By: Kelsey Hevner, Technical Advisor - Syngas at Quest Integrity Group
Steam reformers are critical assets for the successful operation of hydrogen, ammonia, and methanol plants. The steam reformer is also one of the most expensive assets in these facilities. Catalyst tubes inside the reformer are one of the most important and costly components. A typical retube for a 200-tube reformer can cost $4,000,000 USD, not including the cost of catalyst. In addition, procurement lead time for tubes can be 17 weeks or more. A typical syngas plant loses approximately $250,000 per day when shutdown. Coupling retube costs with the plants' production loss can cost the business millions, making managing the integrity of this asset crucial.
An effective integrity management program for steam reformers that focuses on tube integrity encompasses these essential parts: inspection, data analysis, and remaining life assessment. An integrated inspection and assessment methodology has significant economic benefits including minimizing the risk of unplanned shutdowns due to tube failure, allowing reformers to be operated more aggressively, and providing a wealth of diagnostic information.
Numerous inspection and life assessment methods are available to measure the damage accumulated in tubes and determine the life and integrity of the tubes, each with varying levels of confidence. The effectiveness of internal and external inspection technologies varies throughout the tube's life (as shown in Figure 1).
Inspection Technology Comparison
Additionally, reformer tube remaining life assessments are greatly dependent on the calculation method chosen and expert knowledge of each alloys’ unique behavior. Operators should consider all of these factors when building an integrity management program for their steam reformers.
Plant engineers and inspectors are not only responsible for daily monitoring and reformer operation, but also the assets’ long-term reliability. Radiography and ultrasonic inspection methods only give partial information and do not cover the entire length of the tube. Thus, many operators rely on third-party inspection companies who have the technology to quickly and accurately gather equipment condition data and provide it in a manner that allows them to make real-time decisions on its operation and lifecycle management. A quality inspection with repeatable results and a remaining life analysis allows the equipment owner to effectively manage the reformers’ lifecycle, plan for tube replacements, safely extend tube life, and even troubleshoot operational problems.
Detecting and quantifying creep damage in the early stages of an asset’s lifecycle allows equipment owners to proactively manage the reliability of the steam reformer tubes. Tube inspections are valuable after the unit has been in service to detect creep strain damage (e.g. bulging, swelling, etc.), but can be equally important when the tubes are first manufactured (prior to being placed into service). Even though the tubes are manufactured to very high-quality standards and tolerances, baseline inspections will acquire pertinent baseline data and detect manufacturing flaws (e.g. over-boring of the ID, gouges, and excessive weld-root penetration).
Historically, hundreds of syngas plants around the world have applied internal laser-based tube inspection technology when testing tubes to obtain creep strain dimensional information. Shortly after the tubes have been placed into operation, base-line inspection data obtained from laser-based techniques can be applied to identify and quantify expected or unexpected life consumption. As the tubes age, future inspections can gather data which allows the tube's remaining life to be calculated.
The laser-based, internal inspection technique collects millions of data points along the entire length of the tube, as well as 360 degrees around the internal circumference, which can provide invaluable visual data representation. One of the inspection techniques available in the market utilizes a probe that projects a small laser beam on the internal surface and provides accurate radius measurements to +/-0.005in (+/-0.013mm). This inspection method covers 100% of the tube circumferentially along its full length. Internal inspections are completed quickly once the reformer catalyst has been removed, with some inspection tools requiring less than 3 minutes per tube. Access is only needed at the top of the tube and no access to the reformer's radiant section is necessary. One of the key differences between the laser-based internal inspection technology and other tube inspection methods is the fact that the complete tube surface is inspected. This significantly improves the quality of data and allows for detailed reformer analysis, as well as tube condition to be assessed. There are many advantages to using a laser-based internal inspection technology, such as the fabricated condition of the internal tube surface.
Advances in technology have resulted in the development and use of external tube inspection tools, with comparable quality to the internal technology, that allow for inspections between catalyst changes. External crawlers collect data along the full length of the exposed tube in the box. External inspections are carried out quickly during a shutdown and are ideal for use between catalyst changes. Crawlers currently on the market can have as many as 6 dimensional sensors acquiring radial measurements with an accuracy of +/- 0.015in (+/- 0.38mm), which allows for accurate data gathering for potential creep damage. It is critical that an external crawler is capable of fitting into tight spaces such as the coffin area of the reformer, and since the gaps between tubes in the reformers are as tight as 1 in. (25.4mm), it needs to be slim enough to fit into these spaces. Examples of both internal and external inspection tools can be seen in Figure 2.
