Common defects and countermeasures of corrosion protection and cathodic protection of long-distance natural gas pipelines

1. The importance of corrosion protection of long-distance natural gas pipelines
At present, the demand for natural gas is gradually increasing both at home and abroad, among which natural gas has become an irreplaceable important energy source. In this case, before the emergence of new and efficient energy sources and before mass production, the demand for natural gas will continue to rise. To effectively guarantee the supply requirements of natural gas, it is very important to carry out corresponding protection of long-distance natural gas pipelines. Although in recent years, pipelines have been the safest and least wasteful mode of transportation for natural gas, at this stage, the number of pipeline accidents has gradually increased, proving that there are also defects in the process of pipeline transportation. In this process, the most common factor causing pipeline accidents is metal corrosion.

In the actual pipeline transportation process, the hazards caused by pipeline corrosion include the following aspects: Once the pipeline is severely corroded, the corrosion products will mix with the natural gas in it, resulting in impurities mixed in the natural gas, which will seriously affect the quality of natural gas. Second, if the pipeline corrosion is very serious, it is very likely to cause natural gas leakage, which will not only cause serious loss of natural gas resources but also cause serious property losses to pipeline companies. Third, if the degree of corrosion reaches the level of possible leakage, the leaked natural gas will enter the soil, causing serious pollution to the environment, and this kind of damage to the environment is irreversible. At this stage, with the increasingly prominent environmental protection issues, serious pollution to the environment will seriously restrict the development of natural gas resources. Fourth, once the leaked natural gas directly encounters a fire source, it is very easy to cause fire and explosion accidents, which will not only affect the transportation of natural gas but also cause casualties. Fifth, after the pipeline is corroded, the corrosion products will adhere to the inner wall of the pipeline, thereby accelerating the corrosion process of the pipeline. Therefore, in the actual application process of natural gas pipeline transportation, pipeline corrosion is of great significance.

2. Anti-corrosion measures for long-distance natural gas pipelines
In the actual pipeline transportation process, pipeline corrosion is essentially a normal phenomenon and cannot be completely avoided. To reduce the impact of pipeline corrosion on natural gas transportation can only be alleviated by the application of corresponding measures, thereby reducing the corrosion rate of the pipeline. The anti-corrosion of long-distance natural gas pipelines can be studied from two aspects: physical and chemical. From the physical aspect, the main method is to add a coating, and from the chemical aspect, the main electrochemical protection measures are taken. In most cases, the actual pipeline protection will choose to adopt a combination of physical and chemical protection measures.

(1) Adding coatings
The main methods of adding coatings include: First, coal tar enamel. Adding coal tar enamel outside the transportation area is a relatively mature protection measure at this stage. Coal tar enamel not only has a strong anti-corrosion function but also has a certain insulation property. It can prevent the pipeline from being affected by stray currents and plays a very critical role in the protection of the pipeline. Since coal tar enamel has a relatively long service life, it is more economical and most people choose it as the main material for adding coatings. In addition, coal tar enamel also has some disadvantages during use, which are mainly manifested in the following aspects: This technology has very high requirements for pipeline temperature. Once the temperature of the transportation pipeline exceeds the specified temperature of coal tar enamel, it will cause the coal tar enamel to melt, which will not only fail to protect the pipeline, but also may cause environmental pollution, and thus fail to protect the pipeline. Therefore, in the actual application process, it should be noted that coal tar enamel cannot be used for heating transportation pipelines. Second, the mechanical properties of coal tar enamel are in a relatively poor state, and it is very easy to be disturbed by other hard substances in the outside world. If there are many hard stones in the soil near it, it will also cause serious damage to the anti-corrosion layer of coal tar enamel. In this case, coal tar enamel is not suitable for areas with high hardness of underground stones.

Secondly, PE two-layer structure. The addition of a PE two-layer structure on the outside of the transport pipeline is also a pipeline protection measure that is used more frequently at this stage. The PE two-layer structure not only has a high anti-corrosion function but also has a strong milk bacteria function, which can strongly inhibit the interference of bacteria around the pipeline. At the same time, the PE two-layer structure also has a strong water absorption capacity, which can greatly prevent the influence of moisture in the soil on the operation of the pipeline. The price of the PE two-layer structure is not high, so it is very suitable for long-distance natural gas pipelines. However, there are corresponding problems in its actual application process. On the one hand, such materials cannot be placed under the sun, otherwise they will be seriously disturbed by ultraviolet rays and lose their protective effect. On the other hand, such materials are not easy to be closely combined with the pipeline, so the protective effect will be greatly reduced.

