Hydraulic machinery |
New turbine design silences penstock noise
A redesigned turbine distributor at the Melezet hydropower plant has resolved long- standing RSI-induced noise issues in the residential area of Bardonecchia, reducing vibrations and ensuring compliance with environmental noise regulations. By P. Caretti, A. Bramati, F. Ferrari, M. Lauro, A. Cannata, and D. Giors
Figure 1: Main section of original turbine before unit rehabilitation
THE MELEZET HYDROPOWER PLANT, owned by Enel Green Power since 1979, is situated within the residential area of Bardonecchia, a mountain resort town located below Col du Fréjus near the French border. The plant’s penstock extends 390 meters in length, with sections not fully embedded in concrete or rock.
Originally commissioned in the early 1960s by Ferrovie dello Stato (the Italian national railway company), Melezet began commercial operations shortly thereafter. Enel acquired the facility in 1979 and implemented its current configuration. At that time, the plant was equipped with a highly unconventional horizontal-axis Francis turbine, featuring a single inlet for a double runner. This design excluded a crown and included a double discharge, as illustrated in Figure 1.
Although the original design used a conventional configuration of 13 runner blades and 20 guide vanes – typically effective in avoiding resonance issues – acoustic problems still arose due to penstock vibrations. These issues were most prominent in the 250 Hz third-octave band, linked to the runner-stator interaction (RSI) excitation frequency of 216.7 Hz. To mitigate the resulting noise, an anti-noise coating was applied to the penstock.
Renovation and recommission The plant, excluding the penstock, underwent a
comprehensive renovation and was recommissioned in 2019. The new Francis turbine was redesigned with 17 runner blades and 16 guide vanes – a combination theoretically less susceptible to to excite the penstock vibration. This is due to the dominant RSI pressure pulsations shifting toward the outlet of the spiral case, reducing their impact on the penstock.
During the commissioning of the new unit, the following Spectrum (FFT) CH1 Freq. spectrum
0.300 0.250 0.200 0.150 0.100 0.050 0.000
CH2 Freq. spectrum 0
0.350 0.300 0.250 0.200 0.150 0.100 0.050 0.000
0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 [Hz]
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 [Hz]
[Hz] [mm/s] 283.50 0.285 16.50 0.251 33.50 0.084 66.50 0.066 50.00 0.065 14.50 0.051 64.00 0.041 36.50 0.033
[Hz] [mm/s] 283.50 0.324 33.50 0.171 16.50 0.093 67.00 0.053 23.00 0.042 24.50 0.042 50.00 0.041 26.50 0.040
Above: Figure 2: Generator DE bearing vibration spectra with original distributor 26 | June 2025 |
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14 Jan 2020 12.35.35 - BP5.5MWSCRAD 15/01/20 01:36:07
key observations were made: 1. The generator DE bearing, which is close to the overhung Francis runner showed significant vibration amplitudes on RSI main frequency 283.4 Hz, as indicated by the spectra (Figure 2) taken at full load of 5.5 MW (CH1=horizontal, CH2=vertical direction).
2. A dedicated environmental acoustic measurement showed a clear tonal phenomena centered on the 1/3 octave band of 250 and 315 Hz, exactly due to the RSI frequency of 283.4 Hz. In particular, the tonal effect was heard in few houses of the city of Bardonecchia, which have probably a bad location with respect of the penstock. The results of a nighttime noise survey conducted at the penstock outlet near the hydroelectric power plant (HPP) are presented in figure 3.
In an attempt to reduce this acoustic impact, a specialized insulation layer was applied along the entire penstock
mm/s rms
mm/s rms
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