By Michael Valenti, Senior Editor

The Electric Power Research Institute in Palo Alto, Calif., has developed a phased-array ultrasonic inspection technique that enables utilities to inspect their turbine disc blade attachments faster, more accurately, and at lower cost than previous inspection technologies.

GE technicians inspected disc blade attachments at Alliant Energy's Duane Arnold nuclear plant in one-third the time it took previously, by using EPRI's phased array ultrasonic inspection system.

The technique recently proved itself for the first time on General Electric turbines that serve nuclear power plants operated by Arizona Public Service Corp. and Alliant Energy Corp.

Meanwhile, another ultrasonic inspection technology is being developed by Pacific Northwest National Laboratory in Richland, Wash., to detect cracks in the bolts used in nuclear power plants.

The attachments hold the blades to the rotor and are typically arranged in wheels around the rotor shaft. Over time, the disc blade attachments become susceptible to stress corrosion cracking caused by a combination of centrifugal force, moisture, and the chemistry of rotor alloys. These factors create pitting on the rotor attachments that can grow into cracks, which if left undetected could cause catastrophic failure of the turbine. For this reason, during maintenance outages turbine operators inspect their disc attachments for signs of cracking.

Since the 1960s, turbine inspection organizations have used fixed-angle ultrasonic transducers to inspect the turbine disc attachments. This involves using a single ultrasonic transducer to emit a beam at a fixed angle toward locations where cracks are likely. An ultrasonic receiver captures the returning signal for processing and evaluation to detect flaws.

"Because this technique uses a single ultrasonic transducer, it may have to be set up as many as four times to inspect the critical locations on each side of the attachment. For a double flow rotor, with 10 wheels of attachments, this means it may be necessary to position the ultrasonic transducer 50 to 60 times, which can take two to three 12-hour shifts, a lengthy and costly interval," explained Paul Sabourin, project manager at EPRI's Non-Destructive Evaluation Center in Charlotte, N.C.

In addition, the fixed-angle inspection also has a propensity to produce "false calls" due to an inability to distinguish between closely spaced indications. This may entail removing turbine blades and performing a surface sensitive inspection to verify the validity of the indications, another lengthy and costly process.

By contrast, phased array ultrasonic inspection involves using a probe that contains a series of ultrasonic elements, both transmitters and receivers, to create a high-resolution profile of the complex attachment shape. This technology has been used for years in orthopedic medicine to perform fetal scanning, or sonograms. By 1997, EPRI began adapting phased array ultrasonics for disc blade attachment inspection. That same year, General Electric expressed an interest in using the technique to inspect its own turbine disc blade attachments.

Feasibility studies were begun in the spring of 1998, and the first commercial inspection using the phased array ultrasonic system was finished in the fall of last year.

"GE put the ultrasonic system through its own rigorous Six Sigma quality control program during the months prior to commercialization to ensure its performance," recalled Sabourin.

The EPRI system uses an ultrasonic array probe mounted on an adjustable arm, a 32-channel phased array interface, and an IBM-compatible computer. Operators manipulate the arm to aim the 10x15 mm probe at the target surface.

The probe contains a series of 32 ultrasonic elements arranged in a row. Each element is connected to an ultrasonic transmitter and receiver. The operator uses the system computer to program the channels to rapidly emit precise, successive longitudinal or shear wave ultrasonic beams in increments of one degree or less. Using a plexiglass wedge, the system's beam can be scanned over a typical range of 30 to 70 degrees. The reflected sonic beams are captured by the array probe, and directed to the inspection system's interface before going to the computer for evaluation. Built-in software creates a profile of the disc attachments at the same time flaw detection is occurring. The system operator monitors the computer screen online during the inspection to validate data acquisition. Later, the operator evaluates the data to locate and size potential cracks.

The phased array ultrasonic system can inspect a double flow rotor with 10 wheels in 20 setups, taking less than 12 hours, "and with new interfacing expected later this year, we believe we can reduce that to less than six hours," said Sabourin.

According to EPRI, the technique greatly reduces the number of false calls because of the improved resolution created by the one-degree increment sector scan.

Because the phased array ultrasonic technology was designed to meet the requirements for GE turbine blade attachments, it was first used at two nuclear power plants equipped with GE steam turbines during their fall 1999 refueling outages. These were Arizona Public Service's Palo Verde Generating Station and Alliant Energy Corp.'s Duane Arnold plant near Cedar Rapids, Iowa.

EPRI's phased array ultrasonic technology can inspect blade disc attachments for cracks, visible in this cross section, that can cause catastrophic equipment failure if left unattended.

Arizona Public Service is the largest utility in the state. Each of the three units at its Palo Verde site generates up to 1,200 megawatts. GE technicians used the phased array ultrasonic system to inspect seven stages of blade attachments on the plant's low-pressure turbine rotor in October. Sabourin oversaw the GE technicians' activities.

"We were looking for better-quality data to use for evaluating any given indication found in the plant's disc blade attachments, and the phased array system met or exceeded our expectations," said Bill Lehman, a mechanical engineer in charge of in-service inspection for the turbine generator at Palo Verde. Lehman expects the additional data gleaned through phased array ultrasonic inspection to reduce the number of false calls that previously could have resulted from a fixed point ultrasonic inspection. Reducing false calls helps to extend the service life intervals between maintenance outages.

