Abstract
Dangerous runner lifting-up (RLU) accidents regarding Kaplan turbines, which are widely used in low-head hydropower stations, were frequently reported. Three-dimensional (3D) computational fluid dynamics (CFD) was used to simulate the load rejection transients with guide-vane closing to predict the RLU possibility of the fixed-blade Kaplan turbine in an under-construction hydropower station. It was found that using any linear closing rule, the upward axial water force on the runner was larger than the weight of rotating parts that started before the guide-vanes were closed, which indicated a RLU possibility. It was the pumping effect that caused the imbalance, during which the high rotational speed runner propels water downstream with a low discharge. We proposed a piecewise closing rule based on this finding. By keeping the opening unchanged in a period in the closing process, the rotational speed can be reduced by using the braking effect, and the concurrence of high speed and low discharge can be prevented. Simulations verified this effective measure and accepted by the manufacturer. Although this study used a fixed-blade Kaplan turbine, the revealed mechanism and verified solution to the RLU problem have reference value for all of the Kaplan turbines.
Highlights
Kaplan turbines are widely used in low-head hydropower stations, because of their high specific-speed, large discharge capacity, and broad performance zone
In China, the Kaplan turbines in Hejiatan [3], Mantianxing [4], and Xiacheng [5] hydropower stations encountered several runner lifting-up (RLU) accidents during load rejection transient processes, which resulted in the destruction of the bearings
The reverse water hammer that caused RLU can be described, as follows: during the load rejection transient process, the rapid closure of guide-vanes causes pressure drop in the draft-tube; the water column can be separated if the pressure at the runner outlet reaches at the cavitation pressure [10]; the rejoining of the separated water columns results in an impact on the runner [11]
Summary
Kaplan turbines are widely used in low-head hydropower stations, because of their high specific-speed, large discharge capacity, and broad performance zone. The reverse water hammer that caused RLU can be described, as follows: during the load rejection transient process, the rapid closure of guide-vanes causes pressure drop in the draft-tube; the water column can be separated if the pressure at the runner outlet reaches at the cavitation pressure [10]; the rejoining of the separated water columns results in an impact on the runner [11]. The pumping effect that caused RLU can be described, as follows: without reverse water hammer during the load rejection transient process, the rotational speed increases while the discharge decreases; the turbine will work in a special mode in which the runner propels water downstream if a high rotational speed encounters a low discharge; if the reaction force of water is large enough the runner can be lifted up. The detailed methods, discussions, and verifications will be presented
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