NDT.net 1998 July,
APPLICATIONS OF LASER PROFILOMETRY FOR
BOILER TUBE INSPECTION
Richard D. Roberts,
QUEST Integrated, Inc.,
21414-68th Avenue South,
Kent, Washington 98032,
In order to safely and efficiently operate process and utility plants, operators must consistently strive to employ the best possible nondestructive testing technology available. Laser-based profilometry inspections systems, used either alone or in conjunction with complementary technologies, show significant promise of benefit to plant operators. The American Society of Nondestructive Testing (ASNT) has established a Laser Methods Technical Committee as well as an Education and Qualifications Committee. Laser profilometry is considered one of the cornerstones of laser methods in nondestructive testing. Because it is rapid, accurate, and quantitative, laser profilometry is finding a place among more conventional technologies as a reliable and useful tool for today's plant operators.
Industrial applications of laser-based profilometry have rapidly evolved over the past 15 years. Employing miniature optics, high-speed digital signal processing electronics, and computer-graphic data presentation, systems have been developed for a broad spectrum of NDT and QC applications. These tools are not only capable of providing a high-resolution three dimensional profile of the test surface but also a monochrome "laser-video" image of the surface. These devices are now being used for the inspection of tubular goods with inner diameters ranges from 0.44-inch I.D. to 3.94-inch I.D., rifled gun tubes, and various sizes of process piping & tubing. In addition, the technology has been extended to operation underwater and to the profiling of complex surfaces such as nuclear recirculation nozzles and solid rocket motors. This paper presents an overview of this rapidly growing NDT method and provides examples of recent industrial applications.
The principal of optical triangulation employs the use of a light source (in most cases a coherent laser), imaging optics, and a photodetector. As shown in Figure 1, a diode laser is used to generate a collimated beam of light that is then projected onto a target surface. An imaging lens projects the spot of light onto a photodetector, which generates a signal that is proportional to the spot's position in its image plane. As the target surface height changes, the imaged spot shifts due to the parallax. To generate a three-dimensional image of the part surface, the sensor is scanning in two dimensions, thus generating a set of radius data that represents the surface topography of the part.
Figure 1. Laser Triangulation
PRACTICAL APPLICATION OF LASER PROFILOMETRY
Tubing and Piping
Internal corrosion, erosion, open cracking, and pitting in tubing and pipes are common problems in both power generation and process plants. Oxygen attach and hydrogen embrittlement often result in complex internal flaw configurations that are difficult to quantitatively assess using conventional NDT methods. By scanning the internal surface using a high-speed rotating laser profilometer, high resolution maps can be generated of a tube's internal surface. Selecting the proper combination of sensor rotational rates, and axial drew rates, pits as small as 0.025 inch in diameter and 0.002 inch in depth can be reliably and accurately mapped. A cross-sectional display, such as that shown in Figure 2, allows the operator to view the pits both in cross-sectional and axial perspectives. The cursor can be used to measure the depth of a pit at any location. In the case shown in Figure 2, the pit depth is 0.058 inch and its location is 1.82 inch into the tubes elbow. The line of the left side of the figure is the axial profile of the flaw at the axial position dictated by the computer cursor. Figure 3, illustrates a split in the boiler tube wall due to freezing.
A key advantage of using laser profilometry for the location and measurement of internal pitting is the fact that a 40' long boiler tube can be scanned very quickly (normally in less than two or three minutes) to obtain detailed, dimensional information without the negative influence of external features such as fins or changes in metallurgical content.
Figure 2. Cross-Sectional Display of Pit in Boiler Tube Elbow
Figure 3. Cross-Sectional Display of Split in Boiler Tube
The external surfaces of critical components are occasionally subjected to erosion, wear, or ablation. To accurately assess the condition of the surface, a two-dimensional scanning unit can be used to acquire quantitative information regarding the geometry of flaws. One such example is a system developed for the inspection of section nozzles in a boiling water reactor. In this case, the surface of the nozzle had experienced cavitation-induced pitting. A custom scanning unit was developed that was capable of operating 80 feet underwater. After removing the external corrosion, the scanner was lowered into place where it attached itself onto the nozzle bore and automatically generated a three-dimensional map of the irregular contoured surface. This map allowed operators to establish a baseline against which future inspections could be compared.
Laser profilometry has proven useful for the inspection and measurement of a wide variety of critical components. Examples include small-diameter tubing (less than 0.25 inch I.D.) solid rocket motor nozzles, and aircraft components. It has been used to measure hypervelocity track rail geometries, chrome loss in 120-mm cannons, and sludge-pile topography in nuclear steam generators. In some cases, laser profilometry is combined with other conventional NDT technologies. One such example is a system currently being developed that combines laser profilometry with ultrasonics. The resulting data provides a three-dimensional image of not only the internal surface profile but the remaining wall thickness as well.
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