The California Department of Water Resources improved unit availability and reliability and increased efficiency by as much as 6.5 percent at its 645-MW Edward Hyatt pumped-storage project through a comprehensive rehabilitation. The increased efficiency equates to a value of $168 million over 30 years.
By Farshid Falaki and Soheil Loghmanpour
Lake Oroville, with a gross capacity of more than 3.5 million acre-feet, is the principal water storage facility for the State Water Project (SWP) in California. The SWP conserves water and delivers it to more than two-thirds of the state’s population. In addition to water supply, Lake Oroville is operated for electricity generation, flood control, recreation, and fishery and wildlife habitat enhancement.
The bedrock below Lake Oroville in northern California is the location for the 645-MW Edward Hyatt Powerplant, a pumped-storage hydroelectric facility owned and operated by the California Department of Water Resources (DWR). The powerhouse cavern is the size of two football fields. Construction began in 1964, and the plant went on line in 1969. With three turbines and three reversible pump-turbines, Hyatt produces enough electricity to supply about 600,000 homes each year.
Hyatt maximizes electricity production through a pumped-storage operation. During periods of peak power demand, water is released (in excess of local and downstream requirements) for generation. This water is pumped back into Lake Oroville during off-peak periods. However, DWR has drastically curtailed pumpback operation at Hyatt in recent years because of the high cost of the electricity and environmental issues. For example, because the lower reservoir of the plant is relatively shallow compared with the upper reservoir, water temperature in the lower reservoir rises quickly. As a result, any pumpback operation can change the ecology of the upper reservoir for fish habitation.
Deteriorating situation at Hyatt
Since Hyatt began operating, repairs on the six units have been performed on as-needed basis. Repair frequency increased as the units aged. Repairs for cracks on the runners and/or stay vanes were being performed annually. However, repairs for cavitation and corrosion damage were performed less frequently, based on available time during usual outage periods of October through May.
Over the years, results from routine annual inspections by DWR staff and consultants raised serious concerns regarding the metallurgical and hydraulic condition of all six units. The turbines had severe cavitation damage on the carbon steel runners and wicket gates. The pump-turbines had fatigue cracking in the runner blades and stay vanes.
Years of repairs to the runners to account for this cavitation and corrosion damage changed their hydraulic profiles. Without templates, it was difficult to perform repair work to reshape the runners to the original design. Turbine efficiency degraded from 93 percent in 1970 to a weighted average of 91.49 percent in 2003, as measured by DWR’s engineering and test branch. Over the same time period, efficiency of the original pump-turbines degraded from 89.9 percent to 85.8 percent.
In addition, several other issues were critical to the long-term viability of plant operations. First, all units had rough areas of operation that DWR had to avoid. Second, excessive vibration during periods of low head restricted operation of the units and reduced power generation. Third, turbine units had rough operation areas at certain gate settings, which varied with head. Finally, pump-turbine units had a rough operation area at low lake elevations in both turbine and pump modes that prevented the units from being operated under these conditions.
In 1997, DWR performed a comprehensive feasibility study and developed two alternatives to ensure continued serviceability of the project. The alternatives were major repair, consisting of repairing all components, or refurbishment, consisting of replacing the major components (runners, wicket gates, and facing plates) and repairing the rest. These alternatives were presented to DWR executive management and the state water contractors that are stakeholders of the project. Based on an economic analysis, DWR executive management chose the refurbishment alternative.
Refurbishment work performed
DWR developed the design for the rehabilitation work to be performed at Hyatt, including preparation of the plans and specifications. Refurbishment of the turbines and pump-turbines were awarded as two separate contracts through a competitive bidding process. Turbine refurbishment work was awarded to VA Tech Hydro of Austria in February 1999.
As soon as the design of the turbine contract was completed, DWR started to prepare plans and specifications for the pump-turbine contract. This contract was awarded to GE Hydro of Canada in November 2001.
The two contracts allowed DWR to refurbish one unit per year for six consecutive years. Having two contracts also reduced DWR’s risk exposure in case of unforeseen situations in one contract.
