River Operators Hoping Dworshak Turbine Fixed
Dworshak Dam's operators are hoping that a new problem that emerged nearly two week's ago to force the shutdown of the facility's largest turbine unit is really an recurring problem that could be fixed, at least for the short term, relatively quickly.
U.S. Army Corps of Engineers on May 22 noted excessive water leaking into the turbine pit of the dam's No. 3 generating unit. An inspection with an underwater camera indicated that the leak could well be coming from the vicinity of the turbine's head cover. The unit had to be shut down for safety reasons.
A check of the dam's maintenance records showed that leaks had sprung at the head cover previously. In 1982 sealant had been applied to stop a leak resulting from a deteriorated O-ring that seals the head cover-turbine connection. A similar procedure was necessary in 1985 and again in 1986 but "since 1986 there has been no repair," the Corps' Dave Tucker told the Technical Management Team Wednesday.
The cool water stored behind Dworshak is an important tool for fishery managers in late summer. It is used on-demand to help bring down temperatures in the lower Snake River to improve conditions for migrating fall chinook and steelhead spawners. The TMT's federal, state and tribal fishery and hydro managers help to make those calls. The dam sits on west-central Idaho's North Fork of the Clearwater River, which flows into the Snake.
A continued shut down into July and August would hinder efforts to cool the water downriver. The large turbine unit controls more than half of the powerhouse flows - about 5,500 cubic feet per second of a 10.5 kcfs total.
With all units running, the dam's outflow can be lifted to as much 15 kcfs before the "total dissolved gas" levels rise to legal limits. That maximum flow includes full powerhouse generation with additional water released through spill gates and/or the dam's "regulating outlets." The powerhouse flows create little gas, but the spilled water and RO releases stir up TDG quickly in the dam's tailwaters.
The large turbine unit's flows could only be replaced with more gas-causing spill. Without that unit operating, outflows this summer would likely be limited to about 7-8 kcfs before reaching the TDG cap, Corps officials say.
"It's going to have an effect on fish and water quality and power generation" if the shutdown continues, the Corps' Rudd Turner told the TMT. Last week the Corps estimated repairing the leak would take anywhere from a few weeks to more than a year, depending on the cause of the problem.
The O- ring deterioration would likely be the best-case scenario. If, after a visual inspection, it is determined to be the cause of the leak it could probably be resealed in about a week, Tucker said.
The O-ring could be replaced later "at a time that is less detrimental" for desired operations, Tucker said. Actually replacing the O-run would require the "unstacking" of the turbine units and likely take "months" to complete.
The Corps won't know for sure what the problem is until a required structural inspection is completed of an emergency gate that is designed to seal off water coursing down the long penstock from the top of the dam to the turbine unit.
If the gate passes the safety inspection it would be lowered into the unit to provide a safe work environment for those sent down to determine the cause of the leak.
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