PennEnergy reported this last week:
“By Jay Pickett, John Stender and Linda Fulsaas

The city of Centralia, Wash., sought a 15- to 20-year maintenance solution in early 2010 for its 12-MW Yelm Hydroelectric Project, a run-of-river facility on the Nisqually River about 20 miles east of Olympia, Wash. The city has routinely maintained this 1930s facility, which supplies nearly a third of the city’s electrical needs.

The project consists of a 20-foot-high concrete gravity dam that diverts water to a downstream powerhouse via a 9.1-mile-long canal. There is no impoundment at the diversion dam, which has a hydraulic height of only 4 feet and during high stages is almost completely submerged. (The difference between headwater and tailwater is less than 1 foot.) The diversion dam, which includes a fish bypass, was constructed in 1930, expanded in 1955 and reconstructed in 1985.

The powerhouse contains three vertical Francis units that operate at 208 feet of head and are fed by two 7-foot-diameter penstocks. Units 1 and 2, both 3-MW machines built by Pelton Water Wheel, share a penstock. Unit 3, a 6-MW unit built by S. Morgan Smith, is fed by the other penstock. The first two units were installed in the 1930s, and Unit 3 was added during an expansion in 1950. Discharge from the powerhouse is returned to the Nisqually River several miles downstream from the diversion dam.

Through a competitive bid process, the city sought services to perform repairs for known conditions and inspections to evaluate the condition of Units 1 and 3.”