Common faults of rotating machinery

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There are many common faults in rotating machinery, including imbalance, misalignment, shaft bending and thermal bending, oil film whirl and oil film oscillation, steam excitation, mechanical looseness, broken and detached rotor blades, friction, shaft cracks, rotational stall and surge, mechanical deflection, and electrical deflection. 1 Imbalance: Imbalance is the most common fault in various rotating machinery. There are various reasons for rotor imbalance, such as unreasonable structural design of the rotor, deviations in mechanical machining quality, assembly errors, uneven material properties, and poor dynamic balancing accuracy ； Change in the relative position of the coupling during operation ； Defects in the rotor components, such as: corrosion, wear, uneven scaling due to the medium, or detachment during operation ； Under the action of fatigue stress, the components of the rotor (such as impellers, blades, shrouds, struts, etc.) suffer local damage or detachment, resulting in fragments being ejected. 2 Misalignment: Rotor misalignment generally refers to the degree of inclination or deviation between the axis lines of adjacent rotors and the center lines of the bearings. Rotor misalignment can be divided into coupling misalignment and bearing misalignment. Misalignment of couplings can be further divided into three types: parallel misalignment, angular misalignment, and combined parallel-angular misalignment. The vibration frequency is twice the rotor’s power frequency in the case of parallel misalignment. The misalignment of the angles induces a bending moment in the coupling, in an attempt to reduce the angle between the centerlines of the two shafts. With each rotation of the shaft, the direction of the bending moment changes once; therefore, the misalignment of the deflection angle increases the axial force on the rotor, causing it to experience power-frequency vibrations in the axial direction. Parallel deflection misalignment is a combination of the above two conditions, causing radial and axial vibrations in the rotor. Bearing misalignment actually reflects the deviation between the bearing seat elevation and the axis center position. Bearing misalignment causes the load in the shafting to be redistributed. Bearings under heavy loads may experience high-order harmonic vibrations, while bearings under lighter loads are prone to instability; this also causes a change in the critical speed of the shafting system. 3-axis bending and thermal bending: Axis bending refers to a condition in which the center line of the rotor is not straight. Rotor bending is divided into two types: permanent bending and temporary bending. Permanent bending of the rotor refers to a permanent arch-shaped deformation of the rotor’s shaft. This occurs due to an unreasonable rotor design, large manufacturing errors, uneven material quality, improper storage of the rotor over time, failure to turn the rotor properly during hot shutdowns, poor thermal stability of the rotor, or an increase in the natural bending of the shaft as a result of long-term operation. Temporary bending of the rotor is caused by factors such as a large preload on the rotor, improper warm-up procedures during startup, too rapid acceleration, and uneven thermal deformation of the shaft. Permanent rotor bending and temporary rotor bending are two different types of faults, but their failure mechanisms are the same. Whether the rotor experiences permanent or temporary bending, it will generate rotational vector excitation forces similar to those resulting from mass eccentricity. 4 Oil film whirl and oil film oscillation: Oil film whirl and oil film oscillation are self-excited vibrations in sliding bearings caused by the dynamic properties of the oil film. Oil film vibration is generally caused by factors such as excessive bearing wear or clearance, inappropriate bearing design, and changes in lubricant parameters. Oil film vortexing can be easily identified from the vibration spectrum; its vibration frequency is close to half of the rotational speed. As the rotational speed increases, the ratio of the fault characteristic frequency of oil film vortexing to the rotational speed remains constant, and this is often referred to as half-speed vortexing. Oil film eddy currents and oil film oscillations are two distinct concepts; they differ from each other yet are also closely related. Oil film oscillation occurs when oil film vortices appear in the machine, and the frequency of these vortices equals the system’s natural frequency. Oil film oscillation can occur only when the operating speed of the machine is greater than twice the critical speed of the rotor. When the rotational speed rises to twice the critical speed, the vortex frequency becomes very close to the rotor’s critical speed, resulting in resonance and significant vibration. Generally, once oil film oscillation occurs, no matter how much the rotational speed increases, the vortex frequency will always remain at the rotor’s first-order critical speed frequency. When oil film oscillations occur in the rotor, they generally exhibit the following characteristics: ① The time waveform becomes distorted, appearing as an irregular periodic signal, usually with a low-frequency signal of large amplitude superimposed on the power-frequency waveform ； ②In the spectrum, the amplitude of the frequency component at the rotor’s natural frequency ω0 is the most prominent ； ③Oil film oscillation occurs when the operating speed exceeds twice the first-order critical speed; thereafter, even as the operating speed continues to increase, the characteristic frequency of the oscillation remains essentially unchanged ； ④The occurrence and disappearance of oil film oscillations are sudden and involve inertial effects; in other words, the speed at which oil film oscillations occur during acceleration is higher than the speed at which they disappear during deceleration ； ⑤When the oil film oscillates, the vortex direction of the rotor is the same as