May-2024

Steam methane reformer tube lifecycle improvement best practices

Reformer tube life assessment can be done via online monitoring, non-destructive testing, and tube harvesting with historical operating, monitoring, and inspection data.

Richard D Roberts and Grant Jacobson
Becht

Article Summary

Steam methane reformers (SMRs) are an integral asset operated globally within methanol, hydrogen, ammonia, and direct reduced iron (DRI) plants. Providing plant owners/operators with the ability to extend tube life within the steam reformer is essential in maximising the use of an owner’s capital investment. Most of the tube’s material content is made up of nickel, ultimately driving the overall economics of fabrication. The cost of purchasing and installing a single reformer tube can be upwards of USD 25,000. However, in today’s highly competitive markets, the effect of unplanned downtime in reducing the plant on-stream factor is far greater than the installed cost of a single reformer tube.

It is essential that plant engineers have access to highly accurate online monitoring and inspection data, allowing them to better manage the reformer tubes and, where circumstances allow, extend the tube life expectancy beyond the typical prescribed life expectancy. One hundred thousand operating hours is a typical design life guideline practice used by furnace design engineers.

Today, plant owners and tube manufacturers have become accustomed to inspecting tubes with the latest technologies, even before they ship from the tube fabrication facility. This proactive approach enables quick detection and quantification of manufacturing flaws, enabling repairs to be made if necessary. Additionally, this early inspection approach provides plant owners with ‘baseline’ inspection data for future use over the life of the tube.

Tube inspection instrumentation technologies often apply non-destructive testing (NDT) methods such as laser profilometry (LT), eddy current (ET), ultrasound (UT), radiography (RT), and electromagnetic acoustic transducer (EMAT). Each method has its strengths and weaknesses. Therefore, some instruments have coupled two or more NDT methods to maximise the accuracy of test result outputs and inspection area coverage.

QA/QC and ‘baseline’ inspections
Both internal and external inspection approaches (see Figure 1) can be utilised to facilitate QA/QC screening at the tube manufacturing facility. Internal inspection methods tend to be preferred as they allow the tubes to be stacked on top of one another in the horizontal orientation, thus requiring a small footprint within the facility. External methods require tubes to be spread out to allow for adequate space around the exterior, thus requiring a much larger space to carry out the inspections. Both approaches often apply one or more of the previously described NDT methods.

Conducting QA/QC inspections prior to the tubes being shipped from the manufacturer’s facility allows necessary repairs to be made in advance of shipping and often tight installation deadlines. Flaws such as dimensional over-boring, machine gouges, and excessive weld-root penetration are damages that may reduce tube life downstream if not addressed prior to placing into service.

Data collected as part of the QA/QC inspection can also be archived as ‘baseline’ data and applied during future routine inspections. Even tubes fabricated well within design tolerances naturally contain variations within the internal bore dimension. Precise bore dimensions can be obtained and documented during this exercise. Future routine inspections can leverage these dimensional data rather than applying design values. This approach ultimately produces much more precise creep strain monitoring over the life of the tube, which directly impacts the accuracy of remaining life assessment calculations.

‘Online’ monitoring techniques
Infrared (IR) technology has been used as an ‘online’ temperature monitoring approach within steam reformers for many decades. IR is a non-destructive, non-intrusive, non-contact method for mapping thermal patterns on the surface of tubes and refractory within the reformer during operation. Understanding the tube metal temperature defines the performance capability and inherent reliability of the reformer and, ultimately, the risk of a tube rupture failure.

IR is accomplished with two primary instrument types, such as thermal imaging cameras and pyrometers. The thermal imaging camera forms a two-dimensional thermal image of the target surface, while the pyrometer provides only a single target point temperature. Each instrument has advantages and disadvantages, and effective programs will apply both to leverage the strengths. The imaging camera provides meaningful images and measurements for a historical record, which can be used to assess tube creep damage rates and long-term performance changes. The pyrometer should be used for accurate field measurements to compare specific tubes and troubleshoot real-time performance issues. Pyrometers also require adherence to well-written procedures to maintain the accuracy and consistency of the temperature measurements.

An effective IR program is an absolute necessity to monitor the steam reformer tubes, as well as provide a wealth of diagnostic information that may be used to evaluate the performance and reliability of other reformer components (tubes, tube supports, burners, and refractory).

Radiation signals are strong functions formed from many sources within an operating reformer. Planck’s Radiant Function and the Stefan-Boltzmann equations set the theoretical foundation for these temperature measurements (see Figure 2). The complexity of the measurement environment, coupled with differing materials that change over time, makes accurate temperature measurements difficult but possible with technical know-how and properly applied measurement techniques.

To confirm temperature measurements in a reformer, a contact pyrometer, aka ‘Goldcup’ (see Figure 3), can be used. This water-cooled tool allows temperature measurements to +/-2°C of mid-wall temperature when properly applied. To maintain data accuracy, one must be very cautious about properly calculating the cooling effect via shielding and contact by the Goldcup when in use. After a field survey of a reformer with a Goldcup, these in-situ temperature calibration locations can be used to improve the measurement accuracy of easier-to-field tools (IR cameras and pyrometers). With proper procedures and training, the basic optical pyrometers can approach the Goldcup accuracy after correction factors are applied via the data sets obtained with the Goldcup and proper procedures with training are implemented for the pyrometer use.

‘Offline’ inspection methodologies
During routine plant shutdowns, ‘offline’ tube inspections can be facilitated. Whether the catalyst is being extracted from the tubes or not will dictate which internal or external inspection option can be applied. The internal approach can only be applied once the tube is fully evacuated of catalyst and preferably the catalyst support cone (if present) at the bottom of the tube, providing access to the full tube length, top flange to bottom cone or flange, depending upon tube design.

Inspections facilitated from the tube interior surface while applying an internal probe mechanism often provide more accurate test results, as the internal bore of the tube is machined smooth with tight profile tolerances, as opposed to the rough ‘as cast’ exterior tube surface profile. The full tube length can also be accessed from the tube’s interior bore. Often, many obstructions near the tube’s exterior, such as floor and roof refractory penetrations and narrow tunnel walls, limit an external crawler’s travel, ultimately leaving a portion of the tube uninspected where obstructions exist.