Work is under way to replace the single turbine at the 45.2-MW Stockton Power Plant on the Sac River in Missouri. The original unit suffered from blade failure in February 2009 that was attributed to rough operation. The new unit will increase capacity of the facility to 53.5 MW and be commissioned in mid-2014.
By Ryan Sollars, Daniel McGinnis and John Kinard
The new single Kaplan turbine at the 45.2-MW Stockton Power Plant on the Sac River in southwest Missouri should be operating this summer, after a multi-year effort to replace the old unit, which suffered a blade failure in February 2009.
The U.S. Army Corps of Engineers, owner and operator of the facility, suspects (but cannot confirm) that the original turbine unit, which had been operating since 1973, was not designed for this particular facility. As a result, the unit has experienced rough operation for its entire life that eventually led to the blade failure.
The new unit, being supplied by Voith Hydro under a $30.8 million contract awarded in April 2010, will have an increased capacity of 52 MW and be custom-designed for the unique site characteristics. The unit is currently being assembled on site and is scheduled to be commissioned in mid-2014.
The Stockton Power Plant was authorized under the 1944 Flood Control Act. Stockton Dam, near the town of Stockton, creates a lake with an area of almost 25,000 acres. Construction of the dam and associated powerhouse began in 1963. The flood control project was placed in operation in December 1969, while the power plant was placed in service in March 1973. The powerhouse contained a single 280-inch six-bladed Kaplan unit designed by Newport News Shipbuilding and Dry Dock Co. and a General Electric generator rated at 52 MW. The powerhouse and lake are operated by the Kansas City District of the Corps.
This original machine was oversized for the hydraulic conditions of the site. A review of original model test documentation showed that the runner was designed for a much larger output than the generator, but the reason for the mismatched parameters was not known. In fact, the turbine appears to have been originally designed and manufactured for installation at another hydro plant, although the Corps cannot verify this.
The operating characteristics at Stockton differ enough from the original design conditions of the turbine to cause rough operation, which eventually led to a blade failure during normal operation in February 2009. The unit was exhibiting very high levels of vibration (both audible and visual observation), and personnel quickly shut the unit down and performed an inspection. About three-quarters of one of the six Kaplan blades was missing, and several other blades showed evidence of cracking, ranging in size from 0.5 to 2 inches, originating at the blade trunnions.
|Repair work was undertaken, which consisted of welding the failed blade section back in place, to get the unit operating again while the U.S. Army Corps of Engineers planned a runner replacement.|
Stockton is one of 24 hydroelectric plants included within the grid of hydroelectric power marketed through the U.S. Department of Energy, Southwestern Power Administration. The Stockton Power Plant is a peaking plant that produces 55 GWh of average annual energy with benefits valued at $8.3 million.
The Corps’ response to the blade failure was two-fold. The first step was to return the powerhouse to operation as quickly as possible by repairing the broken blade. The second step involved planning a long-term rehabilitation of the powerhouse. The Kansas City District contacted the Corps’ Hydroelectric Design Center, its center of expertise for hydropower, in 2009 to provide support services and plans and specifications for this effort.
A diving contractor was hired to locate the missing blade section. The section, which weighed 9,000 pounds, was found and hoisted to dry land. It amounted to three-quarters of the entire blade.
Peak Hydro Services (now called Voith Hydro Services) was awarded a contract in 2009 to weld the broken blade section back onto the runner. Peak developed a weld procedure and performed the weld repair and post-weld heat treatment on the broken blade, as well as performed repairs on the additional blade cracks. The generating unit was then returned to service in 2010.
Replacing the unit
Concurrent with the weld repair, the HDC worked with the Corps’ Kansas City District to create a contract for refurbishment of the entire Stockton powerhouse to ensure reliability. The contract would include hydraulic modeling testing; design of a 71,700-horsepower Kaplan turbine runner; a generator rewind; supply of a digital excitation system, surface air coolers, and digital governor; and typical refurbishment of the mechanical accessories.
