Design, construction and operational experience at the Torpa air cushion surge chamber

The Torpa Power Plant, one of the 2 power stations in the Dokka project, features an air cushion surge chamber. The total volume of this chamber is 17,400 m 3 and the air volume 10-12,000 m 3 . In the surge chamber area, the rock overburden is 225 m while the water head is 450 m. To prevent air leakages, a water curtain above the chamber has been constructed. The water curtain pressure is between 2 to 5 bar higher than the chamber pressure. One of the main design criteria imposed, was that the curtain pressure must be less than the minimum principle stress in the rock, in order to prevent hydraulic fracturing. Extensive hydraulic fracturing tests have therefore been performed. The air cushion surge chamber has been built in the shape of a doughnut. The water curtain holes were drilled from a “manifold” chamber above the surge chamber. The installed air compressor capacity is 940 Nm 3 /h. Two different control systems for monitoring the water level in the chamber, are also installed. After 2 years in operation, tests indicate that there is no air leakage through the rock mass, so long as the water curtain excess pressure exceeds 2 bar.