Project 2: Vibration control of the Sallent reversible Hydroelectric Power Plant (90’s)

#ICR 25th year anniversary project review.

 

The Cabdella lakes are a group of 27 lakes connected by underground tunnels at the head of the Vall Fosca, in the LLeida Pyrenees. Its function is to supply water to the Estany Gento, from where two hydroelectric power stations are supplied, the Cabdella Hydroelectric Power Plant and the Sallente Reversible Hydroelectric Power Plant, in the Sallente reservoir.

 

The reversible hydroelectric power station takes advantage of the low cost of electricity during the night to pump the water from the lower reservoir of Sallente to the upper Estany Gento and the high cost of electricity during the day to generate energy by turbining the water from the upper reservoir to the lower one.

 

After the construction of the Sallente power plant in 1985, excessive vibration was detected in the two main pipes that feed the four turbines of the plant. For this reason the plant could not operate with a group load greater than 50%, because of the possible risk of pipe breakage due to vibration fatigue.

 

ICR worked to diagnose the problem and propose solutions to solve it. Vibration measurements, finite element models and ODS (Operational Deflection Shape) visualization were carried out to study the vibration modes of the pipe both at idle condition and in a forced regime under the effect of water. Mechanical fatigue studies were also done (with the collaboration of ST Mecánica Aplicada).

 

As a result of the study, it was detected that the second harmonic of the pressure pulsations, caused by the passage of the impeller blades in front of the turbine pump distributor, caused the vibration of the free section of the forced pipe to enter into resonance with one of its own modes, causing excessive movements.

 

This was solved by modifying the own modes of vibration with the placement of some straps on the force pipes, thus achieving full performance. Since then, its position, tightness and vibration level are periodically checked.