1. Imbalance

Rotor unbalance is a fault caused by mass eccentricity of the rotor component or defects in the rotor component. It is the most common fault of rotating machinery.

Unreasonable structural design, manufacturing and installation errors, mass eccentricity caused by uneven materials, and local damage and falling off of parts due to corrosion, scaling, alternating stress, etc. during the operation of the rotor will cause the rotor to be affected by the centrifugal force during rotation and cause abnormal vibration.

The main vibration characteristics of unbalanced rotor:

(1) The vibration direction is mainly radial, and the unbalanced cantilever rotor may exhibit axial vibration;

(2) The waveform is a typical sine wave;

(3) The vibration frequency is the power frequency, and the phase difference between the horizontal and vertical vibrations is close to 90 degrees.

Case: The vibration intensity of the coupling side of the pump bearing box of a certain device is horizontal 13.2 mm/s, vertical 11.8 mm/s, and axial 12.0 mm/s. The vibrations in all directions are power frequency components, and the horizontal and vertical waveforms are sine waves. The horizontal vibration spectrum is shown in the figure 1, and the horizontal vibration waveform is shown in the figure.

Then a dual-channel phase difference measurement was performed on the horizontal and vertical vibrations, which showed that the phase difference was close to 90 degrees. The diagnosis is an unbalanced fault, and the unbalance is likely to occur at the coupling.

No obvious wear of parts was found during disassembly and inspection, but the coupling was detected to have mass eccentricity. During the dynamic balancing operation, the corresponding parts of the coupling were polished and corrected, and the vibration was reduced to 2.4 mm/s.

2. Misalignment

Rotor misalignment includes two situations: shafting misalignment and bearing misalignment. Shaft misalignment means that the axes of each rotor are not on the same straight line after the rotors are connected.

Bearing misalignment means that the journal is deflected in the bearing, and the axis of the journal and the bearing hole are not parallel to each other. Usually the misalignment refers to the misalignment of the shaft system.

Vibration characteristics of misalignment:

(1) The maximum vibration is often on the bearings on both sides of the misaligned coupling, and the vibration value increases as the load increases;

(2) Parallel misalignment mainly causes radial vibration, and the vibration frequency is 2 times the power frequency. There are also power frequencies and multi-fold frequencies, but the main ones are power frequency and 2 times the power frequency;

(3) The phase difference of the radial vibration of the misalignment at both ends of the coupling is close to 180 degrees;

(4) During alignment, the axial vibration is large, the vibration frequency is power frequency, and the axial vibration phase difference at both ends of the coupling is close to 180 degrees.

Case: The vibration of a horizontal high-speed pump reaches 16.0 mm/s. It can be seen from the vibration spectrum diagram that 50 Hz (motor power frequency) and its 2 multiplier amplitude are significant, and the 2 multiplier amplitude is significantly higher than the power frequency. It is initially determined to be a misalignment fault. Then measure the phase difference between the corresponding parts of the pump bearing box and the motor bearing seat and find that it is close to 180 degrees.

The disassembly inspection found that 2 connecting bolts of the coupling were broken, the upper radial bearing bush of the high-speed shaft had metal falling off, and the bearing bush clearance was too large; the thrust surface of the high-speed shaft was worn, and the clearance between the thrust bush and the inert shaft bearing bush was too large.

After overhauling and replacing the high-speed shaft bearing bush, the inert shaft bearing bush and the coupling connecting bolt, the vibration dropped to area A.

3. Looseness

When there is looseness in machinery, a small imbalance or misalignment can cause large vibrations. There are generally three types of mechanical looseness.

The first type of looseness refers to the structural looseness of the base, platen and foundation of the machine, or the lack of solid cement grouting and deformation of the structure or foundation. The vibration spectrum displayed by this type of looseness is mainly 1x.

The second type of looseness is mainly caused by loosening of the fixing bolts of the machine base or cracks in the bearing seat. In addition to 1X, its vibration spectrum also contains a considerable 2X component. Sometimes it also stimulates 1/2X and 3X vibration components.

The third type of looseness is caused by improper fit between components, which produces many vibration harmonic components, such as 1X, 2X...nX, and sometimes fractional harmonic components such as 1/2X, 1/3X... etc. The looseness at this time is usually the looseness of the bearing bush in the bearing cap, excessive bearing clearance, or loose parts on the rotating shaft.

