reformer tube

Click here to load reader

Post on 10-Oct-2014

359 views

Category:

Documents

38 download

Embed Size (px)

TRANSCRIPT

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 Reference(3). The technique relies on changes in electric circuit conditions; the circuit being the instrumentation, cables, sensing coil, and the item under test. As the mechanical properties of the test materials change, a change in overall circuit impedance occurs, which is displayed on an oscilloscope. By monitoring these changes, it can be inferred that creep damage is present, based on observation of the signal parameters in comparison to similar changes that occurred on known creep-damaged materials. The depth of penetration of eddy currents is primarily influenced by frequency, conductivity, and relative permeability. Eddy Current coil design is important to obtain adequate sensitivity and signal to noise ratio. Some tubes, such as HP40 and similar materials that have a high percentage of nickel, require the use of specialized coils to reduce the effects of material permeability variations. This improves the signal to noise ratio so a reliable test result is obtained, allowing adequate discrimination of creep damage from general material property characteristics. Referring again to Figures 1, 2, 3, and 4 that depict varying degrees of damage within a reformer tube, the eddy current responses to these samples are as shown in Figure 9. Note the differences in response to the various stages of damage. The eddy current operator evaluates these changes in signal response. Other factors that the operator considers are: Ultrasonic The primary ultrasonic technique utilized for the detection and estimation of creep damage is: Through transmission ultrasonic attenuation Varying lift-off, influencing the signal response, scale and welds being typical examples Overheating that causes chromium migration, scale formation, and a significant eddy current response in terms of phase and amplitude changes(4) Variations in material permeability

The through transmission technique is shown in Figure 10. The basis is a pitch-catch technique, and it relies on ultrasonic attenuating and scattering due to the presence of c scattering is assumed to be a function of the amount of damage present. Referring again to Figures 1, 2, 3, and 4, that depict varying degrees of damage, the images outlined in Figure 11 depict the four samples and their responses to the ultrasonic examination. The primary disadvantage of this technique is the influence of tube surface condition, which can vary from smooth, dimpled, tightly-adhering scale, to loose scale, or a combination of them all, that affects the ultrasonic signal and gives the impression of creep damage. This can be clearly demonstrated by referring to the two samples outlined in Figure 6, which depict as-cast and fired samples from the same tube. Figures 12 and 13 display the response from the ultrasonic attenuation technique; however, the response from the fired coupon would indicate much less damage than the new or as-cast coupon. This is caused by the signal attenuation due to the surface condition of the as-cast tube. Careful evaluation of a suitable ultrasonic technique is required to demonstrate its suitability for the examination of cast materials. Using an incorrect ultrasonic attenuation technique as a stand alone assessment tool in this case could lead to a significant false call. COMBINING NDE TECHNIQUES - 'H' SCAN TECHNOLOGY Review of the NDE techniques outlined above illustrates some of the advantages and disadvantages associated with each individual technique. Extensive trials have been conducted to determine the viability and optimization of the various techniques. It is currently concluded that no one technique can in all cases provide stand-alone information that will allow complete quantitative assessment of tube condition. (2,13) It is therefore prudent to combine NDE techniques to improve the overall reliability of reformer tube condition evaluation. The optimum combination of NDE techniques is dependent on: Type of material Type of suspected damage Surface condition of material Time frame allowed for data analysis Cost

The applicability of a particular nondestructive examination technique to detect and characterize damage must carefully be considered and demonstrated. The three key ingredients in this review are a full understanding of the damage morphology, its effect, and lack of effect on the material under test, i.e. how the damage manifests itself, and the practical employment of individual techniques.

The techniques must also be employed in a way that the resulting information is meaningful and can assist in the assessment of remaining life. Table 1 outlines the various techniques and their applicability. Acquisition of NDE data is accomplished by the use of a powered carrier mechanism that traverses the length of a tube. The following NDE sensors can be loaded onto a carrier mechanism for simultaneous data collection:

Ultrasonic (attenuation, scattering and wall thickness) Eddy Current Profilometry

Figure 14 shows an IESCO 'H' SCAN assembly of carrier and sensors. It takes about one hour to set up such a system on-site and two to four minutes per tube for data collection and to assign a provisional condition status. The NDE specialists evaluate each tube and assign a damage grade per tube determined on the worst section of tube. These grades are assigned based on comparison of each tube to the NDE responses obtained from samples subjected to metallography confirmation. Final evaluation tube grading and dimensions are then transferred automatically to a life assessment software.(14) CONCLUSIONS Tube condition cannot be determined by one stand-alone technique, as the degree of damage within a particular tube may not lend itself to that specific NDE technique. The reliability of NDE evaluation of reformer furnace tube condition can be improved by combining a variety of advanced NDE techniques ('H' SCAN Technology) that individually monitor differing physical parameters. The advantages and disadvantages of each technique, when compared against each other, reduces the occurrence of false calls, improves tube condition assessment and can increase overall furnace reliability.

