Dynamic Absorber Aplication on Condensate Pump (Vertical Pump)

Power generation with rated output 400 MW operate two condensate pump to transfer water form condenser hotwell to low pressure heater. Theese pump are 6 stages vertical pump. If one of the pump is out service it would derated 50 % from the design output. The plant operator reported high vibration one of the pump. We were invited to conduct vibration analysis. Figure 1 shows machine diagram and picture :

Figure 1. Machine diagram

Vibration data ( table 1) were taken at motor outboard an inboard bearing.

Vibration	Motor		Pump
mm/s rms	Out	In	In	Out
Horizontal	7.0	2.5	0.8	--
Vertical	1.9	1.3	0.6	--
Axial	0.6	--	0.3

Table 1. Vibration data

From table 1 it was noted that vibration were dominant at horizontal direction ( peroendicular with pump discharge ) and the dominant frequency was at 1x order as shown in figure 2. Amplitude ratio between horizontal and vertical nearly 3 : 1.

Figure 2. Vibration spectrum at Outboard bearing-horizontal

Our hypothesis was we have resonance problem at horizontal direction, bump test also indicated that the pump natural frequency was very close with pump operating speed. To avoid resonance condition we have to move pumps natural frequency but this would cause the pump out service and decreased plant output. The only available option was to install Dynamic Vibration Absorber. This was made form rectangular bar and a mass adjusted to get natural frequency equal with pump natural frequency. Table 3 show vibration amplitude after absorber installation

Vibration	Motor		Pump
mm/s rms	Out	In	In	Out
Horizontal	2.1	0.9	0.6	--
Vertical	1.9	0.9	0.6	--
Axial	0.8	--	0.3

Table 3. Vibration after Absorber istalation

Figure 3. Vibration spectrum after absorber installation

After absorber installation vibration dropped from 7 mm/s to 2.1 mm/s.

Figure 4. Dynamic Vibration Absorber Instalation