State-of-the-art of materials and inspection strategies. reformer tubes and outlet components

Transcription

1 Paper no. 2B State-of-the-art of materials and inspection strategies for reformer tubes and outlet components by : R. Gommans Dr. D. Jakobi Dr. J.L. Jiménez Gommans Metallurgical Services Schmidt + Clemens S-C Spain S.A. Stevensweert, NL Kaiserau, D Murieta, SP prepared for presentation at the 46 th Annual Safety in Ammonia Plants and Related Facilities Symposium in Montreal, Quebec, Canada on January 14-17, 2002 copyright R.J. Gommans, GMS and D.Jakobi, S+C Group UNPUBLISHED AIChE shall not be responsible for statements or opinions contained in papers or printed in it s publications - 1 -

2 State-of-the-art of materials and inspection strategies for reformer tubes and outlet components ABSTRACT Catalyst tubes in steam reformers are made of high-alloy spun cast materials. Alloys of the present and past are presented with their specific advantages and disadvantages. The development of spun cast alloys for outlet parts (e.g. manifolds) are presented in comparison to wrought alloys. Also, new alloys for reformer tubes and outlet parts are presented. The main damage mechanisms for catalyst tubes and outlet parts will be discussed. For catalyst tubes these are relaxation of thermal stresses by start/stopcycles and creep by steady-state operation. For outlet parts the main damage mechanisms are creep by steady-state operation and creep-fatigue interaction resulting from hindered thermal expansion during start/stop-cycles. Also, failure cases will be presented to demonstrate the failure mechanisms mentioned above. The influence of materials choice on tube wall thickness, tube weight, catalyst volume and tube life will be discussed. Because of the damage mechanisms involved tube life does not always profit from a thicker tube wall! Also, the life assessment method and inspection strategy has to take these damage mechanisms into account, thus providing a solid basis for a Risk Based Inspection (RBI-) strategy. The preferred life assessment method and an overview of the pro s and con s of the various inspection techniques are presented for catalyst tubes. In many cases it has been observed that inspection of outlet parts is overdone a guideline is given to cut inspection costs

3 1. Introduction Steam reformer units are critical to many processes in refining and chemical plant. They are used in the production of ammonia, but also for the production of hydrogen for oil refining, and direct iron reduction, hydrogen and carbon monoxide for nickel reduction and purification, and syngas as a basis for producing methanol, acetic acid and various other chemicals. The costs for the reformer furnace are a substantial part of the investment of the complete plant. For a typical 1500 metric tons ammonia plant this is about 20%. Also, the materials and assembly costs for the reformer tubes and outlet components make up a substantial part of the reformer furnace (for the above mentioned plant size this is about 10 Million USD). For economic reasons, but mainly because of maintaining high levels of safety, reliability, and structural integrity, end-users want to use state-of-the-art materials and inspection strategies for their reformer tubes and outlet components. This paper describes these aspects. 2. State-of-the-art and new materials 2.1. Reformer tube assemblies Because of the severity of the operating conditions, reformer tube assemblies are fabricated from centrifugally cast, thick section materials. For a high creep strength these alloys contain around 0.4% carbon. A precondition for manufacturing wrought ttubes is the malleability of the material, which ends at a carbon content of ~0.15%. At higher carbon contents, the tubes must be produced by centrifugal casting. Compared to wrought tubes, centrifugal cast tubes show subsequent advantages : - high purity level because of the centrifugal casting process ; - all diameters and wall thickness combinations can be made economically (also in smaller quantities) ; - concentricity is guaranteed by the centrifugal casting and the pull-boring process; and - larger grain size for better creep strength. After centrifugal casting tubes are internally machined by pull-boring to remove casting porosity and impurities that have been driven to the ID by the centrifugal casting process. The internal boring also prevents carburisation of the tube material by decreasing the active surface for carburisation. Internal boring of the tubes also saves tube weight and decreases fuel costs (less thermal resistance). Because of the necessary oxidation resistance at temperatures up to 1000 C the chromium content is about 25%. This chromium content is the standard for all modern centrifugal cat alloys used as radiant tubes. Centrifugal cast tubes are available to the market since the early 1950 s and have found increasing use at the expense of wrought tubes since then. Since the mid-1960 s centrifugal cast tubes were used for nearly 100% of all new reformers. Alloys of the past are : - HK40 (used since the early 1960 s), - IN519 (used since early 1970 s) ; and - HP-Nb (used since the mid 1970 s)

