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Rotor rub can result in serious damage such as a shaft failure. Bending moments associated with shaft bow caused by rubs can lead to high stresses on the shaft. Vibration-based condition monitoring systems can be employed to identify rub severity and approximate its location.
The study of rotor-to-stator rub phenomenon in turbomachines is necessary because of the relatively small clearances present between the rotor assembly and various static parts.
Heavy rubbing can cause impacts, chaotic motions, sub-synchronous vibrations (1/2×, 1/3×, 1/4×, and so on.), and super-synchronous vibrations (2×, 3×, 4×, and so on.). Light partial-arc rubs and full annular rubs, on the other hand, often give rise to progressive changes in synchronous vibrations (1×).
And depending on the mechanical and thermal characteristics of the machine as well as the shaft rotating speed, stable or unstable synchronous spiral vibrations can sometimes occur.
Such rub phenomena can be prevented by properly modifying design and operating parameters. Modifications of the shape and wideness of rotor orbits (and consequent changes in the rotor centerline position inside bearings and seals) may allow sufficient radial clearance to avoid rubs. When rotor-to-stator rubs occur during a transient situation such as a start-up or shutdown, the shaft bow evolution can become more complex.
Friction forces generated during heavy contacts and rubs can produce a considerable amount of heat. Depending on rub characteristics, this heat can sometimes be transmitted to the rotor through a small portion of the circumferential surface.
Localized heating can create a shaft thermal bow as well as sudden changes in synchronous vibration. Additionally, heat due to friction forces can induce a timevarying thermal bow to the shaft.
This can generate transient synchronous vibrations. Time-varying characters, therefore, should be considered in analytical models and condition monitoring exercises. The magnitude of bending induced by rubbing can offer helpful information about the thermal and mechanical properties of the rotor.
The heaviest rubs usually occur on portions of the rotor train where the smallest clearances are located. The shaft can then be affected by one (or more) local bows. A higher vibration can then be expected at the rotor side nearest the rub location. Its precise whereabouts can be identified by comparing vibrations on both sides of the rotor.
A 21 MW steam turbine can serve to illustrate this phenomenon. A high pressure (HP) steam turbine driver was directly coupled to a centrifugal compressor. The operating speed was 4,100 rpm. Four oil-film journal bearings supported the shaft train.
The first lateral critical speed of the steam turbine driver was close to 1,983 rpm. The first startup after an overhaul resulted in the appearance of rubbing. When the rotating speed reached around 1,900 rpm (near the first lateral critical speed), the amplitude of steam turbine vibrations increased.
Within a short time, vibration levels reached abnormally high values. Simultaneously, amplitudes of 2× and 3× showed up. For the non-drive-end (NDE) bearing, 1×, 2× and 3× vibrations were reported around 108 microns, 85 microns and 29 microns, respectively.
For the drive-end (DE) bearing, 1×, 2× and 3× vibrations were recorded about 58 microns, 21 microns and 16 microns, respectively. These high-shaft vibrations indicated the presence of a serious machinery fault. The control system canceled startup, at which point vibration levels measured at the bearings peaked at more than 340 microns.
Investigation revealed too small a radial clearance between seals and the steam turbine shaft. When the rotating speed approached the first critical speed upon startup, vibrations caused partial arc or full annular rubs. The heaviest rubs happened near the NDE bearing in the seal. A visual inspection of the steam turbine after the shutdown found serious damage to the seals and journal bearings.