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

Activity: Talk or presentationTalk at conference or symposiumScience to science


Surge tanks are required to enable power and frequency governing of hydraulic machines in hydropower plants with pressure tunnels. Surge tanks decouple the water inertia in the pressure tunnel and the pressure shaft to mitigate the water hammer and decrease the acceleration time of the water masses. To improve the hydraulic performance and reduce the construction costs of surge tanks, the 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 instead of on the invert 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 improve 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) for an upgrade with an additionally 250 MW. In the presented case study, the semi-ACST is proposed as an extension of one of the three exiting hydraulically coupled surge tanks. The main contributions in the paper is the presentation of this new surge tank concept, its design principles, limitations and advantages.
Period27 Jun 2022
Event title31st Symposium on Hydraulic Mmachinery and Systems
Event typeConference
LocationTrondheim, NorwayShow on map
Degree of RecognitionInternational


  • Surge Tank
  • Air Cushion surge tank
  • Transient Hydraulics
  • 3D CFD
  • Sustainable Hydropower
  • Retrofitting