Anti-deformation and anti-pollution manufacturing process for graphite thrust pad of large shielding motor
By adopting anti-deformation and anti-pollution technology in the manufacturing process of graphite thrust tiles of large shielded motors, the problems of thrust tile deformation and chloride ion contamination are solved, and the processing accuracy and motor life are improved.
Patent Information
- Application Number
- CN202411347603.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-26
- Publication Date
- 2025-01-17
AI Technical Summary
Large shielded motor graphite thrust tiles are prone to deform during manufacturing and may release chloride ions to contaminate the thrust disc, resulting in a shorter motor life.
The anti-deformation and anti-pollution manufacturing process is adopted, including rough machining, water pressure, heating, cooling, stress relief, grinding and flushing to ensure that the graphite thrust tiles are processed and stored in a wet state to avoid deformation and pollution.
It effectively controls the deformation of the graphite thrust shoe, ensures processing accuracy, and reduces chloride ion pollution through the flushing step, extending the service life of the motor.
Smart Images
Figure CN119321447A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a deformation and pollution prevention manufacturing process of a graphite thrust bearing of a large shielded motor, and in particular to the field of graphite thrust bearing manufacturing. Background Art
[0002] As the country vigorously develops industry, innovation and development in the field of motors have made motors larger and larger, which in turn has led to the various components within the motors becoming larger and larger. As the size of some workpieces increases, the corresponding processes also need to be changed and innovated.
[0003] In the past, water-lubricated graphite thrust bearings were widely used because of the advantages of graphite such as self-lubrication, high temperature resistance, and corrosion resistance. Shielded motors have also been using water-lubricated graphite thrust bearings as thrust bearings. The graphite thrust bearings and thrust discs are a pair of friction pairs that support the rotation of the rotor. Their processing accuracy is crucial to the operation of shielded motors. For small motors, the processing difficulty is relatively small due to the small size of their thrust bearings. However, for large shielded motors, as the size of their thrust bearings increases, they are easily deformed by temperature and stress. To ensure their processing accuracy, it is crucial to control the deformation of graphite thrust bearings during the manufacturing process. At the same time, it is also crucial to control chloride contamination of graphite thrust bearings during the operation of the motor to prevent the graphite thrust bearings from releasing chloride ions to contaminate the thrust disc, causing pitting or failure of the thrust disc and affecting the life of the motor. Summary of the invention
[0004] The purpose of the present invention is to provide a large shielded motor graphite thrust bearing anti-deformation and anti-pollution manufacturing process, which controls the deformation of the thrust bearing during the manufacturing process, ensures the dimensional stability of the graphite thrust bearing, and ensures the processing accuracy. Controlling chloride contamination of the graphite thrust bearing, avoiding the graphite thrust bearing releasing chloride ions to contaminate the thrust plate during the operation of the motor, causing pitting or failure of the thrust plate and affecting the life of the motor.
[0005] The technical solution of the present invention is:
[0006] A large shielded motor graphite thrust bearing anti-deformation and anti-pollution manufacturing process:
[0007] a. Rough machining of the graphite thrust pad (1): Place the graphite thrust pad (1) on a surface grinder to grind the reference surface (2) of the graphite thrust pad (1) first, and then grind the working surface (5) of the graphite thrust pad (1). At this time, both the reference surface (2) and the working surface (5) of the graphite thrust pad (1) have machining allowances. After grinding, use a dust-free cloth dipped in alcohol to clean the graphite thrust pad (1);
[0008] b. Cleaning tools: clean the thrust washer bracket (3), heating water tank (6), cooling water tank (7) and transfer storage water tank (4). After cleaning, place the thrust washer bracket (3) in the transfer storage water tank (4), add deionized water to the heating water tank (6), cooling water tank (7) and transfer storage water tank (4), and remove the deionized water after 12 hours if no rust is produced, and wipe dry with a dust-free cloth;
