Maintenance of medium plate mill_ Improvement of hydraulic fine adjustment system
Maintenance of medium plate mill_ Improvement of hydraulic fine adjustment system
The hydraulic fine adjustment system of medium plate mill is an important facility to improve the accuracy of plate thickness, control the shape of plate and improve the hit rate. After the hydraulic fine-tuning system of 2500 medium plate mill was put into use, the servo valve was jammed in less than one month due to design and use reasons, so it was replaced frequently. Through the analysis and corresponding improvement, the stable, controllable and high-quality operation of the system is ensured. The specific process is as follows.
(1) Main problems
There are several problems in the hydraulic fine adjustment system of medium plate mill
① The filtration accuracy can not meet the target accuracy requirements, the filtration ratio of the filter element is small, and the filter element is replaced frequently;
② There is not enough time to flush the system after installation and maintenance, so that the servo valve and hydraulic cylinder are polluted;
③ The charging pressure and installation position of the accumulator are not proper, which does not play its due role, and the impact and vibration of the pipeline are large;
④ The oil temperature is too high;
⑤ The hydraulic oil in the rod cavity of the fine adjustment cylinder is easy to be aged and deteriorated, and the cylinder block and piston rod are locally worn;
⑥ The spherical pad in the fine adjustment cylinder has poor lubrication and internal and external leakage;
⑦ Poor contact of electrical circuit and plug.
(2) Improvement measures
① To ensure the target cleanliness of the system, solid particle pollutants are the main causes of valve core sticking, wear and seal failure. The hydraulic fine-tuning system of plate mill is a high-pressure and large flow servo control system, which is very sensitive to solid particles. Especially for MOOG servo valve with large flow and high response, the radial clearance of valve core is 2 ~ 5 μ m. The filtration accuracy must be better than nas6. The measures taken are as follows.
a. An auxiliary oil tank is added to filter and store spare oil during refueling.
b. The precision of the filter element in front of the servo valve is increased from 10tlm to 5pm.
c. All filters should be replaced by filter elements with high pollutant carrying capacity and high filtration ratio to prolong the replacement cycle of filter elements and reduce the pollutants that may be brought in due to the replacement of filter elements.
d. When replacing hydraulic components, clean kerosene must be used to clean new components, tools and joints. Precision components such as servo valve, pressure sensor and force sensor must also be cleaned with precision electrical cleaning agent to prevent new pollutants.
e. When repairing or installing pipelines and joints, mechanical cutting and blanking shall be used, sub arc welding shall be used for backing, and then common surfacing shall be used to prevent welding slag and weld bead from entering into the pipes. Cutting and welding at will is strictly prohibited, and relevant standards for installation and flushing of hydraulic system pipelines shall be strictly implemented.
f. Before installing the servo valve, the flushing plate should be installed to flush the system for 3 ~ 4H.
② The hydraulic fine-tuning system of medium and heavy plate mill is a servo system with high pressure and large flow. The servo valve responds frequently, and the hydraulic cylinder adjusts quickly, resulting in great flow impact. The measures taken are as follows.
a. Check the charging pressure of the accumulator regularly. In the hydraulic station, as an auxiliary energy, the flow of accumulator should be greater than the difference between the maximum flow of the system and the working flow of the pump, and its charging pressure is about 0.80 times of the working pressure of the system; In front of the servo valve, an accumulator is set to absorb the pressure impact, and its charging pressure is about 0.85 times of the working pressure of the system; The main oil return pipe is equipped with an accumulator to absorb the flow pulsation, and the charging pressure is about 0.85 times of the system oil return pipe pressure.
b. In the pipeline design, the pipeline length to the servo valve should avoid the natural frequency of the pump as much as possible, which is an odd multiple of 1.14M, and the length difference between the two branches behind the double cylinder pressure filter in the station should be 1.14M, so as to realize H-type filtering vibration elimination.
c. The instability of the system is the main reason for the shock vibration of the pipeline. By adjusting APC adjustment coefficient and AGC adjustment parameters reasonably, the system has good stability and fast response.
d. Tighten the pipe clamp to make the pipe connection firm and reliable.
e. Cancel the two check valves behind the pressure filter in the station. The working characteristic of hydraulic fine-tuning is to adjust frequently near the target position, resulting in large flow and pressure fluctuations. As shown in Figure 4-25, when the servo valve reverses, the flow and pressure fluctuation generated by the servo valve propagates along the pipeline to check valve 3. Due to the isolation effect of the check valve, the accumulator loses its original damping effect. At the same time, the check valve is closed frequently, which not only produces larger noise, but also shortens its service life. After the improvement, a check valve is added, so that the flow and pressure fluctuation generated by the servo valve will be absorbed by the accumulator along the check valve 6, which increases the stability of the system.
③ The system has high working pressure, large flow and high ambient temperature, so the heat balance temperature is high. Strong cooling facilities must be adopted to force cooling, and the energy loss caused by overflow valve or safety valve should be minimized. The measures taken are as follows.
a. The pressure of medium pressure system is low (generally 3-5mpa) and the flow is small. If it shares the oil source with high pressure system, it is necessary to set pressure reducing valve to cause energy loss. Therefore, it is better to set oil supply system separately for medium pressure system.
b. The pipe type direct acting safety valve is easy to overflow, so the pilot type relief valve is used instead.
c. The station is equipped with water-cooled air conditioning to reduce the ambient temperature.
d. The maintenance of heat exchanger is a problem that is easily ignored. Due to the high temperature of oil before heat exchange, the quality of cooling water is the key to ensure the cooling efficiency. However, in general medium plate plants, there are many impurities in the common clean circulating water, and it is easy to scale at 40 ~ 50 ℃ for a long time, which greatly reduces the cooling efficiency. Therefore, the heat exchanger should be cleaned at least once a year to ensure cooling efficiency. It is better to set plate or tubular cooler with high heat dissipation coefficient, and select the cooler with appropriate heat dissipation area according to the heat balance conditions to ensure that the oil temperature does not exceed 550c.
e. Select the pump with excellent performance to reduce the energy loss caused by pump leakage.
