Teresa Hansen, Section Editor
The Three Gorges hydropower plant in China has been under construction since 1994 and is still about two years away from completion. The Three Gorges is China’s biggest construction project since the Great Wall. A gravity type dam, it will retain the water from 2,800 miles of the Yangtze River. Stretching one and a half miles, it is expected to generate 18,000 MW of electricity. Providing an estimated one-ninth of China’s energy, it will replace 40 to 50 million tons of raw coal combustion each year.
 Workers install the hydropower turbine in the Three Gorges dam. Photo courtesy of Trelleborg Sealing Solutions. |
Construction of the dam itself was completed about a year ago and generating equipment is being installed. Alstom Power is supplying almost half of the hydro turbines and generators for the Three Gorges. Their runners are 23 percent larger than any of the hydro turbines and generators Alstom Power previously produced. They are 35 feet in diameter and 17 feet tall.
For hydro turbines on the right bank of the Three Gorges Dam, German-based Trelleborg Sealing Solutions is supplying Orkot bearings made of a reinforced polymer material. The bearings are fitted on wicket gate housings within the Alstom hydro turbines. These gates will be adjusted to control water flow and the bearings will aid their movement.
 Orkot bearings will be used in the Three Gorges hydropower plant motor-generators. Photo courtesy of Trelleborg Sealing Solutions. |
The composite technology offers significant advantages over the traditional bronze solution. In particular, the friction characteristics of Orkot results in longer bearing life.
Composite versus Bronze
Bronze bearings are a traditional solution in hydropower turbines but they require lubrication. During operation, the lubricant enters the water, having a potentially detrimental effect on wildlife. Environmental concerns mean that greaseless “fish friendly” bearings, such as Orkot, must now be fitted on hydro turbines.
As an alternative to proven lubricated bronze bearings, Trelleborg recommended that Alstom use Orkot advanced polymer bearing technology. Although Orkot appeared to be a cost effective solution, Alstom was skeptical about the product’s ability to work effectively. However, independent test results supported Trelleborg’s performance claims.
The tests undertaken were specific to bearings in hydropower applications. The Orkot performance was measured against bronze bearings and competitive composite solutions. In all cases, Orkot outperformed the other products. The bearings’ friction characteristics mean that they have a low coefficient of friction in dynamic situations and especially when movement begins after extended periods of rest. In fact, the coefficient of friction proved to be almost as good at start up as it is in the dynamic state. This is important in the Three Gorges turbines where once the optimum flow of water is established, the bearings within the runners will be quasi-static for a considerable period of time, perhaps several hours.
The Orkot TXMM and TLMM materials used in these bearings are a polymer material whose elasticity is significant to the bearings’ functions. The Orkot material is relatively soft compared to a metal bearing. When a load is applied to the material, it gives slightly, deflecting or deforming in shape. This material property improves the life of the bearing by reducing wear. It has the further benefit of accommodating misalignment within equipment housings.
Test Results
Dinorwig power station, a pumped-storage power station with six generator-motors located in Northern Wales, was the site of the independent study that reviewed alternative bearing designs. The power station conducted the study because its bearings were showing significant wear due to difficult operating conditions. Coupled with this, the facility needed to reduce grease use, which increased maintenance costs and was detrimental to the environment. The tests concentrated on greaseless bearings. Two samples, including Orkot, were supplied by a number of manufacturers and fitted to a single hydropower unit which was monitored for two years.
Following is a description of the generator-motor used in the test, as well as the service conditions under which the bearings were tested.
- Generator-motor description:
- Type - Vertical shaft, salient pole, air cooled
- Generator rating - 330 MVA
- Motor rating - 312 MVA
- Terminal voltage - 18kV
- Excitation - Thyristor rectifier
- Starting equipment - Static variable frequency
- Type - Vertical shaft, salient pole, air cooled
- Service conditions:
- Guide vane shaft material to BS 1630 Grade B stainless steel, 200 HB hardness
- Journal size - 8 inches
- Bearing - 8 inches ID, 9.25 inches OD, 4 inches long
- Housing material - Mild steel 1 inch thick
- Load - 20N/mm2 max
- Temperature - 50 F to 59 F
- Shaft speed - 0.625 rpm
- Arc of rotation - 30 degrees
- Guide vane shaft material to BS 1630 Grade B stainless steel, 200 HB hardness
After 12 months in service, the bearings were removed. Of the nine different bearing types, Orkot showed the second lowest amount of wear at 0.04 mm over the period. After two years this ranking was retained with wear on one Orkot sample at 0.055 mm and the other even less at 0.0425 mm.
The report concluded that the Orkot bearing wear was lower than lubricated bronze bearings and all but one of the competitive greaseless bearings. The Orkot bearing’s life expectancy based on 12 months of service was around four times longer than greased lubricated bronze bearing. The study results also indicated that greaseless bearing performance is more predictable than that of lubricated bronze bearings, because the greased bearings are dependent on the consistency and effectiveness of their lubrication. Finally, the study showed that, unlike metal bearings, non-metallic bearings allow for some shaft misalignment.
Now that they are a proven solution for Alstom, Orkot bearings will be fitted on a number of turbine runners in the largest hydropower project ever built.
Fly Ash System Technology Improves Opacity
PacifiCorp Energy is one of the lowest-cost producers of electricity in the United States, serving 1.6 million residential and business customers with a total power generation capacity of 10,400 MW. The Dave Johnston Power Plant, located beside the North Platte River just east of Glenrock, Wyo., is one of 10 thermal plants operated by PacifiCorp Energy’s Rocky Mountain Power division. The plant is fueled by sub-bituminous, low-sulfur coal and carries an 817 MW rating. With four units in operation, Units 1, 2 and 3 have electrostatic precipitator systems and Neundorfer controls and precipitator optimization system (POS) software to control environmental emissions and comply with all state and federal requirements.
