Cement ball mill discharge end cover replacement and bearing heating treatment

Liang Liqiang Rong Jianyong

(Hebei Luquan Jinyu Dingxin Cement Co., Ltd.)

Abstract: Ф4m×13m When replacing the discharge end lining plate of the cement mill, it was found that there was a long penetrating crack in the screw passage hole of the end cover plate. The discharge end cover was replaced, a heat insulation sleeve was installed in the hollow shaft of the grinding end, and cooling air was introduced to cool down, which effectively controlled the problem of high temperature of the grinding end bearing bush.

1 Mill problems and causes analysis

1#, a branch of our company, has cement mill specifications Ф4m×13m, double-silo tube mill, and circle-flow production. During the production, the machine was shut down for maintenance due to severe vibration, and it was found that the discharge end cover plate had a circumferential penetrating crack at the edge of the screw hole (see Figure 1). The total length of the crack was about 2.5m, and it had become a circle, accounting for nearly 1/3 of the entire ring.

After comprehensive analysis, it is believed that there are internal casting defects during the casting process of the components, and local stress concentration occurs during the operation of the mill, causing fatigue cracks. This grinding system has a high frequency of start and stop due to peak adjustment, and the residual stress and alternating loads used can easily lead to fatigue of the base material and cracks and other problems.

Figure 1 End cover plate cracks

2 problem handling

2.1 end cover plate including barrel nipple customization and replacement construction

Customized new end cap plates. According to the specifications of the ball mill barrel, the outer edge of the discharge end cover plate is circumferentially welded to the barrel, and the discharge end direction of the barrel is welded with steel plates of two thicknesses: 60mm and 46mm respectively. Select the barrel nipple specification Φ4.12m×1.18m, wall thickness 60mm, material Q245R; end cover plate Φ4.12m, thickness 120mm, material Q245R.

For the overall construction of the replacement of the end cover plate, because the ball mill is placed in the factory building, the space is limited and a crane cannot be used. The winch is reasonably set up according to the characteristics of the building for large-scale hoisting operations. In the early stage, the original data such as the radial and axial runout of the hollow shaft of the ball mill are measured and kept. Because the new pup joint has its own Pyramid brace device near the interface, and the pre-cut of the old barrel is closer to the compartment frame, there is no longer a separate anti-deformation Pyramid brace near the interface.

(1) Marking the shell cutting line. The cutting line on a ball mill shell must be marked according to the actual relevant dimensions of the replacement part, allowing for the bevel gap and the difference in the width of the shell section to be removed so that the line is positioned accurately. First mark the cutting line: at 1 m intervals around the intended cut, calculate the precise distance as described above and measure from the machined face of the end flange to mark each point. Connect the points to form the cutting line. Because the mill can slide axially during rotation, using the auxiliary drive to rotate it slowly while marking is not recommended. Then use the cutting line to mark the reference line, bevel line, and cutting-machine track line in sequence.

(2) Barrel cutting. A jack grinding device and a dial indicator are set up near the pre-notch of the ball mill. A dial indicator is also set up on the hollow shaft to monitor the pressure-bearing condition. After jacking up to a reasonable position, a safety frame is set up. Make sure there is enough axial space for lifting the pup joint, and then use a flame type pipe cutting machine to cut the cylinder according to the cutting line. Remove the old end cover plate including the barrel nipple, and then use the baseline to review the cut surface to prevent horseshoe mouth.

(3) Hoist the new cylinder (see Figure 2). After the new cylinder is initially in place, the axial adjustment screws and radial adjustment wedges are evenly divided into groups at 8 points on the inner surface for fine adjustment (see Figure 3). The misalignment of the circumferential welds on the inner wall is controlled to be ≤1 mm, and the gap between the two sides is left (2±1) mm. This ensures that the axes of the bolt holes of the cylinder lining plates on both sides of the interface are consistent.

(4) The hollow shaft of the grinding body falls back. After the adjustment of the interface between the old and new cylinders is completed, the connecting plate can be welded for reinforcement. Operate the top grinding device, lower the grinding body hollow shaft back to the spherical pad, reinstall the bearing oil supply device, etc., use the auxiliary transmission circumference equally divided 8 point to detect and control the relative radial circular runout tolerance of the two hollow shaft journals ≤ 0.2mm (if the value exceeds the tolerance requirements during the detection process, the corresponding comprehensive fine-tuning barrel interface adjustment component can be used.

