Causes and Effects of cavitation

InHydraulic power

plants

By hosam alryes mechanical engineering department

DATE : 17/12/2013

INTRODUCTION

Hydroelectric power is a form of sustainable green energy, by use moving water to produce electricityThis power is generated by converting the kinetic energy of flowing water into mechanical and then electrical energyIf we want to achieving high electrical energy, we need a

higher efficiency of hydro turbines with time ,but the turbines show declined performance after few years of operation as they get severely damaged due to various reasons, One of the important reasons is cavitation

Hydraulic turbines are divided into two groups:

Occurrence of Cavitation:

Different types of cavitation in Francis's turbine:

leading edge cavitation: travelling bubble cavitation

draft tube swirl: inter-blade vortex cavitation

: PHYSICAL DESCRIPTION OF CAVITATION

Thoma Suggested a dimensionless number called as Thoma’s Cavitation factor :σ

Ha is the atmospheric head pressure

Hs is the suction pressure at outlet of reaction turbine Hv is vapor pressure headH is the net head on the turbine

Ns is thespecific speed of the turbine

The values of from equation (1) is compared with the value of c from equation (2) σ σand (3) and if value of is greater than c, cavitation will not occur in that turbineσ σ

Reducing Cavitation:

The figure below shows that a high strain, work hardening austenitic stainless steel produces superior resistance to cavitational erosion of weight loss (mg)

Conclusions

Cavitation is a phenomenon of formation of vapor bubbles in low pressure regions and collapse in high pressure regions, high pressure is produced and metallic surfaces are subjected to high local stresses. Cavitation can present different forms in hydraulic turbines depending on the machine design and the operating condition. As a result, high vibration levels, instabilities and erosion can occur this invalidates the machine operation and cause damage. It is difficult to avoid cavitation completely in hydraulic turbines but can be reduced to economic acceptable level.

Thank you for your attention