Wang Yanhua
(Jiangshan Southern Cement Co., Ltd.)
Abstract: The piping and valve arrangements at the slide-shoe lubrication stations of two Φ4.2m×13m mills presented significant safety hazards and operational inconvenience. The arrangements were optimized to eliminate hazards that could burn the slide-shoe bearing bushes, reduce inspection staff operating and maintenance workload, and improve equipment reliability. The modification was implemented and put into service in April 2020, achieving the expected results.
1 Oil supply status of skid oil station and existing safety hazards
1) oil supply status of oil stations and sliding shoes
Our company has two Φ4.2m × 13m sliding shoe mills. Each mill is equipped with an oil station. Each oil station has a working oil pump that supplies oil to the sliding shoe bearings in three ways. Two of them supply oil to the two sliding shoes, and the other one supplies oil to the rolling ring at the top of the sliding shoe. The main outlet pipe of the working oil pump is divided into three branch pipes leading to each oil supply point (see figure 1).
Figure 1 Working principle of the oil station before transformation
Each branch pipe has an internal nominal diameter of DN25. The oil receiving points for the two slide shoes are both oil trays, into which the branch pipes deliver oil. Oil is supplied to the upper riding ring through multiple holes distributed across its width. Consequently, resistance at all three oil supply points is very low (oil pressure is below 0.1 MPa when the branch ball valves are fully open). To ensure sensitive loss-of-pressure protection at the lubrication points by the lubrication station's PLC control system, the main machine is generally permitted to start only when oil pressure is greater than or equal to 0.18 MPa. Therefore, before shipment, an adjusting ball valve is fitted to each of the 3 branch pipes at the lubrication station's supply outlet. By partially closing these valves, oil pressure is artificially raised above 0.18 MPa to meet PLC control requirements. The valves also allow the oil flow to each branch to be adjusted.
2) problems
Oil station operations are difficult and require high skills from employees. The newly installed oil station has been commissioned and put into operation. In order to prevent the ball valves on the 3 branches from being misoperated, the ball valve handles are removed during normal production operations. However, in the following two situations, it is necessary to re-operate and adjust the ball valves on the 3 branches:
First, in the low temperature season in winter, the oil in the oil station needs to be heated to improve its fluidity before the mill is started. By closing the 1, 2, and 3 valves on the branch, opening the pressure relief valve 4, and starting the oil pump to form an internal circulation of the oil in the oil station, the lubricating oil can be heated up;
Second, during the grinding shutdown and maintenance process, it is also necessary to operate 3 branch ball valves during the process of replacing the oil in the oil tank and filtering impurities in the crude oil.
In the above two cases, after the relevant work is completed, the oil volume distribution and oil supply pressure of the three branches need to be readjusted to ensure the safe operation of the mill, which is very inconvenient.
During the operation of the 3 branch ball valves, due to employee operating skills and other individual factors, it is possible that one of the 3 oil supply points is not opened, or is only opened a little to meet the bearing lubrication needs. However, the pressure of the pressure transmitter on the oil pump main pipe is within the normal range (≥0.18MPa), and the oil station allows the start signal to be sent out, which may easily cause a burning accident during the operation of the mill.
2 Oil station renovation plan
The goal of the modification is to make the valve easier to operate and increase the convenience of operation and maintenance. Therefore, we make our own pressure regulating valve to stabilize the oil pressure; we also add a pressure gauge on the oil line to facilitate maintenance operations.
1) homemade oil pressure regulating device
A self-made pressure regulating valve is made by simply modifying the original DN25 union joint on each branch, and reducing the inner diameter of the joint to regulate pressure, as shown in the picture 2. Turn the protruding part of the semi-union joint flat so that the sealing gasket can be placed smoothly after the two semi-union joints are closed, and replace the original O-ring with a PTFE gasket seal. First, a 4.5mm PTFE gasket with an inner diameter of Φ20mm was used to fully open the 3 original DN25 ball valve. After starting the oil station, it was found that the oil pressure in the oil line was only about 0.12MPa, which was low.
Finally, the inner diameter of the gasket was reduced to regulate the pressure, and the test was re-tested. After many debuggings, when the inner diameter of the gasket was finally Φ14mm, the pressure gauges of each branch showed that the oil pressure was 0.22MPa, which met the requirements. After the self-made pressure regulating valve is completed, the oil pressure of the three branches is adjusted to the normal control value of 0.20~0.25 MPa, completely replacing the DN25 ball valve on the branch.
Figure2 Union joint (pressure regulating device) before and after modification
2) Oil station fuel supply pipeline renovation
Figure 3 shows the operating principle of the modified oil station. Remove the original DN25 ball valves in the branch pipes and replace them with the homemade pressure-regulating valves described above (a, b, and c in Figure 3). Add 0.6 MPa dial pressure gauges 6, 7, and 8 to the 3 branches, respectively, so that oil supply to each branch can be checked easily during on-site inspections. Add a main ball valve 5 to the oil-supply outlet header, upstream of the pressure transmitter and downstream of pressure relief valve 4.
During normal mill production, valve 5 is fully open. The pressure regulating devices on each branch keep oil circuit pressure within the values permitted by the automatic control system, and the lubrication station operates normally. When the oil in the tank needs heating or replacement and the lubrication station must operate in internal circulation mode, simply open valve 4 and close valve 5. The pressure transmitter then indicates 0, and no main-machine operation permission signal is present. The mill cannot start because it is protected by the lubrication station's controls. To restore normal lubrication station operation, reverse the positions of valves 4 and 5.
Figure3 Working principle of the oil station after renovation
3 The effect after transformation
(1) Safety hazards in the slide-shoe bearing lubrication system were eliminated, greatly improving oil-supply reliability. Before modification, the pressure transmitter was upstream of the 3 branch ball valves. If one or more branch valves were closed, the slide-shoe bearings could receive insufficient oil or none at all, yet the transmitter would show no pressure drop and the automatic system would not trigger protection. After modification, valve 5 is upstream of the transmitter. If it is closed or insufficiently open, the transmitter immediately indicates loss of pressure, triggering the control signal. The main motor is then prevented from starting or, if the mill is running, de-energized to stop it.
(2) The valve is easy to operate, reducing the workload of operation and maintenance workers. In winter, when the temperature is low, the oil station needs to perform internal circulation heating before starting the machine. The two valves 4 and 5 can be operated, eliminating the need to re-adjust the size of the 3 ball valves in each branch before modification.
4 Conclusion
As an important mechanical equipment for cement production, the operational reliability of the main mill is very important. The safe and reliable operation of the oil supply system of the oil station is the key to ensuring the reliable operation of the mill. The transformation has been put into operation for nearly two years. It is easy for employees to operate. The oil supply system protects the operation of the mill and is very reliable. It has received good results and has been simultaneously promoted and transformed on other mills.
Source: "Cement" 2022 (9)
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