Cavitation in hydraulic structures : with special reference to hollow jet valve type energy dissipators and ukuwela outlet works

Abstract

Cavitation is a major source of damage to hydraulic structures and hydraulic machinery and leads to failure of function. In all forms of cavitation (fixed, travelling, vortex and vibratory) low ambient pressures and high velocities are conducive to cavitation. Low pressures can be caused by narrowing down or abrupt change of curvature of flow passages, separation of flow from wall surface, vorticity or pulsation and stable cavities are formed when local pressures drop close to vapour pressure. Cavities thus formed, collapse when they move into high pressure zones with tremendous implosive violence causing the damage to guiding surfaces . Theoretical analyses of "Bubble Dynamics” have been made and the theories verified experimentally. The influence of factors such as surface viscosity, tension tensile strength of liquids, solvates etc. have also been analysed. Elaborate equipment such as water tunnels with facilities for observing cavitation on models, controlling and simulating influencing conditions such as velocity, pressure, temperature and measuring functions such as torque and thrust, while visually observing and "listening in" on cavitating components have been evolved. Stroboscopic lighting and high speed motion photography have be:.n developed to record and study such phenomena. Equipment such as Rotating Disc and Drop Impact Device have been made to study cavitation resistance of various materials. Study of cavitation through models and comparison of performance and ensuring dynamic similarity are achieved by use of a dimensionless coeffithe Cavitation Farameter, which is a ratio of energy available to collapse a cavity to the energy available to produce it. The computation of this parameter takes a form appropriate to the hydrodynamic context. The Cavitation Parameter is also used as an index to demarcate cavitation inception, threshold, advanced stage, etc. cient Stilling basins are structures under potential cavitation hazard. Ukuwela Power House Irrigation Outlet Stilling Basin of Jet Diffusion type with Twin Hollow Jet valve has been selected for case study, since the structure commissioned in 1976 suffered cavitation damage in several forms. Model tests for this structure have been conducted by the Irrigation Department Hydraulic Laboratory earlier. The model test results and "Wet test" results on the Prototype are reviewed in the light of Cavitation Theory studied in depth. Conclusions regarding situations conducive to cavitation and methods of preventing same by appropriate design and computation have been drawn from the above study. Thus recommendations for cavitation free design and construction of hydraulic structures (especially stilling basins) have been made for future guidance.