Field test and leakage cause analysis of a high frequency leakage gas pipeline
In recent years, with the increasingly complex underground service environment of the buried gas pipeline network, the frequent occurrence of gas pipeline corrosion and leakage accidents has brought serious consequences, such as causing the loss of residents’ property, threatening the safety of people’s lives, and also causing serious social harm . According to incomplete statistics, 3356 urban gas accidents occurred in China from 2011 to 2014, resulting in a large number of casualties and economic losses [2,3,4,5,6,7]. In recent years, there are more than 200 leakage accidents caused by corrosion in Beijing gas pipeline network every year, 90% of which occur in the pipeline without cathodic protection, mainly concentrated in the low-pressure community pipeline network. In a high frequency leakage area, there are many corrosion leakage accidents of gas pipeline in half a year. Such frequent leakage has posed a threat to the safe production of gas pipeline network, and also brought many inconveniences to the life of residents.
At present, the vast majority of low-pressure pipe networks in residential areas only use anti-corrosion coating for external corrosion protection, without cathodic protection [8,9,10,11]. Due to the poor quality of the anti-corrosion coating, and with the increase of operation time, the performance will decline year by year. In addition to the complex distribution of buried metal facilities in the community, in addition to the buried gas pipeline, there are also metal facilities such as water pipeline, heating pipeline, building lightning protection and grounding system, and many metal structures, which may affect the corrosion of underground gas pipeline [12, 13, 14, 15, 16]. At present, although there are some reports of corrosion and leakage of gas pipeline in residential areas at home and abroad, there is no detailed analysis on the specific causes of leakage.
The gas pipeline of a residential area started to operate in 2005, which is a low-pressure pipeline without cathodic protection. From December 2017 to December 2018, more than ten corrosion leakage accidents occurred in the pipeline. Such frequent leakage has caused great potential safety hazards to the lives of the majority of residents. After testing, the corrosion rate of the pipeline in the community is 0.32 mm / A, but generally, it is only the case of soil corrosion, the corrosion rate is lower than 0.1 mm / A, and the actual corrosion rate is much higher than the value. In this paper, the field test and laboratory test are carried out on the high-frequency leakage pipe section of the community, the potential of the metal structure in the community is measured, the pipe ground potential of the gas pipeline and the electrical connectivity between the gas pipeline and the ground network are measured, and the main reasons for the frequent leakage of the pipeline in the community are clarified.
Test method and content
The whole test content is divided into field test and laboratory test. The measurement of corrosion potential of leaking pipe section, the measurement of corrosion potential of independent samples of the same material, the measurement of soil resistivity and the check of electrical connectivity were carried out on site. The corrosion morphology and depth were measured in the laboratory.
In the field test, the saturated copper sulfate electrode is used as the reference electrode to measure the corrosion potential of the failed pipe section, and the soil resistivity, and then the electrical connectivity of the grounding grid and the gas pipeline is measured. In order to facilitate the comparison, in addition to the potential of the failed pipeline itself, the self corrosion potential of other steel structures in the community was also measured. In the laboratory test, the corrosion morphology and corrosion depth of the failed pipeline are measured to determine the type of corrosion, so as to further find the cause of failure.
In the field test and laboratory test, the measurement of pipe ground potential, soil resistivity and electrical connectivity are mainly carried out. The measurement methods refer to GB-T 21246-2007 measurement method of cathodic protection parameters of buried steel pipeline. The pipe ground potential is measured by reference potential method, multimeter and copper sulfate reference electrode; the soil resistivity is measured by ground resistance meter (zc-8, the error is not more than 3%), and the quadrupole method is used; the electrical connectivity is measured by potential method, which is further introduced in combination with the field situation.
The electrical connection between gas pipeline and grounding grid is tested by potential method, and the measurement wiring is shown in Figure 1. Keep the position of saturated copper sulfate electrode unchanged, use fluck289 high impedance voltmeter to measure the tube ground potential E and other steel structure potential E structures, and use udl2 high frequency data recorder to record the potential, according to the size and change of e tube ground and e structures, we can judge whether they are electrically connected. Through this method, the electric connection between the underground gas pipeline and the ground grid, and between the gas pipeline and the water pipeline are investigated. The results show that the potential of the latter is obviously different, but the potential of the former is similar.
Fig.1 Electrical connectivity test diagram
Laboratory analysis results
The leakage section was taken back to the laboratory and the corrosion morphology and depth were measured.
