Pressure Regulation Valves for Energy Dissipation and Power Control in a Hydropower Plant Case Study

Pressure regulation valves (PRVs) are optional safety devices, used in hydropower conduits, to dampen pressure surges and dissipate excessive energy subsequent to discharge variations. The operation can reduce the risk of severe pressure surges and the negative environmental effects of hydropeaking of run-of-river hydropower plants (HPP) through a turbine bypass. For instance, sleeve pressure regulation valves (SPRVs) have been installed in the five Francis turbines at Tonstad HPP, a 960 MW high-head power plant in Norway, to minimize the pressure surge and runaway speed following the emergency turbine guide vanes closure. In this paper, the transient operation of the SPRVs was investigated by computational fluid dynamics (CFD) tool as a case study. The energy dissipation, the downstream and the draft tube end head fluctuations, and the risk of cavitation in the valves were assessed. The results indicate that high velocity gradients can lead to local cavitation wear in case of longer operation. Consequently, a study was conducted to evaluate design improvements to avoid cavitation. One alternative is to install a submerged hooded fixed-cone valve (FCV). The principal hydraulic effect has been studied theoretically for Tonstad HPP. According to the CFD simulation results, the installation of the FCV does not cause noticeable head fluctuation at the tail-race tunnel end. The average local loss coefficient of the SPRV and FCV are 1.08 and 1.05 [s2/m5], respectively. PRVs are demonstrated to efficiently bypass water and dissipate energy to allow significant reduction of pressure rise and runaway speeds for hydropower turbines.