4 Their composition is given in table 1. HK40 uses apart from chromium no carbide forming elements. HK40 develops primary M 7 C 3 -type carbides (during casting, which are transformed into M 23 C 6 upon ageing) and secondary carbides of the M 23 C 6 - and M 6 C-type (during ageing). The newer alloys IN519 and HP-Nb also precipitate Nb carbides (NbC, both primary and secondary carbides). Because of the fine precipitation of secondary Nb carbides, dislocation movement is effectively hindered, which increases creep strength. These NbC-forming alloys have been used with much success. Since the investigations by Zaghloul [ref.1] and the early work of some tube manufacturers so-called micro-alloys have been developed since the early 1980 s. When strong carbide forming elements are added, creep strength increases about 15-20% at temperatures of interest for reformer tubes. This is because MC-type carbides are formed that are more resistant to ageing (less Ostwald ripening) than NbC carbides. Such strong carbide forming elements are Nb, Ti, Zr and others. The effect of this mix of elements is higher than their individual addition, therefore, these alloys are called synergistic hardened alloys. Because only small amounts of carbide-forming elements are used, the alloys are called micro-alloys. The most successful alloy is the micro-alloy based on the HP-composition: HP-micro alloy or HP-MA. Although the micro-alloy based on HK40 has been developed some time ago, unfortunately this alloy has not gained much appreciation. This was due to the success of HP-MA, which is obviously the stronger alloy, and the formation of σ-phase below 900 C. On the other hand HK-MA is nearly as strong as HP-Nb and it can be a low-cost alternative for existing reformers that need re-tubing. This needs to be considered case by case. Table 1 Nominal chemical composition of centrifugal cast materials for reformers material type S+C Märker G / Centralloy CA Fe Ni Cr CFE * high-carbon alloys ** for radiant tubes HK < IN < Nb HP Nb 4852 < Nb HK-MA 4848 micro **** < Nb, Ti, Zr HP MA 4852 micro < Nb, W, Ti, Zr low-carbon alloys *** for outlet components 32/20+Nb 4859 < Nb HP LC H 101 < Nb HP LC MA H 101 micro **** < Nb, Ti, Zr * CFE = carbide forming elements ** high carbon alloys contain typically % carbon *** low-carbon alloys contain typically % carbon **** new S+C alloy S+C Schmidt + Clemens - 4 -

5 2.2. Outlet component assemblies Outlet components such as manifolds, T-pieces, and cones can be made by centrifugal casting. These components need to cope with expansion stresses, therefore, ductility is of primary importance. Creep strength is of secondary importance. Because of these two reasons the carbon content is generally limited to about 0.15%. The cast variety of Alloy 800H, thus of the 32Ni/20Cr type, has become very widely used in Europe and Asia since the early 1970 s and later also in the America s. The cast alloy with 32Ni/20Cr and Nb has a creep strength which is about 50% higher at the temperatures of interest, while ductility after ageing is on nearly the same (high) level as the wrought variety. The chromium content of 20% limits the oxidation resistance and, therefore, the maximum operation temperature is C (which is also dependent on the lifetime). For some applications more oxidation resistance is required. For instance this is the case for thinsection components after long operating times, or for modern designs that have high outlet temperatures. For these cases the chromium and nickel contents have been increased up to the HP-composition (35Ni/25Cr). However, the ductility after ageing of HP-LC is less than 32/20+Nb. By increasing the chrome content from 20% to 25%, the carbide amounts are increased which cause a decreased ductility after ageing [ref.2]. Small diameter pigtail tubing is usually made of wrought alloys such as Alloy 800HT. However, dependent on pigtail dimensions, such small diameter tubing can be manufactured by centrifugal casting in cast 32/20+Nb and HP-LC materials as well. Table 2 Minimum 10 5 hours creep rupture strength (in MPa) for low-carbon alloys material S+C name 800 C 850 C C 1000 C Alloy 800HT (wrought) ** /20+Nb G 4859 * HP-LC H 101 * HP-LC MA H 101 micro *** * source : S+C. ** source : ECCC [ref.3]. *** source : S+C. Preliminary values. S+C has developed a new alloy with high oxidation resistance and high creep strength. This HP low carbon version contains 25%Cr for high oxidation resistance, while a modified composition and micro-alloy additions improve both high creep strength (see Table 2) and ductility after ageing. The ductility after ageing of the HP-LC micro-alloy is much better than that of the standard HP-LC. This alloy is specially recommended for use at higher operating temperatures ( C) in order to profit from the increased oxidation resistance. Furthermore, the new H101-micro can be used at lower temperatures as well

