Hydraulic Model Studies of the Circulating Water System for the Sheburne County Generating Plant - Units 3 and 4

The Sherburne Generating Plant of the Northern States Power Company is located 40 miles NW of Minneapolis. Plant Units 1 and 2 were placed in service in 1976 and 1977. Proposed Units 3 and 4 of 800 Mw capacity each were scheduled for commercial operation in the early 1980's. Proposed design improvements in the circulating cooling water systems.for Units 3 and 4 involved complex hydraulic flows. To assure performance, hydraulic model studies were undertaken. This report details the studies of a 1:12 model which were made at the St. Anthony Falls Hydraulic Laboratory. In normal operations, heated water from the condensers is elevated to the top of forced draft cooling towers where it is released to spray downward for cooling. Start-up during sub freezing weather could lead to damaging freeze-up in the cooling tower. An added 84 inch diameter temporary cold weather by-pass drops the 168,000 gpm flow a distance of over 40 feet and requires a dispersed manifold port system assuring good energy dissipation and a stable, well-distributed flow into the tower basin. Based on initial model studies, several small changes were made in the original design to achieve the desired flow quality. Flow in the coaling system is maintained by two 5,000 HP pumps drawing from a pump basin supplied by two cooling towers. The basin serves to route flow from either or both towers to either or both pumps. There was a concern that flow asymmetries or vortices detrimental to pump performance might be generated in the pump basin. Extensive tests at the model pump suction using pitots and vortex sensors establish that good flow conditions existed for all routings and no modification of the basin design was needed. Separate considerations were given to the influence of trash screens and fire water pumping and the dispersion of additive acid in the pump basin.