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1. The harm of poor insulation and leakage fault of protective pipeline
After the cathodic protection station is put into operation or the sacrificial anode protection is put into operation for a period of time, the phenomenon that the output current increases but the pipeline protection distance is shortened under the specified power on point potential occurs, or in the sacrificial anode system, the output current of the sacrificial anode group increases, its value has exceeded the protection current demand of the pipeline, but the protection potential still fails to reach the specified index ¡£ The main reason for the above situation is the "short circuit" between the protected metal pipe and the unprotected metal structure. This phenomenon is called cathodic protection pipeline leakage, or "grounding fault".
Grounding fault makes the cathodic protection current of the protected pipeline flow into the unprotected metal body, forming a "leakage point" at the "short connection" of the two pipelines, which will cause the increase of cathodic protection current, overload of cathodic protection power supply and interference caused by cathodic protection.
In addition, open circuit of anode ground bed, open circuit of cathode and open circuit of zero position connection of cathode will lead to the failure of cathodic protection. For example, in Golmud station and gansen station, the cathodic protection system could not work normally due to the open circuit of the anode cable in 1993. When judging the open circuit of the connecting cable of the anode ground bed, the following measures can be adopted: (1) measure the output current, turn on the potentiostat, and connect a current meter at the anode output end of the potentiostat. If the current is zero, it indicates that there is an open circuit. (2) Disconnect the anode output line behind the potentiostat and connect it to the temporary ground bed or other grounding devices. If there is output voltage and current, it can be concluded that the anode ground bed connecting line is open circuit. Cement well or mark for connection shall be set at the connection between anode cable and ground bed anode.
2. Causes of pipeline leakage
(1) If the construction is improper, the spacing of crossing pipelines is not in accordance with the standard. That is, when two pipelines are crossed, one is cathodic protection pipeline and the other is unprotected pipeline. The construction requires that the vertical clear distance between pipelines shall not be less than 0.3m, and the pipeline shall be specially insulated within a certain length before and after the crossing point. If the above regulations are not strictly followed during construction, a section of pipeline shall be buried After time, under the action of soil stress, the pipelines may be overlapped together, which will cause the insulation layer to be damaged, and the metal to metal connection will form the leakage point.
(2) Insulation flange failure or leakage, insulation flange quality is not good, after using for a period of time, the insulation parts are damaged or deteriorated, so that the flange is no longer insulated, so that the two flange sides no longer have insulation performance, cathodic protection current is no longer limited; or there are some electrolyte impurities in the transmission medium to make the insulation flange conduction, no longer have insulation performance. From the above reasons, the leakage point can only occur at the intersection of the protective pipe and the unprotected pipe, or the insulating flange of the protective pipe, so finding the leakage point has the above limitations. However, if the underground pipe network is complex and the protected pipeline crosses several lines, it will make the search of leakage point complicated. According to the actual situation of the site, repeated measurement, multi-directional inspection and comprehensive judgment can find the real leakage fault point.
3. Search for leakage point
(1) The method of finding the damaged point of pipeline insulation layer is used to determine the leakage point or short contact point of pipeline. This method first sends the pulse signal to the measured pipeline. If the pipeline has good anticorrosion and insulation layer, the current flowing into the pipeline is very weak, and the instrument does not display it. If there is a leakage current in the soil from the damaged soil, there will be an electric current flowing through the soil around the pipeline. When the detector holds two reference electrodes to detect and walk directly above the pipeline, the voltmeter will vibrate obviously. When the pointer of the voltmeter stops shaking, the position of the leakage point of the anticorrosive coating is located in the middle of the two reference electrodes. This method is simple, feasible and accurate, and it is the internationally recognized leakage detection method (DCVG).
(2) The leakage point can be found by measuring the current in the pipe, because the current in the pipe flows from the far end to the electrified point in the cathodic protection pipeline without branch. When the unprotected pipeline is connected, the branch circuit will be formed, so that the protection current will become smaller after passing through the leakage point. Therefore, this method can be used to find the location of leakage point. When this method is used to measure the current in the pipe, the IR voltage drop method or the compensation method (see the relevant instructions) can be used to measure the current in the pipe in turn. The potential of leakage point is determined by comparing the current of each point.
(3) Insulation flange leakage measurement. When the leakage of insulating flange leads to the failure of cathodic protection system, the pipe ground potential can be measured on both sides of the insulation flange. If the protection side is the protection potential and the non protection side is the natural potential, then the insulation flange is normal. Otherwise, there are problems. The potential to ground at the end of the flange can also be measured on the unprotected side. If the potential is negative than that of the unprotected pipe or other metal structures, the insulation flange will leak.
Measuring the current flowing through the insulating flange can also be used to determine the performance of the insulating flange. If the current can be measured at the unprotected end of the insulation flange, the flange will leak; if the current can not be measured, the insulation flange will not leak. (4) CIPS
The measurement of protection potential on the test pile can only reflect the overall protection level of the pipeline, and can not indicate that all points of the pipeline have been protected. The short distance measurement method is to measure the pipe ground potential every 1-2 meters along the pipeline, which can accurately detect the unprotected pipe section.
4. Anode earth fault
Another common fault of cathodic protection is caused by anode grounding. Anode grounding resistance is closely related to the design and construction quality of anode ground bed. "Frozen soil" will increase the resistance of anode ground bed several times to more than ten times, and "air resistance" will also increase the resistance of anode ground bed. When the anode is used for a period of time, the current barrier will be caused by serious corrosion and uneven surface dissolution.
Therefore, the phenomenon that the potential increases and the protection current decreases will appear on the cathodic protection instrument. At this time, it is necessary to measure, replace or overhaul the anode ground bed to make the cathodic protection work normally. Another weak link is the sealing and insulation between the anode cable and the anode joint. Improper construction will cause corrosion and open circuit at the joint. Make the cathodic protection current open circuit and unable to input to the pipeline.
Shandong Linyi Bosi conductive materials Co., Ltd. is a high-tech enterprise specializing in the production of MMO flexible anode, conductive polymer flexible anode and titanium coated anode, mainly providing cathodic protection technology, engineering design, installation and technical services.
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