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There are many common faults in rotating machinery, including imbalance, misalignment, shaft bending and thermal bending, oil film whirling and oscillation, steam excitation, mechanical loosening, rotor blade breakage and detachment, friction, shaft cracks, rotating stall and surge, mechanical deviation and electrical deviation, etc.

out-off-balance

Imbalance is the most common fault in various rotating machinery.

The causes of rotor imbalance are multifaceted, such as unreasonable structural design of the rotor, mechanical processing quality deviation, assembly error, uneven material, and poor dynamic balance accuracy; Changes in the relative position of the coupling during operation; Defects in rotor components, such as corrosion, wear, uneven scaling, and detachment during operation; The fatigue stress on the rotor causes local damage and detachment of rotor components (such as impellers, blades, shrouds, braces, etc.), resulting in fragments flying out.

Misalignment

Misalignment of rotors usually refers to the degree of inclination or deviation between the axis lines of adjacent rotors and the centerline of bearings.

The misalignment of the rotor can be divided into misalignment of the coupling and misalignment of the bearing. Coupling misalignment can be divided into three situations: parallel misalignment, misalignment, and parallel misalignment. When parallel misalignment occurs, the vibration frequency is twice the power frequency of the rotor. The misalignment causes the coupling to attach a bending moment in an attempt to reduce the misalignment of the two shaft centerlines.

Every time the shaft rotates, the direction of the bending moment changes, so misalignment increases the axial force of the rotor, causing it to generate power frequency vibration in the axial direction. Parallel misalignment is a combination of the above two situations, causing the rotor to experience radial and axial vibrations. The misalignment of the bearing actually reflects the deviation between the elevation of the bearing seat and the position of the shaft center.

Misalignment of bearings redistributes the load on the shaft system. Bearings with larger loads may experience higher harmonic vibrations, while bearings with lighter loads are prone to instability and can also cause changes in the critical speed of the shaft system.

Shaft bending and thermal bending

Shaft bending refers to the condition where the centerline of the rotor is not straight. The rotor bending is divided into two types: permanent bending and temporary bending.

Permanent bending of rotor refers to the permanent bow shape of rotor shaft, which is caused by unreasonable rotor structure, large manufacturing error, uneven material, improper long-term storage of rotor and permanent bending deformation, or improper or delayed turning during hot shutdown, poor thermal stability of rotor, and increased natural bending of shaft after long-term operation.

Temporary bending of the rotor refers to significant preload on the rotor, improper warm-up operation during startup and operation, excessive acceleration, and uneven thermal deformation of the rotor shaft.

Permanent bending and temporary bending of the rotor are two different types of faults, but their failure mechanisms are the same. Whether the rotor undergoes permanent or temporary bending, it will generate a rotational vector excitation force similar to the mass eccentricity situation.

Oil Film Whirl and Oil Film Oscillation

Oil film vortex and oil film oscillation are self-excited vibrations caused by the dynamic characteristics of the oil film in sliding bearings.

Oil film whirling is generally caused by factors such as excessive bearing wear or clearance, inappropriate bearing design, and changes in lubricating oil parameters. It is easy to identify oil film whirl based on the vibration spectrum, and the vibration frequency at which it occurs is close to half of the speed frequency. As the speed increases, the ratio of the fault characteristic frequency of oil film whirl to the speed frequency remains constant at a fixed value, often referred to as half speed whirl.

Oil film vortex and oil film oscillation are two different concepts, which have both differences and close connections.

When the machine experiences oil film whirling and the frequency of oil film whirling is equal to the natural frequency of the system, oil film oscillation occurs. Oil film oscillation can only occur when the operating speed of the machine is greater than twice the critical speed of the rotor. When the speed increases to twice the critical speed, the vortex frequency is very close to the critical speed of the rotor, resulting in resonance and causing significant vibration. Usually, once oil film oscillation occurs, no matter how much the speed continues to rise, the vortex frequency will always remain at the first critical speed frequency of the rotor.

When oil film oscillation occurs in the rotor, it generally has the following characteristics:

① The time waveform undergoes distortion, manifested as irregular periodic signals, usually with low-frequency signals with large amplitudes superimposed on the power frequency waveform;

② In the frequency spectrum, the natural frequency of the rotor ω The amplitude of the frequency component at 0 is the most prominent;

③ Oil film oscillation occurs when the operating speed is greater than twice the first critical speed. After this, even if the operating speed continues to increase, the characteristic frequency of the oscillation remains basically unchanged;

④ The occurrence and disappearance of oil film oscillation are sudden and have inertia effects, which means that the speed at which oil film oscillation occurs during acceleration is higher than the speed at which oil film oscillation disappears during deceleration;

