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1¡¢ Preparation and acceptance of cathodic protection before putting into operation
£¨1£© Inspection of pipeline to be protected before cathodic protection is put into operation
1. Inspection of pipeline to ground insulation
From the introduction of the principle of cathodic protection, it has been known that there is no protection without insulation. In order to ensure the normal operation of cathodic protection, the insulation measures of pipeline must be correct before applying cathodic protection current. It is necessary to check whether the insulation performance of the insulation flange of the pipeline is normal; the facilities arranged along the pipeline, such as valves, pump tanks and gate wells, should be well insulated from the soil; the pipeline and fixed pier, crossing tower and crossing casing should also have correct and effective insulation treatment measures. There should be no "short circuit" fault between the pipeline and other metal structures underground.
There shall be no leakage points on the anti-corrosion coating on the pipeline surface. All defects and damages caused during the construction period shall be detected by DCVG leak detector during construction acceptance, and backfilled after repair.
2. Pipe conductivity check
The pipeline to be protected shall have continuous conductivity.
3. The inspection of the insulation state of the old pipeline to the ground should be handled according to the design requirements. Whether to repair anti-corrosion coating and eliminate grounding fault (such as anti-static grounding electrode) should be determined according to technical and economic conditions. The electrical conductivity of the pipeline still needs to be checked according to the above requirements.
£¨2£© Acceptance of cathodic protection construction quality
1. Whether the installation of all electrical equipment in the cathodic protection room meets the requirements of "electrical equipment installation regulations", and whether various grounding facilities are completed and meet the requirements of drawing design.
2. Whether the material selection and construction of cathodic protection outside the station are consistent with the design. The construction and connection of electrified point, test pile, anode ground bed and anode lead shall strictly meet the requirements of the specification. In particular, the anode lead is connected to the positive pole, and the pipe confluence point is connected to the negative pole. It is strictly forbidden to connect the electrode reversely.
3. The drawings and design data are complete.
2¡¢ Cathodic protection put into operation
1. Organize personnel to measure natural potential, soil resistivity and grounding resistance of anode ground bed of each station. At the same time, we should have a more detailed understanding of the pipeline environment, and these data should be separately recorded, sorted out and filed for future use.
2. Cathodic protection station put into operation
According to the operation procedures of DC power supply (rectifier, potentiostat, storage battery, etc.), power is supplied to the pipeline to keep the potential at - 1.30 v. after cathodic polarization of the pipeline for a period of time (more than four hours), the output current, voltage, potential at power on point, protection potential along the pipeline and protection distance shall be tested. Then, according to the measured protection potential, adjust the potential at the energizing point to the specified value, and continue to power the pipeline to make it fully polarized (usually more than 24 hours). Repeat the first test and make a record. If the protection potential of individual pipeline section is too low, it is necessary to adjust the electrified point potential to meet the cathodic protection potential index of the whole line.
3. Control of protective potential
The control value of the potential at the power on point of each station shall ensure that the protection potential of the pipe section between the two adjacent stations can reach -0.85 v. meanwhile, the most negative potential at the energizing point of each station shall not exceed the specified value. When adjusting the potential of the electrified point, the connection between the adjacent cathodic protection stations on the pipeline shall be strengthened to ensure the potential balance of each station.
4. When the pipeline reaches the minimum cathodic protection potential index, the operation is completed. Each cathodic protection station enters the normal continuous working stage.
3¡¢ Daily maintenance and management of Cathodic Protection Station
1. Routine maintenance of cathodic protection facilities
Regular technical inspection of electrical equipment. The inspection of electrical equipment shall not be less than once a week, including the following contents:
1) Check the electrical equipment circuit contact firmness, the correctness of the installation, individual components whether there are mechanical obstacles. Check whether the power wires connected to the cathodic protection station, the wires connected to the anode ground bed and power on point are in good condition, and whether the joints are firm.
2) Check whether the fuse of the fuse on the distribution board is properly connected according to the regulations. When the fuse of the fuse in the AC circuit is burned out, the cause shall be found out and the power supply shall be restored in time.
3) Observe the electrical instrument, record the output voltage, current and potential value of electrified point on the special form, and check whether there is any change compared with the previous record (or on duty record). If not, find out the reason and take corresponding measures to make the whole pipeline reach cathodic protection.
4) The working grounding and arrester grounding should be checked regularly, and the grounding resistance should not be greater than 10 ohm. Attention should be paid to lightning protection in thunderstorm season.
5) Do a good job in cleaning the equipment in the station, pay attention to keep the indoor dry and well electrified to prevent the instrument from overheating.
2. Maintenance of Potentiostat
1) Generally, two sets of cathodic protection potentiostat are provided for each other, so they should be switched regularly according to the management requirements. When using standby instruments, one observation and maintenance should be carried out immediately. During the maintenance of the instrument, it is not allowed to plug or pull out the connectors and printed circuit boards with electricity.
2) Observe whether all parts are normal, whether the components are corroded, whether there is desoldering, false soldering and damage, whether the connection points are reliable, whether the circuit is faulty, whether the fasteners are loose, and whether the fuse is in good condition. If there is a fuse, the reason should be found out before replacement.
