The Semi-Air-Cushion Surge Tank: Design Principles and Case Study

Surge tanks are required to enable power and frequency governing of hydraulic machines in hydropower plants with pressure tunnels. To improve the hydraulic performance and reduce the construction costs, a new concept of semi-air-cushion surge tank (semi-ACST) is proposed. The semi-ACST can be particularly effective for upgrading and retrofitting of existing hydropower plants, when additional hydraulic machine capacity is added to an existing waterway. The key design element is a crown throttle, constructed as an inverted weir placed in the crown of the chamber. The purpose is to intentionally trap an air pocket during filling of the lower chamber. The air is released slowly through defined air pipes so that the air in practice contributes equivalent to added volume in the upper chamber. Thus, the semi-ACST improves the dampening of mass oscillation, without increasing the surge tank volume. The semi-ACST has been investigated and developed with multiphase 3D CFD simulations with RANS turbulence modelling. The design principles have been tested on the case-study Tonstad hydropower plant (960 MW). In the presented case study, the semi-ACST is proposed as an extension of one of the three exiting hydraulically coupled surge tanks.