6 3. Damage mechanisms 3.1. Reformer tubes The main damage mechanism for reformer tubes is the combination of thermal stresses across the tube wall and internal pressure stresses. This combination causes that creep damage typically develops at the inner diameter or just below the ID surface. Also, the creep damage occurs over the complete circumference (or at least a large part of the circumference) and over a longer (axial) part of the tube. The damage process results in diameter increase and creep damage (cavitation) at the inner diameter. Final rupture occurs in a longitudinal direction. Another main damage mechanism can be overheating by catalyst degeneration or by operating upsets. Typically, catalyst degeneration results in creep damage over a small part of the circumference and over a short (axial) part of the tube. This means bulging and the final rupture occurs also in axial direction. Besides these two main damage mechanisms, other damage mechanisms can be important as well. These are, for example, metal dusting (in cold areas with T<800 C), galvanic corrosion between cast austenitic material and forged ferritic flanges, formation of brittle phases at the fusion line between austenitic and ferritic material. Sometimes bending can be a problem because of a failure in the tube hanging system. The last mentioned damage mechanisms result in failure in circumferential direction Outlet component assemblies The damage mechanisms of outlet components (manifolds, T-pieces, cones) are generally much simpler than that of reformer tubes, because the outlet components are not subject to firing conditions. Thermal gradients across the tube wall are not significant and do not cause thermal stresses. The main damage mechanism for outlet component is hindered thermal expansion. The outlet system cannot expand (or shrink) freely and causes Low-Cycle Fatigue (LCF) problems during start-up and shut-down. Very often, there is an interaction with creep, because of the long hold-times involved. The damage starts at the outer diameter and concentrates near the welds. The final rupture occurs in circumferential direction. Another damage mechanisms is creep under internal pressure resulting in diameter increase and creep damage (cavitation) at the outer diameter. Final rupture occurs in longitudinal direction. Sometimes, Ni-base welding consumables cause problems because of the difference in thermal expansion coefficient. The Ni-base alloy has an expansion coefficient of 2/3 of that of the base metal. This may cause a problem by hindered thermal expansion

7 4. Influence of materials choice on reformer tube assemblies By increasing the materials creep strength the tube wall thickness of the reformer tube can be decreased. This saves money by : - decreased thermal gradient over the tube wall resulting in : - increased tube life - decreased fuel costs - increased resistance to thermal shock - extra catalyst volume (if the OD is kept the same) resulting in : - increased output and/or increased efficiency Sometimes the end-user decides the leave the dimensions unchanged. The stronger alloy will permit the new tubes to operate at higher pressures and/or temperatures than before. The minimum sound wall thickness (MSW) of the reformer tubes can be calculated by simple equations, such as the ones given in API RP-530. This takes account of the internal pressure stresses, but not for the thermal stresses caused by start/stop-cycles. The equations given in API RP-530 are used here as a guideline. Further optimisation of the MSWcalculation can be obtained by taking the thermal cycling stresses into account (see paragraph 5.1). This subject is out of the scope of this paper. P OD The MSW is given by the equation : MSW = CA ( 2 Sa ) + P Here is : P design pressure [MPa] OD outer diameter [mm] Sa allowable stress [MPa] CA corrosion allowance [mm] Some operations require a corrosion allowance (CA), but generally this is not applied. The allowable stress (Sa) is defined in API RP-530 as minimum 100,000 hours creep rupture stress. The minimum is 80% of the mean stress or the lower limit of the 95% confidence interval (if available). Using this equation for the alloys of interest, the MSW can be calculated for each alloy by taking the OD constant. Thus the ID and the catalyst volume increase and the tube weight decreases. As an example, two scenarios are presented with HK40 as a base case (100%). Case A : Ammonia reformer : P = 4.0 MPa ; T = 925 C ; and OD = mm (4") Case B : Methanol reformer : P = 2.0 MPa ; T = 975 C ; and OD = mm (3½") The results are given in table 3 and figure 1. As can be observed in these tables and graphs, considerable improvements can be achieved. For instance, by changing from HK40 to HP micro-alloy the catalyst volume can increase by more than 50%! Wall thickness and tube weight also reduces significantly. For new reformer tubes the strongest alloy (HP-MA) is the primary choice. For re-tubing of existing furnaces also HK-MA could be an attractive solution

8 Table 3 Influence of tube materials on MSW and other parameters Case A : Ammonia reformer : P = 4.0 MPa ; T = 925 C ; and OD = mm (4") Case B : Methanol reformer : P = 2.0 MPa ; T = 975 C ; and OD = mm (3½") CASE A Sa * MSW ID catalyst tube weight [MPa] [mm] [mm] volume [kg/m] HK IN % 35.9 HP-Nb % 28.3 HK-MA % 29.7 HP-MA % 25.1 CASE B Sa * MSW ID catalyst tube weight [MPa] [mm] [mm] volume [kg/m] HK IN % 22.9 HP-Nb % 17.8 HK-MA % 17.7 HP-MA % 15.5 * source : S+C Figure 1 Influence of tube materials on MSW and other parameters for case A CREEP STRENGTH WALL THICKNESS CATALYST VOLUME TUBE WEIGHT Sa (MPa) MSW (mm) increase (%) decrease (%) HK40 IN519 HP-Nb HK-MA HP-MA HK40 IN519 HP-Nb HK-MA HP-MA HK40 IN519 HP-Nb HK-MA HP-MA HK40 IN519 HP-Nb HK-MA HP-MA - 8 -

