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 390m 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 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 (figure 4). However, the intervention did not achieve compliance with Italian environmental noise regulations. These laws mandate nighttime noise levels below 50 dB(A) and daytime levels below 60 dB(A) in residential zones, with an additional 3 dB(A) penalty for tonal components—regardless of the unit’s output power.
A dedicated analysis was conducted to investigate pressure pulsations within the spiral case and associated phase resonance phenomena. The findings suggested that modifying the RSI behaviour would be a more practical and cost-effective solution than replacing the penstock entirely.
Specifically, the analysis proposed replacing the distributor to reduce RSI effects while retaining the current runner (including its identical spare). The recommended design involves increasing the number of guide vanes from 16 to 24, which was found to yield the lowest synchronization factors of RSI pressure pulsations at the spiral case inlet – thereby mitigating vibration and noise transmission along the penstock.
Figures 5 and 6 illustrate a significant reduction in the RSI resonance factor at the spiral case inlet with the proposed distributor modification. This factor, ranging from 0.0 (ideal) to 1.0 (worst case), directly correlates with the severity of vibration and noise issues.
The analysis revealed a significant reduction in the resonance factor – from the original value of 0.136 to 0.047 – achieved through the implementation of the new distributor configuration. Additionally, the 24-wicket gate solution offers a further advantage: the shorter gate length increases the distance between the gate ends and the runner inlet, resulting in pressure pulsations with lower peak-to-peak amplitudes. These positive results provided a solid foundation for designing a new distributor with 24 wicket gates.
Designing the new distributor posed several challenges, primarily due to the following reasons:
1. The limited space available on the head cover, originally intended for 16 guide vanes, required precise optimization of the lever and link dimensions to accommodate 24 guide vanes.
2. The Francis turbine is equipped with a relief valve whose oil servomotor is directly connected to the distributor servomotor. Consequently, the new distributor configuration had to be fully compatible with the existing relief valve system.
Techydro undertook the design of the new distributor assembly, carefully minimizing the impact on existing turbine components. At the same time, additional enhancements were implemented, including: A redesigned shaft seal arrangement to reduce water leakage; enlarged relief pipes to minimize axial thrust by improving seal flow control; and upstream covers split into two parts to facilitate maintenance and dismantling.
Comparative drawings (Figure 7) illustrate the differences between the new 24-wicket gate configuration and the previous 16-gate design.
By optimizing the linkage dimensions and verifying the resulting loads, it was possible to retain the existing servomotor without modification. Commissioning of the unit confirmed not only proper turbine operation but also validated the numerical analysis results. Notably, it eliminated vibration excitation in the penstock, thus preventing tonal effects. This is supported by noise measurements taken during the day at the most exposed nearby residence, showing a level of Leq = 49.6 dB(A), with corresponding spectra (Figure 8).
Furthermore, vibration spectra from the DE bearing at full power (5.5 MW) showed that the RSI pressure pulsation component at 283.4 Hz was entirely eliminated (Figure 9).
Transient tests validated the proper functioning of the new distributor in conjunction with the existing relief valve. These tests also determined the optimal final stroke of the distributor servomotor, which was set at 65% to precisely deliver the concession flow of 3.00 m3/sec.
References
Den Hartog – Mechanical vibration – McGraw-Hill Book Company, Inc. 1956
P.Caretti, S.Cartapani, L.Papetti – Penstock resonance due to RSI pressure pulsations on Francis turbines: a case study in a revamped plant – Graz- Hydro 2024
P.Caretti, A.Cannata, A.Prato – Mompantero anni 1938-2018. Perchè le turbine Francis possono fare “cantare” le condotte forzate – Padova – Hydromatters 2019
Author details
Paolo Caretti graduated in 1987 in mechanical engineering at the Politecnico of Milano. In 1991 joined Riva Hydroart (from 1993 Voith Hydro Milano) working as hydraulic and mechanical engineer in engineering dept. In February 2016 he started up his own hydro-mechanical activity (HMSS of Paolo Caretti) and operate as a consultant on hydro machinery.
Andrea Cannata graduated at the Politecnico of Milano in mechanical engineering in 2008. In 2009 joined Enel Produzione, from 2013 to 2017 head of Verampio and Pallanzeno Hydro power plants (7 HPP, ~330MW in Toce basin), then from 2017 to 2024 head of Northern Western Area Maintenance (110 HPP, ~2.700 MW in Piedmont Region). Nowadays head of HPP commissioning unit of O&M Hydro Italy. Author of several publications about hydroelectric industrial history and technical publications.
Adalberto Bramati graduated in 1976 in mechanical engineering at the Politecnico of Milan. In 1977 joined Hydroart (then Voith Riva Hydro), Milan, and in 1992 became Director of the Technical Office and in 1999 member of the Management Committee of the Voith Group. Since 2001 he started up as Managing Director and Technical Manager of Techydro S.r.l., Vimercate (MB):
Fulvio Ferrari graduated in 1978 in mechanical engineering at the University of Genoa. From 1980 to 2001worked in the R&D and in the Technical Dept. of Hydroart (then Voith Riva Hydro), Milan. From 2002 to 2019 worked as Technical and Quality Manager at Pompe Cucchi S.r.l. (gear pumps), Opera (MI). Since 2020 collaborates with Techydro S.r.l., Vimercate (MB).
Marco Lauro graduated in 1989 in Mechanical engineering at Università di Genova. From September 1990 to June 1992 in charge to Aeritalia, now Alenia, Space Division as technician for scientific satellites. From 1992 to 2010 technical expert and internal reference for O&M technical support. From 2011 to 30/12/2015 within GLOBAL GENERATION – Hydro Technical Line. Then Head of Genova Coal Thermal Power Plant. From 30/09/2017 within O&M Hydro as technical support manager for Enel worldwide hydro power plants
Diego Giors graduated from ITIS Enzo Ferrari in 2004, from 2007 to 2019 he was a technical assistant at Enel Green Power as a technical employee of the Operating Unit operating in Val di Susa. From 2019 to today in Enel Green Power spa – O&M Hydro. Within the Hydro Technology Line, he is responsible for the Venaus Operating Unit with a fleet of 12 plants and 15 generating units and 3 large dams.