The leakage pipeline is a low-pressure pipeline without cathodic protection, and its self-corrosion potential is – 0.61 vcse. The analysis results of corrosion morphology are as follows.
Figure 2 shows the overall appearance of the failed pipe section, which has an inner diameter of 50.11 mm, an outer diameter of 57.43 mm, a wall thickness of 3.80 mm, and two holes. There is a long strip-shaped perforation in the middle of the pipe section, and a four sided perforation at the other end. The perforation part is from the outside to the inside, and the hole wall is relatively rough. Figure 3 shows the morphology of the perforation part, and the specific perforation size data is shown in Table 1. According to the thickness of the pipeline and the operation time of the gas pipeline, the corrosion rate of the pipeline in this area can be calculated to be about 0.32 mm/a.
Fig.2 Overall picture of the invalid pipeline
Fig.3 Local morphologies of perforation site: (a) No.1, (b) No.2
|Number||Maximum diameter / mm||Minimum diameter / mm|
Field analysis results
After the test, the pipe ground potential and the ground potential of the grounding grid of the gas pipeline are -0.39 vcse, while the measured potential of other steel structures is -0.49 ~ -0.51 vcse, among which the potential of the buried steel water pipe is -0.50 vcse, the potential of the lighthouse ground and the excavated pipe are -0.51 and -0.49 vcse, respectively. The ground potential of the gas pipeline is far higher than its self corrosion potential.
In order to further determine whether there is electrical connection between the two, the reference potential method was used for testing, and the potential fluctuation of the two was recorded, as shown in Figure 4. It can be seen from the figure that the potential fluctuation of the two is basically the same, so it can be determined that there is an electrical connection between the gas pipeline and the grounding grid, forming an electric couple. As the anode of the pipeline, the potential rises, the current flows out, and the corrosion rate greatly increases, reaching 0.32 m m / A; the soil resistance rate is 35 Ω· m, and the corrosion is not high, so the natural corrosion rate of the gas pipeline in this soil is far away Lower than the actual corrosion rate. Therefore, the galvanic corrosion between the gas pipeline and the grounding grid is the main reason for the frequent leakage of the gas pipeline in the residential area.
Fig.4 Potential of gas pipeline and grounding grid
Suggestions for corrosion protection of low pressure gas pipeline in urban area
In this paper, field test and laboratory test are carried out on the high-frequency leakage section of a residential area, and the cause of high-frequency leakage of gas pipeline is found. To some extent, this case reflects the general situation of high-frequency leakage of low-pressure gas pipeline network in urban residential areas. In view of the problems existing in the urban low-pressure gas pipeline network, the following management suggestions are put forward:
- (1) The low-pressure gas pipeline shall be equipped with cathodic protection system, especially in the area with serious stray current interference.
- (2) Effective insulation shall be done at the end of low-pressure gas pipeline to ensure all buried metal pipelines are protected. Insulation shall be carried out by welding insulated joints before entering the household pipe and leaving the ground in the new community, and an effective insulation joint free from welding shall be designed for the old community that cannot weld insulated joints.
- (3) Keep a certain distance between gas pipeline and other underground metal components, such as water pipeline, to prevent overlapping.
- (4) In order to deal with the electrochemical corrosion of buried metal pipelines, the selection of anticorrosive materials should be strengthened, the construction should be strictly in accordance with the specifications, and the construction quality of anticorrosive coating should be supervised.
- (5) The regional cathodic protection system can be implemented in the area where conditions permit, so as to reduce the galvanic corrosion between different cathodic protection systems.
Based on the laboratory analysis of the corrosion morphology and depth measurement of the failed pipe section, the measurement of the corrosion potential of the leaking pipe section, the measurement of the corrosion potential of the independent sample of the same material, the measurement of the soil resistivity and the investigation of the electrical connectivity, the following conclusions are drawn:
- (1) The corrosion rate of gas pipeline is much higher than that of natural corrosion in soil, and the actual corrosion rate is 0.32 mm / A.
- (2) The potential of gas pipeline is stable and positive shift, the size is – 0.39 vcse, but its self corrosion potential is – 0.50 vcse, stable and positive shift is 100 mV.
- (3) There is galvanic corrosion between the gas pipeline and the grounding grid of the residential area, which leads to the stable positive displacement of the pipeline potential, the greatly increased corrosion rate and frequent leakage.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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