Laser Profilometry as an Inspection
Method for Reformer Catalyst Tubes

Early detection of corrosion or creep damage in catalyst tubes is essential for safe plant operation. The oil industry has conventionally been using eddy current and ultrasonics to detect creep damage. However, the advances made in optics, laser hardware, and signal processing made the use of laser profilometry a viable NDT method. The applications of laser profilometry in evaluating "catalyst dusting" in reformer tubes at a methanol processing plant, and in detecting fluid level corrosion in a reformer furnace at a fertilizer plant, are described in this month's article. The data collection procedures and the unique advantages of the method are discussed. G.P. Singh Associate Technical Editor

Figures 1-2
Figures 3-4

Introduction
Reformers can be found in hydrogen, ammonia, and methanol process plants around the world. Typically, reformers contain several hundred vertically orientated straight tubes, referred to as catalyst tubes. The inner diameter (ID) of these particular tubes is generally 75-125 mm (3-5 in.). During plant operation, the tubes are filled with catalyst while gases pass through the catalyst at extremely high temperatures. One of the major concerns is that the tube material in the catalyst tubes is susceptible to a failure mechanism referred to as "creep." This condition exists due to the harsh environment to which the tubes are exposed, including elevated temperatures and mechanical loading cycles. Being able to identify and locate such damage in its early stages of growth is essential for safe plant operation.

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Laser profilometry has gained worldwide recognition over the last 15 years as an effective NDT method.

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For several years, many process plants have employed conventional nondestructive testing (NDT) methods such as eddy current (ET) and ultrasonics (UT) in an attempt detect creep prior to tube failure. Recently, laser profilometry has also gained acceptance as a method for early detection of creep. Process plants in New Zealand, South America, and North America have successfully used the laser profilometry method. During the initial stages of creep damage, the ID of the tube begins to expand or swell. Using 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 (that is, 2-3 percent) are potential indications of the 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.

 

Laser Profilometry
Laser profilometry has gained worldwide recognition over the last 15 years as an effective NDT method. Thanks to miniature optics, higher speed signal processing electronics, and computer graphic data presentation software, systems have been developed for a broad spectrum of profilometry NDT applications based on laser technology. Laser profilometry commonly employs a principle referred to as optical triangulation. Optical triangulation uses a light source (commonly a diode laser), imaging optics, and a photodetector. As shown in Figure 1, a diode laser is used to generate a collimated beam of light, which is then projected onto a target surface. A lens focuses 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's surface, the sensor scans in two dimensions, generating a helical set of radius data that represent the inside surface topography of the tube. Software then generates a color 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 time. For example, with a properly configured automated laser profilometry system, a catalyst tube 15 m (50 ft) long can be inspected in approximately three minutes while acquiring well over a million radius readings. Large data files of this sort must be manageable and easy to analyze if any substantial benefit is to be gained from them. Software has been designed that automatically compresses and arranges the data for easy viewing and quick analysis processing, substantially reducing the size of the raw data files.

Over the last few years our firm and a large worldwide methanol producing company have formed a partnership resulting in the development of several custom laser mapping inspection probes, which 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, because the laser mapping probe must pass through the tube to gather the ID information. Probes have been designed to inspect catalyst tubes with inner dimensions between 75-135 mm (3-5.3 in.). These laser mapping probes were designed to be compatible with the firm's existing laser optic tube inspection system. The software was already capable of handling large quantities of data produced by the laser mapping probes and assembling them into smaller, manageable data files. The software then arranges the collected inspection data into several 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 75 mm/s (3 in./s). 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 gathered for each revolution of the laser head. In approximately three minutes, the probe reaches the top of a 15 m (50 ft.) long catalyst tube. The inspection technicians then move on to the next tube to be inspected.

After the data collection process is completed, the inspection data are available to be analyzed. The entire catalyst tube's interior surface is viewable in a straightforward format that allows damage to be located and easily quantified. Three different views (contour, three dimensional isometric, and cross sectional) can be viewed to allow for easy and accurate interpretation of discontinuities (see Figure 2). In addition to the color graphical presentations, the software generates a separate file that provides diametric readings in axial increments as tight as 0.25 mm (0.01 in.). The diameter output file is in ASCII format to allow modeling or trending with several commercially available spreadsheet software packages (see Figure 3).

 

Performance
The two firms first employed the technology at a large methanol processing plant in New Zealand. The plant had experienced a phenomenon referred to as "catalyst dusting" in the lower 635 mm (24 in.) of the reformer tubes. Catalyst dusting occurs when dust from the catalyst vertically works as a sandblaster and erodes interior tube material, eventually resulting in failure. Typically, the catalyst dusting damage is isolated to a 360 degree circumferential band around the catalyst tube's interior surface. This particular catalyst tube design contained flanges at the lower section of each tube which allow the laser mapping probe access from the bottom. The laser mapping probe was inserted vertically 914 mm (36 in.). The interior surface of the reformer catalyst tube was mapped while the laser mapping probe was being extracted. Upon completion of each scan, the extent of damage could be quickly assessed on a portable laptop computer. This provided the plant engineers with precise information which allowed them to make immediate decisions regarding repairs. In addition to quantifying the catalyst dusting damage, inner diameter readings were acquired every 1.3 mm (0.05 in.) down the tube. This information could then be compared to the manufacturer's tube specifications to determine if inner diameter increase was occurring.

The technology was also applied in a reformer furnace at a fertilizer plant in northern Texas. These particular tubes ran horizontally, which required use of an air mover to transport the probe to the far end of the tube. Several 15 m (50 ft) long tubes were inspected to detect fluid level corrosion. In this particular case, the damage had occurred at the gas and air interface line. Damage ran down the axial length of the tube, which provided an ideal application for laser profilometry (see Figure 4). After completion of the data collection, the damages depth and precise axial positioning were clearly quantified. Based upon that information, the fertilizer plant engineers were able to make necessary modifications to the unit design and operating conditions.

 

Conclusion
Laser profilometry has proven to be an effective inspection method for catalyst tubes in reformers. In most cases even a large percentage of the catalyst tubes interior surface is inspected.

 

* Quest Integrated, Inc., 21414 68th Avenue South, Kent, WA 98032; (253) 872-1275.

Copyright © 1999 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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