Several typical failures of rotating machinery in centrifugal units

There are many common faults in rotating machinery, including unbalance, misalignment, shaft bending and thermal bending, oil film swirl and oil film oscillation, steam excitation, mechanical loosening, rotor blade breakage and shedding, friction, shaft cracking, rotating stall and wheeze, mechanical deviation and electrical deviation, etc.

1 Unbalance

Unbalance is the most common fault in all kinds of rotating machinery. The causes of rotor unbalance are many, such as the structure of the rotor design is not reasonable, machining quality deviation, assembly error, material uneven, dynamic balance precision is poor; operation of the coupling relative position change; rotor parts damage, such as: operation due to corrosion, wear, media uneven scaling, shedding; rotor by fatigue stress caused by the role of rotor parts (such as impeller, blade, around the belt, tension bars etc.) are locally damaged and fall off, resulting in fragments flying out, etc.

2 Misalignment

Rotor misalignment usually refers to the extent to which the axis lines of two adjacent rotors are tilted or offset from the bearing centre line.

Rotor misalignment can be divided into coupling misalignment and bearing misalignment. Coupling misalignment can be divided into parallel misalignment, off-angle misalignment and parallel off-angle misalignment. In the case of parallel misalignment the vibration frequency is twice the working frequency of the rotor. Off-angle misalignment causes a bending moment to be added to the coupling in an attempt to reduce the deflection of the two shaft centrelines.

The direction of the bending moment changes once per week of shaft rotation, so the misalignment increases the axial force on the rotor and causes the rotor to vibrate in the axial direction. Parallel misalignment is a combination of the above two conditions, causing the rotor to vibrate both radially and axially. Bearing misalignments actually reflect deviations in bearing seat elevation and shaft centre position.

Bearing misalignment redistributes the load on the shaft system. Bearings with higher loads may experience high harmonic vibrations, bearings with lighter loads are prone to instability and also cause the critical speed of the shaft system to change.

3 Shaft bending and thermal bending

Shaft bending means that the centreline of the rotor is in an unstraight state. Rotor bending is divided into permanent bending and temporary bending two types.

Rotor permanent bending refers to the rotor shaft is permanently bowed, it is due to the rotor structure is unreasonable, manufacturing errors, material is not uniform, the rotor long-term storage improper and permanent bending deformation, or hot state parking is not timely pan or pan improper, the rotor’s thermal stability is poor, long-term operation of the shaft natural bending increase and other reasons.

Temporary bending of the rotor refers to a larger pre-load on the rotor, improper warm-up operation when running, too fast speed, uneven thermal deformation of the rotor shaft and other reasons.

Permanent bending and temporary bending of the rotor are two different kinds of failure, but the mechanism of failure is the same. Whether the rotor is permanently bent or temporarily bent, it will generate a rotational vector excitation force similar to the mass eccentricity situation.

4 Oil film vortex and oil film oscillation

Oil film vortex and oil film oscillation are self-excited vibrations caused by the dynamics of the oil film in a sliding bearing.

Oil film swirl is generally caused by excessive bearing wear or clearance, unsuitable bearing design, changes in lubricant parameters, etc. The oil film vortex is easily identified according to the vibration spectrum and occurs at a vibration frequency close to half the speed frequency. As the speed increases, the ratio of the fault characteristic frequency of the oil film vortex to the speed frequency also remains constant at a fixed value, often referred to as half-speed vortex.

Oil film vortex and oil film oscillation are two different concepts, which are both different and closely related.

Oil film oscillation occurs when there is oil film swirl in the machine and the frequency of the oil film swirl is equal to the inherent frequency of the system. Oil film oscillation can only occur when the machine is running at a speed greater than two times the critical rotor speed. When the speed rises to two times the critical speed, the vortex frequency is very close to the critical rotor speed and therefore resonates and causes a lot of vibration. Once oil film oscillation has occurred, the vortex frequency will always remain at the first order critical rotor speed frequency, regardless of the speed increase.

