Discussion on the installation of vibration sensor

Discuss the installation of vibration sensor

whether the installation of vibration sensor is correct or not is not directly related to whether the correct and valuable measurement data can be obtained, and incorrect installation will also cause damage to the vibration sensor. When using portable vibration instruments to measure the vibration of equipment, due to the short-term temporary measurement, we often do not pay attention to the correct installation of vibration sensors, resulting in significant test errors, which brings trouble to vibration fault diagnosis and shafting balance. In order to improve the correctness and reliability of equipment vibration test. Next, we will introduce the correct installation methods and key points of some vibration sensors

vortex type vibration sensor

the installation of vortex type vibration sensor is the most complex one, and the following points should be paid attention to:

1. Working temperature

generally, the maximum allowable temperature of vortex type vibration sensor should be less than 180 degrees. At present, the maximum allowable temperature of domestic vortex type vibration sensor is mostly below 120 degrees. In fact, when the working temperature exceeds 70 degrees, not only its sensitivity will be significantly reduced, And it will also cause damage to check whether there is a glued sensor. Therefore, when measuring the vibration of high, medium and low rotating shafts of steam turbine, the vibration sensor must be installed in the bearing bush, and only the special high-temperature eddy current sensor is allowed to be installed near the gas seal

2. Avoid cross induction and lateral clearance

when two vibration sensors installed vertically or in parallel are close to each other, cross induction will occur between them, which will reduce the output sensitivity of the sensor. In order to avoid cross induction, the two sensors should not be too close. For different types of eddy current vibration sensors, the distance between the two sensors is required to be different. Too small lateral clearance is mainly due to the existence of conductors on both sides of the sensor head, which significantly reduces the output sensitivity of the sensor. The correct lateral clearance B should be greater than or equal to D (coil diameter at the top of the sensor). The direction finding clearance should consider not only the cold state, but also the expansion change of the cylinder and rotor after heating. Generally, there is no specific regulation on the exposed height C of the sensor head, but according to the field use, too small C will significantly reduce the sensitivity of the sensor

3. Avoid resonance and looseness of the structural support

the vibration measurement frequency of the support of the sensor must be higher than the frequency corresponding to the maximum speed of the equipment, otherwise the measurement result will be distorted due to the resonance of the support. CTC factory of the United States stipulates that the natural vibration frequency of the vibration sensor support in the vibration measurement direction should be higher than the maximum working frequency of the machine, which is often difficult to achieve in practical operation. Generally, the natural vibration frequency of the support in the vibration measurement direction is higher than the rotating speed working frequency of 2~3 times, which can basically meet the vibration measurement requirements

in order to improve the natural vibration, the price of the former is lower than the frequency of the latter. The structural support is generally made of 6~8mm thick flat steel, and its cantilever length should not exceed 100, but our company has given a very positive evaluation mm; When the cantilever is long, profile steel, such as angle iron, I-beam, etc., should be used to effectively improve the natural vibration frequency of the support

in order to prevent the support or vibration sensor from loosening during the test, the support must be fastened to the support component with good stability, preferably fixed on the bearing bush or bearing seat. The connection between the sensor and the support should be tapped on the support, and then tightened with nuts. Do not drill holes on the support and tighten with double nuts

there have been many accidents in China that cause the lateral resonance of the side crossbar of the eddy current vibration sensor, resulting in abnormal shaft vibration display. This resonance mainly occurs on the medium pressure, low pressure, generator rotor and bearing bush with long side bars. There are two forms of vibration measuring rod resonance. One is that in the process of speed increase (generally the speed reaches more than 24000r/min), the vibration of a certain measuring point of shaft vibration increases rapidly with the increase of speed, resulting in tripping, but there is no significant change in the other measuring point of the bearing and bearing vibration; Another form is unstable resonance at 3000r/min speed. It is easy to judge the resonance of the vibration measuring rod during speed increase, and the unstable resonance at 3000r/min speed is caused by the unstable connection between the vibration measuring rod and the bearing cover and the lateral natural vibration frequency of the vibration measuring rod is close to 50Hz, which causes the amplitude of the vibration measuring rod to increase and decrease at 3000r/min speed, making the amplitude of shaft vibration fluctuate greatly. However, sometimes the shaft vibration is normal, so people often mistakenly think that the equipment vibration is abnormal

4. Correct initial clearance

various models of eddy current type vibration sensors should be under a certain clearance voltage (the clearance between the top of the sensor and the measured object, indicating the general voltage on the instrument) value, so their readings can have a better linearity, so the appropriate initial clearance must be adjusted when installing the vibration sensor. The static maximum range of the eddy current sensor cannot be greater than 2.5mm. In order to obtain better linearity under dynamic conditions, its working gap should be within the range of 0.3~2.8mm, that is, the gap voltage indicated by the instrument is 2~16v

after the rotor rotates and the equipment is loaded, the rotor will displace relative to the sensor. If the vibration sensor is installed on the top of the bearing, its clearance will be reduced; If it is installed in the horizontal direction of the bearing, its clearance depends on the rotation direction of the rotor; When the rotation direction is fixed, the clearance depends on whether it is installed on the right or left. In order to obtain a suitable working clearance value, the rotor speed from static to working should be estimated during installation, and the shaft neck elevation is about half of the top clearance of the bearing bush; The horizontal displacement is 0.20mm related to the form of bearing bush, the clearance on both sides of bearing bush and the sliding pin of the unit. The sensor is installed at the right horizontal position, and the gap C increases after the rotor rotates; It is installed on the left side, and D is reduced

the displacement of the journal in the bearing bush is not only related to the speed, but also related to the active load of the equipment. For the high-pressure rotor of steam turbine with small mass and the rotating shaft with reducer, under the action of partial steam admission and gear transmission torque, the cervical spine of the shaft will be pushed to the side of the bearing bush, and its displacement value may be close to the diameter clearance of the bearing bush

when adjusting the initial clearance of the sensor, in addition to the above factors, the maximum vibration value and the original runout value of the rotor should also be considered. The initial clearance of the sensor shall be greater than the maximum amplitude that the rotating shaft can occur and one-half of the original swing value of the rotating shaft

5. Axial position selection

from the requirements of measuring shaft vibration, the measuring point of shaft vibration should be as close to the center of the bearing bush as possible, but it is often limited by the installation position of eddy current vibration sensor, and sometimes it has to keep a certain distance from the bearing bush. According to the field vibration test, the shaft amplitude will increase with the increase of the distance between the shaft vibration measuring point and the bearing bush. The axial position of the shaft vibration measuring point also needs to consider the machining accuracy of the rotating shaft and whether the magnetic conductivity on the surface of the rotating shaft is uniform. Generally, before formal installation, it is best to use a dial indicator to check the wobble value at this point

if the runout value is greater than 50 m, another measuring point should be selected, otherwise the shaft vibration runout value at low speed will be too large, which will make the shaft vibration display value at high speed appear untrue. The uneven magnetic conductivity on the surface of the shaft cannot be directly judged by the naked eye. Only after sending out that the excessive swing value of the shaft is caused by the excessive mechanical swing on its surface, can it be determined that the excessive vibration swing of the shaft at low speed is caused by the uneven magnetic conductivity on the surface of the shaft. To eliminate this fault, only another measuring point position can be selected

6. Radial position selection

according to the requirements of ISO dis7919/2 specification, the installation of shaft vibration sensor should ensure that the two bearing sensors are in an axial plane and perpendicular to each other