Boilers, O&M, Renewables

Ending Excessive Vibration Response at a Nuclear Power Plant

Issue 4 and Volume 113.

By William Marscher and Eric Olson, Mechanical Solutions Inc.

Premature bearing and seal failures were occurring in a safety-related decay heat removal (DHR) pump in a nuclear power plant. The DHR pump is a single-stage, end-suction volute pump.

Operating deflection shape (ODS) test results showed the pump casing badly distorted dynamically, with a dominant frequency of impeller vane pass versus the casing’s single volute tongue. The system’s motion at this frequency involved a vertical “bouncing” of the oversize discharge pipe, which tended to “jam” the pump casing through the dynamic force that it imparted to the pump discharge nozzle.

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An impulse modal test determined that a structural natural frequency dominated by vertical motion of the discharge pipe was nearly coincident with the first harmonic of the vane pass frequency, causing a resonance. The resonance was poorly damped, based on the “half power” width of the resonant response peak of the frequency response function for this mode, as obtained from the modal test.

Plant management and the Nuclear Regulatory Commission asked Mechanical Solutions Inc. (MSI) to verify that the resonance issue was the only reason for the increased vibration, and if so, to determine how best to eliminate the problem to end the chronic failures. This was problematic, as there were no piping taps near the pump discharge. Furthermore, if the piping was violated by installing a tap, it would need to be re-qualified for system operation in a safety-related situation.

Verifying the Problem Source

MSI applied a technique used successfully in other plants where pressure pulsation measurements were required in pump or compressor systems and where many axial pressure readings were needed to evaluate acoustic natural frequency mode shapes. The method consisted of using a minimum of four uniaxial accelerometers attached perpendicular to the pipe wall at 90-degree intervals around the periphery of the pipe, at each location where pulsation readings were needed.

Away from stiffening components such as flanges or piping supports, this technique allows the pressure pulsation that makes the pipe expand radially to be separated from the piping gross structural vibration, which merely translates the pipe as a relatively rigid body unless shell modes are suspected. The latter occur at high frequencies on the order of 1 kHz and higher, depending on the pipe thickness divided by the diameter squared. If shell modes are a possibility at the frequency of interest, they can be sorted out by taking a larger number of measurements around the periphery of the pipe.

MSI measured the pressure pulsation one pipe diameter downstream of the discharge nozzle. It then demonstrated with both manual calculations and finite element analysis (FEA) that the pulsation level was close to that required to drive the discharge piping resonant response at the vibration level that had been detected. This verified it as the “problem source.”

To perform “what if” analyses of possible alternative solutions, MSI used the cross-confirmed FEA model. Adding mass to the pipe, using a piping damper or shock absorber, stiffening the pipe supports or lengthening the pipe were all explored as potential solutions and were found to be viable. However, each of these solutions would require piping re-qualification. To avoid that, MSI designed a low weight dynamic absorber in the form of a low-mass, thin-walled pipe “clamshell” to surround the discharge pipe vertical leg. This absorber also would be able to move vertically—yet separately—from the discharge pipe. The dynamic absorber was attached to the discharge pipe through clamping plates that featured tuned and adjustable stiffness in the vertical direction.

After analysis and testing, the assembly was found to have quenched resonance at the vane pass acoustic frequency. Because the assembly was of sufficiently low mass and did not penetrate the discharge pipe, only the dynamic absorber required analysis and review/approval.

Authors: William D. Marscher is president and technical director of Mechanical Solutions Inc. He wrote the Turbomachinery Structural Design & Analysis chapter of the Third Edition of “Sawyer’s Gas Turbine Handbook.” Eric Olson is the director of sales and marketing at MSI. He is a multi-year short-course speaker at the Texas A&M International Pump Symposium.

On-site Machining

By Andy Becker, Climax Portable Machine Tools

When flanges are welded onto wind turbine tower tubes during manufacturing, the high heat can cause the flanges to warp out of specification. To correct this, manufacturers are turning to portable circular mills or flange facers, a solution that shaves time and cost off traditional re-machining.

Recently, 500 wind turbine towers shipped from overseas to a U.S. port were found to be out of specification. Rather than return them, ship them to a job shop or risk rejection by the customer, the manufacturer contacted Derek Dodge of Dodge Machining, a service company with 25 years experience in on-site machining.

Dodge worked with the port to set up an area where the towers could be offloaded and machined. He rented two FF8000 flange facers from Climax Portable Machine Tools that were capable of fast metal removal for up to 120-inch welding diameters, with finishes as fine as 125 Ra. With set-up taking less than 30 minutes, machining was underway within 48 hours. All of the 8-ft. diameter flanges were restored to specified flatness tolerances at the port and then shipped inland to the wind farm location.