the direction of rotor rotation, resulting in positive precession ； ⑥When the oil film oscillates violently, the oscillations cease as the oil film is disrupted; once the oil film is restored, the oscillations occur again. If this continues, the journal and the bearing will keep rubbing against each other, producing knocking sounds, and the pressure of the oil film inside the bearing will experience significant fluctuations ； ⑦When the oil film oscillates, its axis trajectory shows an irregular divergent pattern; in the event of rubbing, the axis trajectory takes on a petal-like shape ； ⑧The smaller the bearing load or the lower the eccentricity, the more likely oil film oscillation will occur ； ⑨When the oil film oscillates, the vibration phases of the bearings at both ends of the rotor are essentially the same. 5 Steam excitation: There are usually two reasons for steam excitation. One is due to the sequence in which the control valves are opened; high-pressure steam generates a force that lifts the rotor upward, thereby reducing the bearing pressure ratio and causing the bearings to become unstable ； Secondly, the uneven radial clearance at the tip of the blade generates tangential forces, as does the tangential force resulting from gas flow within the end shaft seal, which causes the rotor to experience self-excited vibration. Steam excitation generally occurs on the high-pressure rotors of high-power steam turbines. When steam oscillation takes place, a key characteristic of the vibration is that it is highly sensitive to the load, and its frequency coincides with the first-order critical speed of the rotor. In the vast majority of cases (when steam-induced excitation is not severe), the vibration frequency is dominated by the half-frequency component. When steam oscillations occur, changing the bearing design is sometimes ineffective; only by improving the design of the steam seal flow passages, adjusting the installation clearance, significantly reducing the load, or changing the sequence in which the main steam inlet control valves open can the problem be resolved. 6 Mechanical loosening: There are usually three types of mechanical loosening. The first type of looseness refers to structural looseness in the machine’s base, platform, and foundation, or insufficient cement grouting, as well as deformation of the structure or foundation. The second type of looseness is mainly caused by loose fixing bolts of the machine base or cracks in the bearing housing. The third type of looseness is caused by an improper fit between the components; in this case, the looseness usually manifests as looseness of the bearing shells inside the bearing cover, excessive bearing clearance, or looseness of the impeller on the shaft. The vibration phase of this looseness is very unstable, with a large range of variation. The vibration during loosening is directional; in the direction of loosening, the decrease in restraining forces leads to an increase in the amplitude of the vibration. 7 Rotor blade breakage and detachment: The failure mechanisms of rotor blade breakage, component detachment, or scale detachment are the same as those of dynamic balance failures. Its characteristics are as follows: ① The amplitude of the fundamental vibration suddenly increases instantaneously ； ②The characteristic frequency of vibration is the operating frequency of the rotor ； ③The phase of the power-frequency vibration also undergoes a sudden change. 8 Friction: When the rotating parts and fixed parts of a rotating machine come into contact, radial friction or axial impact between the moving and stationary parts occurs. This is a serious fault that can cause the entire machine to be damaged. When friction occurs, it is generally divided into two situations: The first is partial friction, in which the rotor comes into contact with the stationary part only occasionally, and this contact lasts for only a fraction of a full cycle of the rotor’s precession; this usually results in relatively low levels of damage and risk for the entire machine ； The second type represents a more serious situation, especially in terms of the destructive effects and hazards posed by machines; this refers to cyclic friction throughout the week, sometimes also known as \"full friction\" or \"dry friction,\" and it mostly occurs within seals. During the entire week of cyclic friction, the rotor maintains continuous contact with the seal, and the friction force generated at the point of contact can cause a drastic change in the direction of the rotor’s precession, shifting it from forward precession to backward precession. Friction is highly harmful; even short-term friction between the rotating shaft and the bearing shells can lead to serious consequences. 9-axis cracks: The cause of rotor cracks is mostly fatigue damage. If the rotor of a rotating machine is poorly designed – including improper material selection or unsound structure – or if the manufacturing methods are inadequate, or in the case of older units that have been in operation for a long time, microcracks can form at locations on the rotor where stress corrosion, fatigue, creep, and other factors are present. Due to the continuous action of large and varying torques as well as radial loads, these microcracks gradually expand and eventually develop into macrocracks. The original initiation points usually appear in areas where stress is high and the material has defects, such as stress concentration points on the shaft, scratches left from machining, dents, and locations with minor material defects (such as inclusions). In the early stages of cracks appearing in the rotor, their propagation speed is relatively slow, and the amplitude of radial vibration increases only slightly. However, the crack propagation rate accelerates as the crack depth increases, resulting in a rapid increase in amplitude. In particular, the rapid increase in the amplitude of the second harmonic and changes in its phase can often provide diagnostic information about cracks; therefore, the trends in the amplitude and phase of the second harmonic can be utilized to diagnose rotor cracks. 