In addition to work on the generating units, DWR chose to replace the mechanical governors with digital governors. The contract for this work was awarded to Sulzer of Germany in November 1999 through a competitive bidding process. Sulzer was later acquired by VA Tech Hydro, which took over responsibility for designing the governors.
Turbine model testing and construction
Turbine refurbishment work performed by VA Tech Hydro included designing, manufacturing, and delivering new runners, wearing rings, wicket gates, greaseless bushings, guide bearings, facing plates, and shaft sleeves. VA Tech Hydro personnel disassembled the three units and installed all the new components. The company also sandblasted, inspected, tested, repaired, and coated water passages and other components, such as the spiral casing, head cover, operating ring, and shafts. Finally, VA Tech Hydro assembled the unit and performed start-up tests.
In December 1999, VA Tech Hydro conducted model tests for the new runner and wicket gate designs at its laboratories in Linz, Austria. These tests showed that the refurbished turbines would meet the guaranteed efficiency of 95.04 percent and would provide cavitation-free performance over the entire range of operation.
As soon as the model tests were completed, VA Tech Hydro began manufacturing the components. Once manufacturing was complete, VA Tech Hydro disassembled the first unit in preparation for refurbishment to begin in October 2001.
The material for the runners, rotating wearing ring, and wicket gates is A743 CF3-MN, an austenitic stainless steel with superior corrosion resistance and excellent resistance to cavitation damage. The material has exceptional characteristics for ease of weld repair, which requires no post-weld heat treatment. One big advantage of this material is that its mechanical property and chemical composition, such as carbon content, stay intact in the weld or heat-affected zones. To maximize unit efficiency, blade thickness was kept to a minimum while maintaining safety factor requirements per American Society of Mechanical Engineers (ASME) Section 8. That decision resulted in higher stresses in the blades and in the welds connecting the blades to the band and crown. Finite element stress analysis, combined with rigorous non-destructive examination by VA Tech Hydro and inspections by DWR were used to ensure integrity of the runners.
The new runners installed in the three turbines at the 645-MW Hyatt pumped-storage project are as much as 5 percent more efficient than the old runners and are designed to avoid cavitation.
The 14-foot-diameter turbine runner crown and band were cast and then welded to the blades, which were previously machined using pressing plates. VA Tech Hydro pressed the blades at room temperature by calculating the precise amount of bounce-back after pressing. Because all machining was completed prior to pressing the plates, no further machining or heat treatment were required. This method provided a consistent manufacturing process from blade to blade or runner to runner. The most notable problem during the manufacturing process was a large crack encountered on the first crown during the casting process. VA Tech Hydro discarded this crown and redid the casting.
The runners and wicket gates were manufactured to a tolerance that is half of that required by International Electrotechnical Commission (IEC) code 60193. Overall, this tighter tolerance is well worth the effort, considering the reliability it adds to the geometric similarity check between the model and the prototype.
One special feature of the original runner that needed to be incorporated into the design of the new runner was a series of conduits installed inside the nose cone to reduce down thrust. These relief holes reduce pressure under the inner head cover and hence reduce down thrust by allowing water to flow to the lower-pressure draft tube through the nose cone. However, during start up of the first unit, DWR discovered that down thrust still exceeded the thrust bearing capacity of the unit.
Figure 1: Comparisons of turbine efficiency for Unit 3 at the 645-MW Edward Hyatt pumped-storage plant before and after refurbishment show significant improvement.
To overcome this problem, VA Tech Hydro installed a balance line intended to drain the head cover. However, because of the excessive noise and vibration in the new balance line, the contractor removed the line. Instead, VA Tech Hydro welded channels inside the nose cone to guide the water from the relief holes to the center of the runner nose cone, where the pressure was significantly lower. Because the centrifugal forces on the water discharge have less effect on creating back pressure, the pressure under the inner head cover can drop, reducing the down thrust to below the thrust bearing capacity. This method proved very effective at reducing down thrust, with the only drawback being increased maintenance to ensure the integrity of the welds holding the channels in place.
