Common defects and countermeasures in anticorrosion and cathodic protection of long distance natural gas pipelines



1. The importance of anticorrosion of long-haul natural gas pipelines

At this stage, whether domestic or foreign, the demand for natural gas is gradually increasing, of which natural gas has become an irreplaceable important energy. In this case, the demand for natural gas will continue to rise until new and efficient energy sources are available and cannot be mass-produced. In order to ensure the supply of natural gas, it is very important to protect the long distance natural gas pipeline. In recent years, in the transportation of natural gas, pipeline is the safest and least costly transportation mode of natural gas, but at this stage, the number of pipeline accidents has gradually increased, which proves 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 process of pipeline transportation, the harm that pipeline corrosion may cause includes the following aspects: First, once the pipeline is seriously corroded, the corroded products will be mixed with the natural gas, resulting in impurities 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 seriously lose natural gas resources, but also cause serious property losses for the pipeline company. Third, if the degree of corrosion has reached the extent of possible leakage, the leaked natural gas will enter the soil, causing serious pollution to the environment, and such damage to the environment is irreversible, at this stage, environmental protection problems are becoming increasingly prominent, serious pollution to the environment will seriously limit the development of natural gas resources. Fourth, once the leaked natural gas directly encounters the fire source, it is easy to produce 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 anticorrosion has a very important significance.

2. Long distance natural gas pipeline anticorrosion measures

In the actual process of pipeline transportation, in essence, pipeline corrosion is a normal phenomenon, and can not be completely avoided. In order to reduce the impact of pipeline corrosion on natural gas transportation, it can only be mitigated through the application of corresponding measures, so as to reduce the pipeline corrosion rate. The anticorrosion of long distance natural gas pipelines can be studied from both physical and chemical aspects. From the physical aspect, the method of adding coating is mainly adopted, and from the chemical aspect, the electrochemical protection measures are mainly adopted. In most cases, the actual pipeline protection will choose to use a combination of physical and chemical protection measures.

(1) Add coating

The main methods of adding coatings include:

First, coal-tar enamel. Adding coal tar enamel outside the transportation area is a relatively mature protective measure at present. Coal tar enamel not only has strong anti-corrosion function, but also has certain insulation property. It can prevent the pipeline from being affected by stray current, which has a very key role in the protection of the pipeline. Due to the long service life of coal tar enamel, its economy is high, and most of them choose it as the main material of additional coating. In addition, coal tar enamel in the use of the process, there are also some shortcomings, mainly in the following aspects: First, this technology for the pipeline temperature requirements are very high, once the temperature of the pipeline exceeds the specified temperature of coal tar enamel paint, it will cause the melting of coal tar enamel paint, not only can not play a protective role in the pipeline, and is likely to cause environmental pollution, and then can not play a protective role in the pipeline, so in the actual application process, It should be noted that coal tar enamel is not suitable for heating piping. Second, the mechanical properties of coal tar enamel are relatively poor, it is very easy to be disturbed by other substances with high hardness outside, if the soil near it contains more hard stones, it will also cause serious damage to the coal tar enamel anticorrosive layer, in this case, for the area with high hardness of underground stones, coal tar enamel is not applicable.

Secondly, PE two-layer structure. The addition of PE two-layer structure on the outside of the transportation pipeline is also a pipeline protection measure that has been applied more frequently at this stage. PE two-layer structure not only has an efficient anti-corrosion function, but also has a strong milk bacteria function, which can play a strong inhibitory role in bacterial interference around the pipeline. At the same time, PE two-layer structure also has a strong water absorption capacity, can greatly prevent the soil moisture in the pipeline operation caused by the price of PE two-layer structure is not high, so for long-distance natural gas pipeline, very suitable, but in its practical application process, there are corresponding problems, on the one hand, such materials can not be placed in the sun, Otherwise, it will be seriously disturbed by ultraviolet rays and lose its protective effect. On the other hand, such materials are not easy to combine closely with pipes, so the protective effect will be greatly reduced.