When oil film oscillation occurs in the rotor, it is generally characterised by the following:

① distortion of the time waveform, which is manifested as an irregular periodic signal, usually a low frequency signal with a large amplitude superimposed on top of the waveform of the working frequency;

(ii) The amplitude of the frequency component at ω0, the intrinsic frequency of the rotor, is most prominent in the spectrogram;

(iii) oil film oscillations occur at operating speeds greater than two times the first order critical speed, after which the characteristic frequency of the oscillations remains essentially unchanged even if the operating speed continues to increase;

(iv) The onset and disappearance of oil film oscillation is sudden and has an inertial effect, i.e. the speed at which oil film oscillation is generated at speed rise is higher than the speed at which it disappears at speed fall;

⑤ when oil film oscillation occurs, the direction of rotor vortex is the same as the direction of rotor rotation, which is positive feed;

⑥When the oil film oscillation is intense, with the destruction of the oil film, the oscillation stops, and after the oil film is restored, the oscillation occurs again. If this continues, the journal and the bearing will keep touching the friction, producing a crashing sound, and the oil film pressure in the bearing has a large fluctuation;

(7) when the oil film oscillation, its axial trajectory is irregular dispersion, if the occurrence of touch mo, then the axial trajectory is petal-shaped;

(8) the smaller the bearing load or the smaller the eccentricity, the more likely oil film oscillation will occur;

⑨ oil film oscillation, bearing vibration phase at both ends of the rotor is basically the same.

5 Steam Excitation

Steam excitation is usually generated for two reasons, one is due to the opening sequence of the regulating valve, high pressure steam generated a force to lift the rotor upwards, thus reducing the bearing specific pressure, thus making the bearing unstable; the second is due to the uneven radial clearance at the top of the leaf, resulting in tangential force, as well as the end shaft seal within the gas flow when the tangential force generated by the slip, so that the rotor produces a self-excited vibration.

Steam excitation generally occurs in the high-power turbine high-pressure rotor, when the occurrence of steam oscillation, the main characteristics of vibration is vibration is very sensitive to the load, and the frequency of vibration? coincides with the first-order critical rotor speed frequency. In most cases (steam excitation is not too severe) the vibration frequency is dominated by the half-frequency component.

In the occurrence of steam oscillation, sometimes change the bearing design is not useful, only to improve the design of the steam seal flow-through part, adjust the installation gap, significantly reduce the load or change the main steam into the steam regulating valve opening sequence in order to solve the problem.

6 Mechanical loosening

There are usually three types of mechanical loosening.

The first type of loosening refers to the presence of structural looseness in the base of the machine, the table plate and the foundation, or poor cement grouting and deformation of the structure or foundation.

The second type of loosening is mainly caused by the loosening of the machine base fixing bolts or cracks in the bearing housing.

The third type of loosening is caused by an improper fit between the components, which is usually a loose bearing pillow in the bearing cover, excessive bearing clearance or the presence of a loose impeller on the rotating shaft. The vibration phase of this loosening is very unstable and varies considerably. The vibration when loose is directional and will cause an increase in vibration amplitude in the direction of loosening due to the reduction in binding force.

7 Rotor broken blade and shedding

Rotor broken blade, parts or scale layer off the failure mechanism and dynamic balance failure is the same. Its characteristics are as follows:

① a sudden increase in the amplitude of the through-frequency of the vibration in an instant;

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

③ The phase of the working frequency vibration also changes abruptly.

8 Friction

When the rotating parts of the rotating machinery and fixed parts come into contact, radial friction or axial friction of the moving and static parts will occur. This is a serious fault, which can lead to damage to the entire machine. When friction occurs there are usually two types of situation:

The first is partial friction, when the rotor only occasionally touches the stationary part, while maintaining contact only in a fractional part of the rotor into the moving whole cycle, which is usually relatively less destructive and dangerous for the machine as a whole;

The second, and especially more serious situation in terms of destructive effect and danger to the machine, is the circumferential ring friction, sometimes called “full friction” or “dry friction”, which is mostly generated in the seal. When full circumferential ring friction occurs, the rotor maintains continuous contact with the seal and the friction generated at the point of contact can lead to a dramatic change in the direction of rotor movement from forward to backward.

Friction is so dangerous that even a short period of friction between the rotor shaft and the shaft tiles can have serious consequences.

9 Shaft cracking

The cause of rotor cracks is mostly fatigue damage. Rotating machinery rotor if improperly designed (including improper material selection or unreasonable structure) or improper processing methods, or is the operating time of the old unit, due to stress corrosion, fatigue, creep, etc., will be in the rotor originally existed at the location of the trigger point to produce micro-cracking, coupled with the larger and changing torque and radial load continued to act, micro-cracking gradually expanded, and eventually developed into macro-cracking.

The original initiation points are usually found in areas of high stress and material defects, such as stress concentrations on the shaft, tool marks left during machining, scratches, areas with minor material defects (e.g. slagging), etc.

In the early stages of cracking in a rotor, the rate of expansion is relatively slow and the increase in radial vibration amplitude is relatively small. However, the speed of crack expansion will accelerate with the deepening of the crack, and the amplitude will increase rapidly. In particular, the rapid rise in the diphthong amplitude and the change in its phase can often provide diagnostic information about cracks, so the trend in diphthong amplitude and phase can be used to diagnose rotor cracks.

