The case study pertains to 4 x 80 hydro station (Fig. 7.49) having high vibrations and intense noise problem. The machines, 428.6 rpm, operated at a net head of 251 m,. the number of runner vanes were 11 and the number of guide vanes were 24. The vibrations and noise occurred predominantly at 2 x blade-passing frequency. Interference calculations (Equation 7.26) showed that the calculated speed factor S = 0.070 and (KZr – Zg)/KZr = 0.09 were in the near neighbourhood of each other. Changing the number of runner vanes to 13 was suggested.
There are certain problems of vibration / noise in the components of hydro unit which arise out of poor tail-race design. In one particular case where four units of 80 MW were operating, the tailrace boundary in front of the first unit was too close which seriously affected the evacuation of water from that unit. This caused a serious vibration and noise and thrust / guide-bearing failure problem on that unit. The other units had not experienced this problem since the tail-race boundary was at a sufficiently long distance from these units. Based upon the detailed model studies, the tail-pool boundary in front of Unit 1 was shifted by about 5 metres resulting into smooth operation of Unit 1.
In hydro units, the penstock vibrations, at times result into vibration / noise of the machine. In one case, the flutter of butterfly between penstock and the spiral casing had caused both penstock as well as machine vibrations. These case studies do show that apart from the normal causes of vibrations such as unbalance and misalignment, there can be several other reasons which require an in-depth study of system parameters, lay out and so on. It is needless to emphasize that these problems cannot be solved by popular methods such as balancing and alignment. For such problems, the solution lies in eliminating or minimizing the hydraulic perturbation forces.
