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Integrated Steam Reformer Tube Inspection and Remaining Life Assessment Methodologies Enable Methanol Plants to Operate Reliably Between Turnarounds

By: Rich Roberts, Vice President & General Manager, Quest Integrity Group
As seen in the November 2012 issue of Nitrogen + Syngas Magazine

During recent years, the global demand for methanol has continued its upward trend.  Steam reformers remain a critical operating asset in the methanol plant, and reformer tubes are one of the most critical and expensive components to operate and maintain.  In today’s highly competitive markets, the effect of unplanned downtime in reducing the on-stream factor is far greater than the replacement and installation costs of a single reformer tube.  Combined with the steady increase in methanol demand, reliable operation of methanol plants between turnarounds is of paramount importance.

Common Problems Affecting Tube Life

There are several common problems that affect reformer tube life, and thus adversely impact methanol plant operations.  These include premature tube failure, pigtail failure and header system damage.  Personnel at the plant level responsible for day-to-day monitoring and operation of steam reformers commonly rely on external experts to conduct specialized reformer tube inspections and remaining life assessments.  The quality of such data can positively or adversely affect the operational decisions made by plant engineers.  Therefore, a solid inspection and remaining life assessment program with a proven track record is essential for reliable operation.

Technology Solutions

Globally for more than a decade methanol plants have relied on the quality inspection data produced by the Laser-Optic Tube Inspection System (LOTIS®) technology.  Further complimenting LOTIS is the external tube crawler technology, known as MANTIS™.  A combined internal and external comprehensive tube inspection approach has provided clients with the ability to inspect steam reformer tubes, regardless of whether catalyst was present in the tubes.

Over the last few years, MANTIS has undergone a major enhancement redesign.  To further complement the precise creep strain data already being collected, an innovative technique called permeability cross-canceling eddy current was added, providing crack detection capabilities.  For years, conventional eddy current has been applied to inspect reformer tubes with limited success.  The challenge associated with conventional eddy current is that permeability variations within the reformer tube material will often produce erroneous inspection results.  This can result in premature retirement of expensive tubes which were in good condition, or worse, leaving tubes in the steam reformer which are at the end of their useful life. 

For decades the nuclear industry has dealt with permeability variations when inspecting critical high alloy tubes.  Due to the enormous risk associated with component failure in nuclear power plants, the industry developed a unique eddy current approach which eliminates the effects caused by this phenomenon.  In addition, the need to place high powered saturation magnets near the eddy current sensors is also eliminated.  MANTIS leverages this unique approach and applies it within the methanol industry to help operators improve overall reliability.  

The eddy current functionality added to MANTIS was developed specifically to provide crack detection capability.  The prime method of detecting normal service creep damage remains measurement of diametrical expansion. However, the presence of cracks is not the only factor that may influence the eddy current signal and the data may be interpreted to identify these influences. These include:

  •  Cracking - the principal reason for employing eddy current technology
  •  Minor effects caused by creep void formation prior to crack formation
  •  Material changes caused by the extended exposure to service temperatures (aging)
  •  Manufacturing defects such as inclusions, porosity and cold-shuts
MANTIS allows confident interpretation of cracking. The other influences however, may be more difficult to identify and are referred to as metallurgy changes in this article. While positive identification of these eddy current effects may be difficult in isolation, in combination with the diametrical expansion data the risk associated with these readings can be assessed (see Figure 1).

 


Figure 1 ”" Cracking Detected By MANTIS Eddy Current Technique

 

Inspection Data

Detecting and quantifying creep strain in its early stages allows plant engineers to proactively make operational or physical design changes to the steam reformer to extend tube life.  Three-dimensional modeling of creep strain damage in both individual tubes as well as the full population of tubes provides powerful visual aids in understanding the root cause of the damage (see Figures 2 and 3).  Both LOTIS and MANTIS technologies provide full 3D modeling of inspection data within minutes of collection.

