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Reformer Tube

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REFORMER TUBE INSPECTION USING A MULTIPLE TECHNIQUE APPROACH FOR CONDITION ASSESSMENTBrian Shannon, MSc MinstNDT FIAQP Reyaz Sabet-Sharghi, Ph.D. IESCO, Inc. 3445 Kashiwa Street Torrance, CA 90505 USA E-Mail: [email protected] E-Mail: [email protected]

ABSTRACT Centrifugally cast materials, namely HK40, HP Modified, and Micro-Alloy materials, are used for tube materials in steam reformer tubes. The material undergoes various stresses resulting in damage which can manifest itself in several ways. The quantification of damage is of vital importance if tube life is to be predicted accurately. A comprehensive inspection system has been developed to assess the exact degree of damage. The technology utilizes several NDE techniques, including ultrasonic and eddy current examinations. The combination of techniques provides valuable date for the prediction of the remaining life of tubes. Field experiences and findings are discussed in the paper. Keywords: reformer tubes, reformer furnaces, diametrical growth, creep damage INTRODUCTION Reformer tubes normally used in the refining, petrochemical and fertilizer industries are manufactured by the centrifugal casting process and heat-resistant austenitic alloys such as HK40, HP40, and HPNiobium modified materials. A design life of 100,000 operating hours has been the normal time-based criteria for considering retirement of tubes. Many operators of furnaces using such tubes desire to change their maintenance philosophy for tube retirement to condition-based assessment rather than time-based assessment. At a cost of several thousands of dollars per tube and a retubing cost of $1MM$4MM U.S. Dollars, a significant amount of capital can be inadvertently applied if tubes are retired either too early or too late. There are many reformer furnaces remaining in service beyond the 100,000 operating hours criteria. Metallurgical examination of tubes removed from such service has typically indicated carbide agglomeration, but no discernable creep voids or fissures.(1) This provides the opportunity to improve reformer furnace life-cycle value by lifeextension of the tubes, using condition-based criteria.

Rather than remove tubes from service for sectioning and metallurgical examination at every plant turnaround, it is advantageous to use NDE techniques to screen tube condition for environmental damage such as creep. Operational data required for estimating tube condition by analysis are usually not available. Proper determination of tube condition and its ultimate life requires specific insitu examinations. The disadvantages in removing tubes from service on a sampling basis to determine tube integrity include: Catalyst removal Early retirement of serviceable tubes Late removal of non-serviceable tubes, impacting turnaround critical path duration if it is found that all the tubes need to be renewed Maintenance costs The advantage of removing tube(s) from service to determine condition includes: True metallurgical condition of that particular tube is known. However, the condition of the sample tube may or may not be a representative of the total number of tubes in the furnace. For an operating facility to change from a time-based to condition-based philosophy requires confidence in the methods and techniques used to determine tube condition. Extracting tubes at a turnaround close to the end of their design life and subjecting them to metallurgical investigation would appear to be fairly well accepted practice. Some facilities have also embraced the use of certain NDE techniques to trend changes in tubes. The actual technique used is heavily dependent upon the following: Costs Individual plant preferences (limited knowledge of technologies) Historical experiences at the specific location Turnaround duration Availability of analyzed data from reformer tube testing Knowledge of the different NDE technologies (strengths and weaknesses) Availability of specialist services

To reduce the occurrences of furnace tube removal for condition-based assessment and to improve overall reliability of tube life, the use of NDE techniques on a regular basis during reformer furnace turnarounds is beneficial. The condition of a reformer tube is inferred from the response of a NDE sensor to a change in material properties. As such, there are certain limits on detectability, sizing and characterization of flaws that are heavily dependent on the overall test system characteristics, comprised of the environment, instrumentation, sensor, material under test and, of course, the operator.

