Before rehabilitation work could begin on the five units at the 456-MW Noxon Rapids project, owner Avista Utilities needed a method for tipping the turbine and generator shafts into a horizontal position. Although this facility, on the Clark Fork River in Montana, was equipped with a shaft tipper, the existing equipment had limitations that made it impractical for use during this rehab. Avista designed and fabricated a new tipper that allowed the work to be performed quickly and effectively.
Need for a new tipper
This facility has been operating since 1960, and four of the five units dated from that time. The fifth unit was installed in 1979. Avista determined turbine replacement was required for the four older units, to extend the life of the plant, enable the utility to qualify for federal renewable energy tax credits, and help meet state renewable portfolio standards. Toshiba International supplied the runners. Other work being performed at this time included major mechanical overhaul, refurbishing bearings, replacement of wicket gate operating system bearings with greaseless bearings, and replacement of stationary wearing rings. The stay vanes of all units were modified to reduce losses and optimize efficiency of the turbine. Work on Unit 1 also included replacing the stator core and winding, which was performed by Voith Hydro.
|This shaft tipper, in use to tip the turbine shaft of one of the units at the 456-MW Noxon Rapids plant, was designed to facilitate replacement of four of the five turbines.|
However, before this work could begin, Avista would need to have systems and tools in place to complete the replacements safely and in a timely fashion. One important piece of equipment that came under review was the shaft tipper. Upon their removal, the turbine and generator shafts needed to be tipped into a horizontal position so they could be moved around the plant. The turbine shaft would be sent off-site for refurbishment, while the generator shaft would be stored on-site for the duration of the work.
Although this tool already existed at the Noxon Rapids plant, it had several limitations. For example, the existing shaft tipper had to be fastened to the generator deck in one particular spot, making this the only location at the plant where tipping of the shafts was possible. In addition, alignment of the shaft to the tipping device and axis of the powerhouse crane hook was a time-consuming process that had to be accurate for the shafts to tip correctly and safety. Finally, the bolt pattern on top of the generator shaft varied between units. This misalignment caused the crane hook to be put into a slight bind when the shaft was tipped, causing the hook to not be in its natural vertical state and putting uneven stress on the crane cables.
Avista personnel knew of a shaft tipper of a different design that was in place at the 1,038-MW Wanapum project on the Columbia River in Washington. Project owner Grant County Public Utility District allowed Avista to review that design, and Avista personnel determined to design and build a similar tipping device for use at the Noxon Rapids plant.
This new tipper would allow personnel to tip the turbine and generator shafts anywhere that load limits on the powerhouse deck would allow. The new tipper had a turntable feature, which allowed for radial movement of the shaft and/or tipper for alignment purposes, either before or during tipping.
Fabrication of the new tipper was done in-house by skilled Avista machinists. All materials were sourced locally, and, using provided drawings, the tipper was fabricated in a matter of only a few weeks.
|The new shaft tipper design features a turntable to allow for radial movement of the shaft and/or tipper for alignment purposes.|
Using the new tipper
Each turbine replacement was carried out by Avista mechanics and electricians. Complete disassembly and reassembly was handled in-house, as was project management and engineering. Turbine replacement work at Noxon Rapids began with Unit 1 in 2008.
The process was very similar for all four turbines. Once a shaft was ready to be tipped, personnel bolted it to the tipper using four 1.5 inch all-thread studs. Once joined together, the whole assembly – tipper, shaft and all – is lifted from the deck. By hand, the tipper is aligned in the tipping direction. Once this alignment is complete, the assembly is placed back on the deck, and the shaft is spun to align in the tipping direction (if needed).
Typically, both of these alignment maneuvers are very slight and are quick and easy. The turntable of the tipper is locked down before tipping via a bolt and plate on the underside of the tipper. However, it can be left loose during tipping to allow the shaft to rotate naturally as it is being tipped.
Once both are aligned in the tipping direction, the crane is then in control of the tipping operation. By traveling the crane in the tipping direction and lowering the crane hook, tipping is achieved. The shaft is tipped to the final horizontal position and set on cribbing. Once the shaft is in the horizontal position, the lifting device is unbolted from the top of the shaft, and the tipper is unbolted from the lower end of the shaft. The shaft then can be rigged and moved about the plant as necessary.
The tipping operation, from attaching the tipper to the shaft flange to setting the shaft on cribbing in the horizontal position took only about 45 minutes. With the old tipping arrangement, it would take personnel the better part of half a day to perform the tipping operation. Avista plans to re-use this tipping device at its other Clark Fork River plant in the coming years when an overhaul of one of the four units at that facility is performed.
Work on the final unit, Unit 4, was completed in May 2012. In total, this was a $45 million project and increased plant capacity by about 32 MW.
– By P.J. Henscheid, P.E., mechanical engineer, Avista Utilities