For newer wind turbines with diameters greater than 10 feet, service companies can use the Climax circular mill. One major wind tower manufacturer recently incorporated the circular mill into its manufacturing process so flanges can be re-machined on the assembly line.

Author: Andy Becker is vice president of business development and marketing at Climax Portable Machine Tools, Newberg, Ore.

Changing IX Resin Cut Acid/Caustic Needs

By Cliff Lebowitz, Indumark

After changing the ion exchange (IX) resin and regeneration procedure at the Puerto Rico Electric Power Authority (PREPA) Aguirre thermoelectric plant, operations and chemistry management groups reported a reduction in acid, caustic and man hours for boiler feedwater deionization/demineralization (DI). Man hours for DI wastewater treatment were also cut.

Costs for acid and caustic had gone up because of increasing hardness in the deep well boiler feedwater source, but no new capital investment was available to address the problem. Changing the water source was not an option, even though PREPA owns a lake, because of the expense required for the channel, pipeline, pumps and ultrafiltration pretreatment. Changing the IX resin was identified as a possible solution.

Changing the ion exchange resin lowered costs at PREPA’s Aguirre station.
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To protect turbine blades and minimize deposit buildup in steam piping, the plant requires boiler feed water silica at < 125 parts per billion (ppb) and complete removal of chloride, magnesium and calcium. Since the installation of the new resin, silica is running at < 50 to 60 ppb, and chloride, magnesium and calcium are non-detectable.

The DI systems perform eight to 10, 12,000-gallon regenerations a month, generating about 6 million gallons of wastewater. Before the resin change, the plant needed 11 pounds of acid per cubic foot of resin and 10 pounds of caustic for regeneration. After the resin change, acid requirements were cut by 50 percent and caustic by 40 percent.

An IX DI system serves each of the plant’s two 450 MW boilers. Each boiler uses 1,950 gallons a minute of feed water and requires 6,000 gallons an hour of makeup water. Water consumption for regeneration has now been cut by 25 percent or about 100,000 gallon a day, helping the plant avoid water supply problems.

Some $750,000 in chemical cost savings were realized in the first year, growing to more than $1 million in the second. In addition, due to the longer service cycle time, 40 percent fewer man hours are needed for demineralization and 20 percent fewer for DI wastewater treatment.

Changing the ion exchange resin is now recommended as the best way to improve the performance of the demineralization system at other PREPA plants that have the same problem with their deep well sources.

Author: Cliff Lebowitz heads Indumark, an independent technical case history reporting firm with offices in New York, N.Y., and Doylestown, Pa. He holds a B.S. in biology from Rutgers University.

Engineering and Manufacturing Improve Solar Collecting Trough Technology

By Alan Bennett, Hydro Aluminum

Nevada Solar One (NS1), a 64 MW concentrating solar power (CSP) generating facility that came online in July 2007, is among the first large-scale solar stations in 17 years to use a parabolic mirror trough design. The concept has been in commercial use since 1985, but simply wasn’t cost efficient enough to compete with fossil fuels during the last two decades. Once power generation cost structures began shifting and environmental awareness received renewed emphasis, the technology reemerged as potentially cost competitive.

For most parabolic trough power plants, the solar field represents 50 percent of the total investment cost. So to further close the cost-competition gap with fossil fuels the trough technology required significant refinements in design engineering, materials performance and cost of construction. These advancements were achieved through a joint effort by concentrated solar power developer Solargenix Energy, solar technology provider Gossamer Space Frames and Hydro Aluminum the manufacturer of the solar troughs. For Nevada Solar One, the net effect is an annual output increase of 4 percent, or approximately 2.5 MW, over the expected performance due to the frame’s design and manufacture.

Sunlight and blue sky reflect off parabolic mirrors at the 400 acre Nevada Solar 1. For scale, the nearby 500 MW El Dorado gas-fired power plant can be seen on the left side of the photo. Power Engineering magazine photo (both).
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Built at a cost of $266 million, Nevada Solar One occupies 1,618,742 m2 (about 400 acres) in Boulder City, Nev., southeast of Las Vegas. The solar field covers 357,200 m2 (more than 88 acres). Its 760 solar concentrator structures (frames) comprise 9,120 space frames, each assembled from 36 extruded aluminum parts. Each frame is 8 m (about 26 ft.) long and 3.6 m (12 ft.) high. The concentrator structures support 182,400 mirror segments.