DISCUSSION Reformer tube condition can currently be inferred in-situ by qualitative NDE assessment using the following techniques: Diametrical Growth (diameter change with creep in some cases) Wall Thickness Measurement (apparent decrease in wall thickness with creep) Replication (final stages of creep damage; i.e., macrocracking) Radiography (final stages of creep damage; i.e., macrocracking) Eddy Current (responds to chromium migration due to overheating and conductivity changes) Ultrasonic (responds to attenuation and scattering)

Diametrical Growth The principal rationale behind this technique is that, as creep damage occurs, the tube bulges. Each material type has its own nominal value of diameter change where creep is considered to have occurred. The following rules of thumb have been reported by various operators over the years. As an example: HK40 23% HP45 57% Yet, recent findings show that in some cases, significant growth may be apparent, but the tube may show the absence of internal damage.(1) Using diametrical growth (O.D. and I.D.) may provide a very general indication of tube condition; however, using diametrical growth as a stand alone method for measuring creep damage, or lack of damage as the case may be, may lead to a significant false call on the actual condition of the tube. The issue is further complicated by the fact that no tolerance is given by the manufacturer for tube O.D. measurement; and the tube I.D., while machined, can vary greatly over the length of the tube segment. In fact, the machining process may produce a given I.D. dimension, but because of the variation in the machining process, the tube may see a significant reduction in wall thickness on one side of the tube while having an abundance of material on the other. While four different samples from the same tube (Figure 1, 2, 3 and 4) had significant changes in creep damage, it is only when the tube reached macrocracking that a noticeable change in the O.D. or I.D. dimension occurred (Figure 5A). The above scenario is not always the case, as is demonstrated in Figure 6. These tube segments represent fired and unfired samples from the same tube. Significant diametrical growth (6%) is noted at both the O.D. and the I.D., well within the guidelines for tube replacement.

Note the total degree of damage is much less than expected (Figure 7). Isolated and aligned voids extend approximately 60% through the wall thickness. Only through the application of other techniques was the true condition of the tube determined. To assess diametrical growth, manual strapping of the tube is often performed, and the results are tabulated per tube, at specific locations on the tube (normally at burner locations). As this technique tends to be tedious, time-consuming and requires scaffolding, automated techniques have been developed. Current automated techniques include eddy current proximity sensors and displacement sensors. The 'H' SCAN displacement sensor is attached to a scanning head that traverses an insitu tube and records the diameter measurement at pre-determined intervals indicating the precise location of suspect diameter changes. The output of the tool is input directly into the software spreadsheet for data recording and analysis. A typical finished chart is shown in Figure 5B. Note the difference in O.D. measurements of the three tube segments. This is a result of the manufacturing variations. Due to these variations, it is preferable if baseline date can be obtained on the tubes when initially installed so accurate trends may be developed. Wall Thickness Measurement As creep damage occurs, an apparent decrease in wall thickness is evident. As an example, average wall thickness measurements were obtained from a tube that had been sectioned at 1.3', 3.3', 23', 36.5' and/or (0.4M, 1M, 7M, 11M) positions; the metallographic condition is depicted in Figures 1, 2, 3, and 4, respectively. (2) There is an apparent decrease in wall thickness for these four sections of tubes, as shown in the graph of average wall thickness in Figure 8. A typical finished chart is shown in Figure 5B, note the difference in O.D. measurements of the three tube segments. This is a result of the manufacturing variations. Due to these variations, it is preferable if baseline data can be obtained on the tubes when initially installed so accurate trends may be developed. Replication Replication is useful for in-situ assessment of reformer tube outside surfaces, to detect overheating that causes microstructural changes. Replication is a "spot" type assessment and is normally used as a supplemental technique. Only the advanced stages of creep damage can be assessed utilizing in-situ replication. Radiography Random radiographic examination is normally used as a supplementary technique to confirm the presence of severe cases of creep damage. It is reasonable to expect to locate

such damage when it has extended 50% in the thru-wall direction, when the tubes are filled with catalyst and isotopes are used instead of an X-ray tube. Although using an Xray tube provides an improved quality image, it is not normally employed, because of practical conditions on site. Eddy Current Eddy Current techniques have been used for a number of years on HK40 and HP45 tubes. The basic principles of the technique can be found in