Wearing rings were designed and manufactured with tight tolerances to minimize leakage and increase efficiency. ASTM A240, type S21800 (Nitronic 60) stainless steel was used for the stationary wearing rings to avoid galling while providing excellent wear resistance.
With the optimized runner and wicket gates, the units can generate the same power with less water and are free of cavitation throughout a much greater range of operation.
To transport the new runners for the three pump-turbines into the underground powerhouse at the 645-MW Hyatt pumped-storage project, the contractor used a special truck.
Facing plates were fabricated from Nitronic 60, again for its galling resistance. Facing plates have bronze sealing strip at their midpoint to provide a tight seal between the wicket gates and both the upper and lower facing plates when the gates are closed, to minimize leakage. In addition, it eliminates the need to close the 114-inch-diameter turbine shut-off valve when the unit is shut down for short durations, allowing much faster response time to bring the unit on line.
The air admission systems used by the original turbines to reduce vibration, pressure pulsation, and cavitation damage were disconnected because the new turbine units operate with minimal vibration, pressure pulsation, and no cavitation.
The spiral casings, draft tubes, discharge rings, stay rings, bottom rings, and inner and outer head covers were sand-blasted, tested, weld repaired, machined, retested, and coated to improve performance and to increase unit longevity. Surfaces of the upper and lower stay rings stay vanes, draft tube within 18 inches of the discharge ring, outer head covers exposed to water above the facing plates, discharge rings, and bottom rings were coated with ceramic composite from Chesterton Coating. The draft tubes, spiral casings, inner head covers, other surfaces of the outer head cover, and wicket gate operating assemblies were coated with EnduraFlex high solids epoxy.
Greaseless bushings in the new turbines eliminate all lubricants from waterways. Devatex II bushings, suitable for both wet and dry application, were selected for the wicket gates, including all wicket gate linkages. All wicket gate linkage pins were replaced to provide suitable mating surfaces for the new bushings. Tenmat provided the curved wear plates for the operating ring assembly after the contractor discovered that the flat wear plates could not be easily bent in place. Lip seals were installed on the intermediate and lower wicket gate stem bushings to prevent ingress of abrasives.
Before refurbishment, the wicket gate thrust bearing consisted of a bronze thrust disk and a steel thrust collar to carry both the weight of the gate and the uplift due to pressure from a 2,000 pounds per square inch (psi) grease pump. As part of the refurbishment, the grease pump was disconnected and the thrust bearings were replaced with Devatex self-lubricated bushings.
Work on the turbines was completed in March 2004. The new turbine runners and wicket gates have resulted in the new turbines being up to 5 percent more efficient than the original ones over a wide range of operation. (See Figure 1.) Because of the way DWR is structured, revenue from the sale of electricity from the Hyatt units offsets the cost of operating the units. Thus, the new turbines reduce plant operating costs by $72 million over 30 years and greatly minimize the need for annual repair of the units.
Pump-turbine model testing and construction
The scope of the work for the pump- turbine refurbishment contract was similar to the turbine contract. This contract also required the replacement of major components, such as the runner and wicket gates, and repairing the remaining components to their original design dimensions.
GE Hydro, the contractor for the pump-turbine refurbishment, used the latest design and computational fluid dynamic (CFD) technologies and physical models to design the runners and wicket gates. Model testing of the redesigned pump-turbine runner and wicket gates was conducted in Trondheim, Norway, in November 2003. The pump-turbines were designed using the same approach, workmanship quality, and material technologies as the turbines.
The wicket gates on the turbines and pump-turbines at the 645-MW Hyatt pumped-storage facility were replaced during the powerhouse modernization project.
Notable features of the pump-turbine work include:
– The pump-turbine runner crown and band, which are 19 feet in diameter, were cast and then welded to the blades, which were formed by hot-pressing plates. The heating process GE Hydro used avoids prolonged exposure to temperatures that would potentially sensitize the metal and change its mechanical properties. These runners weigh three times more than the turbine runners.