[0009] c. Pumping water pressure on the graphite thrust washer (1): placing the thrust washer bracket (3) in the water pressure well (10), and then placing the graphite thrust washer (1) on the thrust washer bracket (3) in the water pressure well (10), installing the well cover (9) and fixing it with bolts (8), filling the water pressure well (10) with deionized water, and using a water pressure pumping device to pressurize the water pressure in the water pressure well (10) to 15 MPa. After 10 minutes, check the pressure in the water pressure well (10). If the pressure in the water pressure well (10) does not decrease, it means there is no leakage. The graphite thrust washer (1) maintains the water pressure at 15 MPa for at least 24 hours. After depressurization and drainage, remove the bolts (8) and the top cover (9), and transfer the thrust washer bracket (3) with the graphite thrust washer (1) from the water pressure well (10) to the transfer storage water tank (4). Deionized water is injected into the transfer storage water tank (4) for storage. During the manufacturing process, the graphite thrust washer (1) is always placed vertically on the thrust washer support (3), and the graphite thrust washer (1) is always immersed in deionized water to keep the graphite thrust washer (1) in a wet state;
[0010] d. Heating and cooling to relieve stress: inject deionized water of room temperature into the heating water tank (6) and the cooling water tank (7), place the graphite thrust washer (1) and the thrust washer bracket (3) into the heating water tank (6), start the power supply, heat the deionized water in the heating water tank (6) to 90°C-100°C, and keep the graphite thrust washer (1) in the required temperature range of 90°C-100°C for at least 15 minutes. After the graphite thrust washer (1) is kept in the hot water for at least 15 minutes, the graphite thrust washer (1) and the thrust washer bracket (3) are removed from the heating water tank (6) together and immediately placed in the cooling water tank (7) for cooling. The graphite thrust washer (1) is kept in the cooling water tank (7) for at least 60 minutes. After cooling for at least 60 minutes, the graphite thrust washer (1) and the thrust washer bracket (3) are removed from the cooling water tank (7) and placed in a transfer storage water tank (4) filled with deionized water. The deionized water in the heating water tank (6) and the cooling water tank (7) is drained, so that the graphite thrust washer (1) completes one heating and cooling stress relief, and the above operation is repeated once for the second heating and cooling stress relief. During the manufacturing process, the graphite thrust washer (1) is always placed vertically on the thrust washer bracket (3), and the graphite thrust washer (1) is always immersed in deionized water to keep the graphite thrust washer (1) in a wet state;
[0011] e. Grinding the graphite thrust washer (1): Place the graphite thrust washer (1) on a grinding and polishing machine to grind the graphite thrust washer (1). Only one graphite thrust washer (1) is allowed to be ground at a time. First, grind the reference surface (2) of the graphite thrust washer (1), and then immediately grind the working surface (5) of the graphite thrust washer (1). During the manufacturing process, the graphite thrust washer (1) is always placed vertically on the thrust washer bracket (3), and the graphite thrust washer (1) is always immersed in deionized water to keep the graphite thrust washer (1) in a wet state;
[0012] f. Graphite thrust washer (1) flushing: Place the graphite thrust washer (1) on the thrust washer support (3) in the heating water tank (6), add deionized water to the heating water tank (6), take 100 ml of water sample before flushing in the heating water tank (6) and measure the chloride ion concentration in the water sample, which is recorded as the chloride ion concentration a before flushing, start the power supply, heat the deionized water in the heating water tank (6) to 55-65°C, keep the graphite thrust washer (1) in the temperature range of 55-65°C for at least 24 hours, take 100 ml of water sample after flushing in the heating water tank (6) and measure the chloride ion concentration in the water sample, which is recorded as the chloride ion concentration b after flushing. If the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is not greater than 1.0 ppm, the flushing is considered qualified. If the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is greater than 1.0 ppm, it is necessary to flush again until the required value is met. The graphite thrust washer (1) and the thrust washer support (3) are removed from the heating water tank (6) and placed in a transfer storage water tank (4) filled with deionized water. During the manufacturing process, the graphite thrust washer (1) is always placed vertically on the thrust washer support (3), and the graphite thrust washer (1) is always immersed in Class A water to keep the graphite thrust washer (1) in a wet state;
[0013] g. Cleaning and packaging: After rinsing the graphite thrust washer (1) in step f, use alcohol and a dust-free cloth to clean the graphite thrust washer (1), and then package and protect the thrust washer.