⑤ When the oil film oscillates, the vortex direction of the rotor is the same as the direction of rotor rotation, which is a forward precession;

⑥ When the oil film oscillates violently, as the oil film is destroyed, the oscillation stops. After the oil film recovers, the oscillation occurs again. If this continues, the journal and bearing will continuously collide and rub, producing a collision sound, and the oil film pressure inside the bearing will fluctuate significantly;

⑦ When the oil film oscillates, its axis trajectory shows an irregular divergent state. If there is friction, the axis trajectory shows a petal shape;

⑧ The smaller the bearing load or the smaller the eccentricity, the easier the oil film oscillation will occur;

⑨ When the oil film oscillates, the vibration phase of the bearings at both ends of the rotor is basically the same.

Steam excitation

There are usually two reasons for steam excitation. Firstly, due to the opening sequence of the regulating valve, high-pressure steam generates a force to lift the rotor upwards, thereby reducing the bearing specific pressure and causing bearing instability; The second reason is that due to the uneven radial clearance at the blade tip, a tangential component force is generated, as well as the tangential component force generated by the gas flow in the end shaft seal, which causes the rotor to generate self-excited vibration.

Steam excitation generally occurs on the high-pressure rotor of high-power steam turbines. When steam oscillation occurs, the main characteristic of the vibration is that the vibration is very sensitive to the load, and the frequency of the vibration is consistent with the first critical speed frequency of the rotor. In the vast majority of cases (steam excitation is not too severe), the vibration frequency is mainly the half frequency component.

When steam oscillation occurs, sometimes changing the bearing design is useless. Only by improving the design of the steam seal flow passage, adjusting the installation clearance, significantly reducing the load, or changing the opening sequence of the main steam inlet regulating valve can the problem be solved.

mechanical looseness

There are usually three types of mechanical loosening.

The first type of looseness refers to the structural looseness of the machine's base, platform, and foundation, or inadequate cement grouting, as well as deformation of the structure or foundation.

The second type of looseness is mainly caused by loose fixing bolts on the machine base or cracks in the bearing seat.

The third type of looseness is caused by improper fitting between components, usually due to looseness of the bearing pads in the bearing cover, excessive bearing clearance, or looseness of the impeller on the shaft. This loose vibration phase is very unstable and has a large range of changes. The vibration during loosening has directionality, and in the loosening direction, the decrease in restraining force will cause an increase in vibration amplitude.

Broken and detached rotor blades

The fault mechanism of broken rotor blades, component or scale detachment is the same as that of dynamic balance faults. Its characteristics are as follows:

① The direct frequency amplitude of vibration suddenly increases in an instant;

② The characteristic frequency of vibration is the working frequency of the rotor;

③ The phase of power frequency vibration can also undergo sudden changes.

friction

When the rotating parts of a rotating machinery come into contact with fixed parts, radial friction or axial friction will occur between the moving and stationary parts. This is a serious malfunction that may cause complete damage to the machine. There are usually two situations when friction occurs:

The first type is partial friction, where the rotor only accidentally contacts the stationary part and maintains contact only for a fraction of the entire rotor precession cycle. This is usually less destructive and dangerous for the overall machine;

The second type, especially for the destructive effect and danger of the machine, is a more serious situation, which is the circumferential friction, sometimes also known as "full friction" or "dry friction", which mostly occurs in the seal. When circular friction occurs throughout the entire circumference, the rotor maintains continuous contact with the seal, and the frictional force generated at the contact can cause a drastic change in the direction of the rotor's precession, changing from a forward forward precession to a backward reverse precession.

The harm of friction is significant, even if the shaft and bearing pads rub for a short period of time, it can cause serious consequences.

Axial crack

The causes of rotor cracks are often fatigue damage. If the rotor of rotating machinery is improperly designed (including improper material selection or unreasonable structure) or processed in an improper way, or the old unit has been running for a long time, due to stress corrosion, fatigue, creep, etc., microcracks will occur at the location where the rotor originally had an induced point. In addition, due to the continuous effect of large and varying torque and radial load, microcracks will gradually expand and eventually develop into macro cracks.

The original triggering points usually appear in areas with high stress and material defects, such as stress concentration points on the shaft, tool marks left during processing, scratches, and areas with minor defects in the material (such as slag inclusion).

In the early stages of rotor cracks, their propagation speed is relatively slow, and the amplitude of radial vibration increases relatively small. But the propagation speed of cracks will accelerate with the deepening of crack depth, and correspondingly, there will be a phenomenon of rapid increase in amplitude. Especially the rapid increase in the amplitude of the second harmonic and the change in its phase can often provide diagnostic information for cracks, so the trend of changes in the amplitude and phase of the second harmonic can be used to diagnose rotor cracks.