④ The main causes of internal and external leakage of the system are the stuck valve core and seal failure, especially the internal and external leakage of hydraulic cylinder, servo valve and pressure sensor, which directly affects the stability and control accuracy of the system. The measures taken are as follows.
a. Hydraulic cylinder and other key seals are combined seals with high temperature and high pressure resistance and low friction.
b. The flange connection should be reduced as much as possible and welding should be used. Meanwhile, the flange and valve seat should be sealed with pure copper gasket.
c. The pipe type direct acting safety valve and the check valve behind the pilot pressure regulating valve for the accumulator in the station are canceled, and one of the pilot pressure regulating valves on the pressure regulating valve station is changed into the system safety valve.
d. The pressure sensor used to be sealed with cone surface inner hole, but the inner and outer joints of cone surface seal were not considered in the design of the system, and the end face combined gasket seal was directly used in the installation. Because the end face sealing surface is uneven, the contact surface is too few, and the wrench position is not enough, oil leakage often occurs. The manufacturer is required to improve the overall dimension of the pressure sensor, and change the combined gasket seal to red copper gasket seal (see Fig. z), the effect is very good.
⑤ The hydraulic oil in the rod cavity of the fine adjustment cylinder is aged and deteriorated, and the cylinder block and piston rod are worn locally. The system only makes hydraulic fine adjustment, not hydraulic reduction, so the liquid column is basically unchanged. The contact position between the piston rod and the cylinder block is in the same place for a long time, which causes local wear between the piston rod and the cylinder block, causes the leakage of the hydraulic cylinder, affects the stability and control accuracy of the system, and even makes the hydraulic cylinder scrapped. At the same time, the hydraulic oil with rod cavity can not be replaced in time, which leads to oil aging and deterioration, affects the service life of seals, and causes pitting damage on the surface of piston rod and cylinder block. The measures taken are as follows.
a. Change the height of hydraulic cylinder once a month.
b. Two high-pressure stop valves (see Fig. a) are set on the oil inlet pipe of the rod cavity to realize the replacement of the hydraulic oil in the rod cavity. The method is as follows: the hydraulic fine-tuning system opens the parking space, sets the hydraulic cylinder liquid column value to the maximum, closes the oil inlet stop valve, opens the oil discharge stop valve system working position, exhausts the hydraulic oil with the rod cavity to the oil tank, opens the oil inlet stop valve, closes the oil discharge stop valve, opens the hydraulic fine-tuning system parking space, completes the oil change operation.
⑥ The function of spherical pad is thrust and self-aligning. Under the action of over balance force, the hydraulic cylinder and the screw down screw are in reliable contact through the automatic centering of spherical pad. Due to the limitation of installation position, the spherical pad can only be placed in the piston rod of fine-tuning cylinder. At the same time, due to the wear of the screw down screw rod and screw down nut, and the screw down screw rod and spherical pad, a large number of copper filings are accumulated at this place, resulting in the screw down screw rod and spherical pad biting and bonding, and the spherical pad losing its original function. If it is serious, the piston rod of fine adjustment cylinder will rotate, the seal and displacement sensor will be damaged, and the mechanical screwdown device will be worn prematurely, as shown in Figure B. Therefore, it is necessary to remove the aged lubricating oil, copper scraps, oil sludge and other pollutants regularly. This is a very trivial work, but it has to be done, because it is very important. In addition, due to the very bad environment in this area, although the telescopic dust cover is set up, the dust, smoke, water vapor and other debris are still easy to enter the rod cavity of the fine-tuning cylinder, which accelerates the wear of the rod cavity and the damage of the seal. By injecting clean compressed air into the dust cover, it can effectively prevent the entry of dust, smoke, water vapor and other debris.
⑦ Improvement of poor electrical circuit grounding and plug contact poor electrical circuit grounding and plug contact will seriously affect the stability and control accuracy of the system, resulting in roll gap drift and abnormal vibration of hydraulic system pipeline. Regularly check whether the electrical circuit is grounded and whether the plug is in poor contact. In addition, the system should be equipped with voltage loss protection power supply to prevent the loss of the whole computer system program due to sudden power failure.
(3) Effect of improvement
After the improvement, the cleanliness of the oil is controlled above nas6 level for a long time, and there is no fault such as servo valve stuck, hydraulic pump and seal premature damage caused by oil pollution. The impact vibration of the pipeline is obviously reduced, and the pipe collapse and leakage caused by the impact vibration are rare. When the oil temperature is controlled between 40 ℃ and 50 ℃, the service life of the hydraulic pump, servo valve and fine-tuning cylinder can reach more than 2 years, the hit rate of the same plate difference of 10 mm steel plate controlled within + 0.10 mm can reach more than 95%, and the start-up rate of the system can reach more than 99%, which realizes the controllable, reliable, low consumption and stable operation of the system.