Because Arizona Public Service made improved data its top priority for the phased array system, the utility did not use the detection system specifically to reduce inspection time in October. "However, we do expect to reduce future disc blade attachment inspections by approximately two-thirds the time required by fixed-angle ultrasonic inspections," said Lehman.

Alliant Energy cut its disc blade attachment inspection by that two-thirds margin when it conducted a phased array inspection on November 1 at its 565-MW Duane Arnold plant. Alliant also avoided what would have been a costly surface refinishing treatment of a low-pressure turbine rotor.

Each low-pressure turbine section at Duane Arnold has 14 stages with an integral mono-block design; that is, dovetail connections are machined out of the rotor itself, rather than being separate components installed on the rotor shaft. When blades are removed from mono-block rotors for repair or replacement, the blades and attachments can be damaged.

Alliant Energy had planned to remove all the low-pressure blades and shot peen, then machine, the entire disc attachment surface. Shot peening and machining is a proven surface treatment technique that improves the stresses in the blade attachment by increasing the dovetail radius and placing the surface material in compression to be more resistant to cracking.

However, shot peening the blade attachment surfaces would have required a costly 19 days of work during the scheduled refueling outage in November. "In October, I witnessed a presentation by Paul Sabourin describing the benefits of phased array ultrasonic inspection and convinced our management to implement this technique on our turbine," said Mark Huting, manager of program engineering at Duane Arnold. "I felt that phased array inspection would provide an accurate baseline of our turbine prior to starting the machining and shot peening."

The Duane Arnold inspection succeeded beyond Huting's expectations. With Sabourin's assistance, GE personnel inspected all the low-pressure disc blade attachments in a single 12-hour shift, instead of the 36 hours typically required by fixed-angle ultrasonic inspection. "The results were excellent, particularly differentiating between pitting and cracks," said Huting.

In order to test the accuracy of the phased array system, the Duane Arnold engineers removed the blades from an area where a crack had been found and performed magnetic particle inspection. This inspection procedure involves applying ferromagnetic particles suspended in a fluorescent solution to the cracked area. The particles are attracted to the crack, making it visible.

"The phased array system pinpointed the crack to within 20-thousandths of an inch, extraordinary accuracy for an ultrasonic based system," noted Huting.

So strong is Alliant's confidence in the accuracy of the phased array inspection system, that the company scrapped plans for shot peening and machining of the blade attachments when trouble developed trying to remove the blades. "Using the phased array technology, we can extend the inspection interval to possibly eight years, compared to four years using fixed-angle ultrasonic inspection," Huting said.

"I think the utilization of phased array in other nuclear power components, including piping welds, reactor vessels, and internal reactor components, will revolutionize the ultrasonic inspection industry."

Nuclear power plant operators may soon extend ultrasonic inspection to their bolts and fasteners, thanks to a system taking shape at Pacific Northwest National Laboratory. The inspection system, being developed for the U.S. Department of Energy, is designed to detect cracks in bolts more accurately and more efficiently than previous inspection systems. This capability will enable nuclear utilities to schedule maintenance during planned outages, similar to the preventive maintenance provided by EPRI's phased array ultrasonic technique.

Pacific Northwest National Laboratory in Richland, Wash., is developing an ultrasonic system to detect cracks in bolts and fasteners on site in nuclear power plants.

Typically, nuclear reactor operators remove bolts and visually inspect them for cracks and wastage. Another option is to inspect the bolt in place by using cylindrical guided ultrasonic waves, which involves either placing a transducer on the exposed bolt end, or positioning a transducer so that a Rayleigh wave from a crack would be detected behind the thread root. These methods are time-consuming and labor intensive, and can miss very small cracks near the thread root, an area prone to cracking.

"This inaccuracy is due to background noise caused by the bolt threads, the nut, or collar of the bolt, as well as the coarseness of the bolt material itself," explained Morris Good, a staff engineer in PNNL's sensors and measurements systems group. If left unattended, bolt degradation can cause part failure and unplanned downtime to replace the fasteners.

PNNL designed its system to take advantage of the synthetic aperture focusing technique for ultrasonic inspections to enhance image resolution. The PNNL inspection system uses an ultrasonic scanner with a shield placed over the bolt head so that the scanner's transmitter, specifically designed by PNNL, emits its ultrasonic beam into the bolt shaft and not the nut. The returning wave, which reflects off features in the bolt, is captured by the scanner's ultrasonic receiver and digitized by a computer.

The computer uses PNNL software to create an image of the bolt profile that is presented on a display screen for inspectors. Cracks in the bolt thread are shown extending toward the center of the bolt shaft from the thread root.

PNNL engineers tested their ultrasonic system on an Inconel fastener in August 1999, with a 2.1-cm diameter and a coarse helical thread with a pitch of 3 mm per revolution. The ultrasonic system displayed notches as shallow as 0.3 mm near the thread root.

The PNNL researchers are striving to commercialize their ultrasonic technology to work in the field, as well as the laboratory. "By using the PNNL technology to examine bolts on-site, nuclear power plants would eliminate the need to remove the bolts and reduce the cost of inspecting bolts," noted Good.


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