Hydraulic automatic roll gap control, pressure pulsation and noise elimination

1. pressure pulsation and noise generation

Hydraulic automatic roll gap control (AGC) system used in hot rolling mill and cold rolling mill in metallurgical system. The response frequency of hydraulic cylinder is generally 10~25Hz, and the stroke of hydraulic cylinder is usually 20~50mm. Also have long stroke hydraulic cylinder, but generally not more than 200mm. The system adopts constant pressure variable pump, constant pressure oil source of overflow valve, setting pressure of overflow valve is 20~30Mpa, and the diameter of hydraulic cylinder varies from 500~1500mm. The pipeline between each section of the system is different, and the speed of each section of the pipeline is below 16m/s.

When the valve current is transmitted to the electro-hydraulic valve, the displacement of the slide valve of the servo valve is proportional to the signal current. If the input signal is a sinusoidal signal current, then the spool valve is closed at the same frequency as the sinusoidal signal. In the servo shut down process, the pressure in the tubing changes before the servo valve and the servo valve. Oil pressure servo valve closed, the kinetic energy of oil pipe is converted into pressure energy, so close to the servo valve before the tubing pressure rise; servo valve opening pressure of oil, if the hydraulic cylinder load, is close to the servo pressure before the tubing down stuffy. When the servo valve closes the oil circuit, the pressure in the oil pipe is close to the front of the servo valve due to the inertia of the oil flow; when the servo valve opens the oil return circuit, the pressure in the oil pipe is raised close to the servo valve. Similarly, in the hydraulic pipe, because in the rolling process, frequent movement of the hydraulic cylinder, servo valve is also a frequent opening and closing, pipeline pressure change frequently, and a band followed by a band to occur frequently, namely pressure servo valve before and after the pressure wave propagation along the pipeline to form.

At the scene, the general hydraulic cylinder and the servo valve are installed in the mill, away from the oil source are between 60~80m, and the pressure wave speed is 1300m/s, this calculation, when the frequency response of the servo valve 1Hz, servo valve 1s in the opening and closing time, without considering the energy loss 1s, the pressure in the pipeline along the AGC spread a dozen times, reducing the oil can be excluded and the pipeline will cause energy loss, the pressure wave will gradually disappear with the liquid flow. But because in the rolling process, the hydraulic AGC is on the strip thickness control, as long as the strip thickness deviation detection, the feedback signal will immediately transmitted to the servo boring, servo valve will immediately adjust the valve opening and the opening and closing direction, in order to adjust the hydraulic cylinder pressure volume. Therefore, there are many individual wave and reflection waves in the system pipeline, that is, any pressure on the pipeline is a composite wave superimposed by multiple incident waves and reflected waves. In the pipeline, some small amplitude wave position, but in some locations and a large amplitude, resulting in considerable pressure fluctuation, pressure pulsation and large amplitude once the effect will cause severe mechanical vibration in the pipeline, and spread through the air, and violently noise.

2. measures to eliminate pressure pulsation and noise

The vibration and noise of hydraulic AGC system is quite a problem. Elimination of vibration and noise in the system, usually in the design and practice of the following methods.

(1) as far as possible to shorten the distance between the servo valve and the hydraulic cylinder, in order to eliminate the vibration and noise of the pipeline, when the hydraulic AGC system is used, if the field conditions permit, the servo valve can be installed directly on the hydraulic cylinder. According to the information, when the pipe length is shortened from 6m to 3m, the frequency response of system under pressure can be increased from 10Hz to 15Hz, and reduce the pressure pulsation and pressure pipe shrinking and expanding the shock, thereby reducing the vibration and noise of pipeline.

(2) the accumulator is installed with different sizes of accumulators before and after the valve of the servo valve, and the accumulator is charged with different pressure according to the installation position. Practice has proved that the accumulator installed on the branch pipe of the oil return pipeline can reduce the vibration and noise. This is because the servo valve switch oil return pipeline, pressure pulsation of the servo valve after the oil return pipeline has the large value, the pressure pulsation and back pressure pipeline of the average value of large, more important is the change of the pressure pulsation pressure amplitude rate of dPdt is quite large, so with the same pressure pipeline, a have strong vibration and noise on the return line. An accumulator is arranged on the branch pipe of the return oil pipeline which sends out noise and vibration, and can effectively absorb the pressure pulsation. Similarly, in the servo valve before and after the installation of different sizes of accumulators, can make vibration and noise significantly reduced.

(3) design flow capacity of the filter in order to ensure the performance of the servo valve and meet the oil cleanliness requirements in the hydraulic AGC system, servo valve and the system back to the oil pipeline have chosen different specifications of the filter. However, due to the pressure pulsation before the servo valve and oil return pipeline caused by large amplitude, when the filter through the filter, if the flow capacity is limited, will also pressure pulsation and noise amplitude generated in the filter. Therefore, in the hydraulic quasi AGC system, the flow capacity of the filter before the servo valve should be greater than 2 times the maximum flow of the pump. On the oil return line, the flow capacity of the filter shall be greater than 2 times of the maximum flow of the return line. This reduces the pressure shock caused by the pressure change caused by the filter.

(4) one of the effective methods to reduce the pressure pulsation of the hydraulic AGC system and to absorb the pressure fluctuation of the system is to set up a filter in the pipeline. The principle of increasing the volume of a pipe is similar to that of an accumulator. When the flow of the pipeline exceeds the average flow, the liquid is compressed, and the filter unit stores excess liquid. When the flow is below average, the excess liquid is released. This increases the volume of the pipe to absorb the pressure pulsation caused by the flow pulsation. At present, the filter device used to increase the volume of the pipeline, in addition to accumulators, there are various filters and Quineck tubes designed by interference principle. However, the most widely used forms of practice are their combinations. It has been proved that in the hydraulic AGC system, the installation of Quineck tube with the accumulator in the near position of the inlet and outlet of the servo valve makes it possible to absorb the effects of pressure pulsation and noise.

(5) a pulsating damper is added to the pipeline of the hydraulic AGC system. The pulsation damper designed according to the principle of the expansion chamber muffler is shown in figure 3-7. The pulsation damper are connected in series in a hydraulic pipeline, when has the pressure pulsation of the liquid flowing through the large pipes of different diameters and the cavity, the quality of liquid in the conduit form liquid, the sense of sound, sense) and its full chamber compressible liquid form liquid (Acoustic) visible. As in the circuit of LC filter, as long as the liquid damper and fluid volume, well, in a hydraulic pipeline can play very well to filter noise reduction effect.

It has been proved by experience that the pulsation damper with the cut-off frequency below 70% of the pulse frequency is best to be added to the AGC hydraulic system.

(6) the pipeline of hydraulic AGC vibration isolation measures in hydraulic AGC system and the camp between inlet and outlet pipe with rubber damping pad, can be used to alleviate the vibration of the pipeline, the rubber pad is often used, in between the hydraulic cylinder and the servo valve between the pump outlet and the camp road system, and between the pressure pulse of relatively large pipeline use; hose connection, can also play the purpose of reducing vibration.