**3.1.1 Unbalance**

Unbalance is common to all rotating elements. Primary symptom of this malfunction is the 1 *f*0 component of absolute vibration in a direction perpendicular to the turbine shaft line. They are, however, many other possible malfunctions (some of them quite common) that produce similar vibration patterns; additional procedures are therefore usually needed for a correct diagnosis.

In general, a 'pure' unbalance, be it static, quasi-static or dynamic, produces a 1 *f*0 component that remains almost constant in amplitude and phase during steady-state operation and disappears at low rotational speed. As rotor systems are non-linear, this component is typically accompanied by higher harmonics (*n* > 1), with amplitudes decreasing as *n* increases. Shaft orbits usually are quite regular and nearly circular or slightly elliptical. If such vibration pattern is present, the probability of unbalance being the root cause is high. Proper rotor balancing will usually reduce the residual unbalance to an acceptable level.

Rotor systems will always respond to balancing. Step changes of the 1 *f*0 component not related to any maintenance activities (but occurring mainly after turbine shutdown and subsequent startup) may be indicative of a loose rotor disk. Similarly, sudden and dramatic change may result from a broken rotor blade; such step changes are often big enough to enforce turbine tripping. Much slower, but continuous increase is often indicative of a permanent rotor bow (see also sub-section 3.1.3). An example is shown in Fig.3; it is easily

Fig. 3. Time history of the 1 *f*0 component with permanent rotor bow present: 230 MW unit, rear intermediate-pressure turbine bearing, vertical direction. Arrows indicate balancing sessions.

seen that balancing results in a considerable decrease of the 1 *f*0 component, but the improvement is only temporary. If this component is comparatively high at low rotational speed, coupling problem (offset rotor axles) is a possible root cause, especially in turbines with rigidly coupled rotors.
