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For several years many process plants have utilized conventional nondestructive methods such as Eddy Current (ET) and Ultrasonics (UT) in an attempt detect creep prior to tube failure. Recently, the use of laser profilometry has also gained acceptance as a viable inspection method for early detection of creep. Process plants in New Zealand, South America, Canada, and the United States have successful used the laser profilometry method. During the initial stages of creep damage, the ID of the tube begins to expand or swell. Use of laser profilometry allows mapping and quantification of a tube's ID as several hundred thousand diameter readings can be acquired down the tube's length. Since small diameter increases on the tube's interior (i.e. 2-3%) are potential indications of early stages of creep, it is essential to gather data with such accuracy. This is a quick process, requiring little or no preparation to the tube's interior surface, since reformer tubes are inherently clean.
Figure 1. Laser Triangulation
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. A lens images the spot of reflected laser light onto a photodetector, which generates a signal that is proportional to the spot's position on the detector. As the target surface height changes, the image spot shifts due to the parallax. To generate a three-dimensional image of the part surface, the sensor scans in two dimensions generating a helical set of radius data that represents the inside surface topography of the tube. Software then generates a user friendly color graphical image of the inside surface of the tube.
A laser profilometry inspection system has the ability to acquire substantial quantities of inspection data in a very short period of time. For example, with a properly configured automated laser profilometry system, a catalyst tube 15 meters (50 ft.) in length can be inspected in approximately three minutes while acquiring well over 1,000,000 radius readings. Of course, large data files of this sort must be manageable and easy to analyze for any substantial benefit to be gained from them. Software has been designed which automatically compresses and arranges the data for easy viewing and quick analysis processing making the initial large data files a non-issue.
Over the last few years QUEST Integrated, Inc., and a large worldwide methanol producing company have formed a partnership resulting in the development of several custom laser-mapping inspection probes. These custom laser-mapping probes were designed to inspect catalyst tubes prior to being placed into service or during a catalyst change out. Access to the interior of the tube is essential since the laser-mapping probe must pass through the tube to gather the ID information. Probes have been designed to inspect of catalyst tubes with inner dimensions between 76 mm (3.0 inches) and 134 mm (5.3 inch). These laser-mapping probes were designed to be compatible with QUEST's existing Laser-Optic Tube Inspection System (LOTIS(tm)), Model 400N. The LOTIS-400N software was already capable of handling enormous quantities of data produced by the laser mapping probes and assembling into small manageable data files. The LOTIS software then arranges the collected inspection data into several user-friendly color graphical presentation formats. The inspection starts by inserting the laser-mapping probe into the upper section of the catalyst tube. An automated probe pusher inserts the laser-mapping probe downward in the vertically orientated catalyst tube until it reaches the bottom. Once at the bottom of the catalyst tube, the probe pusher automatically extracts the laser-mapping probe with speeds up to 76 mm (3.0 inch) per second. While the laser-mapping probe is being mechanically extracted through the catalyst tube, the probes laser head spins at 1800 rpm. Up to 360 laser data samples are being gathered for each revolution of the laser head. In approximately three minutes the probe reaches the top of a 15 meter (50 ft.) long catalyst tube. The inspection technicians then move on to the next tube to be inspected.
Figure 2: LOTIS Views
(Cross-Sectional, Isometric & Contour)s
Figure 4. Axial Grooving Damage in Reformer Furnace Tube apparatus