Figure 7.21 shows the vibration data obtained on a large boiler feed pump operating on a re-circulation mode. The boiler feed pumps, at times, are run in a recirculation mode, when turbine trip takes place and restart of the turbine / boiler is expected to be done shortly.
The data shows very high velocity / acceleration levels in horizontal directions, both on DE and NDE bearings. Note that the displacement levels are, although high, are not alarming. In situations where the overall displacement and acceleration levels are not high, the vibration spectrum will have lowfrequency peaks dominating. But in situations, as shown in Fig. 7.21, where the acceleration levels are very high when compared with the displacement levels, we must look for the presence of highfrequency components when spectra are seen in the acceleration mode. In fact, in such situation, one should always see the vibration spectrum in NDE bearing, the highest levels are at 7 × running speed.
The pump has seven vanes in the impeller and hence, any defect in radial / axial setting of the pump or flow irregularities, will produce high vibration at blade or vane-passing frequency. Figure 7.22 shows the acceleration spectrum where one can notice hardly any significant peak at 1x, 2x, and so on, and a lone peak at vane-passing frequency. In this particular case, it was noticed that the pump-impeller setting was asymmetrical and also that radial-tip clearances had been kept too tight. In many situations, the pump-impeller stages are staggered so that the excitation at vane-passing frequency minimized (by staggering, it meant that the angular position of vanes for various stages are not the same though each stage has same number of vanes).
The case study shows that while we note down vibration details, we should also include pertinent information of the machine such as number of blades / vanes, guide blades / vanes, number of stages, machine sketch, the operating parameters, and vibration / noise at various operating conditions. The pertinent information of the machine will vary from machine to machine—for example, it is an electrical motor we need to know information about whether the motor is AC or DC motor, rpm of motor, current drawn, number of poles, the type of bearings used, and so on.