– DWR used Nitronic 60 for both the stationary and rotating wearing rings. This material provides excellent wear and resistance to galling when used for mating surfaces.
– GE Hydro did not experience down thrust problems with these units, mainly because these units were equipped with balance lines. These lines allowed the water between the runner and head cover to drain much more efficiently.
– GE Hydro replaced the discharge ring in the draft tube near the eye of the runner with stainless steel discharge rings because some cavitation may occur near the rings in the pump mode.
– As part of the refurbishment, the grease pump used to operate the wicket gates was disconnected and the wicket gate stem bushings and thrust bearings were replaced with self-lubricated bushings. GE Hydro used Devatex bushings on two of the units and Orkot bushings for the wicket gates on the third unit.
– The new pump-turbines and wicket gates have resulted in efficiencies up to 6.5 percent higher than the original units over a wide range of operation. Revenue generated as a result of the new pump-turbines operating in the turbine mode reduce operating costs by $96 million over 30 years. This mode of operation also minimizes wear and tear on the units, greatly reducing the need for annual repair.
– The size of the entrance tunnel limited the size of the components that could be brought into the powerhouse. That is why the original runners were designed in two pieces. However, GE Hydro manufactured the new runners in one piece after confirming via a successful mock test that it could overcome the tunnel limitations by transporting the new runners on a special truck.
– DWR used ceramic composite coating for both the spiral case and the draft tube in the pump-turbines to allow better wear in pumping mode.
Disassembly of the first pump-turbine unit began in October 2004, and refurbishment of all three units was completed in September 2007.
Performance testing the new units
DWR personnel conducted field performance tests for the turbine units according to ASME Performance Test Code (PTC) 18. Results showed a weighted average efficiency of 95.56 percent (including 1.5 percent instrument inaccuracy allowance) for the refurbished turbines, exceeding the guaranteed weighted average efficiency of 95.04 percent as predicted by model tests. The efficiency tests were performed using the plant acoustic flowmeter, manufactured by Ferranti, O.R.E.
DWR personnel also conducted field performance testing for the pump-turbines in turbine mode, again according to ASME PTC 18. Results showed a weighted average efficiency of 93.37 percent (including instrument inaccuracy allowance), exceeding the guaranteed weighted average efficiency of 92.2 percent as predicted by model tests. The efficiencies of the refurbished pump-turbine in turbine mode are more than 6.5 percent higher than pre-refurbishment efficiencies. Efficiency testing in the pump mode will be performed when the lake level is at the ideal elevation to perform this test.
Over the eight-year course of this refurbishment program, DWR learned valuable lessons in several key areas: disassembly, preparing for refurbishment, performing the work, and commissioning new units.
Before disassembly begins, the hydro project owner should field verify all information contained in drawings of the existing unit. The contractor should then receive these drawings and any other pertinent information before disassembly begins. The contractor also should verify all dimensions before beginning design and manufacturing of any equipment.
During disassembly, any missing fasteners discovered should be listed and, if needed for reassembly, ordered immediately. Any fasteners whose quality or reliability is questionable also should be listed and inspected. Finally, identify the need for machining work for the embedded components and any extra work on generator shafts, bearings, etc., as soon as possible, and schedule this work as far ahead of time as possible.
Preparing for refurbishment
Good communication between the owner and contractor(s) is essential. Contractors should provide timely schedules, plans, drawings, information, and measurements to the owner. All parties should discuss any major decisions required to resolve problems. Unilateral decisions could prove costly for all parties involved. The contractor should give the owner an accurate and timely inventory of parts and components on site.
The refurbishment and assembly area should contain enlarged detailed drawings of the parts placement, including the weights of all parts and weight rating of lifting devices. The need for any special tools and/or equipment for the refurbishment work should be identified during this stage.