It can be seen from the vertical vibration spectrum diagram of the induced draft fan foot 03 that 1X and 2X are larger and have more harmonic components.

After tightening the anchor bolts, the maximum vibration of the bearing box dropped to 4.2 mm/s, which is still too high. It is analyzed that there is still loose fit in the bearing or the parts on the shaft. 2012 8 After the induced draft fan was disassembled and inspected, it was found that the tightness between the bearing and the gland was insufficient. After adding copper gaskets to adjust the tightness of the gland, the vibration dropped to 2.7 mm/s.

4. Fluid disturbance

When the fluid in a high-speed centrifugal pump flows out of the flow channel of the impeller and enters the diffuser or volute, if the flow direction of the fluid is inconsistent with the angle of the blades, great boundary layer separation, mixed flow and reverse flow will occur in the flow channel, and the fluid will impact the diffuser blades and volute tongues.

The fluid will cause large pressure pulsation and unstable flow in the pipeline. This pressure wave may be reflected to the impeller, stimulating the rotor to vibrate. The vibration frequency is the number of impeller blades multiplied by the rotation speed (called blade flow frequency) or its multiples.

When the process flow deviates greatly from the pump's rated flow or the impeller outlet and volute are poorly aligned, the overflow frequency will vibrate significantly, which is called fluid disturbance. Generally, increasing the distance between the outer edge of the impeller and the starting point of curling can alleviate the pressure pulsation and reduce the amplitude.

Case: The vibration of a certain pump exceeded the standard, and the vibration of the pipeline was also large. The monitoring found that the maximum vibration of the front bearing was 19.4 mm/s, and the amplitude of 4 times the power frequency was the largest. This component is the pump overflow frequency. The pump vibration spectrum is shown in Figure 6. After calculation, it was found that the pump size was too large. After the impeller was disassembled and cut, the vibration dropped to within the standard.

5. Static and dynamic friction

In rotating machinery, serious deformation of the shaft center due to shaft bending, rotor misalignment, etc., or bending and deformation of non-rotating parts, may cause friction between the rotor and fixed parts, causing abnormal vibration. Vibration characteristics of dynamic and static friction:

The frequency spectrum is dominated by the power frequency component, with a small amount of low frequency or multipliers present. When friction is severe, the low frequency and multiplier components are more obviously reflected. There may be unilateral clipping on the waveform graph or a sawtooth shape near the maximum amplitude.

6. Rolling bearing failure

Regarding vibration analysis theory, when a rolling bearing is damaged or worn, the high-frequency demodulation value will generally increase, and the fault characteristic frequencies of the bearing outer ring, inner ring and other components are often visible. When the bearing wear reaches an advanced stage, the characteristic frequency of the bearing failure may disappear, but the vibration value usually increases, and the vibration spectrum diagram becomes a series of spectral lines.

Case: The pump vibration of the gas separation device reaches 18.2 mm/s. It can be seen from the horizontal vibration spectrum diagram of the front bearing that in addition to the power frequency, there are dense high-frequency components. It can be seen from the front bearing horizontal demodulation spectrum that the prominent spectral component is only the power frequency, but the bottom line of the spectral line is higher.

It is analyzed that the pump bearings are severely worn and have developed to the point where the characteristic frequency of bearing failure has disappeared. After disassembly and inspection, it was found that the bearing cage on the coupling side had broken. After replacing the bearing, the vibration dropped to area B.

Case: The vibration of the induced draft fan increases, with a maximum vibration of 8.1 mm/s. It can be seen from the vibration spectrum diagram of the front bearing of the induced draft fan that the amplitude of the high-frequency component is significant. It can be seen from the demodulation spectrum of the front bearing of the induced draft fan that the frequency amplitude of about 5.8 times the rotational speed is prominent. This component is close to the characteristic frequency of the outer ring fault of the bearing 22316 of the machine, and the diagnosis is bearing wear.

Disassembly verified that the bearing was indeed worn. After replacing the bearing, the maximum vibration was 1 mm/s and the demodulation value was maximum 0.8 g.

(Source: Petrochemical Equipment Maintenance and Overhaul Network)