Overview The seawater circulating cooling water pipeline in the No. 1 and No. 2 unit of Huaneng Dandong Power Plant is based on the original design of the outside party, and uses seamless carbon steel pipes with exposed inner walls. The seawater cooling water pipeline is mainly composed of heat exchanger shaft cooler). The pipeline is divided into three sections: the first section is the 500 inlet pipeline before the suction port of the booster pump, with a total length of about 88m; the second section is the outlet of the booster pump to The heat exchanger suction section 450 pipe has a total length of about 68 m; the third section is the heat exchanger outlet section 450 pipe, with a total length of about 55 m. In addition, there are 0 150 seawater recirculation pipes 12 m, 0 m below zero meters, and in and out Condenser vacuum pump 0100 pipeline. Pipes are mainly connected by welding, and some pipe sections are connected to titanium or stainless steel equipment.
The cooling water is seawater at the inlet of the Yalu River, the electrical resistivity is 0.311 "m, and the flow rate is 3 Ws. The main corrosion forms of carbon steel pipe inner wall in seawater are total corrosion, under-scale corrosion and galvanic corrosion. It is more serious. According to the operating experience of similar circulating cooling water systems in coastal power plants, to ensure the normal operation and service life of the pipeline, it is technically feasible and economically reasonable to adopt the method of joint protection of coating and cathodic protection on the inner wall of the pipe.
Because the foreign party has not put forward design requirements for the corrosion protection of the inner wall of the seawater circulating cooling water pipeline in the power plant building, and considering that there are many pipe elbows and welding connections, it is difficult to apply anti-corrosion coatings to the installed pipeline inner wall. For some cathodic protection engineering practices, it was decided to use a reinforced external cathodic protection scheme with no internal anti-corrosion coating for pipes with a diameter greater than 150mm.
1 Main parameters and technical indicators of cathodic protection 1.1 Main parameters 1.1.1 Minimum protection current density p According to engineering experience and test results, regarding In3, the initial p value of the bare tube is determined to be 500mA / m2, and the stable p value is 100 ~ 200. Silver / silver chloride electrode and zinc electrode are used as electrodes: the protective potential of the pipeline near the titanium equipment. All the protective potential reference electrodes close to the pipeline of the titanium equipment are less than 0.2m from the titanium equipment (measured value). 9 Seek control in: 1.2 Main technical indicators 1) After the protection is implemented, the potential of the cooling water pipe is energized, relative to the silver / silver chloride reference electrode: near the titanium equipment -0-70-0.80V, other arbitrary points -0- 75——1-15V; relative to zinc reference electrode: titanium design 2) Design life of auxiliary anode is greater than 10a. 2 choice of protection facilities 2.1 auxiliary anode 2.1.1 anode type According to the requirements of technical indicators, select the long Lifetime titanium ruthenium auxiliary anode, anode model CP32-60, working surface area 60cm2 ASA membrane provided by Northwest Nonferrous Metal Research and Design Institute.
2.1.2 Anode performance The main performance of titanium ruthenium auxiliary anode: working current density 100-transition potential in seawater, relative to Ag / AgCl electrode) 8V; rated output working current! 6A. 2.1.3 Anode system The anode system should be easy to install, not block water, and do not affect the safe operation of the pipeline. Therefore, the outer tube titanium ruthenium anode is selected. See structure.
2.1.4 The anode insulation adopts the overall insulation method: according to the shape and size of the anode, a special mold is made. Except for the anode working surface, the remaining parts are sealed and insulated with the whole mold of the mold to make a special tube-type external anode.
2.2 The reference electrode used for reference electrode control is a silver / silver chloride electrode and a high-purity zinc electrode that are durable, accurate in potential measurement, and have good reproducibility. The high-purity zinc electrode is used for the fixed monitoring reference electrode. The reference electrode is externally mounted and its structure is similar to the auxiliary anode.
2.3 Control power supply and wiring According to the working conditions, select the manual control indoor cathodic protection DC power supply. The main requirements for the power supply equipment are: the output current and voltage are continuously adjustable within the design requirements, can run for a long time, have a certain anti-interference ability, and can work normally when the working conditions such as environment and climate change. The system wiring should meet the basic requirements of cathodic protection design. The anode wire and cathode wire use YC copper core rubber sheathed wire, and the reference wire and measurement wire use RVV shielded wire.
3 System Composition and Electrode Arrangement 3.1 Subsystems Each unit's circulating cooling water pipeline cathodic protection system is composed of 4 relatively independent subsystems, which are controlled by 4 power sources respectively.
3.2 Auxiliary anode arrangement Pipe installation The anode separation is generally 2-3m, and the anode installation separation between the elbow connection and the connection between the pipe and the titanium or stainless steel equipment is less than 2m. 3.3 Confluence point and reference electrode arrangement Each sub-system is provided with 2 confluences Point and 2 measuring line contacts. Install four reference electrodes, one of which is used for system control, and one is used for system control backup. The leads of these two electrodes and the corresponding measurement wires are led to the corresponding terminal of the cathodic protection power cabinet. The other two reference electrodes of each sub-system are used for on-site fixed monitoring, but no wires are required. Except that the inlet and outlet pipe sections of the heat exchanger of the third sub-system are respectively equipped with one silver / silver chloride electrode for control and fixed monitoring, the other reference electrodes are all high-purity zinc electrodes. The auxiliary anode, reference electrode, bus point and measuring line contact arrangement of each subsystem is shown in Table 1. 3.4 System wiring leads the auxiliary anode, reference electrode and bus point lead wires and measuring line to the power cabinet, and connects to the corresponding terminals, Then each sub-system is connected to the corresponding terminal of the card by the corresponding main line.