[0014] The technical effects of the present invention are:
[0015] 1. During the manufacturing process of graphite thrust bearings, use alcohol and dust-free cloth to clean the graphite thrust bearings to prevent the coolant used in the processing from staying on the surface of the graphite thrust bearings and contaminating the graphite thrust bearings. Clean the thrust bearing bracket, heating water tank and transfer storage water tank before use to prevent foreign objects from contaminating the graphite thrust bearings. 2. Because there are microscopic pores in the graphite thrust bearings, the inside of the graphite thrust bearings is still in a dry state after rough processing, while the pores near the surface are occupied by the coolant used for processing and are wet. In this state, the graphite thrust bearings expand or deform unevenly, which will affect the processing accuracy of the reference surface and working surface of the graphite thrust bearings. Apply water pressure to the graphite thrust bearings so that all the pores inside the graphite thrust bearings are occupied by water. In this state, the graphite thrust bearings expand or deform evenly, which does not affect the processing accuracy of the reference surface and working surface of the graphite thrust bearings. At the same time, the internal pores of the graphite thrust bearing are occupied by water, which prevents coolant and other foreign substances from entering the interior of the graphite thrust bearing and contaminating the graphite thrust bearing; 3. The graphite thrust bearing is heated and cooled to eliminate stress. First, the surface stress generated during the rough processing of the graphite thrust bearing is eliminated to avoid deformation of the graphite thrust bearing due to surface stress. Second, the air in the microscopic pores inside the graphite thrust bearing is further eliminated, and harmful elements that can be precipitated in the graphite thrust bearing, such as chloride ions, are eliminated to avoid contaminating the graphite thrust bearing; 4. The graphite thrust bearing is always kept immersed in deionized water to keep the graphite thrust bearing in a wet state. The environment of the graphite thrust bearing in water when it is working, the processing and manufacturing environment are consistent with the working environment, ensuring that the size of the graphite thrust bearing before the motor starts running is consistent with the processed size, thereby improving the manufacturing quality of the graphite thrust bearing. Graphite thrust bearings are thin-walled parts. During the manufacturing process, graphite thrust bearings are always placed vertically on the thrust bearing bracket to reduce deformation caused by uneven support after long-term horizontal or tilted placement of graphite thrust bearings, which affects the processing accuracy. 5. When grinding thrust bearings, only one graphite thrust bearing is allowed to be ground at a time. First, grind the reference surface of the graphite thrust bearing, and then grind the working surface of the graphite thrust bearing immediately. This avoids the time interval between grinding the reference surface and grinding the working surface being too long, and the ground reference surface is deformed before the grinding of the working surface. The shape and position tolerance accuracy between the working surface and the reference surface of the graphite thrust bearing is improved. 6. Flushing the graphite thrust bearing can precipitate pollutants such as coolant and hands that are in contact during the processing and manufacturing process from the graphite thrust bearing, especially chloride pollution. Avoid the graphite thrust washer releasing chloride ions to contaminate the thrust plate during motor operation, causing pitting or failure of the thrust plate and affecting the life of the motor; 7. Determine the cleanliness of the graphite thrust washer by evaluating the difference between the chloride ion concentration b after flushing and the chloride ion concentration a before flushing, using a quantitative value. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 This is a schematic diagram of the graphite thrust washer on the thrust washer bracket.
[0017] Figure 2 This is a schematic diagram of the graphite thrust pad in the transfer storage tank.
[0018] Figure 3 It is a schematic diagram of the graphite thrust bearing in the heating water tank and the cooling water tank.
