Finding the Flaws
Ask key players in nondestructive testing industry how things are going these days and you can bet they won't shrug their shoulders and say, "Oh, business as usual." When it comes to ultrasonic flaw detection for the energy industry, business not only continues to grow, it is seeing more bells and whistles added every day.
Ultrasonic testing (UT) has been a staple in the energy industry-both conventional and nuclear-for decades. To ensure safety, it is critical that pressure vessels and associated piping remain free of leaks and breaks. UT is widely used for locating and sizing flaws at power plants, including nuclear power facilities where small defects in steel parts can increase under a number of stresses, including those caused by repeated pressure and temperature changes.
As leaders in this market segment look to the future, they are focusing on the need to avoid the formation of flaws both during manufacturing and service. That's in large part because recent legislation and heightened public awareness have increased the financial, criminal and corporate image risks associated with catastrophic failure of aging industrial infrastructure, according to Jeffrey Stetson, global product manager-UT systems for GE Inspection Technologies LLC (Hurth, Germany).
"Asset owners, unwilling to accept this risk, are increasingly implementing automated methods to assess the integrity of their mission-critical infrastructure," Stetson says. "Traditional flaw detection techniques, while having the necessary resolution, may not provide adequate inspection coverage required to completely understand asset integrity."
Bells and whistlesA typical UT inspection system uses high-
frequency sound energy to conduct evaluations and take measurements. It is made up of several components, including a pulser and receiver, transducer and display devices. The pulser and receiver produces a high-voltage electrical pulse while the transducer generates high-frequency ultrasonic energy. Reflection and refraction occur when the sound waves interact with interfaces of different acoustic properties. The vibrational energy picks up discontinuities-such as a crack-in the wave path, reflecting back some of the energy from the flawed surface. The transducer transforms the wave signal to an electrical signal and displays it on a screen.
While the theory remains the same, ultrasonic flaw technology has improved, expanding its uses and capabilities. "Modern ultrasonic flaw detectors are small, portable, microprocessor-based instruments suitable for both shop and field use," says Tom Nelligan, senior applications engineer at Panametrics-NDT, A Business of R/D Tech Instruments Inc. (Waltham, MA). He adds that newer detectors "typically capture a waveform digitally and then perform various measurement and analysis functions on it. A clock or timer will be used to synchronize transducer pulses and provide distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude vs. time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance and amplitude correction and trigonometric calculations for angled sound paths.
"The display may be a CRT, liquid crystal or electroluminescent display," Nelligan explains. "The screen typically will be calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance. Internal data loggers can be used to record full waveform and setup information associated with each test, if required, for documentation purposes, or selected information like echo amplitude, depth or distance readings, or the presence or absence of alarm conditions."
Erich Henjes, product manager for Panametrics-NDT, says that the trend in flaw detector technology comes down to ease of use. "We try to make the inspectors' lives a bit easier. They're looking for instruments that are durable, smaller and lighter. We introduced the EPOCH 2002, the world's first portable flaw detector, in 1984. That first generation weighed 17 pounds. Now they weigh about 2 pounds and have more capabilities." Today, the company's EPOCH 4 series of flaw detectors feature just that. For example, the company's new EPOCH 4PLUS flaw detector features a multicolor LCD and combines flaw detection and measurement capabilities, extensive data storage and the ability to transfer detailed inspection data to the personal computer via a high-speed USB port.
Turnkey designAt GE Inspection Technologies, the IAS 50 provides a complete turnkey system designed for automatic integrity assessment of pipelines and other critical structures. The IAS 50 base ultrasonic testing system is a high-performance, five-channel system featuring two-axis motion control and data acquisition capabilities with application-specific imaging and analysis software. It also features a mechanized two-axis automatic scanning robot. The IAS 50 comes with a series of ultrasonic probes and probe holders to support a variety of automatic inspection on both curved and flat surfaces. When inspection productivity is required, the system is easily set up to scan with a five-channel probe that reduces inspection time by 80% over a single probe configuration.
