There are many common failures of rotating machinery, including steam excitation, mechanical loosening, rotor blade breakage and shedding, friction, shaft cracking, mechanical deviation and electrical deviation, etc.
Steam Excitation
There are usually two reasons for steam excitation, one is due to the opening sequence of the regulating valve, the high pressure steam produces a force that lifts the rotor upward, thus reducing the bearing specific pressure and thus destabilizing the bearing; the second is due to the uneven radial clearance at the top of the lobe, which produces a tangential component force, as well as the tangential component force generated by the gas flow in the end shaft seal, causing the rotor to produce self-excited vibration.
Steam excitation generally occurs in the high-pressure rotor of high-power turbines, when steam oscillation occurs, the main characteristic of vibration is that the vibration is very sensitive to the load, and the frequency of vibration coincides with the first-order critical rotor speed frequency. In the vast majority of cases (steam excitation is not too serious) vibration frequency to half-frequency components.
In the event of steam oscillation, sometimes it is useless to change the bearing design, only to improve the design of the through-flow part of the steam seal, 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.
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, table and foundation of the machine, 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 seat.
The third type of loosening is caused by the unsuitable fit between the parts, when the loosening is usually the loosening of the bearing tile pillow in the bearing cover, excessive bearing clearance or the existence of loosening of the impeller on the rotating shaft. The vibration phase of this loosening is very unstable and varies greatly. The vibration when loose has a directional nature, in the direction of loosening, due to the decline in binding force, will cause the vibration amplitude to increase.
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 vibration of the through-frequency amplitude in the instant sudden increase.
② the characteristic frequency of vibration is the rotor's operating frequency.
③The phase of the working frequency vibration will also change abruptly.
Friction
When the rotating parts of rotating machinery and fixed parts come into contact, radial friction or axial friction of moving and static parts will occur. This is a serious failure, it may lead to the entire machine damage. There are usually two cases when friction occurs.
The first is partial friction, when the rotor only accidentally 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, especially for the destructive effect and danger of the machine is the more serious case, which is the full circumferential ring friction, sometimes called "full friction" or "dry friction", they are mostly generated in the seal. When 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 motion, from a forward positive motion to a backward negative motion.
Friction is so harmful that even a short period of friction between the rotor shaft and the shaft shank can have serious consequences.
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 an old unit with long operating time, due to stress corrosion, fatigue, creep, etc., will produce micro-cracks at the location of the original rotor inciting point, coupled with the continuous action of the larger and changing torque and radial load, micro-cracks gradually expand and eventually develop into macro-cracks.
The original initiation points are usually found in areas of high stress and material defects, such as stress concentrations on the shaft, tool marks and scratches left during machining, and areas with minor material defects (e.g., slagging).
At the initial stage of cracking in the rotor, the expansion rate is relatively slow and the growth of radial vibration amplitude is relatively small. But the crack expansion speed will accelerate with the deepening of the crack, the corresponding will appear amplitude rapidly increased phenomenon. In particular, the rapid rise of the diphthong amplitude and its phase change can often provide diagnostic information of cracks, so the trend of diphthong amplitude and phase change can be used to diagnose rotor cracks.
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 coincides with the rotational frequency of the rotating part. The cause of imperfectly machined cutting surfaces is usually generated by worn bearings in the machine tool where the final machining took place, dulled tools, too fast feeds or other defects in the machine tool, or by the wear of the lathe thimbles. Unsmooth or other defects on the journal surface, such as scratches, pits, burrs, rust scars, etc. will also produce deviation output.
The easiest way to check this error condition is to check the runout value of the journal 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 field present as long as the field is 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 only has a low magnetic field, this can cause electrical deviations. This is due to the change in sensor sensitivity caused by the magnetic field from the eddy current sensor acting on such journal surfaces.
In addition, uneven plating, uneven rotor material, etc. can also cause electrical deviations which cannot be measured and confirmed with a percentage meter.