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 between 76.2 mm (3.0 inches) and 127 mm (5.0 inches). During plant operation the tubes are filled with catalyst, with gases are passing 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 in which the tubes are exposed, such as 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.

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.

LASER PROFILOMETRY

The use of Laser Profilometry has gained worldwide recognition over the last 15 years as a viable NonDestructive Testing (NDT) method. As a result of miniature optics, higher speed signal processing electronics, and computer graphic data presentation software, systems have been developed for a broad spectrum of laser-based profilometry NDT applications. Laser profilometry commonly utilizes a principle referred to as optical triangulation. Optical triangulation employs the use of a light source (in most cases a diode laser), imaging optics, and

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 3

After the data collection process is completed the inspection data is readily available to be analyzed. The entire catalyst tube's interior surface is viewable in a straight forward format which allows damages to be located and easily quantified on or off-site. Three different views (contour, 3D isometric and cross sectional) can be viewed to allow for easy and accurate interpretation of flaws (see Figure 2). In addition to the color graphical presentations, the LOTIS software generates a separate file providing diameter readings in axial increments as tight as 0.254 mm (0.010 inch) or ten diameter readings per inch. The diameter output file is in ASCII format to allow modeling and/or trending with several off-the-shelf software packages such as MS Excel^(tm), MATLAB^(tm), etc. (see Figure 3).

PERFORMANCE

QUEST and its methanol partner first utilized the LOTIS 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 inch) of the reformer tubes. Catalyst dusting is a condition where dust from the catalyst vertically works as a sandblaster and erodes interior tube material, resulting in failure over time. Typically, the catalyst dusting damage is isolated to a 360° circumferential band around the catalyst tube's interior surface. This particular catalyst tube design contained flanges at the lower section of each individual tube which allow the laser mapping probe access from the bottom. The laser-mapping probe was inserted vertically a total of 914 mm (36 inch). The interior surface of the reformer catalyst tube was mapped while the laser-mapping probe was being extracted. Immediately upon completion of each scan the amount of damage could be quickly assessed on a portable laptop computer. This provided the plant engineers with precise information allowing them to immediately make decisions with regards to repair requirements. In addition to quantifying the catalyst dusting damage, inner diameter readings were acquired every 1.3 mm (0.050 inch) down the tube. This information could then be compared to the manufacturer's tube specifications to determine if inner diameter growth was occurring.

Figure 4. Axial Grooving Damage in Reformer Furnace Tube apparatus

On another occasion QUEST applied the LOTIS technology in a reformer furnace at a fertilizer plant in northern Texas. These particular tubes were orientated horizontally, which required use of an air mover to transport the LOTIS probe to the far end of the tube. Several 15 meter (50') long tubes were inspected for the detection of 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). Immediately upon completion of the data collection the results clearly quantified the damages depth and precise axial positioning. Based upon that information, the fertilizer plant engineers were able to make necessary modifications to the unit design and operating parameters. Figure 3. LOTIS Diameter Output File

CONCLUSION

Laser profilometry has proven to be a viable inspection method for catalyst tubes in reformers. In most cases even a large percentage of the catalyst tubes interior surface in inspected. QUEST will continue research to determine other applications of laser profilometry within the reformers tubing network systems.