Whether the vibration sensor is installed correctly is not only directly related to whether it can obtain correct and valuable measurement data, but also the incorrect installation will cause damage to the vibration sensor. When using portable vibration instruments to measure the vibration of the equipment, because it is a short-term temporary measurement, it often fails to pay attention to the correct installation of the vibration sensor, causing significant test errors, which causes troubles in vibration fault diagnosis and shafting balance. In order to improve the accuracy and reliability of equipment vibration testing. Below we will introduce the correct installation methods and key points of some vibration sensors.
Eddy current type vibration sensor
The installation of the eddy current transmission type vibration sensor is the most complicated one, and the following points should be noted:
1. Working temperature
The maximum allowable temperature of the general eddy current type vibration sensor should be less than 180 degrees. At present, the maximum allowable temperature of domestic vortex type vibration sensors is mostly below 120 degrees. In fact, when the operating temperature exceeds 70 degrees, not only its sensitivity will be significantly reduced, but Will cause damage to the sensor. Therefore, when measuring the high, medium and low shaft vibration of the steam turbine, the vibration sensor must be installed in the bush, and only the special high-temperature type eddy current sensor is allowed to be installed near the air seal.
2. Avoid cross induction and lateral clearance
When two vertically or parallel mounted vibration sensors are close to each other, cross-sensing occurs between them, which will reduce the output sensitivity of the sensor. To avoid cross-sensing, the two sensors cannot be too close. For different types of eddy current vibration sensors, the value of the distance between the two sensors is required to be different. Too small lateral clearance is mainly due to the presence of conductors on both sides of the sensor head, which significantly reduces the output sensitivity of the sensor. The correct lateral gap b should be greater than or equal to d (the diameter of the coil at the top of the sensor). The direction-finding gap must not only consider the cold state, but also the expansion change of the cylinder and rotor after heating. The exposed height c of the sensor head is generally not specified, but according to the field use, too small c will also significantly reduce the sensitivity of the sensor.
3. Avoid resonance and loosening of the structural support
The vibration frequency of the sensor bracket must be higher than the frequency corresponding to the highest speed of the device, otherwise the measurement result will be distorted due to the resonance of the bracket. The US CTC factory stipulates that the natural vibration frequency of the vibration sensor bracket in the vibration measurement direction should be higher than the maximum operating frequency of 10 times of the machine. This is often difficult to achieve in actual operation. The general vibration frequency of the bracket measurement vibration direction is higher than 2 ~ 3 times The working frequency of the speed can basically meet the vibration measurement requirements.
In order to improve the natural vibration frequency, the structural support is generally made of 6-8mm thick flat steel, and the length of the cantilever should not exceed 100mm; when the cantilever is long, the steel should be used, such as angle iron, I-beam, etc., in order to be effective Increase the natural frequency of the bracket.
In order to prevent the bracket or vibration sensor from loosening during the test, the bracket must be fastened to a stable support member, preferably fixed on the bearing pad or bearing seat. The connection between the sensor and the bracket should be tapped on the bracket, and then twisted with a nut Tighten, do not use double nuts to tighten the holes on the bracket.
There have been too many accidents in China due to the lateral resonance of the side bar of the eddy current type vibration sensor, which caused the abnormal display of the shaft vibration. This resonance mainly occurs on the medium- and low-voltage, generator rotors and bushes with long side bars. There are two forms of vibrating rod resonance. One is during the speed-up process (generally the rotation speed reaches 24000r / min or more). The vibration of a certain measuring point of the shaft vibration increases with the rotation speed and increases rapidly to produce a jump, but the tile There is no big change in the other measuring point and tile vibration; another form is to cause unstable resonance at 3000r / min speed. The resonance of the vibrating rod is relatively easy to judge during the speed increase, and the unstable resonance at 3000r / min is due to the unstable connection between the vibrating rod and the bearing cover and the lateral natural vibration frequency of the vibrating rod is close to 50Hz, thus at 3000r / min. The amplitude of the vibrating rod is large and small, causing the amplitude of the shaft to fluctuate greatly. But sometimes the shaft vibration is displayed normally, so people often mistakenly think that the equipment vibration is abnormal.
4. Correct initial gap
Various models of eddy current type vibration sensors should have a good linearity at a certain gap voltage (the gap between the top of the sensor and the measured object, indicating the general voltage on the meter), so the vibration is installed. The sensor must be properly adjusted for the initial gap. The static maximum range of the eddy current sensor cannot be greater than 2.5mm. In order to obtain better linearity under dynamic conditions, the working gap should be within the range of 0.3 ~ 2.8mm, that is, the gap voltage indicated by the meter is 2 ~ 16V.
After the rotor rotates and the equipment is loaded, the rotor will move relative to the sensor. If the vibration sensor is installed on the top of the bearing, the gap will be reduced; if installed in the horizontal direction of the bearing, the gap depends on the direction of rotation of the rotor; when the direction of rotation is fixed, the gap depends on whether it is installed on the right or left. In order to obtain a suitable working clearance value, the rotor should be estimated from static to working speed during installation. The journal lift is about one-half of the top clearance of the bearing bush; the horizontal displacement and the form of the bearing bush, the clearance on both sides of the bearing bush and the unit sliding pin 0.20mm. The sensor is installed in the right horizontal position. After the rotor rotates, the gap c increases; when it is installed on the left side, d decreases.
The removal of the journal in the bearing shell is not only related to the speed, but also to the active load of the equipment. For low-pressure steam turbine high-pressure rotors and rotating shafts with reducers, under the action of part of the steam inlet and gear transmission torque, the cervical spine will be pushed to the side of the bearing pad, and the displacement value may be close to the diameter gap of the bearing pad.
When adjusting the initial gap of the sensor, in addition to these factors, the maximum vibration value and the original sway value of the rotor must also be considered. The initial gap of the sensor should be greater than half the maximum amplitude that the shaft can occur and the original sway value of the shaft.
5. Axial position selection
From the requirements of measuring shaft vibration, the shaft vibration measurement point should be as close as possible to the center of the bearing bush, but it is often limited by the installation location of the eddy current type vibration sensor, and sometimes has to maintain a certain distance from the bearing bush. According to the on-site vibration test, as the distance between the shaft vibration point and the bearing pad increases, the shaft amplitude value will increase. The axial position of the vibration measurement point of the shaft should also be considered to change the processing accuracy of the rotating shaft and whether the surface magnetic permeability of the rotating shaft is uniform. Before the formal installation, it is better to check the sway value at this point with a dial indicator.
If the sway value is greater than 50Î¼m, another measurement point should be selected, otherwise the oscillating value of the low-speed shaft vibration will be too large, and the displayed value of the high-speed shaft vibration will not appear. The magnetic permeability of the surface of the rotating shaft is uneven, and it cannot be judged directly with the naked eye. Only after the large value of the shaft sway is sent out due to the excessive mechanical sway, can it be determined that the excessive vibration of the shaft at low speed is caused by the surface If the magnetic permeability is not uniform, to eliminate this fault, only the location of the measuring point is selected.
6. Radial position selection
According to the requirements of the ISO DIS7919 / 2 specification, the installation of the shaft vibration sensor should satisfy that the two bearing sensors are on an axial plane and are perpendicular to each other.
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