Systems such as the IAS 50 help to ensure asset integrity. In the energy industry, equipment can fail because of corrosion, inadequate welds, gear tooth failure and more. "Because of changes in regulation, increased environmental awareness and increases in security issues, coupled with the fact that infrastructure in the United States is aging, these plant owners can't have their existing assets fail," David Jankowski, GE Inspection Technologies' general manager for ultrasonic products says. "They need to be confident that their inspection system locates defects and provides a simple method to assess
GE Inspection Technologies' IAS 50 is configured to automatically scan to accurately locate and size flaws within inspected components. The operator sets up the instrument to concurrently display the A-scan of each ultrasonic channel, complete with distance amplitude correction and time corrected gain curve, evaluation gates, gain and other key channel settings to create an inspection plan. Once the inspection plan is uploaded to the system, the IAS 50 will automatically scan a part and provide a C-Scan image of the scanned area for further evaluation and analysis.
Ultrasonic flaw detection, in the end, is a comparative technique that uses reference standards along with an understanding of sound wave propagation and accepted test procedures, Nelligan says. The technique uses straight beam testing to find cracks or delaminations parallel to the surface of the test piece and angle beam testing for cracks located perpendicular to or tilted relative to the surface of a test piece.
For a complete list of ultrasonics providers, visit the Online Buyers Guide at Quality Online, www.qualitymag.com. NDT
sidebar: UT Provides Clues to Dovetail CracksIn 2000, General Electric Co. (Fairfield, CT) turned to the Electric Power Research Institute (EPRI) to assist in investigating an alternative ultrasonic inspection technique for potential dovetail cracking in GE's generator rotor fleet. Past techniques required partial disassembly of the rotor components; GE's and EPRI's goal was to avoid disassembly, thereby reducing inspection time and costs. The solution: Phased- array ultrasonic inspection.
"This project was aimed at solving this specific generator rotor problem," EPRI Project Manager Paul Zayicek says. "It was of immediate concern to EPRI utility members who own GE generator rotors. It does, however, have wide application. EPRI has used phased-array ultrasonic inspection for piping systems, pressure vessel and other rotating equipment applications."
The key advantage of phased-array ultrasonic inspection, which stems from medical ultrasound technology, is it can do a sector scan using a probe made up of 32 individual ultrasonic elements. Each of these elements can be electronically controlled to steer the ultrasonic beam through a wide variety of inspection angles. "This provides multiple looks at the potentially flawed area," Zayicek says. "In this case we conducted a manual exam using a phased-array inspection probe placed in the area of interest in the generator rotor."
EPRI teamed with Structural Integrity Associates (San Jose, CA) to develop dimensional information using representative mock-ups of GE dovetail geometry obtained from in-service rotors. The team fabricated mock-ups from retired rotor material and used a wide range of electrodischarge-machined notches for inspection development.
Past and future
GE was prompted to conduct the investigation after identifying a potential dovetail cracking problem on rotors that had experienced a negative sequence of events, or extensive cycling or turning gear operation. GE later expanded its inspection scope to cover large steam turbine-generator units in service from the late 1960s to late 1970s, as well as the GE Model 324 rotors (Lynn design), which include both two-pole and four-pole generator designs. GE recommended that inspection of units in service for more than 25 years be completed before January 2006. Newer units were required to undergo inspection during the next major outage and before their 30-year service date.
GE had developed an eddy current inspection technique for generator rotor dovetail cracking but it required the removal of at least one retaining ring and the rotor wedges to gain access to the dovetail geometry surface. Although traditional pulse-echo and time-of-flight diffraction techniques also were investigated for this application, researchers determined phased-array inspection is most applicable to four-pole GE generator rotors. The technique also can be developed for two-pole rotors to address surface geometry issues that limit inspection access.
"We liked the phased-array ultrasonic technique because it was easier to implement, giving us better flaw sizing and flaw detection capabilities," Zayicek says. "Generator rotor geometry on the outer diameter area can vary among different vintages and sizes of generator rotors. As you get into more complicated outer surface geometries, the time-of-flight technique, for example, is more complicated to use because it requires two transducers and it's difficult to place the probes. The phased-array technique is more flexible in that different types of transducers and probe wedge configurations are available to optimize the inspection."
Structural Integrity Associates plans to provide the phased-array ultrasonic inspection services to the electric utility industry this year. To learn more about the investigation, contact EPRI at (925) 609-9169 for the report, "Investigation of an Ultrasonic Inspection Technique for Generator Rotor Dovetail Cracking," (Report 1008353), December 2004. The report features a generic inspection procedure useful to utility ultrasonic technicians interested in applying the technique.