With technical assistance from EPRI, Australian power provider CS Energy successfully applied a cutting edge ultrasonic inspection technique, enabling the replacement of a major component at one of its power plants, shortening the outage schedule, and potentially avoiding millions of dollars in future maintenance costs.
A Better Way to Inspect Boilers
Inside power plant boilers are tall walls comprised of water-filled tubes that are welded together. The boiler’s furnace heats the tubes, and the water inside turns to steam. Historically, many power plants around the world have used radiography to examine boiler tube welds and assess repair needs. This involves shutting down the boiler, directing X-rays at a weld or other boiler materials, and capturing the X-rays on radiographic film after they penetrate the materials.
Radiography has downsides. To minimize worker exposure to radiation, other maintenance work in the area must stop during the inspection, increasing costs. As a result, radiography is typically performed only during short periods late at night, when other technicians are not on site. The film must be processed and interpreted in a lab to identify welds and other components in need of repair. These activities can take days or even weeks.
With recent technology developments, phased array ultrasonic inspection has emerged as a potentially safer, faster option for weld inspection in boilers. A hand-held phased array device simultaneously directs multiple ultrasonic beams into a weld and combines the reflected beams to create an image of any flaws or defects. Unlike radiography, ultrasonic beams do not pose human health risks, so other maintenance work can proceed during the inspections. The results are available immediately, enabling technicians to analyze defects and immediately repair welds.
A Deep Dive into Ultrasonic Inspection at Kogan Creek
CS Energy’s 750-megawatt, coal-fired Kogan Creek Power Station is located near Chinchilla, a small Australian town about 200 miles northwest of Brisbane. Kogan Creek maintenance staff discovered significant corrosion in the boiler’s reheater and planned to replace the damaged section during the next major plant maintenance outage. The reheater replacement would require inspecting about 5,000 new boiler tube welds and repairing ones found to be defective. Historically, Kogan Creek has used radiography for testing thin-walled reheater tube welds, but the staff were concerned about potentially costly work stoppages that would be required during radiography inspections. They knew that EPRI had previously helped another power plant 100 miles away (Millmerran Power Station) implement phased array inspections and requested similar technical assistance. EPRI has a long-running program to help power plants improve their inspection techniques.
“Reheater replacements require accuracy and precision,” said Jay Richardson, an EPRI expert on inspection of power plant components. “The tube walls are just a few millimeters in thickness. Scratches and other defects measuring less than one millimeter could lead to boiler failures, which can result in several million dollars in replacement costs. Phased array ultrasonic technology can provide the accuracy needed to eliminate defects. The ultrasonic technician and the welder can do their work simultaneously. As the technician inspects new welds and generates ultrasonic images, the welder can refine welding processes and materials based on the images. If a problem is identified with a weld, it is replaced.”
CS Energy’s engineering manager wanted EPRI to review the phased array protocol proposed by the plant’s nondestructive evaluation (NDE) vendor (Intertek) because the tube dimensions were outside the range where phased array inspections had been applied in the past.
EPRI’s Richardson reviewed the vendor’s proposed protocol and recommended refinements based on the latest advances in ultrasonic technology. He fabricated mockups of the plant’s boiler tubes and brought them to the vendor’s testing facility in Brisbane, where the team demonstrated the refined technique. Using computer simulation, Richardson analyzed the results and recommended further refinements (such as different equipment settings) to improve the accuracy of defect detection. The vendor successfully implemented the technique at the plant during the reheater replacement, with assistance from Richardson in interpreting ultrasonic images. Plant technicians used radiography and destructive methods to examine numerous welds, confirming the presence of defects identified by the phased array inspection.
“The replacement of the Kogan Creek horizontal reheater was a large logistical exercise involving removal and replacement of 588 reheater elements,” said Ian Rawlings, who manages power plant boilers for CS Energy. “While phased array inspection has been used on previous tube replacements, it hadn’t been used for thin-walled reheater tubes. Our NDE contractor Intertek developed a procedure to detect and size defects in the welds. With input from EPRI, the procedure was revised to enhance its accuracy. Additionally, EPRI supplied sample welds with and without defects, and these samples were used to train and verify the competency of the NDE technicians. Since returning to service in 2019, the plant has not experienced any leaks in the new reheater section.”
Need to Sharpen Your Inspection Skills? EPRI Can Help
In response to a growing share of renewable energy in the electric power system, thermal power plants around the world increasingly cycle up and down or shut down temporarily—modes known as flexible operations. Because many plants were not designed to operate under these conditions, they can lead to damage of critical components. More than ever, it is essential for power plants to have expertise in nondestructive evaluation (NDE)—inspection with technologies that can probe and penetrate metals and reveal defects without damaging plant components. NDE can help plant operators identify emerging problems with components and plan for repairs and replacements.
Since 2012, EPRI has run a program to assess the proficiency of NDE technicians in applying advanced inspection techniques at power plants, with an objective to demonstrate that NDE procedures, equipment, and personnel can achieve a high level of accuracy in detecting and characterizing damage. As part of the program, EPRI technical experts help plant operators and their NDE vendors refine inspection protocols based on the latest technologies, expand application of advanced techniques, interpret data, and better understand damage mechanisms. EPRI’s NDE laboratory in Charlotte, North Carolina has a large inventory of service-damaged plant components that are used for analyzing damage mechanisms and fine-tuning NDE techniques. Nine power companies around the world currently participate in the program, which has completed more than 470 proficiency assessments to date.
“NDE inspections and analysis are critical to the safe and reliable operation of power plants,” said Mike Ruszkowski, who leads research in EPRI’s NDE group. “If improperly executed, they could lead to substantial financial loss and potential safety concerns for both equipment and personnel. An inspection that misses signs of potential component failure could lead to significant unexpected cost in repairs or replacements. On the other hand, an incorrect prognosis of a failure could lead to unnecessary maintenance costs and loss of production due to a plant outage.”
Through its ongoing communications with power plant operators around the world, EPRI’s NDE program is well-positioned to identify emerging problems related to plant cycling and other flexible operations. “Our NDE research team supports power companies globally,” said Ruszkowski. “This allows us to work with multiple utilities on a wide range of issues and identify common problems that require further research.”
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Photo at top of article: Kogan Creek Power Station. Photo courtesy of CS Energy