[0019] Figure 4 This is a schematic diagram of a graphite thrust pad in a hydraulic well. DETAILED DESCRIPTION
[0020] like Figure 1 As shown, a deformation and pollution prevention manufacturing process of a graphite thrust bearing of a large shielded motor, a. Rough machining of the graphite thrust bearing 1: placing the graphite thrust bearing 1 on a surface grinder to first grind the reference surface 2 of the graphite thrust bearing 1, and then grinding the working surface 5 of the graphite thrust bearing 1. At this time, both the reference surface 2 and the working surface 5 of the graphite thrust bearing 1 have machining allowances. After the grinding, the graphite thrust bearing 1 is cleaned with a dust-free cloth dipped in alcohol to prevent the coolant used in the machining process from staying on the surface of the graphite thrust bearing 1 and contaminating the graphite thrust bearing 1;
[0021] b. Cleaning tools: clean the thrust washer bracket 3, heating water tank 6, cooling water tank 7 and transfer storage water tank 4. After cleaning, place the thrust washer bracket 3 in the transfer storage water tank 4, add deionized water to the heating water tank 6, cooling water tank 7 and transfer storage water tank 4, and remove the deionized water after 12 hours if no rust is produced, and wipe dry with a dust-free cloth. Avoid foreign matter in the thrust washer bracket 3, heating water tank 6, cooling water tank 7 and transfer storage water tank 4 from contaminating the graphite thrust washer 1;
[0022] c. Apply water pressure to the graphite thrust washer 1: Due to the presence of microscopic pores in the graphite thrust washer 1, the interior of the graphite thrust washer 1 is still in a dry state after rough machining, while the pores near the surface are occupied by the coolant used for machining and are wet. In this state, the graphite thrust washer 1 expands or deforms unevenly, which will affect the machining accuracy of the reference surface 2 and the working surface 5 of the graphite thrust washer 1. Apply water pressure to the graphite thrust washer 1 so that all the pores inside the graphite thrust washer 1 are occupied by water. In this state, the graphite thrust washer 1 expands or deforms evenly, which does not affect the machining accuracy of the reference surface 2 and the working surface 5 of the graphite thrust washer 1. At the same time, the internal pores of the graphite thrust washer 1 are occupied by water, which prevents foreign substances such as coolant from entering the interior of the graphite thrust washer 1 and contaminating the graphite thrust washer 1. Figure 4As shown, place the thrust washer bracket 3 in the hydraulic well 10, then place the graphite thrust washer 1 on the thrust washer bracket 3 in the hydraulic well 10, install the well cover 9 and fix it with bolts 8, fill the hydraulic well 10 with deionized water, use the water pressure pumping equipment to pressurize the water pressure in the hydraulic well 10 to 15Mpa, check the pressure in the hydraulic well 10 after 10 minutes, if the pressure in the hydraulic well 10 does not drop, it means there is no leakage, and the graphite thrust washer 1 maintains the water pressure at 15Mpa for at least 24 hours. After depressurizing and draining, remove the bolts 8 and the top cover 9. Figure 2 As shown, the thrust washer bracket 3 with the graphite thrust washer 1 is transferred from the hydraulic well 10 to the transfer storage water tank 4, and the transfer storage water tank 4 is filled with deionized water for storage. The graphite thrust washer 1 is a thin-walled part. During the manufacturing process, the graphite thrust washer 1 is always kept vertically placed on the thrust washer bracket 3, which can reduce the deformation of the graphite thrust washer 1 caused by uneven support after being placed horizontally or tilted for a long time, thereby affecting the processing accuracy. And always keep the graphite thrust washer 1 immersed in Class A water to keep the graphite thrust washer 1 in a wet state. The environment of the graphite thrust washer 1 in the water when it is working, the processing and manufacturing environment are consistent with the working environment, ensuring that the size of the graphite thrust washer 1 before the motor starts running is consistent with the processed size, thereby improving the manufacturing quality of the graphite thrust washer 1;