Rotating stall and surge

Rotating stall is the most common unstable phenomenon in compressors. When the compressor flow decreases, due to the increase of incidence angle, boundary layer separation will occur at the back of the cascade, and the flow passage will be partially or completely blocked. In this way, the stall zone will propagate in the opposite direction of the cascade motion at a certain speed.

The experiment shows that the relative velocity in the stall zone is lower than the absolute velocity of the cascade rotation. Therefore, we can observe that the stall zone moves at a speed lower than the power frequency along the rotational direction of the rotor, hence the rotational motion of the separation zone relative to the cascade is called rotational stall.

Rotating stall worsens the flow situation in the compressor, reduces the pressure ratio, and fluctuates the flow rate and pressure over time. At a certain speed, when the inlet flow rate decreases to a certain value, the unit will experience a strong rotational stall. A strong rotational stall will further cause a more dangerous and unstable aerodynamic phenomenon in the entire compressor unit system, namely surge. In addition, during rotating stall, the compressor blades are subjected to a periodic excitation force. If the frequency of rotating stall matches the natural frequency of the blades, it will cause strong vibration and cause fatigue damage to the blades, resulting in accidents.

Severe rotational stall can lead to surging, but the two are not the same. Surge is not only related to the gas flow inside the compressor, but also closely related to the working characteristics of the connected pipeline network system.

Compressors always work together with the pipeline network, and in order to ensure a certain flow rate through the pipeline network, a certain pressure must be maintained to overcome the resistance of the pipeline network. The outlet pressure of the unit during normal operation is phase equilibrium with the pipe network resistance. But when the flow rate of the compressor decreases to a certain value, the outlet pressure will quickly decrease. However, due to the large capacity of the pipeline network, the pressure in the pipeline network does not immediately decrease. Therefore, the gas pressure in the pipeline network is actually greater than the outlet pressure of the compressor. Therefore, the gas in the pipeline network flows back to the compressor until the pressure in the pipeline network drops below the outlet pressure of the compressor.

At this point, the compressor starts supplying gas to the pipeline network again, and the flow rate of the compressor increases, returning to normal working state. But when the pressure in the pipeline network returns to the original pressure, the flow rate of the compressor decreases again, and the fluid in the system flows back. This cycle repeatedly produces a strong low-frequency pulsation phenomenon of gas - surging.

Identification characteristics of surge faults:

① The object of surge failure is gas compressor units or other gas powered machinery with long pipelines and containers;

② When surge occurs, the inlet flow rate of the unit is less than the minimum flow rate at the corresponding speed;

③ When surging, the amplitude of the vibration will fluctuate significantly;

④ When surging, the characteristic frequency of vibration is generally within 1-15Hz; The volume of the pipe network and container connected to the back of the compressor is inversely proportional;

⑤ The unit and its connected pipelines and other attachments, as well as the ground, experience strong vibrations;

⑥ The outlet pressure fluctuates significantly;

⑦ The flow rate of the compressor fluctuates significantly;

⑧ The motor current of the motor driven compressor unit varies periodically;

⑨ When panting, there is a periodic roar, and the size of the roar is proportional to the molecular weight and compression ratio of the compressed gas.

Mechanical and electrical deviations

The reason why mechanical and electrical deviations occur in vibration signals is determined by the working principle of non-contact eddy current sensors.

Imperfect cutting of shaft surfaces (elliptical or different axes) will generate an indication of sinusoidal dynamic motion, with a frequency consistent with the rotational frequency of the rotating component. The cause of imperfect cutting surfaces is usually due to bearing wear, tool dullness, rapid feed, or other defects in the final machined machine tool, or wear of the lathe thimble. Unsmooth or other defects on the surface of the journal, such as scratches, dents, burrs, rust scars, etc., will also result in deviation output.

The simplest method to test this error state is to use a dial gauge to check the runout value of the journal. The fluctuation value of the dial gauge will confirm the existence of errors observed by the non-contact eddy current sensor on the measured surface.

The measured surface of the journal should be carefully protected like the surface of the journal of a sliding bearing. When lifting, the cable used should avoid the surface area measured by the sensor. The support frame for storing the rotor should ensure that it does not cause scratches, dents, etc. on the surface of the journal.

Generally speaking, as long as the magnetic field is uniform or symmetrical, eddy current sensors can work satisfactorily in the existing magnetic field. If a certain surface area on the shaft has high magnetism, while the other surfaces are non magnetic or only have low magnetism, electrical deviation may occur. This is due to the change in sensor sensitivity when the magnetic field from the eddy current sensor acts on the surface of this journal.

In addition, uneven coating and rotor material can also cause electrical deviations, which cannot be measured and confirmed using a dial gauge.