4 Brief design calculation takes a single unit as the calculation unit.
4.1 The calculation formula of the protective area S is shown in Table 1. The auxiliary anode, reference electrode, bus point and measuring line contact of each sub-system are arranged. The diameter of the tube is the protective area. The auxiliary anode reference electrode bus point measuring line is connected to 4.2. The initial value and stable value of the current density are taken as 500mA / m2 and 150mA / m2, respectively. The protection area of ​​the protected pipeline and the calculation results of the required protection current are listed in Table 2. Table 2 The protected area of ​​the pipeline and the required current strength Length / m area / m2 initial current / A stable current / A total 4.3 Anode water resistance 4.3.1 Single anode water resistance, cm; d is anode diameter, cm. When calculating, p is 31.1 4.3.2 anode The total resistance is shown in Table 3. 4.4 Power supply output 4.4.1 Output current is calculated according to the protection current intensity of four sub-systems. The maximum output current of each sub-system power supply is 40 ~ 45A. The calculated value of the sum voltage is the total anode resistance of the system / output current / A output voltage / V source rated value is 50A. 4.4.2 When calculating the output voltage, the sum of the wire resistance and the equivalent resistance of the pipeline is taken as 0.25, and the output voltage calculation formula 4 Subsystem output power Of 11 ~ 13V, the power rating selected to 20V. 5 5.1 Installation requirements and technical solutions auxiliary anode anode installation position specified by the design drawings. First, open a hole with a diameter of 55mm at the pre-defined position of the pipeline, insert the lower base of the anode, spot weld the base outside the pipe, and then weld it twice in a row, and then install the insulating seal with a molded film The anode body of the rubber pad, the anode base is fixed, the bolt is fixed, and the force is evenly tightened, and the anode wire is fixed on the anode terminal. After the inspection is passed, the anode cover is put on (see the installation anode should be noted: 1) must not be used Directly touch the surface of the anode body with your hands to prevent oil and sweat from getting on the work surface; 2) Do not damage the insulating seal between the electrode body and the protected pipe to ensure the insulation and sealing between the anode body and the protected pipe .
5.2 The reference electrode is installed according to the position marked in the design drawings, the method is the same as above.
5.3 Install the busbar base and the measuring line at the base point. According to the drawing, weld the base to the pipeline, peel off the outer skin of the cathode busbar or measuring line terminal by about 3cm, and use sandpaper to connect the terminal and the base The copper nose hole is polished to reveal the metallic luster, the copper wire is coated with solder paste, hung tin, inserted into the copper nose that has been melted into the solder, and left to stand until the tin is completely solidified.
5.4 The control power cabinet is installed in the designated location. The control power supply is fixed on the prefabricated base according to the design requirements.
5.5 Wiring 1) Before wiring, check whether the auxiliary anode, bus base, measuring line contact base and reference electrode are connected with the corresponding wires; detect the connection and disconnection of various wires in the system. ") Measure and record the auxiliary anode and the natural liver path of the pipeline) potential.) Connect the anode lead to the positive pole of the DC power supply, the busbar of the pipe to the negative pole, the reference electrode lead and the measurement line are respectively connected And measurement "connection.
6 Operation and protection effect evaluation 6.1 Operation is connected to the power supply as required, and power is supplied to the protected system. After running for a certain period of time, check the power-on potential of the test point and the output current and voltage of the DC power supply. According to the test results, adjust the current output of the DC power supply from small to large to make the test point potential reach the specified value. Repeat the above process until the potential reaches the required and stable.
6.2 Evaluation of Protection Effect During the commissioning phase of the No. 2 unit's cathodic protection system, the operating parameters of the No. 3 sub-system were adjusted and the pipeline energization potential detection results corresponding to the two test points. In the table! It is the measured value relative to the silver / silver chloride reference electrode control electrode), + is the measured value of the zinc electrode fixed monitoring electrode) as the reference electrode, and the output current and output voltage of the DC power supply, respectively. Adjust the output current amplitude by 10A every 12h), and measure the corresponding potential value after stabilization.
Table 4 The operating parameters of the No. 3 sub-system and the measured value of the pipeline energization potential can be seen from Table 4. As the output current increases, the energization potential changes to a negative direction. When the current increases to 30A, the two test point potential values ​​have reached the technical indicators and Design requirements. At this time, the protection current density is approximately around.
The relationship between the working potential of the titanium ruthenium anode of No. 3 sub-system and the Ag / AgCl electrode) and the output current. It can be seen that when the output current reaches 30A, the anode potential is only about 5V, which is lower than the critical breakdown potential value of 2V), indicating that the anode of the system can work normally at a higher current, and the performance index meets the design requirements.
Within 90 days of the initial operation of the cathodic protection system, the output current of each sub-system of the two units was controlled at about 30A. At present, each sub-system has been running stably for more than 600d, and the output current is controlled at about 25A. Table 5 lists the sub-systems of Unit 2 as the silver / silver chloride electrode relative to the reference electrode for control. Sub-system 3 is silver / silver chloride electrode, and the other sub-systems are high-purity zinc electrodes. The measured value of the power-on potential for 600 days. It can be seen from Table 5 that after each sub-system runs for 90 days, the energization potential value has reached the technical index requirements, indicating that the pipeline is already in a protected state.
Table 5 Potential measurement value of each sub-system of Unit 2 V sub-system open circuit operation 90d operation 600d At present, the cathode protection systems of Unit 1 and Unit 2 are all operating normally. The results of construction, operation and potential testing show that the design of the cooling water pipeline with current cathodic protection is reasonable, the construction is simple, the maintenance is convenient, and the technical indicators meet the design requirements.
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