Rotational stall and wheeze

Rotational stall is one of the most common instabilities in compressors. When the compressor flow rate decreases, the back of the impeller grid will separate at the boundary layer due to the increased impulse angle and the flow path will be partially or completely blocked. The stall zone will then propagate at a certain speed in the opposite direction of the impeller movement.

Experiments have shown that the relative velocity of the stall zone is lower than the absolute velocity of the impeller rotation. As a result, we can observe the stall zone moving in the direction of rotation of the rotor at a speed lower than the working frequency, so this rotational movement of the separation zone relative to the impeller grid is called rotational stall.

The rotational stall deteriorates the flow in the compressor, the pressure ratio drops and the flow and pressure fluctuate with 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 can further cause a dangerous and more unstable aerodynamic phenomenon in the entire compressor system, i.e. wheezing. In addition, rotating stall when the compressor blade is subject to a periodic excitation force, such as the frequency of rotating stall and the blade’s inherent frequency coincides, will cause strong vibration, so that blade fatigue damage caused by accidents.

Rotational stall can lead to a serious wheeze, but the two are not the same thing. In addition to the internal gas flow of the compressor, it is also closely related to the operating characteristics of the pipeline network system to which it is connected.

The compressor always works in conjunction with the pipe network and in order to ensure a certain flow through the pipe network, a certain pressure must be maintained to overcome the resistance of the pipe network. The outlet pressure of the unit in normal operation is in balance with the resistance of the pipe network. However, when the flow rate of the compressor is reduced to a certain value, the outlet pressure will drop quickly, however, due to the large capacity of the pipe network, the pressure in the pipe network does not immediately decrease, so the gas pressure in the pipe network is instead greater than the outlet pressure of the compressor, therefore, the gas in the pipe network flows backwards back to the compressor, until the pressure in the pipe network drops below the outlet pressure of the compressor.

At this point, the compressor starts to supply gas to the pipe network again, and the flow rate of the compressor increases and returns to its normal working state. However, when the pressure in the network returns to the original pressure, the compressor flow is reduced and the fluid in the system flows backwards. This repeatedly produces a strong low-frequency pulsation of the gas – the phenomenon of wheezing.

Identifying features of a wheezing fault:

① the object of the generation of wheezing failure for gas compressor sets or other gas power machinery with long pipelines and containers;

(ii) when the wheeze occurs, the inlet flow of the unit is less than the minimum flow at the corresponding speed;

(iii) The amplitude of the vibration will fluctuate considerably when the oscillation occurs;

④The characteristic frequency of the vibration is generally within 1 to 15 Hz when the surge occurs; it is inversely proportional to the size of the volume of the pipe network and vessel connected behind the compressor;

⑤ strong vibrations occur in the unit and in the attached objects such as pipes and the ground connected to it;

(6) Large fluctuations in outlet pressure;

(vii) Large fluctuations in the flow rate of the compressor;

(viii) The motor current of the motor-driven compressor unit varies cyclically;

(ix) Periodic roaring sound when wheezing, the size of the roar is proportional to the molecular weight of the compressed gas and the compression ratio.

Mechanical and electrical deviations

The reason for the mechanical and electrical deviations in the vibration signal is determined by the operating principle of the non-contact eddy current sensor.

Cutting imperfectly machined shaft surfaces (elliptical or different shafts) produce an indication of sinusoidal dynamic motion with a frequency that corresponds to the rotational frequency of the rotating part. The cause of imperfect cutting surfaces is usually produced by worn bearings in the machine where the final machining takes place, dull tools, too fast feeds or other defects in the machine, or by the wear of the lathe thimbles. Unsmooth or other defects on the surface of the journal, such as scratches, pits, burrs, rust scars, etc. will also produce a deviated output.

The easiest way to check this error condition is to check the runout values of the journals with a percentage meter. The fluctuation value of the percentage meter will confirm the presence of error on the measured surface as observed by the non-contact eddy current sensor.

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

In general, eddy current sensors work satisfactorily in the magnetic fields present, provided that the magnetic fields are uniform or symmetrical. If one surface area on the shaft has a high magnetic field while the rest of the surface is non-magnetic or has only a low magnetic field, this may cause electrical deviations. This is due to the magnetic field from the eddy current sensor acting on such journal surfaces causing a change in sensor sensitivity.

In addition, inhomogeneities in the plating, inhomogeneities in the rotor material etc. can also cause electrical deviations which cannot be measured and confirmed with a percentage meter.

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