Performing the work
The embedded parts of the units will change shape due to ground movement over many years. Schedule any needed in-place machining ahead of time, and machine all embedded parts where applicable to proper size and level before refurbishing components such as the outer head cover to make sure mating surfaces match properly.
When possible, assemble all parts (tower assembly) in the shop to ensure proper fit before shipping to the site.
The facing plates that were finish machined and then installed create uneven surfaces. Facing plates should be machined in place after installation to achieve flat surfaces with a uniform gap between the wicket gate blades and facing plates.
All areas where carbon steel and stainless steel meet should be sealed. This means installing the stainless steel parts before coating, then coating onto or up to the stainless steel. On any surface that will require adhesion of a coating or filler, the needed profile must be achieved by sand-blasting. Carefully measure the clearances of mating parts before coating. Ensure that proper clearance can be maintained after coating is applied. Use single coating to avoid transition between two coatings next to each other. Where coating transitions exist, coating edges should be protected by using proper filler material from the coating manufacturer.
Commissioning new units
Before commissioning a new unit, inspect the spiral casing for hard objects – bolts or small tools – that may have been left inside. During start up, do not operate the unit below synchronous speed. This mode of operation is sometimes used for a bearing heat run. However, at low flow, centrifugal force on the particles is higher than the flow force passing the particles through the runner. This causes hard objects in the water, such as rocks and metal pieces, to get caught between the runner and wicket gates and keep circulating around the runner. This will damage the surface of the wicket gates and facing plates until the particle loses enough mass to allow the flow force to overcome the centrifugal force and pass the particle through the runner.
Messrs. Falaki and Loghmanpour may be reached at California Department of Water Resources, 1416 Ninth Street, P.O. Box 942836, Sacramento, CA 94236-0001; (1) 916-653-4399 (Falaki) or (1) 916-653-9848 (Loghmanpour); E-mail: [email protected] or [email protected] water.ca.gov.
Farshid Falaki, P.E., chief of the mechanical and electrical engineering branch in the division of engineering at the California Department of Water Resources (DWR), was the project engineer/manager responsible for the Hyatt refurbishment. Soheil Loghmanpour, P.E., supervising mechanical engineer in the division of engineering at DWR, was job manager for the refurbishment.
Touring the Hyatt Project
The 645-MW Edward Hyatt pumped-storage project is included in the HydroVision 2008 post-conference technical tour. This tour, to be held Friday, July 18 through Sunday, July 20, features a trip to the California Department of Water Resources’ 762-MW Oroville Facilities in northern California. Oroville Dam impounds Lake Oroville, the upper reservoir for Hyatt.
Tour participants also will visit Yuba County Water Agency’s 315-MW New Colgate Powerhouse and 645-foot-high New Bullards Bar Dam.
– To register for the tour, go to: www.hcipub.com/hydrovision. To request a conference brochure, which includes details on the tour and registration information, telephone: (1) 816-931-1311, extension 129, or e-mail: [email protected] hcipub.com.
Location: Butte County, California
Owner: California Department of Water Resources
Capacity: 645 MW
Rated Head: 615 feet
Annual Generation: 4.9 billion kilowatt-hours
On-Line Date: 1969
Cost of Modernization: $40 million
200 revolutions per minute (rpm)
184,000 horsepower (hp)
Originally manufactured by Allis-Chalmers
Refurbishment by VA Tech Hydro
1,870 cubic feet per second pump capacity at rated head of 592 feet
Originally manufactured by Allis-Chalmers
Refurbishment by GE Hydro
Generators for turbines (3)
12,500 volts, three phase, 60 Hertz
123,000 kilovolt amperes (kVa)
0.95 power factor
Manufactured by Westinghouse
12,500 volts, three phase, 60 Hertz
115,000 kVa (generator), 131,000 kVa (motor)
173,000 hp (motor)
97.75 MW (generator)
0.85 power factor
Manufactured by Westinghouse
Digital (replaced original mechanical governors)
Manufactured by VA Tech Hydro
22 feet in diameter
550 feet long by 69 feet wide by 137 feet tall