 This depiction of a “main screen” provides an overview of the status of all the hoppers and NUVA feeders. |
Cole Harris, an electrical engineer at the plant, used to wonder as he neared the station on his drive into work, how the opacity on Unit 3 would look. Unit 3 experienced problems staying at or below opacity limits set by the state.
One of the problems with Unit 3, which has a 250 MW turbine rating and burns Powder River Basin (PRB) coal, was that the existing system was set up to use ash-conveying blowers and feeders to pull the ash from the collection hoppers. The unit makes use of a Lodge Cottrell precipitator with six fields in the direction of gas flow, each with eight hoppers, for a total of 48 hoppers.
The fly ash control system on Unit 3 is twice as large as Units 1 and 2, but operates at half the frequency, resulting in an inability to properly evacuate hoppers on this unit. Unit 3 used ash-conveying blowers and NUVA feeders to pull ash from the collection hoppers, requiring considerable man-hours to keep so many feeders working at peak performance and to monitor and diagnose poor performing feeders. As configured, the system monitored pressure in the hopper system only and was blind to ash levels, simply using a programmable logic controller (PLC) to open and close hopper gates at regular intervals.
 This detail screen shot illustrates the operating status of the NUVA feeders. |
As Harris describes it, “The system was ‘dumb’ in the sense that it was looking to detect back pressure and assumed there was no ash build up.” In reality, when the plant changed over to PRB coal, ash buildup became a significant issue.
There was so much ash in the front row hoppers that it bridged over and compromised the precipitator’s capacity. Not only did this situation negatively affect the plant’s operations and maintenance budget, but the facility was experiencing ash plugging on the transport line with backups into the NUVA feeders and hoppers that compromised precipitator performance and contributed to emissions that potentially exceeded opacity limits. Specifically, high hoppers caused precipitator sections to short out in some instances, when ash came into contact with the bottom of the discharge electrode system. In other cases, ash was actually carried away or re-entrained into the system’s gas flow.
Harris knew something could be done to help the opacity and cut the workload for the mechanic maintaining the system.
PacifiCorp Energy considered two options to address Unit 3’s ash buildup and high opacity issues. One option was to install additional ash-conveying equipment to ensure that the ash was carried away more efficiently. This would have required a capital investment estimated in the range of $1 million to $3 million, without any guarantees that the larger upstream issues of ash detection and evaluation would also be solved.
Another alternative was to replace the programable logic controller (PLC) on Unit 3 and install a software optimization package for the precipitator fly ash control system, a much more cost-effective approach estimated at less than $500,000-if it worked. Harris had attended a Neundorfer POS Users’ Group meeting and learned about the SmartAsh system, an optimization system for operating and troubleshooting fly ash systems, already installed at 11 other utilities in North America. He learned that the system could provide an accurate, real-time view of fly ash systems to display system trends, logs and alarms to detect high feeder levels before high hopper levels occur. The SmartAsh program measures the amount of ash pulled on each cycle and calculates ash-pulling rates by hopper, creating a visual profile of ash collection in the precipitator and optimizing ash evacuation control. Communicating directly with a plant’s PLC or distributed control system (DCS), the SmartAsh system compares expected results with actual results while measuring hopper ash flow through pressure signal analysis. The SmartAsh software provides performance improvements by:
- Proactively identifying and alarming ash flow interruptions as they occur so action can be taken before hoppers fill
- Continuously monitoring component operation to identify failures
- Measuring hopper collection rates and using these rates to create a hopper evacuation profile
- Using instrument feedback and hopper collection rates to improve system capacity, reducing component wear and optimizing overall system performance.
Harris thought this system might be the correct solution for Unit 3. Working with Dave Sheetz, a plant mechanic, SmartAsh was installed on Unit 3. The plant upgraded Unit 3 to an Allen Bradley PLC with added pressure and level transmitters to each NUVA feeder. The Neundorfer SmartAsh system communicates an accurate view of the ash system to the PLC that displayes real-time diagnostics including system trends, logs and alarms to detect high hoppers before they can occur.
Sheetz worked with the system daily, looking for trouble spots and mechanical problems.
“The results of using the SmartAsh system have been nothing short of amazing,” he said. He is now able to diagnose feeder performance in less than three minutes, where it previously took an hour to cycle through the system.
The anticipated benefits of installing a new PLC and the SmartAsh optimization software were improved ash pulling sequences that activated hopper gates only as needed (rather than automatically), therefore reducing mechanical wear and transport line plugs.
“Having worked on the feeders and knowing what the pressures should be, I can look at the computer and see right away if I have a plugged ash conveying line,” Sheetz said. He uses the feeder pressure to diagnose the problem with the system. Using the SmartAsh maintenance mode, he can operate gates individually while monitoring vessel pressure to diagnose problems.
“All of this helps keep the ash flowing more effectively, with less chance to build ash clinkers while eliminating ash bridging,” Harris said.
Since the upgrade to the Unit 3 hopper evacuation system, there have been no plugged hoppers and the stack opacity-which previously ran in the high 20 percent range-now runs consistently in the 3 percent to 5 percent range.
Without the need for traditional level sensors on precipitator hoppers, the SmartAsh software provides an accurate, instantaneous indication of ash flow throughout the system, enabling Unit 3 operators to optimize the performance and capacity of the precipitator fly ash system.
“There is a noticeable improvement,” said Harris, “and I feel good that we were able to make a positive difference.”
In addition, PacifiCorp Energy has projected that the payback for the plant’s fly ash control system will require just more than five years.