Nondestructive testing (NDT) is a vital activity in the power-generation sector, not only for safety reasons, but to ensure optimum generating efficiency is achieved and maximum operating life is obtained from aging plants. These considerations apply right across the power-generating spectrum, from wind farms to fossil-fuel-fired stations to nuclear power plants. A range of NDT equipment is applied in the power-generation sector.
This article looks at recent advances in inspection technology, particularly in the area of weld inspection and the ways in which smart technologies are improving inspection productivity and, at the same time, meeting the challenges created by the growing shortage of highly qualified NDT technicians.
Repairing Aging Infrastructure
Maintenance, repair, and operations (MRO) budgets apply to all power plants, and MRO costs can typically amount to 30% of the operating costs. In the nuclear sector, this percentage can rise to more than 70%. This is partly due to the nature of the plants and partly because of aging infrastructure. Thirty-five percent of plants in the United States are more than 30 years old, and this is a pattern that is repeated in most developed countries. A significant portion of failures is due to aging infrastructure, and when it is realized that the cost of unplanned shutdowns can range from hundreds of thousands to millions of dollars per day, then some idea of the considerable savings to be made from comprehensive asset management can be gauged.
However, it is not only by improving in-service inspection that savings can be made. It is also important to look at manufacturing processes to ensure that equipment quality on installation is sufficient to meet many years of rigorous service.
NDT during Manufacture
It goes without saying that any component, pipe, tube, pressure vessel, or structure supplied for operation in power generation should be fit for its purpose. Base materials are subject to rigorous quality control, but one of the major areas of in-production inspection is weld inspection. Welding is used for joining components from boiler tubes to wind turbine towers and each application presents different inspection challenges.
Current Inspection Methods
Boiler Tubes
Boiler tubes are welded and, of course, these welds need to be inspected. A boiler contains a large number of tubes, and this necessitates a large number of inspections. Various techniques are used to examine boiler tube welds. For example, butt-joint welds are inspected right after the welding station and a 600-MW boiler typically has about 8500 in-line welds. This means that around 1,500,000 welds in butt joints are inspected every year. These have long been inspected radiographically using a charge-coupled device (CCD) camera and an image intensifier, but the current trend is toward more compact, flat X-ray detector panels.
Another common weld in boiler tubes is the cut-out weld where the tubes are cut and bent and welded to leave an opening in the tube wall. This is an operation usually carried out at the end of the manufacturing stage, and welds are currently inspected using radiography, often using wet film.
Other boiler tube welds currently inspected by radiography include tube on- tube headers and girth weld header assemblies.
General Pipework
Every power-generation plant has miles of pipework, much of which carries fluids under pressure. Most of this is straight pipe, and the conventional practice calls for use of radiography to inspect joints in butt welds.
Wind Turbine Towers
Wind turbine towers are at the other end of the size scale from boiler tubes. These fabricated structures must be welded along their length. Again, radiography has been used, but this involves either moving a very heavy subassembly to a restricted area where radiography can be safely carried out or even shutting down the assembly line to allow the radiographers to do their work.
The Move to Ultrasonic Testing in Manufacturing
Radiographic testing (RT) has long been the NDT technology of choice in the manufacture of pipes, tubes, and components for the power-generation sector, mainly because all standards and inspection procedures specified radiographic testing when originally written.
However, radiography has some limitations and restrictions such as extended film processing times and disposal of waste chemicals. Radiation screening is also essential; this can involve screening online or moving the item to be inspected to a dedicated screening area.
Meanwhile, ultrasonics testing (UT) has made significant advances over recent years with the development of techniques such as time of flight diffraction (TOFD) and phased array ultrasonic testing (PAUT). As a result, ultrasonic procedures are now specified for volumetric weld inspection and can be applied with confidence. Unfortunately, ultrasonic testing data are generally not as transparent as radiographic data. Most radiographs, whether they be wet film or digital, can be readily understood even by non NDT personell.
Ultrasonic testing data needs to be reviewed and analyzed by a qualified NDT technician. Consequently, this can cause staffing, availability, and cost problems in labor-intensive inspections where a number of NDT technicians need to be deployed. Moreover, the problems are magnified because of a growing shortage of suitably qualified ultrasonics testing technicians.
