1 - Alaska Energy Data Inventory

The Svee equations were therefore solved over the entire range ofturbine net heads in the area of the turbine characteristics curve wherethe efficiency term was minimum. The maximum required tank volume forincipient stability was found to occur at a net head of 875 ft with a loadincrease from 46,500 to 47,191 hp. The air volume was 23,838 ft 3 andwater volume was 14,700 ft 3 , yielding a total tank volume for incipientstability of 38,538 ft 3 and a water surface of 3,500 ft2 using minimumlosses for an 11 ft diameter power tunnel.It should be noted that the selected power tunnel diameter of 11 ft isthe design value and for a conventionally excavated tunnel the actualdiameter will be somewhat greater. The construction contract will call fora minimum excavated section corresponding to an 11 ft section. Thecontractor wi 11 not be paid for any excavat i on beyond the 11 ft sect i on.The variance in tunnel diameter from the 11 ft design value is ofimportance both for hydraulic loss factors and for requirements of airchamber volumes. (Tunnel size will effect surge tank diameter in aconventional vented surge tank in the same way.) For the purpose of sizingan air chamber, overbreak will increase the conduit's net flow area abovethe 11 ft design value, and this factor suggests conservatism is requiredto insure an air chamber volume that is adequate for stability. Theapproach taken in analyzing average tunnel areas larger than 11 ft isconservative in that it does not include the added roughness which wouldresult from the cone frustrums blasted from each round. This roughness issignificant and is described in references 5 and 8.The following table shows the air volumes required for various powerconduit diameters as predicted by the Svee equations using expectedhydraulic losses.B4-10