Although the hoists and roller chains used to raise and lower the 18 tainter gates at the 84-MW Tillery hydro project operated reliably for eight decades, they eventually reached the end of their useful lives. A needed tainter gate upgrade project prompted owner Progress Energy to investigate alternatives for replacing the chains to allow for reliable gate operation during this rehab. Progress Energy replaced the roller chain sprocket arrangement with disc layered wire rope drums that wind on themselves and fit the existing footprint of the tainter gate hoists. Nine gates have been retrofit to date, and Progress Energy is preparing to begin its tainter gate upgrade work at Tillery.
The situation at Tillery
The 84-MW Tillery project, on the Yadkin River in North Carolina, features a spillway outfitted with 18 35-foot-wide by 25-foot-tall tainter gates. When the gates were placed in service in 1928, each was operated using a dedicated electric hoist featuring interconnected sprocket arrangements and fabricated roller chains, similar in geometry to a bicycle chain. Despite the fact that it is not unusual to have to replace roller chains one or more times during the life of a hoist, the Tillery roller chains are the originals manufactured by The Maine Electric Co. in 1927.
At the time the gates were installed, roller chain sprocket arrangements were used when gate operating loads exceeded the capacity of typical wire rope hoists that use helically grooved interconnected drum arrangements. For large loads, the cable diameter would be so large that it would result in a correspondingly large-diameter drum of significant length to allow for spooling enough wire rope to raise and lower a gate. The advantage in using roller chain is that it can be designed for high operating loads and the chain sprockets require relatively little space for mounting when compared to conventional wire rope drum arrangements.
However, roller chains manufactured with carbon steel link plates and pins, like those at Tillery, are susceptible to corrosion that causes the link pins to bind and eventually results in uneven gate operation. Over the years, Progress Energy added grease fittings at each pin for lubrication and repainted the chains several times but ultimately came to the decision in the fall of 2007 that the chains had reached the end of their useful life.
Developing a solution
Progress Energy began planning a tainter gate upgrade project in late 2006 that involved painting the gates and replacing the seals. At that time, the company contacted Steel Fab Inc., a gate designer and manufacturer based in Fitchburg, Mass., to obtain an estimate for a floating steel bulkhead. Because the original spillway design did not incorporate upstream stoplog slots, the bulkhead was needed to individually dewater each gate bay and allow the tainter gates to be rehabilitated in the dry. This bulkhead would consist of multiple fabricated steel caissons pinned together after placement on Tillery Lake to form a single gate, then floated to the dam and submerged into place against the concrete piers in front of the tainter gate being repaired.
As part of the planning process for the tainter gate rehabilitation, Progress Energy pursued options for replacing the aging roller chains. Steel Fab had previously provided arrangements consisting of disc layered wire rope drums for large spillway gate hoists at two hydroelectric projects owned by the U.S. Army Corps of Engineers: Ten Killer Dam in Oklahoma and Pompton Lakes Flood Control project in New Jersey. The Corps had moved to this arrangement after experiencing similar operational issues with its roller chain hoists. Steel Fab suggested that Progress Energy pursue retrofitting the existing hoists at Tillery with similar disc layered wire rope drums.
A disc layered drum consists of a series of plates, called separator and root plates, bolted together to allow each wire rope to wind on itself during gate operation. The number of plates depends on the number of wire ropes to be used, and the number and diameter of wire ropes is a function of gate operating load. Multiple wire ropes can be used to address the higher loads experienced with large spillway gates. And, because the cable winds upon itself, in many instances the disc layered drum can occupy the same space as the roller chain sprocket.
To retrofit existing facilities, an adapter plate must be provided to allow for bolting to a spur gear, as was the case at Tillery, or directly to the output shaft of a gearbox. Galvanized steel 6 x 19 IWRC (six outer strands with 19 wires per strand and an independent wire rope core) compacted strand wire rope was used.
During the manufacturing process, the outer strands of this type of rope are flattened, and the internal wires are no longer round. The increased cross-sectional area of this compacted strand configuration offers higher strength and more abrasion resistance than typical wire rope, making it a better choice for applications where the wire rope wraps on itself. Compacted wire rope is no longer available in stainless steel but still is a better option than using typical round stainless wire rope for this application. The lift points on the gate remain in the same locations as those for roller chains but must be retrofit with longer pins to accommodate the multiple cables.
Progress Energy and Steel Fab determined the chain hoists at Tillery could be modified in the field to accept disc layered wire rope drums. The cost to produce the new drums was about half that to manufacture new roller chains. Besides the cost advantage, the operational issues associated with roller chains made it an easy decision to replace the roller chains and sprockets with disc layered wire rope drums. For example, although new roller chains would be manufactured using stainless steel pins, permanently lubricated bushings and high-strength steel links, wire rope drums would require little maintenance and replacement wire rope, if necessary, would be immediately available, compared with five or six months to custom manufacture roller chains.
Completing the work
Steel Fab’s scope of responsibility was to design and manufacture the new drum arrangements and gate attachments. Progress Energy removed the existing spur gear assemblies from each hoist, with chain sprockets attached, and shipped them to Steel Fab’s manufacturing facility, where the sprockets were removed and the spur gear drilled to accept the new disc layered drum. The wire ropes were then wound on to each drum and the assembled drum/spur gear assemblies were returned to Tillery for reinstallation in their original locations. The new gate lifting attachments were installed by divers and the hoists were placed into service within days after receiving the new drum arrangements. The cycle time for manufacturing the new disc layered drum arrangements once the spur gears and sprockets were disassembled was eight weeks on average.
Conversion of the first gate began in late August 2008, and it was completed and operational by December 2008. Eight more hoists have been upgraded since then, with four of the remaining nine gate hoists slated to be completed in 2011. The schedule for converting the final five hoists has not yet been determined, but the entire project should be complete by early spring 2012. The floating bulkhead is now in place, and Progress Energy is ready to start its tainter gate rehab project using operable hoists.
— By David Scott, maintenance supervisor, Progress Energy, and Louis Bartolini, vice president, Steel Fab Inc.