When a wicket gate shaft on Unit 2 at the 456-MW Noxon Rapids plant twisted to the point of failure, plant personnel devised a way to quickly repair the unit and get it back online. Personnel at the plant, owned by Avista, removed the damaged gate and installed a spare gate without dismantling the unit. To accomplish this, they had the leaves cut from the spare wicket gate, installed the shaft in the unit, then welded the leaves back in place. The repair was completed in just three weeks, and the gate has operated normally ever since.
Discovering the problem
Noxon Rapids, on the Clark Fork River in Montana, began operating in 1960. The project is part of the two-powerhouse, 742-MW Clark Fork project. The Noxon Rapids powerhouse contains five turbine-generator units.
The first problem with wicket gate No. 1 in Unit 2 at Noxon Rapids occurred on June 19, 2006. A high-temperature alarm came on in the turbine guide bearing, and personnel took the unit off-line. During the initial inspection of the unit, it was visually apparent that a shear pin had failed on gate No. 1. During this inspection, plant personnel also saw that the stop pin had suffered a violent impact, causing it to partially shear. In addition, the wicket gate shaft was twisted and bent slightly upward. Because June is a peak generating month for Avista, it was imperative to get the unit operating quickly. Thus, nothing was done to address either the cause of the failure or the bent wicket gate shaft. Plant personnel installed a new shear and stop pin and returned the unit to service on June 24.
Then, in mid-May 2007, the shear pin on gate No. 1 failed again. This failure was discovered during a routine daily inspection; again, the cause was unknown. Plant personnel shut down the unit and investigated the failure. During this inspection, personnel noticed the upper part of the wicket gate shaft had yielded and was twisted beyond repair.
Unit 4 at Noxon Rapids was already off-line for generator repair work, so an extended outage was not an option for Unit 2. Therefore, plant personnel decided to replace the gate with a spare, with minimal disassembly of the unit.
Avista asked its generation and production engineers to find a solution to the failure in Unit 2.
Several solutions were considered. One involved maintaining the No. 1 wicket gate in place, locking it in the open position. Avista ruled out this option because a single wicket gate in the open position generally passes enough water to run the unit at synchronous speed. This would mean the unit would have to be constantly motored (a mode of operation also known as spinning reserve). Stopping the unit would not be possible with that amount of flow.
A second option was to weld the wicket gate in the closed position. However, Avista consulted with a turbine designer and learned that this would apply about three times the torque to the shafts on the adjacent wicket gates. This caused concern for failure of the other wicket gates. In addition, it likely would set up severe vibration on the runner due to the pulsations created on the turbine buckets as they pass through the wake created by the closed wicket gate.
A third option was to cut the existing wicket gate out of its position, install a newly machined shaft, and weld the original gate leaves onto this shaft. This option, while possible, was not attractive because of the significant number of man-hours required. A very precise cut would be needed on the existing gate leaves while still in the unit. In addition, a new shaft would have to be machined and doweled to the exact specifications as the existing gates.
A fourth option considered was to quickly cut apart the existing wicket gate in pieces and remove it, then install a spare wicket gate. A machine shop would cut off the leaves on the spare gate and machine the shaft outer diameter to fit through the intermediate bushing. Then the leaves would be welded to the shaft inside the unit.
Avista chose the fourth option because it appeared to require the shortest amount of time. Time was of the essence because Unit 4 was off-line for a rewind and spring runoff was just a few weeks away. Avista had to minimize the lost generation associated with having two units off–line at one time.
Performing the work
As soon as this repair option was selected, Avista shipped a spare wicket gate to Precision Machine in Lewiston, Idaho. The company cut the spare wicket gate leaves off the shaft, machined the leaves 4 inches from the location of the cut, and welded steel extensions on the ends of the leaves. The company then machined these extensions to mate up with the shaft in the correct position and with the same profile as the existing wicket gates.
Precision Machine also machined the shaft down to 8 inches in diameter from the original 10 inches, to allow it to be inserted from above the turbine head cover down through the bushings to the bottom bushing. Finally, the company indexed the gate leaves and shaft to one another before shipping them back to Noxon Rapids. This indexing was needed to provide a precise locating mechanism for the leaves and shaft to mate to one another before welding.
While Precision Machine was working on the spare wicket gate, plant personnel cut the existing wicket gate out of the unit. Personnel then installed the spare wicket gate shaft and leaves and welded them together. Once welding was complete, plant personnel painted the gate.
On June 7, 2007, plant personnel started up the unit. They motored the unit during start up, and the gate closed and sealed as required. No vibrations were felt from the gate lever once the unit was on-line, and the new gate operated normally. Since the repair, there have been no additional issues with this wicket gate.
Avista personnel still do not know the cause of the two shear pin failures. During the second of the two repairs, divers inspected the trashrack for any holes or other access points for large debris, but none were found. In addition, before removing the existing gate, personnel inspected it for signs of significant impact that would have caused the failures. Again, none were found.
Based on this experience, Avista personnel have made preparations for a similar repair in the future. An additional spare wicket gate will be fabricated in three pieces: an 8-inch-diameter shaft and two leaves. Having these pieces on hand will make any repair needed even quicker than the original.
It is imperative to inspect shear pins daily or install a shear pin monitoring system on the unit. If a unit is left running with a "loose" wicket gate, it can lead to turbine bearing heating, increased vibrations, torque on the runner shaft, and other issues.
— By P.J. Henscheid, mechanical engineer (generation and production), Avista Utilities