[0023] d. Heating and cooling to relieve stress: Deionized water at room temperature is injected into the heating water tank 6 and the cooling water tank 7. Figure 3As shown, the graphite thrust washer 1 and the thrust washer bracket 3 are placed in the heating water tank 6, the power is turned on, the deionized water in the heating water tank 6 is heated to 90°C-100°C, and the graphite thrust washer 1 is kept in the temperature requirement range of 90°C-100°C for at least 15 minutes. After the graphite thrust washer 1 is kept in the hot water for at least 15 minutes, the graphite thrust washer 1 and the thrust washer bracket 3 are removed from the heating water tank 6 together, and immediately placed in the cooling water tank 7 for cooling. The graphite thrust washer 1 is kept in the cooling water tank 7 for at least 60 minutes. After cooling for at least 60 minutes, the graphite thrust washer 1 and the thrust washer bracket 3 are removed from the cooling water tank 7 and placed in the transfer storage water tank 4 filled with deionized water. The deionized water in the heating water tank 6 and the cooling water tank 7 is drained, so that the graphite thrust washer 1 completes one heating and cooling stress relief, and the above operation is repeated once for the second heating and cooling stress relief. The graphite thrust washer 1 is heated and cooled to eliminate stress. First, the surface stress generated during the rough processing of the graphite thrust washer 1 is eliminated to avoid deformation of the graphite thrust washer 1 due to surface stress. Second, the air in the microscopic pores inside the graphite thrust washer 1 is further removed, and harmful elements that can be precipitated in the graphite thrust washer 1, such as chloride ions, are removed to avoid contamination of the graphite thrust washer 1. During the manufacturing process, the graphite thrust washer 1 is always placed vertically on the thrust washer bracket 3, and the graphite thrust washer 1 is always immersed in deionized water to keep the graphite thrust washer 1 in a moist state;
[0024] e. Grinding the graphite thrust washer 1: Place the graphite thrust washer 1 on a grinding and polishing machine to grind the graphite thrust washer 1. Only one graphite thrust washer 1 is allowed to be ground at a time. First, grind the reference surface 2 of the graphite thrust washer 1, and then immediately grind the working surface 5 of the graphite thrust washer 1. This avoids a long time interval between grinding the reference surface 2 and grinding the working surface 5, and deformation of the reference surface 2 after grinding before starting to grind the working surface 5. The shape and position tolerance accuracy between the working surface 2 and the reference surface 4 of the graphite thrust washer 1 is improved. During the manufacturing process, the graphite thrust washer 1 is always placed vertically on the thrust washer bracket 3, and the graphite thrust washer 1 is always immersed in deionized water to keep the graphite thrust washer 1 in a wet state;
[0025] f. Graphite thrust bearing 1 flushing: Figure 3As shown, the graphite thrust washer 1 is placed on the thrust washer support 3 in the heating water tank 6, deionized water is added to the heating water tank 6, 100 ml of water sample before flushing is taken from the heating water tank 6 and the chloride ion concentration in the water sample is measured, which is recorded as the chloride ion concentration a before flushing, the power is turned on, the deionized water in the heating water tank 6 is heated to 55-65°C, and the graphite thrust washer 1 is kept in the temperature range of 55-65°C for at least 24 hours, 100 ml of water sample after flushing is taken from the heating water tank 6 and the chloride ion concentration in the water sample is measured, which is recorded as the chloride ion concentration b after flushing. If the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is not greater than 1.0ppm, the flushing is considered qualified. The cleanliness of the graphite thrust washer 1 is determined by evaluating the difference between the chloride ion concentration b after flushing and the chloride ion concentration a before flushing, and a quantitative value is used. If the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is greater than 1.0ppm, it is necessary to flush again until the required value is met. Flushing the graphite thrust washer 1 can remove pollutants such as coolant, hands, etc. that come into contact with the graphite thrust washer 1 during the processing and manufacturing process, especially chloride pollution. Avoid the graphite thrust washer 1 releasing chloride ions to contaminate the thrust disk during the operation of the motor, causing pitting or failure of the thrust disk and affecting the life of the motor. Remove the graphite thrust washer 1 and the thrust washer bracket 3 from the heating water tank 6 and place them in a transfer storage water tank 4 filled with deionized water. During the manufacturing process, the graphite thrust washer 1 is always kept vertically placed on the thrust washer bracket 3, and the graphite thrust washer 1 is always kept immersed in Class A water to keep the graphite thrust washer 1 in a moist state;
[0026] g. Cleaning and packaging: After completing step f, rinse the graphite thrust washer 1, clean it with alcohol and a dust-free cloth, and package and protect it.