REFERENCES 1. Independent Metallurgical Engineering Report N52357 for IESCO client. (July 1996) Shibasaki, T.; Chiyoda Corporation. Private Communication to IESCO. (1996) Electromagnetic Techniques, Volume 4. ASNT Handbook Series. Warren, N.; Summary Report on Study of Prototype EM Inspection Technique for Reformer Tubes. Internal IESCO document. (June 30, 1995) Smith, N.; Non-Destructive Examination of In-Situ Reformer Tubes for Creep Damage. PVP Vol. 336. Structural Integrity, NDE, Risk and Material Performance for Petroleum, Process and Power. ASME (1996) Birring, A. S.; et al. Ultrasonic Methods for Detection of Service-Induced Damage in Fossil Plant Components. EPRI Funded RP-1865-7. Wang, D.; Parra, J.; Internal IESCO document 'H' SCAN development and client sample tubes ultrasonic and metallographic analysis results. (1995) Jaske, C. E.; Viswanathan. NACE Paper #90213. Predict Remaining Life of Equipment in High Temperature/Pressure Service. NACE. Corrosion '90. Mohri, T.; Shibasaki, T.; Takemura, K.; Feature of Creep Rupture Damage of Nb containing Catalyst Tubes for Steam Reformer Furnace. AIChE Ammonia Symposium. (1996) Shannon, B.; Hulhoven, F.; Internal IESCO document, samples and metallography results. (December 1998) Smith, N.; Shannon, B.; Assessing Creep Damage in Cast Furnace Tubes Using Nondestructive Examination 'H' SCAN Technology. AIChE Ammonia Symposium. (1997) Shannon, B.; Evaluating Creep Damage in Catalyst Tubes. Chiyoda Reformer Symposium, Shonan, Japan. (1998) Shell Oil Westhollow Research Center; Private Communication. (1999) Shannon, B.; Jaske, C.; A Practical Life Assessment for Hydrogen Reformer Tubes. AIChE Ammonia Symposium. (2003).

2. 3. 4.

5.

6.

7.

8.

9.

10.

11.

12.13. 14.

TABLES & FIGURESWT Factor 1 1 2 2 Eddy Current OD Yes No No Yes UT (CY1) No No Yes Yes HScan Yes Yes Yes Yes Eddy Current ID No Yes Yes Yes

Damage Attribute Creep Strain (OD) Creep Strain (ID) Initial Damage (30% Wall Volume) ID Cracking OD Cracking Wall Thickness TOTAL

Laser No Yes No No

RT No No No Yes

FMR No No No No

1 1 2 10

No Yes No 4

No No No 1

No No No 4

No Yes No 3

No Yes No 1

No Yes Yes 9

Yes No No 6

Table 1

Macrocracking Severe Damage

Aligned Voids Moderate Damage

Figure 1

Figure 2

Isolated Voids Slight Damage

As Cast Sound Material

Figure 3

Figure 4

Figure 5A Change in Dimension Occurred Due to Macrocracking

Figure 5B Manufacturing Variations

D iam etrical G row th Com parison

5.4 5.2Tube Diameter

5 4.8 4.6 4.4 4.2 4

11

Figure 6 As-Cast And Fired Samples From The

19

13

15

17

1

3

5

7

9

Sample 1 Unfired As Cast

Sample 1 HP Modified Unfired As Cast

Sample 2 Fired 6% OD Diametrical Growth

Sample 2 HP Modified Fired 6% Creep

Figure 7 Significant Diametrical Growth (6%) is Noted at Both the OD and the ID

Figure 8 Average Wall Thickness Decrease in Wall Thickness for Four Sections of Tubes

23%

52%

88%

Figure 9 Eddy Current Responses Note the Differences in Response to the Various Stages of Damage

Figure 10

Grade 1

Grade 2

Grade 3

Grade 4 - 5 Figure 11 Depicts Varying Degrees of Damage

Figure 12 As Cast Response from the Ultrasonic Attenuation Technique

Figure 13 Fired The Fired Coupon Indicates Much Less Damage then the New or As-Cast Coupon

Figure 14 Scanning Process Utilizing 'H' SCAN Technology