1 - Alaska Energy Data Inventory

Asc = Critical (horizontal) area of vented surge tank in ft2.N = Polytropic gas constant.Po = Pressure of air cushion (gauge, steady state). This is the headdifferential between the hydraulic gradient and the water surface elevationin the air chamber in ft of water.Acr = Critical (horizontal) area of air chamber surge tank in ft2.~ = Specific weight of water in Ibs/ft3~Zao = Distance between air chamber roof and water level in the airchamber oj n ft.The first equation is an expansion of the Thoma equation as used forvented surge tank s. The second equat i on converts the Thoma area to thecritical surface area for an air chamber surge tank which is then used tocalculate required air volume for an air chamber surge tank., Minimum head loss values were used for stability calculations. Whileminimum net head is used for stability calculations for vented surge tanks,this may not be the critical condition for determining air chamber surgetank dimensions. The reasons for this are (1) the efficiency termused with the first Svee equation varies with net turbine head and lowervalues of the term produce larger chamber areas and volumes. This term isdependent on the gradient of the turbine efficiency curves as shown onPlate 822. These efficiency gradients are different at different netheads. (2) Since the tank invert is established by maximum downsurge, thiselevation is set; but in calculating air volume required for stability overthe entire range of turbine net heads, it is possible that this requiredair volume plus the volume of water in the tank between the tank invert andthe water surface (which rises as turbine net head rises), will becomemaximum at some intermediate turbine net head corresponding to a reservoirelevation other than minimum pool.84-9