| Hydraulic machinery
Left: Figure 3: Original distributor noise measurement close to penstock
Above: Figure 4: Penstock with insulating layer
Figure 5: Original distributor phase resonance results
(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
Distributor in open position
New arrangement N.24 Wicket gates
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 Section A-A
Old arrangement N.16 Wicket gates
Figure 6: New
distributor phase resonance results
Below: Figure 7: Distributor comparison
Section A-A
Old arrangement N.16 Wicket gates
Distributor in closed position
New arrangement N.24 Wicket gates
www.waterpowermagazine.com | June 2025 | 27
arrangement
New
arrangement
Old
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