10 Rotational stall and surge: Rotational stall is the most common type of instability in compressors. When the compressor flow rate decreases, boundary layer separation occurs on the back side of the cascade due to an increased shock angle, causing the flow channels to be partially or completely blocked. In this way, the stall region propagates in the direction opposite to the flow through the cascade at a certain speed. Experiments show that the relative velocity in the stall region is lower than the absolute velocity of the cascade rotation. Therefore, we can observe that the stall region moves in the direction of rotation of the rotor at a speed lower than the power frequency; hence, this relative rotational movement of the stall region with respect to the cascade is called rotating stall. Rotational stall deteriorates the flow conditions in the compressor, reduces the pressure ratio, and causes fluctuations in flow rate and pressure over time. At a certain speed, when the inlet flow rate decreases to a certain value, the unit will experience severe rotational stall. Severe rotational stall can further induce a more dangerous unstable aerodynamic phenomenon throughout the entire compressor unit system, namely surge. Furthermore, during rotational stall, the compressor blades are subjected to periodic excitation forces. If the frequency of rotational stall coincides with the natural frequency of the blades, it will cause intense vibrations, leading to fatigue damage of the blades and resulting in accidents. Severe rotational stall can lead to surge, but the two are not the same thing. Besides being related to the gas flow inside the compressor, surge is also closely connected to the operating characteristics of the pipeline network system connected to it. Compressors always operate in conjunction with piping systems. To ensure a certain flow rate through these systems, it is necessary to maintain a specific pressure to overcome the resistance present in the pipes. The outlet pressure when the unit is operating normally is in balance with the resistance of the piping network. But when the flow rate of the compressor drops to a certain value, the outlet pressure falls rapidly. However, due to the large capacity of the piping network, the pressure in that network does not drop immediately; as a result, the gas pressure in the network becomes higher than the outlet pressure of the compressor. Consequently, the gas in the network flows back into the compressor until the pressure there drops below the compressor’s outlet pressure. At this point, the compressor begins to supply air to the pipeline network again; its flow rate increases, and it returns to its normal operating condition. But when the pressure in the pipeline system returns to its original level, the compressor’s flow rate decreases again, and the fluid in the system flows back. This cycle repeats, resulting in strong low-frequency pulsations of the gas—surge. Identification characteristics of surge faults: ① The devices prone to surge faults are gas compression units or other gas-powered machinery equipped with long pipes and containers ； ②When surge occurs, the inlet flow rate of the unit is less than the minimum flow rate at the corresponding speed ； ③During surge, the amplitude of the vibration fluctuates significantly ； ④During surge, the characteristic frequency of the vibration is generally within the range of 1–15 Hz ； It is inversely proportional to the volume of the piping network and containers connected behind the compressor ； ⑤The unit, the pipes connected to it, other attached components, and the floor all experienced intense vibrations ； ⑥The outlet pressure experiences significant fluctuations ； ⑦The flow rate of the compressor experiences significant fluctuations ； ⑧The motor current of the motor-driven compressor unit varies periodically ； ⑨Surge is accompanied by periodic roaring sounds, the intensity of which is proportional to the molecular weight of the compressed gas and the compression ratio. 11 Mechanical and electrical deviations: In vibration signals, the occurrence of mechanical and electrical deviations is determined by the working principle of non-contact eddy current sensors. An imperfectly machined shaft surface (elliptical or off-axis) generates a signal of sinusoidal dynamic motion, whose frequency is consistent with the rotation frequency of the rotating component. The reasons for imperfect cutting surfaces are usually due to worn bearings in the machine tool used for the final machining, a dull tool, too fast feed rate, or other defects in the machine tool, or else wear of the lathe center pins. Irregularities or other defects on the surface of the journal, such as scratches, pits, burrs, rust spots, etc., will also result in deviation in the output. The simplest way to check for this error condition is to use a dial indicator to measure the runout of the journal. The fluctuation value of the dial indicator will confirm the presence of errors on the surface being measured, as observed by the contactless eddy current sensor. The surface of the journal to be measured should be protected as carefully as the journal surface of a sliding bearing. When lifting, the cables used must avoid the area of the surface that is being measured by the sensors, and the support frames used to hold the rotor should not cause any scratches or dents on the journal surface. Generally speaking, as long as the magnetic field is uniform or symmetric, eddy current sensors can operate satisfactorily in such a magnetic field. If a certain surface area on the shaft has high magnetism, while the rest of the surfaces are non-magnetic or have only low magnetism, electrical deviations may occur. This is because the magnetic field from the eddy current sensor, when acting on the surface of such a journal, causes a change in the sensor’s sensitivity. Furthermore, unevenness in the coating and unevenness in the rotor material can also cause electrical deviations, which cannot be measured or detected using a dial indicator.