Finally, PE three-layer structure. This PE three-layer structure is a pipeline protection measure that has been produced, and it is also the most effective protection measure at this stage. Compared with the two-layer structure, the three-layer structure adds epoxy powder in the middle link, which not only improves the corrosion resistance but also helps to closely combine the material and the pipeline, thereby fully exerting its protective performance. In addition, since the PE three-layer structure contains epoxy powder, the material will no longer be affected by ultraviolet rays and can be used in the sun.

(2) Electrochemical protection
In the actual electrochemical protection process of long-distance natural gas pipelines, the sacrificial anode cathodic protection method is often used. The principle of this chemical transmission pipeline anti-corrosion method is very simple. In the actual application process, a metal material that is more active than the metal material used in the pipeline is added to the outside of the transmission pipeline to form a primary cell. In this primary cell, the anode is the active metal and the cathode is the pipeline. In the actual process of corrosion, the pipeline will be protected. In the actual application of such anti-corrosion measures, the length, wall thickness, and environment of the pipeline should be comprehensively considered. Then the position and weight of the active metal are calculated in detail.

(3) Cathodic protection method with external power supply
In actual application, this anti-corrosion method for transmission pipelines is rarely used, but it is feasible in theory. In actual application, a power supply should be added to the outer wall of the pipeline to effectively protect the pipeline. The reason for the relatively few applications is that most natural gas is flammable and explosive. When using this method, it is also necessary to perform a simple voltage calculation based on the actual situation.

3. Common defects of cathodic protection of long-distance natural gas pipelines
After a long period of operation, many problems have occurred in the application of cathodic protection systems for long-distance natural gas pipelines. On the one hand, the cathodic protection equipment is old and outdated and can no longer operate normally. The reason for this problem is that the constant potential meter adjustment switch at the first station of the long-distance natural gas pipeline fails due to time problems, and thus the protection potential cannot be adjusted. The constant potential meter converter at the terminal station cannot be efficiently converted, resulting in the machine having output, but the pipeline has no output. The high resistance of the anode bed has a great impact on the divergence of the cathode current, and the sacrifice of the anode cannot play its due role, resulting in most of the pipelines losing protection and causing serious corrosion. On the other hand, the pipeline protection potential is too high, causing serious corrosion in some sections of the pipe. At present, the potential of most long-distance natural gas pipelines is at a high level. Compared with the normal potential, the pipeline protection potential has deviated, which in turn aggravates the corrosion of the pipeline.

4. Countermeasures to improve the cathodic protection of long-distance natural gas pipelines
In the process of cathodic protection of natural gas pipelines, the first method to be selected is the pipeline cathodic protection method. Since long-distance natural gas pipelines usually use sacrificial anodes and forced current cathodic protection methods to prevent corrosion of pipelines, at this time, the buried depth of pipelines and sacrificial anodes is usually kept in the range of 2m to 2.5m, but for long-distance natural gas pipelines, their length is too long. At the same time, due to natural landforms, many pipelines have serious drops at both ends. At the same time, the groundwater level at both ends of the pipeline is different from the geological conditions, and the resistivity of the soil in some pipelines is too high. In this area, the efficiency of the sacrificial anode cathodic protection method is not high. The solution to this problem is to change the sacrificial anode protection method to the forced current protection method for the high soil resistivity section of the long-distance pipeline. Secondly, during the application of the forced current method, the potentiostat can provide a continuously adjustable cathodic protection current to the metal body to be protected. It provides an auxiliary function to the anode in the forced current method and is used to form a loop for the cathodic protection current provided by the potentiostat. Finally, in the actual cathodic protection process of long-distance natural gas pipelines, a professional team with a strong sense of responsibility can be created, and the principle of fixed positions and fixed responsibilities can be adopted for management. While improving the overall quality of the team, the management field can be expanded, thereby improving the cathodic protection of my country's long-distance natural gas pipelines.