sacrificial anode is usually selected based on the soil’s resistivity. Regardless of the type of sacrificial anode used, it is first necessary to measure the average soil resistivity at the location where the pipeline is situated. The varying proportions of components in the soil result in different resistivity values in different locations; even at the same site, the resistivity varies with depth. Therefore, we often use the average resistivity at the depth where the pipeline is located. ②Selection of sacrificial anodes: There are mainly two types of sacrificial anodes, namely magnesium alloy anodes and zinc alloy anodes. Based on the measured soil resistivity (ρ), zinc anodes or magnesium anodes can be selected. Generally, when ρ is 100 Ω·m, a strip-shaped magnesium anode is selected. In moist soil conditions, the application range of zinc anodes can be extended to 30Ω·m. 1.4 Layout of sacrificial anodes ① When arranging sacrificial anodes, ensure that there are no metal structures between the anodes and the pipes. ②The sacrificial anode must be buried below the freezing line. In dry areas where the groundwater level is below 3 meters, the anode should be buried at a greater depth. The downstream anode in a river should be buried in a secure location on the riverbed to prevent damage from floods and dredging operations. ③There are two burial methods for sacrificial anodes: vertical and horizontal. Vertical anodes are installed by drilling, with the anode being buried underground perpendicular to the pipeline at the location where it is to be placed. This method requires no large-scale excavation, but its protective effect is inferior to that of horizontal anodes; it is suitable for existing pipelines. The horizontal anode is installed using the grooving method; it is placed in the same trench as the pipeline during installation, which not only saves on the costs associated with separate excavation but also provides effective protection. Under normal circumstances, the anode is installed 3 to 5 meters away from the outer wall of the pipeline; the minimum distance should not be less than 0.3 meters. However, to facilitate installation in the same trench, the distance is generally kept between 0.3 and 0.5 meters, allowing sufficient space for operations. The burial depth should be such that the top of the anode is at least 1 m above the ground surface. When arranged in groups, the spacing between anodes should be 2 to 3 meters. ④Test piles should generally be installed at the midpoint between two adjacent groups of sacrificial anodes on the pipe section, with the spacing between these test piles not exceeding 500 m. 1.5 Design modifications: After the calculations are completed, when sacrificial anodes are installed on the pipelines, certain special circumstances must also be taken into account to make adjustments to the overall design. For example, at the entry and exit points where a directional drill passes through the ground, if the protection radius provided by the sacrificial anodes is insufficient, it is possible to increase the number of anodes or upgrade their specifications at those points, so as to ensure complete protection of the pipeline passing through. 2 Other considerations for design 2.1 Sleeves When pipes pass beneath railways or highways and sleeves are used, whether they are steel or concrete sleeves, they provide a shielding effect that prevents the external cathodic protection current from reaching the pipes inside the sleeve. This creates a blind spot in terms of cathodic protection; if water enters the sleeve, the pipes in that blind spot will not be protected. For the shielding of casings, strip-shaped zinc anodes are typically used, which are wound around the pipeline in a spiral pattern and welded to the pipeline every 2 meters or so. A test pile should be installed at each casing, allowing for easy testing from the ground through the test wires on the casing and pipes. 2.2 Insulated connections To prevent the cathodic protection current from flowing to unprotected structures that are connected to the ground, the cathodic protection pipeline system must be electrically insulated. This prevents current loss, reduces galvanic corrosion, avoids unnecessary interference, and controls the direction of current flow. The insulation installation should take the following areas into consideration: a. The connection point between the main pipe and the branch pipes ； b. Connection between old and new pipes ； c. Connection between bare pipe and covered pipe ； d. Electrical grounding point ； e. Casing passing section ； f. Supports spanning the pipeline and at the pipeline ； g. Both ends of large penetration and crossing sections. It is also important to note that protective devices against lightning strikes and overcurrents should be installed on both sides of the insulated joint, in order to prevent it from being damaged by sudden current surges. 2.3 AC interference: The high-voltage power lines in urban areas cause AC interference to pipelines. This interference has two main hazards: firstly, the continuous presence of the AC voltage from these power lines can lead to AC corrosion of steel pipelines ； Secondly, when a fault occurs in high-voltage lines, an instantaneous induced voltage is generated, which may break down the insulation devices installed in the pipes and pose a threat to human safety. There are three methods to address communication interference: first, ensure that the installation of the sacrificial anode is carried out all at once after the pipeline construction is completed in phases, in order to establish anode grounding as early as possible ； Second, increase the distance between the pipeline and the grounding electrode; it should be at least 3 meters ； Third, ground batteries are connected in series between the pipeline and the grounding electrode, as well as on both sides of the insulation device; this transfers the momentarily induced voltage to the pipeline, after which the current is dissipated through the pipeline’s grounding device, thereby preventing fault currents from affecting the pipeline. 3 Conclusion In the design of sacrificial anode cathodic protection for buried steel pipelines used in urban gas supply, the number and placement of anodes should be determined based on the average soil resistivity at the location of the pipelines, as well as factors such as the pipeline’s pressure, diameter, length, and anti-corrosion coating. The distribution of anodes is then adjusted taking into account the conditions of the areas through which the pipelines pass, such as the presence of other structures nearby, any crossings or overlaps of the pipelines, and the installation of sleeves. Finally, consideration is given to the installation of insulating devices and grounding cells at the start and end points of the pipelines.