3) Clean the interior and remove foreign matters.
4) In case of instrument failure, timely maintenance shall be carried out, and standby instrument shall be put into operation to ensure power supply. The power on rate should be calculated every year.
3. Maintenance of copper sulfate electrode
1) When using standard products or self-made copper sulfate electrode, the bottom of the electrode should be permeable without leakage and pollution. It should be kept clean after use to prevent large amount of solution leakage.
2) As the signal source of constant potential instrument, the buried copper sulfate reference electrode should be checked once a week in the process of use, and saturated copper sulfate solution should be added in time. Strictly prevent freezing and drying, which will affect the normal operation of the instrument.
3) After using the copper rod in the electrode for a period of time, the surface will adhere to a layer of blue dirt, which should be cleaned regularly to reveal the true color of copper. Pure copper sulfate and distilled water must be used to prepare saturated copper sulfate solution.
4. Maintenance of anode ground bed
1) Anode overhead line: check whether the line is in good condition once a month, such as whether the pole is inclined, whether the porcelain bottle and conductor are loose, whether the connection between the anode wire and the ground bed is firm, and whether the buried signs of the ground bed are in good condition. Timely rectify the problems found.
2) The grounding resistance of anode grounding bed shall be tested once every half a year. When the grounding resistance increases to such an extent that the potentiostat cannot provide the protection current required by the pipeline, the anode ground bed shall be replaced or repaired to reduce the grounding resistance.
5. Maintenance of test piles
1) Check the insulation between the terminal and the earth. The resistance value should be greater than 100 kilo ohm. Measure it with a multimeter. If it is less than this value, check whether the terminal and the outer steel pipe are grounded. If so, it needs to be replaced or repaired.
2) Test piles shall be painted and numbered regularly every year.
3) To prevent the destruction and loss of test piles, and to carry out publicity and education on the protection of state property for urban and rural residents and children along the line.
6. Maintenance of insulating flange
1) The pipe ground potential on both sides of the insulating flange shall be detected regularly. If there is any difference between the potential and the original record, its performance shall be identified. In case of leakage, corresponding measures shall be taken.
2) The maintenance and management of the insulating flange with auxiliary equipment (such as current limiting resistor, overvoltage protection diode, rain proof shield, etc.) shall be strengthened to ensure that they are in good condition.
3) Keep the insulation flange clean and dry, and paint regularly.
7. Cathodic protection management
1) For each cathodic protection pipeline, the cathodic protection operation management regulations in line with the actual situation of the pipeline shall be formulated, so that the daily test, control, adjustment and maintenance of cathodic protection shall be carried out according to this.
2) Strengthen the organization and leadership of cathodic protection. Keep the indoor equipment clean and tidy to achieve no fault, no defect, no rust and no foreign objects. Realize three diagrams on the wall, that is, line trend diagram, protection potential curve diagram and post responsibility system.
3) After the cathodic protection station is put into operation, the wiring of electrical equipment shall not be changed without authorization. If it needs to be changed, the plan shall be made by the competent department and implemented after approval.
4) Check and measure the potential of the power on point every day, fill in the operation log, and report the operation of cathodic protection station to the production dispatching department.
5) The power transmission from cathodic protection station to pipeline shall not be interrupted. The suspension for more than one day shall be reported to the competent department for record. Adjust the instrument by using the method of pipeline power failure, which shall not exceed 2 hours at a time and 30 hours in a year. Ensure that more than 98% of the year's power supply to the pipeline.
6) The cathodic protection potential shall be measured along the pipeline by keeping the potential at the power on point at the specified value once a week at the beginning of the operation of the cathodic protection station, and then once every two weeks or one month. The protective potential measurement records shall be tabulated and plotted and reported to the competent department.
7) The natural potential and soil resistivity along the pipeline shall be measured once a year within the specified time.
8) Check and eliminate the pipeline grounding fault, so that the whole line can achieve complete cathodic protection.
8. Maintenance of sacrificial anode
The daily maintenance work of pipeline sacrificial anode protection is not much. In addition to the protection potential measurement, test pile maintenance, insulation flange detection, grounding fault removal and other work according to the requirements of impressed current cathodic protection, it is recommended to measure the parameters monthly. According to this, the pipeline protection situation is analyzed. If the anode performance deteriorates, corresponding measures should be taken.
4¡¢ Analysis of common faults in cathodic protection system
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 anode cable in 1993
(1) Measure the output current, turn on the potentiostat, and connect an ammeter at the anode output end of the potentiostat. If the current is zero, 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 the anticorrosive coating of the pipeline is damaged, the current will leak into the pipeline through the damaged place from the soil, and the current flow will produce obvious potential gradient in the surrounding soil. 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 tube. Because there is no branch of cathodic protection pipeline, the current in the pipe flows from the far end to the power on point. 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 on one side of the non protective end of the insulating flange, the flange will leak; if the current cannot be measured, the insulation flange will not leak.
(4) CIPS is a close spaced potentiometric method
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.