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According to physical
laws high velocity will lead to low pressure in the flowing medum. If
the flow speed becomes big enough, the pressure locally can fall below the vapour pressure
of water. In these low pressure areasthe water boils and is transformed into vapour
bubbles (cavities). When the vapour cavities gets into areas where the water pressure is
higher than the vapour pressure they can no longer exist but colapse (implode). The
implosions will cause local but extremely intensive pressure shocks. If this happens close
to a metallic surface (for example at the runner blades) erosion damages can arise
(cavitation damages). The cavitation course also often causes vibrations and noise in the
turbine. Cavitation is the comprehensive name for the whole described course (high speed
– low pressure – steam generation – implosion – erosion). Cavitation is a problem for water turbines mainly for three reasons: 1.It limits the size of output from a given turbine. When opening the turbine guide vanes, the flow through the machine increases and accordingly the flow velocity in the runner. The cavitation intensity increases. Finally the runner channels will be blocked by water vapour bubbles to such a great extent that the efficiency falls dramatically, which in turn makes further increase of output impossible. 2. It causes erosion damages on the runner and other parts of the turbine. If the turbine is run with too high load during a long period, damages can incur and grow. Therefore the waterways of the turbine are regularly inspected. Any incurred damages will be repaired by welding and grinding. The repairs must generally be made at site inside the turbine, which is as well complicated as expensive. The repairs and costs for standstill can in certain cases become a considerable part of the maintenance costs for the turbine. Quite decisive for the extension of cavitation is the elevation of the runner compared to the downstream water level (TWL). A high position stimulates the cavitation, while a low position gives an improved cavitation situation. If the height position of the turbine is correct the turbine can be run for many years without cavitation damages, provided that the maximum turbine output allowed has not been exceeded. 3. It causes vibrations and noise in the power station. For these reasons it is important to study and examine the cavitation characteristics of the turbine already during the model tests. Based on the model test results the cavitation limits of the turbine (max output) regarding flow and head are decided.
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| The figure above shows cavitation in
a runner blade. It shows the outer edge of a runner blade in a Kaplan turbine. The
cavitation is developed at the bottom side of the blade where the pressure is lowest.
Cavitation bubbles are produced already at the top of the blade inlet edge. Then the
cavitation intensity increases successively downwards along the blade and culminates at
the outlet edge.
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