Teresa Hansen, Section Editor
Entergy Corp.’s Little Gypsy plant in Montz, La., is a natural gas-fired steam power plant with a generating capacity of 1,250 MW. The plant gathers water from the Mississippi River through three intake channels. Each of the plant’s three units is outfitted with two water screens which, until 2005, were equipped with traditional steel carrier chain and basket screens. Little Gypsy experienced excessive wear with the steel screens due to the abrasive nature of the intake environment where river silt and debris are frequently in contact with the screen.
To experiment with a different screening solution, Entergy partnered with a local company to test Hydrolox traveling screen technology for power plants. The screens are made of an engineered polymer material which is non-corrosive and effectively resists wear, impact and fatigue in abrasive environments. In a variety of other water-screening applications, Hydrolox polymer screens have outlasted traditional steel screen components by two or more times.
Installation of the Series 6000 screen at Entergy’s Little Gypsy plant. Photo courtesy of Hydrolox.
Hydrolox water intake screens also feature some unique design characteristics. Where traditional screens are edge-driven, concentrating the load on carrier chains, Hydrolox screens employ a positive drive system. Sprockets are installed along the entire head shaft to distribute the load across the width of the screen, eliminating uneven wear. Additionally, the Hydrolox screen design features no submerged moving parts, which eliminates the carrier chains and foot-shaft sprockets found on traditional screening technology.
In April 2005, Entergy replaced an approximately 14-foot-wide, 53-foot-deep steel screen with Hydrolox’s Series 6000 screen mesh to test the technology against its biggest circulating pumps. The prototype screen, equipped with debris-handling flights, was installed at the most abrasive point of the intake, where debris is the heaviest.
After installation, Hydrolox monitored the prototype screen mesh for operational feasibility, including its performance flow characteristics and the pressure differential across the screen as compared to the traditional technology. The company also monitored the durability of the screen polymers by analyzing wear patterns and predicted screen life based on that analysis.
The spray bar cleans away debris on the working Series 6000 screen. Photo courtesy of Hydrolox.
A newly designed spray header system was installed on the screen to facilitate debris removal from the polymer material. Plant personnel verifed that the system effectively removed debris from the screen’s mesh holes, which are smaller than traditional screen mesh. The Series 6000 mesh holes are almost square and measure 0.25 inch by 0.30 inch.
In addition to frequent on-site inspections, Hydrolox installed a remote monitoring system to observe the screen mesh’s wear, the active tensioning system that reacts to mesh elongation in temperature changes and the head differential across the screen. The remote monitoring system included a camera for real-time observation with streaming video of debris events. Additionally, sensors were installed to detect any broken screen mesh or debris-handling flights.
In June 2006, Little Gypsy replaced the prototype with a Series 6000 production version, which featured new module design enhancements: the sprocket design and material were modified to extend the life of the mesh, the mesh pattern was redesigned to improve the screen’s load-handling capacity and the debris-handling flights were redesigned to optimize cleanability.
Hydrolox also developed a foot section for better seal and less passage of debris through the bottom of the screen frame.
The plant reports that the new screen installation is successful. Its success is evident in the minimal amount of maintenance the screen has required for operation. The screen’s run frequency varies by season. Since the prototype’s initial installation, the screen has run 2,500 hours maintenance free.
Danny Vicknair is a maintenance technician at Little Gypsy and the “subject matter expert” for the Mississippi River intake equipment. He reports significant benefits from the new screening technology. The polymer screen material surpasses the steel screens in strength and durability. “No real screen wear has been reported,” he said.
Don McCrosky, Little Gypsy’s plant manager, also recognizes the benefits of using this technology. “Based on what we’ve seen, we believe that the Hydrolox screen will last two times longer than our conventional screens. The cost savings from decreased manpower will allow us to use our resources for other projects in the plant.”
In addition, the new screen includes a unique wash-nozzle system that is expected to be superior to traditional systems.
Entergy is pleased with the durability, low maintenance and reliability of the screen, so much so that it plans to replace the other five existing screens on the three intake units with the same technology.
In addition to screen quality and performance, Entergy was also pleased with the easy installation process. Visit http://powereng.hotims.com for more information.
Wear Resistant Lining Extends Pulverizer Rejects Piping Life
Great River Energy’s Coal Creek Station in Underwood, N.D., is fueled entirely by lignite coal. Because the pyrites and ash from the lignite are abrasive to the power plant’s pipe lines, Coal Creek had problems with pulverizer reject line leakage. Plant personnel searched for a way to mitigate the effects of this leaking and extend the pipes’ life. They found that wear resistant basalt- and ceramic-lined piping extended the reject piping’s life and reduced maintenance requirements.