A Solution
Recent developments have provided a solution to problems faced in the application of ultrasonic testing. For example, USM Vision is an ultrasonic testing (UT) weld inspection system from GE’s Inspection Technologies business. Its intuitive, 100% guided operation allows lower-level UT technicians to gather reliable and accurate pipe weld inspection data from one or more systems for subsequent or simultaneous remote assessment by a qualified ultrasonics expert. This permits ultrasonic testing to be used in situations that previously required radiographic testing and removes the constraints associated with radiography. As a result, it facilitates the migration of skills from radiographic testing to ultrasonic testing, reducing the possibility of bottlenecks, providing significant increases in productivity, and improving operational health and safety, as well as making best use of the expertise of the limited numbers of qualified ultrasonics inspection personnel.
This inspection solution can operate in PAUT and TOFD mode and is supplied with its operating software and the company’s analysis software, as well as the probes and wedges necessary to suit the selected codes and pipe ranges. Setup is menu-directed, allowing the operating software to calculate the ultrasonic parameters for each weld and pipe combination and create an easy-to-follow inspection plan. The operator can then scan the weld, with an encoded scanner, using TOFD or PAUT. Inspection data is transmitted to a review station in the industry-accepted DICONDE protocol, allowing advanced analysis tools, such as real-time, volume-corrected imaging, to allow easier and more reliable image interpretation. By using the software platform, inspection data can be reviewed and shared, reports generated, and inspection results archived for tracking or further analysis. The latest equipment features include parallel scanning, linear scanning, and the ability to incorporate phased array probes with up to 128 elements. As a result, it offers increased productivity and greater functionality and can be used on thicker pipes, while satisfying a wider range of inspection codes.
Traceability is an important parameter allowed by today’s new NDT technology. This is important in in-service NDT as well as in manufacture. The new technology is also applicable to the wide range of components manufactured. Special manipulators, which can be used on pipes as small as 0.5 in. in diameter, are ideal for boiler tube inspection. Larger manipulators assist weld inspection on large-diameter pipes and wind turbine towers.
In-Service Inspection
In-service inspection is vitally important to ensure operating efficiency and extend service life. Some inspection is carried out at regular set intervals, but most inspection takes place during planned outages. A wide variety of inspection equipment can be deployed. For example, eddy current can be used to check for boiler tube cracking, and remote visual inspection can be used to view the results of corrosion; however, the most versatile technology is ultrasonics.
Manual Ultrasonic Testing
Manual ultrasonic testing is extensively used throughout power plants. Thickness gauges are used for corrosion measurement. Portable phased array flaw detectors are used for weld inspection in difficult-to-access areas. Unlike radiography, manual ultrasonic testing needs access from just one side and, in nuclear plants, it is unaffected by background radiation. Portable UT equipment has been used to size intergranular stress-corrosion cracking in boiling water reactors (BWRs) in nuclear plants as well as to inspect complex geometry feedwater nozzles in BWRs.
One model of portable UT equipment, the Phasor XS, is part of an Electric Research Power Institute (EPRI) Performance Demonstration Initiative, which aims to validate test methods, NDE equipment, and NDE technicians to specific tasks within a plant. This initiative is yet another attempt to work around the current and growing skills shortage. The equipment has been shown to improve probability of detection, reduce operator training times, and reduce inspection times.
Other NDT systems apply smart technology to improve productivity and optimize the use of qualified inspectors. This is especially the case in boiler tube removal and inspection during planned outages. It has been shown that ultrasonic testing using the newest technology can save up to a week when compared with conventional radiography. This offers the asset owner significant advantages: If the outage period is fixed, the owner can inspect more tubes and, in fact, asset owners are continuously increasing the number of tubes for which they require NDT during shutdowns. With the newest technology, companies can inspect the same number of tubes but free up their inspection teams to carry out other inspection tasks, or, alternatively, they can reduce the length of outages and gain more plant uptime.
Conclusions
NDT is a necessary activity in the power-generating sector, whether that inspection is in manufacture or in service. NDT always needs to be accurate, reliable, and efficient. It should also be traceable and code compliant. As we continue to struggle with the problem of reduced numbers of skilled inspectors, it is even more vital that our inspection technology gets smarter and more versatile.