The fundamental design for CSP parabolic troughs is almost 100 years old. The concentrator structure supports the mirrors and receiver tube, which is connected to the pipe network that circulates heat transfer fluid throughout the plant. At NS1, the parabolic mirrors concentrate the sun’s rays onto the tube at a ratio of 71:1, which heats the fluid to 735 F. Approximately 17,000 gallons a minute are pumped to the generating station where it provides the energy for a conventional Rankine cycle steam turbine built by Seimens.

The concentrator structures provide two other particularly critical functions: they maintain the mirrors’ optical alignment (which affects concentrating efficiency) as well as the mirrors’ tracking alignment (which allows the mirrors to follow the sun’s movement throughout the day). These are areas on which the NS1 development team trained its attention.

Review and Refine

The concentrator structure refinements came about, in part, from a cost-shared R&D contract between Solargenix Energy and the National Renewable Energy Laboratory (NREL) under the U.S. Department of Energy’s USA Trough Initiative.

Solargenix, now part of Acciona, worked with Gossamer Space Frames, which previously developed a new structural system using a proprietary connector design and several frame innovations for specialty applications. This approach was applied in conjunction with advanced Gossamer frame designs to improve performance and reduce costs. According to NREL the new design results in:

  • 50 percent fewer parts than DS-1 (predecessor design)
  • 30 percent lighter
  • 1/3 time required for field assembly.

Previous concentrator structure designs are based on earlier Luz and EuroTrough designs. The Luz system, made of galvanized steel, is used in many solar electric generation power plants. The first structure type, the LS-2, is accurate, but requires a lot of steel and precise manufacturing, which is costly and slow. A later refinement, the LS-3, uses less steel and is somewhat less expensive to produce. But the LS-3 truss design did not provide sufficient torsional stiffness, which led to lower than expected optical and thermal performance.

Seen from 10 miles away near Boulder City, Nev., the 64 MW, 400-acre Nevada Solar One reflects blue sky to the right of the light pole and behind the transmission pylon.
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A European consortium, EuroTrough, later developed a new collector design different than the LS-2 and LS-3 models. It used a torque-box design to provide torsional stiffness using a four-sided truss to reduce the amount of steel needed. This design was workable, but steel is still costly to manufacture, fabricate and ship. The design has been widely used in Europe, where solar energy is heavily subsidized by governments, but not in the United States.

Hydro’s version of the metal specified, 6061-T6, includes 70 to 80 percent recycled content with no loss in performance characteristics. The recycled content is recovered from production scrap and manufacturing it requires only 5 percent of the energy needed to produce virgin aluminum.

Hydro manufactured 36 separate line items at its Phoenix, Ariz. plant, including connectors, fasteners and discrete frame parts. Hydro shipped the extruded parts 385 miles south to the company’s facility in Guaymas, Mexico for punching, multi-spindle drilling and fabrication. The pieces were shipped back to Phoenix for final quality control and shipment to the NS1 site for assembly. Hydro produced 40,000 pounds of aluminum components every day for about 9 months; seven million pounds in all.

While plant construction took 18 months and required 1.5 million total man-hours, the solar arrays were constructed quickly. The concentrator structures were erected in one-third the time needed for Luz or EuroTrough collectors. The time savings came about from three major design improvements.

First, the frame structures use 50 percent fewer parts than predecessor or competitive designs. Second, Gossamer’s proprietary “organic” connector eliminated welding altogether. Third, manufacturing tolerance specification for the frames called for +/- 1/64” over 30 feet on the extruded aluminum. To achieve this, Hydro used tooling bars. This fineness meant the mirrors did not require field focusing after assembly, significantly reducing construction time and significantly improving focal accuracy.

NREL recommended the combined “slope error” (mirror error plus frame-alignment error) at around 3.0 milli-radians or less for solar trough arrays. NS1 appears to be functioning with a combined slope error near 2.0 milli-radians. This translates to a focus improvement of 34 to 38 percent.

This means the trough frames are quite close to theoretically perfect performance. The net effect is a real output increase of 4 percent, or approximately 2.5 MW. During its first peak summer season, NS1 actually had to dump power to keep its system balanced.

During peak summer months, the facility outputs a net 72 MW to the utility. Its annual production is 130,000 MW.

Author: Allan D. Bennett is the vice president, Solar Market Development – Western Region, for Hydro Aluminum’s Extrusion Americas unit.

Want to see other solar technologies? Watch a video tour of the 10 MW El Dorado thin-film solar photovoltaic facility located next to Nevada Solar One by visiting the Power Engineering magazine web site and clicking on Media Center.