Coal Creek Station is North Dakota’s largest power plant, consisting of two units, each rated at about 550 MWs. Unit 1 at Coal Creek Station began commercial operation in 1979 and Unit 2 in 1980. Each unit burns approximately 11,000 tons of lignite per day from the Falkirk Mine, adjacent to the plant.
Lignite coal contains an abrasive portion of non-combustible material that is extremely damaging to piping. Coal Creek had major issues with abrasion in the pyrites rejects pipes, which required them to be rotated annually. Even with the annual rotation, the piping still leaked and began to cause problems within the plant’s pulverizer area. In addition, repairing the leaks and continually rotating the pipes increased the company’s maintenance and labor costs. “It was really getting to be a high maintenance area,” said Louis Simenson, a maintenance mechanic with Great River Energy.
Myron Schell, also a maintenance mechanic with Great River Energy, said, “We had leaks in those areas and, because of the way the previous pipe was designed and installed, we were down in the water a lot. It (the pyrites reject pipe) began to wear out faster so the decision was made to look for other solutions.”
Great River Energy turned to abrasion resistant linings from Abresist Corp., Urbana, Ind., to help extend the piping’s life. Abresist Corp. provided more than 2,500 feet of basalt-lined piping and alumina ceramic elbows for the 16 pulverizers on Units 1 and 2.
The plant’s personnel were already familiar with Abresist Corp.’s lined pipes because the plant’s main 12-inch diameter ash line had previously been replaced with more than 2,000 feet of the company’s 12-inch diameter basalt-lined pipe.
The new lined pipe and fittings were installed in the pulverizer reject areas when each pulverizer was brought down for an overhaul. “We would go in and tear out the existing line and start putting in the Abresist line,” Schell said.
The primary benefit from the basalt-lined piping and ceramic-lined elbows is the wear protection they offer and the resulting life extension. The power plant also realized labor savings, decreased pipe maintenance and less downtime. “In our system, everything is abrasive because we use jet pumps to pump everything out,” Schell said. “We have water that contains abrasive material and we’re basically wet sanding everything at a high velocity.”
With the new lined pipes and elbows in the pulverizer reject line, Schell and Simenson intended to rotate sections of the pipe about every four years to help even out the wear and give the pipe a longer life. Before the lined pipes were installed, the previous pipes had to be rotated more often. “We were rotating the other pipes every year,” Simenson said. Rotating the pulverizer reject section of the line required about two to three days of plant labor. By extending the pipe rotation to every four years instead of annually, the extra days of labor can now be spent more productively.
Eight Years and Counting
The basalt-lined piping and ceramic-line elbows have been in the Coal Creek Station pulverizer reject line for eight years without rotation. An inspection was performed earlier this year and the company was pleased with the outcome. “We found them (the pipes) to be fine, a little bit of wear, but after eight years you’re going to have some wear. We put the pipe back together and were totally satisfied,” Schell said.
“We were anticipating that we were going to have to rotate these lines,” Simenson said. However, plant personnel decided that they can wait four more years before rotating the pipes, resulting in a total of 12 years of service before pipe rotation is needed.
The new lined piping and elbows have also reduced the abrasive wear in critical areas of the line, such as the elbows. The new elbows of the pulverizer reject line, lined with Abresist high alumina ceramics, have mitigated abrasion caused from the material changing direction. “The elbow is a higher abrasive area because it’s changing the flow’s direction and you have an impact point. We will probably have to replace them every five to six years,” Schell said.
Coal Creek Station needed a solution to its wear problems in the pulverizer reject areas, something that could help reduce maintenance costs and labor time. The lined pipes and elbows from Abresist Corp. have proved to be that solution.
They have helped Coal Creek Station reduce its down time, maintenance costs and labor while increasing the life of the plant’s pipes. The lined piping not only stopped the initial leakage problem, but it also offered better overall wear-life on the pipes and allowed the plant to go for longer periods of time without rotating its pipes. “Certain pieces of equipment, like the pulverizer, will come down every four years, so you want to make sure that the other products in the system will last four years, if not longer,” Schell said. “The same thing is true when you bring down a unit that’s been running three years; you want to be sure that the associated products will last for at least three years. So, if you can save some dollars on the material and work with vendors to accomplish all that, you can keep your operating costs down.”