Figure 2 Hoisting the new cylinder

Figure 3 Adjustment components at the interface between the old and new cylinders

(5) Welding the joint between the new and old shell sections. The key to welding is controlling thermal distortion. Use carbon dioxide gas-shielded welding. During welding, retain the radial and end-face dial indicators on the hollow shaft and continuously monitor for deformation or displacement. Adjust the welding positions and methods flexibly according to changes in the indicator readings. The general principle is to weld the outside of the groove first, working progressively in diagonally opposed sections and layers. Then back-gouge, grind and weld the inside of the groove. After welding is complete, perform ultrasonic flaw detection. The quality assessment must meet the requirements of Level II.B in GB/T11345.

In order to improve the quality of the workpiece, reduce the brittleness of the weld and make it have a certain degree of toughness, the cylinder weld is subjected to low-temperature annealing heat treatment (see figure 4) to eliminate the residual stress in the steel parts. Slowly heat to 500~650°C. After a period of heat preservation and then natural cooling in the air, after heat treatment, the service life of the abrasive parts is greatly improved, and the production efficiency is improved.

Figure 4 Weld heat treatment

After the weld is cooled to normal temperature, the workpiece is inspected for flaw, and the relative radial circular runout tolerance of the two hollow shaft journals is re-tested again. After it is correct, the fixing bolts connecting the hollow shaft and the end cover plate are re-tightened according to the design torque requirements, and then the feeding device, cylinder liner and other auxiliary parts are reassembled. Finally, according to the test operation requirements of the ball mill, test run and other acceptance checks are carried out.

2.2 grinding tail bearing bush temperature problem and solution

After replacing the end cover plate, the grinding end bearing bush temperature is high (exceeding 65℃) and the grinding stop occurs. Especially when the temperature is high in summer and the mill is running continuously, when the bearing temperature is set to 70℃, the grinding stop still occurs, which seriously affects the production of the mill. We solve this problem by installing a heat insulation sleeve inside the hollow shaft.

Using 10mm-thick Q235 steel plate, first roll a heat-insulating sleeve body in two halves. Then form a 60° conical flanged inlet ring, whose conical shape reduces resistance and helps material flow, and weld it to the sleeve. Temporarily cut a round or semicircular opening at the corresponding location on the outside of the discharge hood. Make it slightly larger than the sleeve for access. Insert the sleeve through the opening and weld the complete assembly together. Also weld 3~4 helical vanes along a 400mm length of the sleeve's inner wall near the mill end.

The original spiral cylinder blades (1/3 whose length is only the length of the spiral cylinder) do not need to be cut off and can be used as the support part of the heat insulation sleeve.

Generally, a larger insulating gap provides better thermal insulation but affects material passage. The outside diameter of the insulating sleeve can be determined from the screw flight height (80–100mm). An insulating gap of 100mm is appropriate for ease of installation. Replace the original calcium silicate board filling with asbestos to improve insulation. In addition, install a DN50 galvanized pipe in the gap between the original screw barrel and the insulating sleeve as a cooling-air duct. Pass the pipe through the discharge hood and into the insulating sleeve to a length of 2/3 of the original screw barrel, ensuring that it does not touch the screw flights. Weld and secure the other end to the discharge hood.

The fixed cooling duct in this way can naturally introduce outside normal temperature air into the circumferential gap of the hollow shaft heat insulation sleeve that rotates with the mill, so as to reduce the air temperature in the heat insulation gap. After taking the above measures, the temperature of the tail grinding bearing bush was reduced by 5℃, ensuring normal production.

3 Conclusion

The replacement of the discharge end cover plate of the cement ball mill including the barrel nipple effectively increases the overall service life. The overall project implementation provides a good case for the repair, maintenance and transformation of the end cover plate of ball mills with this structure in the same industry. After installing the heat insulation sleeve, the bearing temperature was reduced by at least 5°C, which significantly improved the operating conditions of the mill. The mill did not stop due to high bearing temperature.

Because welding was performed on site and affected by the surrounding environment, manual welding was used and the project was difficult. Despite the uncertainty of welding deformation during the actual work, final dial-indicator measurements showed radial runout deviations at the outer surfaces of the mill-head and mill-tail hollow shafts within ±0.30 mm, meeting operating requirements. A digital ultrasonic flaw detector was used to inspect 100% of the welds, and the quality was acceptable. Strict control of every key stage allowed the work to proceed smoothly and finish successfully, eliminating a major equipment hazard in the cement mill.

In summary, the end cover plate failure problem can be solved by carefully designing according to the stress conditions and different characteristics of the mill end cover plate, with a scientific attitude, strict management, and strictly controlling every step of the end cover plate design, manufacturing, installation, and use.

Source: "Cement"

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