Claims
1. A large shielded motor graphite thrust bearing anti-deformation and anti-pollution manufacturing process, comprising the following steps: a. Rough machining of the graphite thrust pad (1): Place the graphite thrust pad (1) on a surface grinder to grind the reference surface (2) of the graphite thrust pad (1) first, and then grind the working surface (5) of the graphite thrust pad (1). At this time, both the reference surface (2) and the working surface (5) of the graphite thrust pad (1) have machining allowances. After grinding, use a dust-free cloth dipped in alcohol to clean the graphite thrust pad (1); b. Cleaning tools: clean the thrust washer bracket (3), heating water tank (6), cooling water tank (7) and transfer storage water tank (4). After cleaning, place the thrust washer bracket (3) in the transfer storage water tank (4), add deionized water to the heating water tank (6), cooling water tank (7) and transfer storage water tank (4), and remove the deionized water after 12 hours if no rust is produced, and wipe dry with a dust-free cloth; c. Pumping water pressure on the graphite thrust washer (1): placing the thrust washer bracket (3) in the water pressure well (10), and then placing the graphite thrust washer (1) on the thrust washer bracket (3) in the water pressure well (10), installing the well cover (9) and fixing it with bolts (8), filling the water pressure well (10) with deionized water, using a water pressure pumping device to pressurize the water pressure in the water pressure well (10) to 15 MPa, and checking the pressure in the water pressure well (10) after 10 minutes. If the pressure in the water pressure well (10) does not decrease, it indicates that there is no leakage. The graphite thrust washer (1) maintains the water pressure at 15 MPa for at least 24 hours. After depressurization and drainage, remove the bolts (8) and the top cover (9), and transfer the thrust washer bracket (3) with the graphite thrust washer (1) from the water pressure well (10) to the transfer storage water tank (4). Deionized water is injected into the transfer storage water tank (4) for storage; d. Heating and cooling to relieve stress: inject deionized water of room temperature into the heating water tank (6) and the cooling water tank (7), place the graphite thrust washer (1) and the thrust washer bracket (3) into the heating water tank (6), start the power supply, heat the deionized water in the heating water tank (6) to 90°C-100°C, and keep the graphite thrust washer (1) in the required temperature range of 90°C-100°C for at least 15 minutes. After the graphite thrust washer (1) is kept in the hot water for at least 15 minutes, the graphite thrust washer (1) and the thrust washer bracket (3) are removed from the heating water tank (6) together and immediately placed in the cooling water tank (7) for cooling. The graphite thrust washer (1) is kept in the cooling water tank (7) for at least 60 minutes. After cooling for at least 60 minutes, the graphite thrust washer (1) and the thrust washer bracket (3) are removed from the cooling water tank (7) and placed in a transfer storage water tank (4) filled with deionized water. The deionized water in the heating water tank (6) and the cooling water tank (7) is drained, so that the graphite thrust pad (1) completes one heating and cooling stress relief, and the above operation is repeated once to perform a second heating and cooling stress relief; e. Grinding the graphite thrust pad (1): placing the graphite thrust pad (1) on a grinding and polishing machine to grind the graphite thrust pad (1), only one graphite thrust pad (1) is allowed to be ground at a time, first grinding the reference surface (2) of the graphite thrust pad (1), and then immediately grinding the working surface (5) of the graphite thrust pad (1); f. Rinsing the graphite thrust washer (1): Place the graphite thrust washer (1) on the thrust washer support (3) in the heating water tank (6), add deionized water to the heating water tank (6), take 100 ml of water sample before rinsing in the heating water tank (6) and measure the chloride ion concentration in the water sample, which is recorded as the chloride ion concentration before rinsing a, start the power supply, heat the deionized water in the heating water tank (6) to 55-65°C, keep the graphite thrust washer (1) in the temperature range of 55-65°C for at least 24 hours, take 100 ml of water sample after rinsing in the heating water tank (6) and measure the chloride ion concentration in the water sample, which is recorded as the chloride ion concentration after rinsing b, remove the graphite thrust washer (1) and the thrust washer support (3) from the heating water tank (6) and place them in a transport storage water tank (4) filled with deionized water; g. Cleaning and packaging: After rinsing the graphite thrust washer (1) in step f, use alcohol and a dust-free cloth to clean the graphite thrust washer (1), and then package and protect the thrust washer.
2. The anti-deformation and anti-pollution manufacturing process for graphite thrust bearings of large shielded motors according to claim 1 is characterized by: In step c to step f, during the manufacturing process, the graphite thrust washer (1) is always placed vertically on the thrust washer support (3), and the graphite thrust washer (1) is always immersed in deionized water to keep the graphite thrust washer (1) in a wet state.
3. The anti-deformation and anti-pollution manufacturing process for graphite thrust bearings of large shielded motors according to claim 1 is characterized by: In step f, if the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is not greater than 1.0 ppm, the flushing is considered qualified. If the chloride ion concentration b after flushing minus the chloride ion concentration a before flushing is greater than 1.0 ppm, another flushing is required until the required value is met.