Extreme water hammer pressure of spiral case in hydropower station with super long headrace tunnel and surge tank during load rejection: Mechanism analysis and improvement measures

The hydropower station with superlong headrace tunnel (HSSLHT) is a key form of hydropower development. However, HSSLHTs exhibit substantial water flow inertia and large surge wave amplitudes, which make the transient characteristics different from those of conventional short headrace tunnel hydropower stations. Therefore, the conventional water hammer pressure reduction measures for short headrace tunnel hydropower stations cannot be fully applicable to HSSLHTs. This study aims to clarify the mechanism responsible for the maximum spiral case pressure (SCP) in HSSLHTs and to propose corresponding improvement measures. The mathematical model of the HSSLHT is first developed using the method of characteristics and validated through prototype testing. Subsequently, the “double-peak” characteristic of SCP fluctuations is revealed, where the maximum SCP occurs at either the first or second peak. Finally, engineering measures are proposed to mitigate the maximum SCP. Results indicate that the first peak of the SCP occurs earlier, arising from water hammer pressure induced by the rapid closure of the guide vane, while the second peak of the SCP occurs later, coinciding with peak surge in the surge tank resulting from water level oscillations. The first peak can be reduced by enlarging the surge tank shaft diameter, increasing the impedance orifice diameter, extending guide vane closure time, and reducing the headrace tunnel diameter. The second peak can be reduced by enlarging the surge tank shaft diameter and decreasing the impedance orifice diameter. Overall, these findings provide guidance for engineering design and operational safety of HSSLHTs.