The air leaking out of air-cushion surge chambers has to be compensated for by stationary compressors. To predict the air loss, the Norwegian Geotechnical Institute has utilized a method developed by Tokheim and Janbu (1973). The paper presents calculations and measurements made at three sites. The correlation between predictions and observations is reasonable. The observed air losses at the three sites ranged from about 8.3. 10-4m3/sec (at 10°C and 1 bar) to 3.3 10−1m3/sec (at 10°C and 1 bar). The paper also decribes briefly the 120 000m3 air-cushion surge chamber at Kvilldal, West Norway, operating since December 1981.
Luft die aus eine Luftkissenkaverne sickert muss durch Druckluft ersetzt werden. Zur Vorausschatzung der Luftverluste wurde bei Norwegischer Geotechnische Institut eine von O. Tokheim and N. Janbu entwickelte Metode benutzt. Berechnungen und Messungen fuer drei Baustellen werden prasentiert und zeigen eine verhaltnismassig gute ubereinstimmung zwischen berechnete und gemessene Werte. Die gemessene Luftverlusteliegen bei der dichteren Kaverne urn 50 l/min (10°C, 1 bar Normalluft), bei der undichteren urn 20 000 l/min. Eine kurze Beschreibung von die 120 000m3 grosse Luftkissenkaverne des Kraftverkes Kvilldal die seit Dezember 1981 in Betrieb steht, ist auch gegeben.
Les pertes d''air des chambres à coussin d''air sous pression variable doivent être compensees par des compresseurs. Afin de predire ces pertes, l''Institut norvegien de geotechnique a utilise une methode developpee par Tokheim et Janbu (1973). L''article presente les calculs et mesures faits à trois emplacements. La correlation entre predictions et observations est raisonnable. Les pertes d ''air observees aux trois emplacements varient entre environ 8.3.10−4m3/s (à 10°C, 1 bar) et 3.3'' 10−1m3/s (à 10°C, 1 bar). En plus l''article decrit brievement la chambre à coussin d''air de Kvilldal, de dimensions 120 000 m3, en operation depuis decembre 1981.
Developments in tunneling and power plant design have led to the use of deep high pressure head race tunnels. In this new design, the traditional surge shaft is replaced by an unlined rock cavern filled partly by water and partly by air. The aircushion is meant to dampen the pressure oscillations occurring with decreases or increases of the generator input, see fig. 1. The new design required a method to predict air leakage rates as a basis for selection of compressor capacity. The compressor size depends on the maximum time allowable for chamber filling, and the capacity necessary to compensate for air loss through the rock mass. In Norway, the State Power System, uses mainly mobile compressors for filling up, and stationary compressors to compensate for lost air. To predict air loss rates, the Norwegian Geotechnical Institute has used a method developed by Tokheim and Janbu (1973) at the department of Soil Mechanics at the Norwegian Institute of Technology. The method is described in detail by Dr. Tokheim and Prof. Janbu elsewhere in this symposium. chamber Fig. 1. General layout of a power plant with air-cushion surge chamber. At three power plant sites, the Norwegian Geotechnical Institute (NGI) conducted the geological investigations necessary to determine the best site possible for the location of air-cushion surge chambers, and predicted the air leakage according to the method of Tokheim and Janbu. The three power plants owned by the Norwegian State Power System are Lang-Sima, Oksla and Kvilldal. At a fourth location, the Nye Osa power plant, relatively Large air. losses were recorded. The owner, the Energy Authority in the County of Hedemark, provided NGI with permeability measurements which made possible a prediction of the rate of air loss.
The Tokheim and Janbu theory formulates the rate of air leakage through water saturated rock mass, Qaw'' as follows: G Geometry factor, depending on the dimensionless parameters riD and LID. r Chamber radius (idealized geometry). D Distance from cavern center to equipotential surface (= ground water level). Po Reference pressure, 1.0,105 Pa. Ps Maximum absolute air pressure in the cavern, Pa. Absolute air pressure on the dimensioning equipotential surface. This is usually equal to the ground water level and Pe = Po = 105 Pa. Pe As stated by equation (I), the air flow through the saturated rock mass is reduced with a factor Ψ compared to flow through the dry rock mass.
Air leakage rate in water saturated rock mass is a function of the dimensions D, r, L and the absolute air pressures Ps and Pe'' The mean water gradient, im, in the outward direction from the water filled part of the chamber after the surge chamber has been put in operation, is defined according to Tokheim and Janbu: Pr excess pressure of water in the chamber compared to hydrostatic distribution of pore water pressure, Pa. Yw unit weight of water, 10 kN/m3• In the case of air-cushion surge chambers, the gradient im, is generally about 0.5 and never more than 1.0 for free drainage to the surface.