         

Figure 2 ”" 3D model of a
single reformer tube 
Figure 3 ”" 3D model of all
tubes in reformer
                             

 

Remaining Life Assessment

The precise inspection data captured by LOTIS and MANTIS feed directly into Quest Integrity’s proprietary LifeQuest Reformer software, thereby providing remaining life assessment for each inspected tube, following the guidelines developed by API 579 and ASME FFS-1/2007.  Finite Element Analysis using Quest Integrity’s proprietary materials database assesses through-wall creep damage gradient and includes longitudinal and through-thickness stress gradients to accurately model current condition, historical stresses and predicted stresses.

This database includes creep properties in as-cast and aged conditions of HP alloys and microalloys, and represents years of empirical data.  Additional creep testing on both uniaxial and tube samples is ongoing, thereby continually expanding the database.

A proprietary creep model has been developed that captures the unique behavior of HP alloys.  The model incorporates the various stages of creep damage, as well as material aging, formation and growth of microcavities proven by extensive laboratory research and field testing programs.  The tertiary creep stage, where creep rate accelerates prior to rupture, is captured with a continuum damage model where void fraction increases with strain accumulation.

This solution monitors tube life over the entire life cycle, while other techniques that rely on creep cracking damage are typically only useful at or very near end of tube life (when greater than 80% damage has occurred).  This is often too late for remnant life prediction or effective tube replacement management.

The LifeQuest Reformer approach requires the following four data points:
  •  LOTIS or MANTIS tube diameter (strain) inspection data
  •  Total service time of the tube(s) 
  • Operating history (e.g. shutdowns and significant trip events)
  •  Future expected and planned operating conditions

While the above data is required for engineering assessment, additional information such as internal or external temperature history, catalyst performance and past inspection data will also be incorporated into the engineering analysis, subject to availability.

The model effectively calculates a strain-rate or creep curve.  This curve is adjusted by the LifeQuest Reformer model to account for the effects of cyclic operation, such as shutdown cycles and significant trip events.  Of the infinite number of possible creep curves dependent on temperature, pressure and location along the axial length of the tube, only one will pass through the unique value of strain and time as defined by each LOTIS or MANTIS data point.  The definition of such a strain-rate curve allows the failure of the tube to be predicted and hence, the remaining life to be estimated.

The remaining life of each tube is conveyed in a typical LifeQuest Reformer report in tabulated form.  These data can be analyzed in a variety of ways to meet the operator’s requirements.  Such data allows refinement of tube procurement and replacement scheduling (e.g. tube harvesting).

Conclusion

Inspections conducted by LOTIS and/or MANTIS technologies are key to an accurate and comprehensive data collection process.  Critical steam reformer tube inspections can be completed quickly, thereby minimizing furnace downtime.  Preliminary results can be available within minutes following data download from either inspection technology, enabling engineers to make confident, real-time decisions concerning the return of the steam reformer to service.

The major benefit of applying a comprehensive inspection and remaining life assessment program is increased confidence in tube condition coupled with the knowledge of how long it will continue to operate without failure.  This allows for more reliable and confident operations between turnarounds.

References

1. API, RP-573 (2010) Inspection of Fired Heaters and Boilers, Washington DC USA

2. Fisher B., Thomas C., Hill T., (2007) Nitrogen, On-Line Optimization and Reliability Monitoring of Your Synthesis Gas Plant: Fact or Fiction

3. Brightling J., Roberts R., (2005) Nitrogen, An All Inclusive Approach Enabling Maximization of Tube Life Extension For Steam Reformers Utilizing Both Internal and External Inspection Methods Combined With Remaining Life Assessment

4. Roberts R., (2004) AIChE, Maximize Tube Life by Utilizing Internal and External Inspection Devices

5. Roberts R., (2003) NACE, Advancements in Laser Profilometry applied to Reformer Tubes for Detection of Creep Damage Provides a Step Change in Furnace Management

6. Palmer G., (2003) The Progress Report Palmer Industries, Laser Profilometry Applied to Inspect Tubes in Steam Reformers

7. Widrig J., Hill T., Thomas C., (2009) Nitrogen, Automated Inspection, Fitness for Service and Remaining Life Assessment of Reformer Furnace Tubes

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