Abstract
The discharge and power input of Deriaz mixed-flow pumps and pump turbines can be controlled over a wide range without significant loss in efficiency by movement of the blades. The attainment of the highest pump efficiency involves a small region of positive slope in the head discharge graph.
This paper examines the limitations of designing pumps which do not exhibit positive slope and the problems of governing if a positive-slope region is accepted.
It is shown that operation in the positive-slope region can introduce a serious organ-pipe resonant instability in the pipeline.
A simple criterion to determine whether or not such oscillations will occur is given. This stability criterion involves the dynamic characteristics of both the pump and the pipeline.
Finally, computer studies of a particular Deriaz pump installation are presented. These underline the severity of the situation should resonance develop and show how it can be avoided by the use of an air bottle of appropriate design.
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