Cavitation in hydraulic machinery

1. Cavitation in hydraulic machinery

2. Cavitation in hydraulic machinery

3. The collapse of the bobble close to a surface will be asymmetric. • A jet stream will be formed in the center and hits the surface with large impulse. It has been measured pressure pulses up to 1000 bar and velocities around 200 m/s in a collapsing bubble. • The collapse creates local pressure oscillation with a large amplitude. • It is not known if it is the jet stream, pressure pulse or both that causes the damage to the surface.

4. Cavitation over a ving profile Ref. Morten Kjeldsen

7. Types of cavitation in hydraulic machines Saturated water vapor pressure versus temperature Stages of cavitation Ref. Hydraulic Machines, Turbines and Pumps G.I. Krivchenko

8. NPSH Net Pressure Suction Head NPSH Net Positive Suction Head [m] hv vapor pressure head [m] HA atmospheric pressure head [m] z2 Height above ref. line at location 2 [m] z4 Height above ref. line at location 4 [m] c2 mean velocity at location 2 [m/s] s loss coefficient [ - ]

9. 4 z4 Losses

10. 4 z4 Let us introduce the vapor pressure, hv :

11. 4 z4 NPSHNet Pressure Suction Head Atmospheric pressure: HA = h4

12. 4 z4 Suction Head hs

13. Submergence of a turbine NPSH Net Positive Suction Head [m] hv vapor pressure head [m] HA atmospheric pressure head [m] HS Submergence [m] c2 mean velocity at location 2 [m/s] s loss coefficient [ - ]

14. NPSH available This is the NPSH that is given by the site where the turbine is installed NPSH required This is the NPSH that the turbine required for non-cavitating operation NPSH available and NPSH required

15. Law of Thoma Provided that similar hydraulic cavitating flow remain unchanged relative to the flow canals, the relations of hydraulic similar flow, are valid also for flow including cavitation.

16. Thoma’s Cavitation Coefficient Thoma’s Cavitation Coefficient s Speed number W

17. Critical Cavitation Coefficient Dh= 3 % Efficiency, h [ - ] sCritical Thoma’s Cavitation Coefficient, s