Casimir Brenier started work on hydraulic turbines in 1854 to convert the power of flowing water into electrical energy. Today, the company he founded is a subsidiary of Alstom Power and the global Technology Center for its hydropower business.
“The US still has the largest hydropower capacity, but Asia, led by China, is the fastest growing market,” says Jacques Brémond, Mechanical Engineering Supervisor at the Alstom Turbine Technology Center. “In Europe and North America, power generation exceeds the demands of the population, while in China and India increased capacity is desperately needed.”
Filling the energy gap has led to the largest hydropower plant in the world, the Three Gorges on the Yangtze River, China. Alstom is supplying almost half the turbines and generators for the project. The project’s greatest challenge was its sheer size. Compare the width of the Three Gorges at 2.4 kilometers (1.5 miles) to that of a typical dam, which is around 100 meters (328 feet) and the scale of the project can begin to be perceived.
The main part of a hydro turbine is its runner, where the water flow is converted to mechanical energy. “The external diameter of the runner for the Three Gorges was 23 percent larger than any other we had produced before,” says Brémond. “It was a massive 10.6 meters (35 feet) in diameter with a height of 5 meters (17 feet).” It was impossible to produce the runners in the Alstom workshop in Grenoble. When completed they needed to be transported over the town’s bridge, but a single runner’s weight of 425 tons was greater than the bridge’s 300-ton weight limit.
Consequently, the runners were manufactured in a specially constructed workshop in La Ciotat in the south of France. The runners were transported by sea to Shanghai and then transferred from oceangoing vessels to riverboats. It takes six of these just to carry the draft tube elbow.
Alstom started working with Trelleborg, a bearing manufacturer, nearly 10 years ago. “We were unable to specify Orkot bearings on the left bank of the Three Gorges,” says Brémond. “We had little experience of the product at that stage and the customer requested that we design in a known solution.” To support their product, Trelleborg provided details of two independent tests on Orkot specifically for this application. “Based on these reports, we were increasingly confident about the performance of Orkot and decided to trial it in the wicket gate lower bushes during refurbishment of a Francis turbine in the Liu Jia Xia dam in China. After a few more tests in application, the product seemed successful and was first fitted in full scale on turbines supplied to the Alqueva Hydro Power Plant, Portugal commissioned in 2004,” says Brémond.
How the runner works
In a gravity dam, the water flows down from a reservoir into a hydro turbine. It enters the runner from one side via a spiral case, which distributes the water around the turbine. A distributor with adjustable wicket gates in the turbine controls the flow of water circulating in it. The energy of the water (head and flow) transforms into mechanical energy (torque and rotational speed). This then converts to electrical energy within the generator connected to the same shaft as the turbine. Once the water has been through the runner it goes down a draft tube back into the river.
Why build the dam?
The dam is China’s largest construction since building the Great Wall. There were three main reasons for building the Three Gorges dam:
• The first was to regulate the flow of the Yangtze. Its notorious floods have claimed an estimated one million lives in the past one hundred years, regularly disrupt existing infrastructure.
• The second reason was to make the river navigable into the center of China.
• The third reason for the dam was to generate power. The hydropower plant is expected to create as much electricity as 18 nuclear power plants. It will provide an estimated one-ninth of the nation’s energy and replace 40 to 50 million tons of raw coal combustion each year.
Fish friendly?
A gravity type dam, the Three Gorges will retain the water from 4,500 kilometres (2,800 miles) of the Yangtze. Stretching 2.4 kilometres (1.5 miles), it is expected to create as much electricity as 18 nuclear power stations.
Alstom Power, the world’s number one in hydropower generators, will be supplying almost half of the hydro turbines and generators for the Three Gorges. Their runners are 23% larger than any others produced by them before. These are a massive 10.6 meters (35 feet) in diameter with a height of 5 meters (17 feet). For hydro turbines on the right bank of the Three Gorges dam, Trelleborg Sealing Solutions is supplying Orkot® bearings of an advanced reinforced polymer material. The bearings are fitted on wicket gate housings within the Alstom hydro turbines. These are adjusted to control water flow and bearings aid their movement.
Bronze bearings have been a traditional solution in hydropower turbines but to give them sufficient friction to work they require lubrication. During operation, the lubricant enters the water, which has a detrimental effect on wildlife. Environmental pressure means that greaseless ‘fish friendly’ bearings such as Orkot® must now be fitted on hydro turbines. In fact, Alstom was a leader in this practice and have fitted self-lubricating bushes for more than thirty years.
As an alternative to proven products Trelleborg Sealing Solutions offered Alstom Orkot® advanced polymer bearing technology. Orkot® appeared a cost effective solution but Alstom was skeptical about the product’s ability to work effectively. However, Trelleborg Sealing Solutions could support its performance claims for the product with independent reports.
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. Their exceptional friction characteristics mean that they have a very low co-efficient of friction in dynamic situations and especially at start of movement after extended periods of rest. In fact, the coefficient of friction is almost as good at start up as it is in the dynamic state. This was important in the Three Gorges turbines where once the optimum flow of water has been established, the bearings within the runners would be quasi-static for a considerable period of time, perhaps several hours.
The Orkot® TXMM and TLMM materials used in these bearings are unique. A polymer material, the elasticity of the Orkot® product is also significant in the functioning of the bearings. As opposed to a metal bearing, the Orkot® material is relatively soft. When a load is applied to the material, it gives very 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. No other bearing material of this type can offer as low a coefficient of friction along with the same level of elastic properties and load bearing characteristics.
Summary of test results
A review of alternative designs of bearings was undertaken at Dinorwig power station. This was in response to significant wear on bearings caused by difficult operating conditions. Coupled with this they needed to reduce use of grease, which increased maintenance costs and was detrimental to the environment. The tests concentrated on greaseless bearings. Two samples were supplied by a number of manufacturers, including Orkot®, and fitted to a single hydropower unit which was monitored for a period of two years.
After 12 months in service, the bearings were removed. Of the nine different bearing types, Orkot® showed the second lowest amount of wear at only 0.04 mm over the period. After two years this ranking was retained with wear on one Orkot® sample 0.055 mm and the other even less at 0.0425 mm.
Conclusions:
• Wear of Orkot® bearings was much lower than lubricated bronze bearings and all but one of the competitive greaseless bearings.
• Life expectancy based on the 12 month results was around four times longer than greased lubricated bronze.
• Greaseless bearing performance is more predictable than that of greased lubricated bronze bearings as these are dependent on the consistency and effectiveness of their lubrication.
• Non-metallic bearings allow for some shaft misalignment. •
Contact:
Trelleborg Sealing Solutions
Global Marketing & Communications
Handwerkstr. 5-7
D - 70565 Stuttgart
Germany
www.tss.trelleborg.com
Email: olivier.calemard@busakshamban.com