Webbers Falls rehabilitation nearly two-thirds complete
May 29, 2014
Workers at the Webbers Falls Powerhouse have completed more than 60 percent of the required upgrades on turbines and generators scheduled for rehabilitation.
The first turbine was completed and accepted in December 2013 and a second is about 90 percent complete. Once the second turbine is replaced, rehabilitation of the last turbine will begin said Byron Floyd, resident engineer, Tulsa District Army Corps of Engineers.
In addition to turbine replacement, the project includes rehabilitation of four cranes, rewinding the coils on three generators and rehabilitating the roller gates, intake bulkheads and tailrace bulkheads.
The improvements will increase power generating capacity and reduce mechanical failures.
"Some of the problems the plant has encountered over the years included cracked turbine shafts, and failed bolts at major connections that would put each unit out of commission as well as significant amounts of cavitation," said Floyd. "In the early 1980s all of the shafts on the turbines were replaced but this is the first complete rehabilitation since the dam was brought online in the 1970s."
Webbers Falls produces electricity with three Kaplan-type, slant-axis turbines. Slant-axis turbines were popular in Europe when the turbines were installed but the European models were significantly smaller, capable of producing five megawatts or less of electricity. A single turbine at Webbers Falls can generate four times that output but the efficiencies of the smaller European tilt-axis turbines didn't transfer as well to the larger turbines.
"The fatigue stresses weren't properly analyzed when the system was designed," said Ken Lehman, a contract engineer at Webbers Falls. "They didn't have the computer modeling or technology we have today, and they couldn't analyze all the stressors. Now, they can get a better understanding of how the stressors will affect the materials."
An additional challenge not unique to Webbers Falls is cavitation. As the water passes through the turbine blades, the pressure drops below the vapor pressure and bubbles form. When these bubbles implode near or against the component materials, they create powerful shockwaves that act like millions of micro-sized hammers. Over time, these implosions produce pits or cavities in the components that have to be repaired.
"Cavitation is a major concern with any hydroelectric system," said Lehman. "With hydroelectric dams you can't eliminate cavitation, you can only protect against it."
According to Floyd, each turbine is allotted three weeks of annual periodic maintenance, most of which is spent repairing cavitation. Utilizing stainless steel for the various components that have historically shown the most vulnerability to cavitation should significantly reduce that maintenance effort.
"Operation's staff spends about two weeks per year per unit on cavitation repair. The intent is to significantly reduce that time," Floyd said.
Improvements in the insulating material surrounding the copper coils in the generators will permit a higher copper to insulation ratio. Greater copper concentration will increase the electrical output of each generator. The insulation is also more resistant to the heating and cooling of the coils.
The rehabilitation is sponsored by the Southwest Power Administration, an agency of the Department of Energy, and funded by SWPA customers. Upon completion, the project will reduce carbon dioxide emissions and return revenue to the U.S. Treasury.
"The overall benefit SWPA expects for its customers from the rehab project The 60 megawatts of capacity at Webbers Falls has been fully marketed by Southwestern to its customers, so while the units are offline, Southwestern has had to replace that capacity with electricity generated from other sources, usually generating plants that run on natural gas or coal," said Beth Nielsen, the Public Utilities Specialist in the Division of Electric Power Marketing at SWPA. "Following the rehabilitation and return to service of all three units, Southwestern will be able to offset nearly 200,000 tons per year of carbon dioxide and provide almost $6 million a year in revenue to the U.S. Treasury. Of course, it will save our customers money too, because buying Federal hydropower is generally cheaper than buying power on the market."