Figure 2. Internal and External Inspection Tools with 3D Model of Single Reformer Tube
One important thing to note for internal and external inspection technologies is that it is vital that the inspection tools take a minimum of six radial measurements to ensure accurate dimensions. If only diametrical measurements are acquired, bulges are often averaged out and not accurately quantified, resulting in misleading information.
Data collected during inspection can be modeled in 2D line graphs which display dimensional growth, as well as 3D graphical images which display the overall pattern of damage accumulation. The 2D line graphs allow for a clear interpretation of the inspection results and creep growth; they can also be compared to previous inspections to assist in identifying change over time. 3D images provide a more quantifiable interpretation of the creep damage and further help to identify and troubleshoot operational issues, such as flame impingement and flue gas maldistribution (as seen in Figure 3). In a number of cases, the quality of the inspection data combined with the 3D visualization have proven effective in identifying root causes of operational issues and tube problems.
Figure 3. 3D Model of Steam Reformer Tubes
A Level 1 assessment should be performed for each reformer tube following the API 579/ASME FFS-1 2007 Fitness-for-Service standard. API 579-1 is a multi-disciplinary engineering analysis of equipment to determine whether it is fit for continued service, typically until the next shutdown. The first step in the assessment process is a Level 1 grading classification of the creep damage to determine if further (higher level) assessment is required.
For the Level 1 Fitness-for-Service procedure, each reformer tube is graded based upon the extent of through wall creep damage (typically reported in terms of measured inside diameter (ID) hoop strain growth) and damage rate. A Level 1 pass or fail determination is often concluded based upon proprietary grading systems. Three options are available for this example if the reformer tube fails a Level 1 assessment: the tube is either retired, operating practices are adjusted, or an advanced assessment is performed.
A remaining life assessment is an advanced assessment that is another key to a successful integrity management program. Using the inspection data collected, the measurements are uploaded into software programs for each reformer tube; the direct feed decreases the assessment time and eliminates human error in the transfer. Leading assessment methodologies follow API 579 and ASME FFS-1 standards and use finite element analysis coupled with expert knowledge on the reformer tube materials to accurately model HP alloy materials' behavior.
A profound understanding of high temperature damage mechanisms and damage accumulation is essential for an accurate life analysis. Reformer tube materials are unique and a creep property database for HP alloys and microalloys (aged and new tube conditions) is critical to modeling the strain vs. time relationship (as shown in Figure 4). New information regarding creep test information should continually be added to a database to further expand the remaining life expert's materials knowledge and increase remnant life accuracy.
Figure 4. Typical Creep Strain Curve for HP Alloys
The strain vs. time model captures the stages of creep damage and should be verified by field testing and extensive research done in a laboratory. Remaining life analysis should be based upon strain data for the appropriate material. Some remaining life assessments assume older material properties, such as HK40; however, the older material properties cannot be applied to the new microalloys because of differences in the ductility and other pertinent material properties.
An analysis based on strain data allows for the tube life to be monitored over the full lifecycle. Other techniques that depend on cracking damage due to creep can only monitor tube condition at or near end of life which is usually too late for calculating remaining life or managing tube replacement strategies.
Internal and external inspection data (ID/OD measurements), total service time, operating history (such as shutdowns and significant unit upsets), and planned operating conditions are inputs needed for an accurate remaining life assessment. The remaining life output for each tube is presented in a customized report, which allows the engineers and inspectors responsible for long term reliability to manage tube retirement/replacement and plan accordingly for turnaround scopes.
The remaining life data can then be utilized for creating reliability strategies, determining inspection intervals, and assessing the overall health and operating practices of the reformer.
Conducting both internal and external tube inspections which capture the information for the entire tube length, analysis of the inspection data, and accurately calculating remaining life are all essential to an effective integrity management program. The remaining life assessment component of a comprehensive program provides operators with the ability to monitor tube life over the entire lifecycle, from cradle to grave, avoiding tube-related production losses.
The main benefits of applying an integrated inspection and assessment methodology include minimizing the risk of unplanned shutdowns due to tube failure, allowing reformers to be operated more aggressively, and helping operators develop and manage tube replacement strategies.
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T., Fitness-for-Service and Remaining
Life Assessment of HP Alloy Reformer Tubes