The Corps knew the original Newport News turbine at Stockton was not well-matched to site conditions. The unit operated at rated head and full load with the wicket gates partially open and the runner blades at full flat. In other words, the unit was so oversized that the blades would not come off full flat for almost the entire operating range.
In addition, the runner exhibited rough operation between 30 MW and 40 MW, the exact cause of which was not known. The rough operation had historically been significant enough to damage several turbine guide bearings over the first 10 years of operation. In fact, the turbine guide bearing was re-designed to correct this problem in the 1980s. The rough operation was believed to be a contributing factor to the blade failure and cracking.
One limiting factor on the runner replacement work was the fact that the existing turbine was already at the powerhouse crane capacity of 325 tons. Any new turbine design would be constrained by the existing crane capacity.
Voith Hydro was awarded the contract for this work in April 2010 and proceeded with the hydraulic design. The site hydraulic conditions and size of the embedded components required that a seven-blade Kaplan runner be installed.
In the course of the model testing, Voith Hydro found that the geometry of the water passages upstream of the runner resulted in vortices between the wicket gates and runner. Several modifications were studied during the fully homologous model test phase to determine the best solution for the project in terms of runner design. The most cost-effective solution was found to be a lower head cover extension and an increase to the runner hub diameter. These modifications to the hydraulic passageway eliminated the vortices and also resulted in improvements in performance (about 3% efficiency at rated head).
The new runner was based on a bolt-on blade type design. This would allow the runner to be shipped assembled, except for the blades, reducing assembly time at the site. Blade tilt is controlled by an internal runner servomotor and internal bell crank mechanism, which was selected due to the number of blades, the size of the hub, and high internal forces.
Adoption of the bell-crank blade linkage (as compared with the single link design of the old unit) allows the seven-bladed Kaplan linkage to fit inside the turbine hub and was required due to the high linkage forces and tight spacing of the blades (see Figure 1).
Other special considerations of this design include lifting points and covers that are affixed to the trunnion bores and flanges of the hub for protection and lifting. Voith designed a fixture to allow the assembly to be transported on rail and by truck, and this fixture also allowed the assembly to be safely moved from the horizontal position to the vertical position when it arrived at the site. The entire assembly weighs about 165 tons and is 23 feet long and 14 feet high.
During the design of the new runner, the weight of the components was continually monitored because of the powerhouse crane limitations. An example of designing for weight savings is the use of hollow trunnions.
The unit was assembled in Voith Hydro’s facility in York, Pa., and was shipped to the site via rail and 19-axle trailer, arriving in December 2013. It is currently being reassembled and was scheduled to be commissioned in mid-2014. As a result of this replacement, the Corps expects the Stockton Power Plant will continue operating for another 50 years or more.
Funding the work
The $48 million refurbishment of the Stockton Power Plant was funded using $40.7 million received from the American Recovery and Reinvestment Act (ARRA) and supplemental funding of $6.5 million from Southwestern Power Administration’s customers. Since 2010, about 130 jobs have been created as a result of this ARRA project. The power plant’s refurbishment activities have included contracts for intake gate and draft tube bulkhead repairs; in-place blade repair; electrical systems upgrades; 52-MW turbine runner replacement, 161-kV main power transformer replacement; governor and excitation system replacement; emergency diesel generator replacement; rotor and stator winding refurbishment; and asbestos abatement and lead paint removal.
The U.S. Army Corps of Engineers is the largest producer of hydropower in the U.S., representing 24% of total U.S. hydro capability. Hydropower is the largest renewable energy source, which offsets greenhouse gas emissions.
– By Robin Wankum, P.E., and Pete Hentschel, P.E., Kansas City District, U.S. Army Corps of Engineers
Ryan Sollars, P.E., is a mechanical engineer in the U.S. Army Corps of Engineers’ Hydroelectric Design Center. Daniel McGinnis, P.E., supervises the Kaplan turbine engineering team and John Kinard is federal marketing manager with Voith Hydro.