1 - Alaska Energy Data Inventory

4.04 AIR CHAMBER SURGE TANK.A. Operating Requirements - The Snettisham project serves an isolatedload in Juneau. The Crater Lake unit may at times be handling the entireJuneau electrical demand. Such operation requires a plan which has thecapability for rapid load pickup, rapid load rejection, and inherentstability under load changes.B. Need for a Surge Tank - A surge tank is necessary to meet theoperating requirements discussed above. Maximum and minimum water hammerelevations at the turbine without a surge tank are 1,895 ft and 367 ftrespectively, for a 3.5 s wicket gate closing and 5 s opening time.Maximum and minimum water hammer elevations with the air chamber surge tankwere initially calculated to be 1,332 ft and 549 ft, respectively. Thesewater hammer were arrived at by using a conservative approach. After theair chamber design was completed the "WHAMO" computer program becameava,ilable for air chamber surge tanks. The additional check provided by"WHAMO" allowed us to be more certain of design values and use the 1,218 ftand 639 ft values for maximum and minimum water hammer elevations,respectively, as computed by "WHAMO". In addition, a surge tank would berequired as a source of water to fulfill the function of allowing rapidload pickup. Performance of the system without a surge tank on both loadrejection and load acceptance combined with the operating requirementsdisclJssed in the preceding paragraph indicate that a surge tank isnecessary for the Crater Lake project.C. General Description of Air Chamber Surge Tanks - An air chambersurge tank is essentially a large chamber offset from the power tunnel. Itcontains compressed air above a depth of water. Air is provided to thetank by compressors located "in the powerhouse. Once air pressure isbrought up to the design value, there should be little need to use thecompressors for pressure regulation on a regular basis if the rock is ascompetent as expected. (Reference 15 indicates that after 2 years ofoperation, the compressors at the Driva Powerplant have not been usedexcept during the period of filling up the system.the air chamber leaks at the rate of about 6,000 ft 3 per mo.At the Jukla project,leakage rate has left the three compressors idle most of the time.)B4-5This slow