Increasing Hydro Turbine Operation Range and Efficiencies Using Water Injection in Draft Tubes

Abstract

It is a well known fact that most Francis turbines, because of the fixed blade design, faces challenges when running at partial load operation. Especially in the operating range below approximately 50% of the rated output, it is common to observe severe pressure pulsations and surge in the draft tube. These pressure fluctuations are believed to be related to the swirling flow exiting the runner. By using water jets in the draft tube cone directed towards the swirling flow, the swirl strength is believed to be reduced and thereby also the pressure fluctuations produced by the swirl. This system thus has a potential of increasing the turbine operating range. The system can be activated when needed, and will not affect the turbine when running at its best efficiency point.

Based on the main hypothesis, a simplified swirl rig was designed and constructed in order to investigate the nozzle influence on the swirling flow and on the pressure pulsations in a simplified environment. To expand the understanding of the nozzle performance in a Francis turbine, experiments were conducted in a model turbine with a prototype of movable nozzles. To establish a link between laboratory nozzle measurements and full scale nozzle measurements, field measurements were carried out on full scale Francis turbines running at partial discharge. For this purpose the turbines installed at Skarsfjord Power Station and Skibotn Power Station were used, where full scale nozzle injection systems were installed.

The test results suggested that the concept of water injection worked, but not unconditionally. A reduction in pressure fluctuations was achieved both in laboratory and field experiments, as well as a noticeable reduction regarding fluctuations in the shaft run-out at Skibotn. In addition, water injection gave a surprisingly positive effect at overload conditions in the model turbine, even though the nozzle angle was directed in the same direction as the overload swirl.

Ideally, the results from this project should be presented as a linear equation to give the ideal nozzle configuration for a given operating condition. This has not yet been possible because of the complexity of the system and its variable factors. However, several effects from changing nozzle variables individually have been detected, and are summarized in chapter 1 - Conclusions and Achievements.

Efficiency measurements in the laboratory and in the field experiments indicated a possibility of increasing the hydraulic efficiency with the water injection system activated. This positive feature was believed to be caused by improvement of the velocity field in the draft tube. However, the total efficiency was always decreased because of the nozzle water bypass.