Experimental analysis of the vibration on the draft tube of a Francis hydraulic turbine during operation at different power levels

Severe vibrations are present on the draft tube of a 95 MW hydraulic turbine operating at partial load. Frequent failures on structural elements of the power plant are produced, due to the hydraulic instability inside the draft tube; mainly when the machine operates at partial load. Pressure inside the draft tube and stress on the external draft tube wall were measured at several power levels to identify the power where the conditions are critical for the structure. Also, measurements were conducted at partial load with air injection over the stay vanes and pressure relief on the spiral case. It was found that air injection effectively reduced vibrations while the pressure relief on the spiral case did not produce any useful effect.