Finally, PE three-layer structure. This PE three-layer structure is the latest pipeline protection measure, 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 provides help for the tight combination of the material and the pipe, so that it gives full play to its protective performance. In addition, due to the epoxy powder contained in the PE three-layer structure, the material will no longer be disturbed by ultraviolet light and can be used in sunlight.

(2) Electrochemical protection

Cathodic protection with sacrificial anode is often used in the actual electrochemical protection process of long-distance natural gas pipelines. The principle of this chemical transmission pipeline anticorrosion method is very simple, in the actual application process, a metal material is added outside the transmission pipeline that is more active than the metal material used in the pipeline, and then the galvanic cell is formed. In this primary cell, the anode is the active metal and the cathode is the pipe, which will be protected during the actual corrosion phenomenon. In the actual application of such anti-corrosion measures, the length of the pipeline, the wall thickness and the environment in which it is located should be considered comprehensively. Then the position and weight of the active metal are calculated in detail.

(3) cathode protection of external power supply

In the actual application process, this transmission pipeline anticorrosion method is applied less times, but in theory is feasible, in the actual application process, should be in the outer wall of the pipeline to increase the power supply, so that the pipeline is effectively protected. The reason for the relatively small number of applications is that most natural gas is flammable and explosive gas, and when using this method, it is also necessary to perform a simple voltage calculation according to the actual situation.

3. Common defects in cathodic protection of long distance natural gas pipelines

After running for a long time, many problems arise in the application process of cathodic protection system for long distance natural gas pipeline. On the one hand, the cathodic protection equipment is old and aging, and it can no longer operate normally. The reasons for this problem are: Due to the time problem, the potentiostat regulating switch at the first station of the long distance natural gas pipeline failed to adjust the protection potential, and the potentiostat converter at the final station could not be converted efficiently, resulting in output of the machine but no output of the pipeline. The high resistance of the anode ground bed has a great impact on the divergence of the cathode current, and the sacrifice of the anode can not play its due role, which leads to the loss of protection of most of the pipes, resulting in serious corrosion. On the other hand, the pipeline protection potential is high, which causes serious corrosion of some pipe sections. At present, the potential of most long-distance natural gas pipelines is in a higher state, and the pipeline protection potential deviates from the normal potential, which further intensifies the pipeline corrosion.

4. Improve the countermeasures of cathodic protection for long-distance natural gas pipelines

In the process of pipeline cathodic protection of natural gas, the first method to be selected is pipeline cathodic protection. Because the long-distance natural gas pipeline usually uses the sacrificial anode and the cathodic protection law of forced current to prevent corrosion of the pipeline, at this time, the buried depth of the pipeline and the sacrificial anode is usually maintained in the range of 2m ~ 2.5m, but for the long-distance natural gas pipeline, the length is too long. At the same time, due to the reasons of natural landform, the two ends of many pipelines have a serious gap. At the same time, the groundwater level at both ends of the pipeline is different from the geological condition, and the resistivity of the soil where some pipelines are located is too high, so the efficiency of cathodic protection with sacrificial anode is not high in such areas. The solution to this problem is to change the sacrificial anode protection law to forced current protection law for pipelines with high soil resistivity during the process of long distance pipeline. Secondly, in the application of forced current method, potentiostat can provide continuous adjustable cathodic protection current to the metal body to be protected. The anode in the forced current method is assisted to loop the cathodic protection current provided by the potentiostat. Finally, in the actual process of cathodic protection of long-distance natural gas pipelines, we can create a professional team with a strong sense of responsibility, and adopt the principle of fixed post and responsibility for management, while improving the overall quality of the team, expand the management field, and then improve the cathodic protection of China's long-distance natural gas pipelines.

5. Conclusion

To sum up, in the context of the rising status of natural gas, corrosion prevention and cathodic protection of long-distance natural gas pipelines have become the main contents of research in the natural gas pipeline transmission industry. Efficient pipeline protection has a strong role in improving the operational efficiency of long-distance natural gas pipeline transmission. Corresponding measures should be taken to solve the common defects, so as to achieve the improvement of natural gas transmission efficiency.