9 5. Life assessment Any life assessment procedure should take the relevant damage mechanisms into account Reformer tube assemblies Tube life is primarily limited by creep, driven by a combination of internal pressure and through-wall thermal stresses that are generated during start-up cycles and operating transients. Creep life exhaustion is evidenced by progressive grain boundary cavitation which, due to the significant influence of thermal stresses generated during operating transients, initiates within the tube wall towards the bore. Thus, the life assessment technique used, should take both loading mechanisms (steadystate creep and thermal cycling) into account. In the past some attempts were made to incorporate the influence of start/stop-cycles [ref.4,5], but they were not successful. Later attempts were made by DNV [ref.6], MPT [ref.7], and ERA Technology [ref.8]. ERA Technology has developed a model and a software program called REFORM. The model has many specific features to make the model work. These features are described here shortly, but more extensive (and scientific) background can be found elsewhere [ref.9-11]. S+C and ERA have made a collaboration agreement to serve reformer tube users. Materials creep behaviour The simplest established materials model, that simultaneously predicts strain and damage with time, is a continuum damage mechanics model developed by Kachanov and Rabotnov. It reflects the high stress start-up situation as well as the lower stress steady state regime. Creep life consumption A strain based life fraction rule is employed here, which is more realistic than a time based approach. Given the evidence of samples taken from service, see figure 2, which shows that multiple, parallel cracks are formed, all of similar length. Therefore, a damage front propagation model is used rather than creep fracture mechanics. Figure 2 Cross section showing damage front propagation - 9 -

10 Input for the model The REFORM model needs design and operating input data. The operating data need to be available as a function of time. From experience it is known that some parameters have a large influence on the calculated life. Specially tube skin temperatures, and initial dimensions (diameter and wall thickness) have a large influence. If operating data are available from digital control systems or advanced data loggers, this greatly improves the speed at which the REFORM-analysis can be performed. Probabilistic procedure In order to obtain a realistic life prediction for a particular unit, a probabilistic treatment is employed. Statistical distributions of all input variables are determined and sampled using a Monte-Carlo method. The results can be presented as CPcurves (Cumulative Probability curves) for time-to-crack initiation, time-to-failure, time-to reach a certain strain level, and as time to reach a certain damage level. Simulation The greatest advantage of the REFORM model is that it allows simulation of changes in operation before the change is actually effected in the plant. This can save much tube life and thus availability of the furnace. An example of the results of an assessment by REFORM is presented by a Cumulative Probability (CP-)curve for crack initiation and tube failure (see figure 3) [ref.12]. The operating hours at the moment of assessment was 215,000 hours. Each small line represents one year of operation. It can be observed that crack initiation had already occurred, but that tube failure is not to be expected within a short period of time : - the first tube is expected to fail after another 50,000 hours (~6 years); - 10% of the tubes are expected to fail after another 200,000 hours (~25 years); and - 50% of the tubes are expected to fail after another 400,000 hours (~50 years). From these results it is clear that with similar operating conditions in the future replacement of the reformer tubes are not to be expected for a long time. Furthermore, with proper inspection techniques the reformer tubes can be replaced on schedule. Figure 3 CP-curves for time to crack initiation and tube failure % Cumulative Probability 10.00% 1.00% 0.10% Initiation Time 95% Confidence Interval Failure Time 95% Confidence Interval Current Operational Hours Subsequent Years Of Operation 0.01% Total Service Time (Hours)

11 In some particular cases the combination of process conditions, tube dimensions and material properties appear such that compressive effects in the outer part of the tube are sufficient to dominate over tensile effects towards the tube bore. In such cases the compressive stress field at the OD wins from the tensile stress field at the ID. This causes that the OD decreases, which should not be seen as negative creep [ref.12]. With the above mentioned results of ERA's REFORM model recommendations can be made about : - advised future inspections (inspection moment, frequency and moment of inspection, and inspection method) ; - advised availability of spare parts - availability of the catalyst tubes and the reformer furnace as a whole Outlet component assemblies Remaining life calculations of outlet components can be done by an inverse design procedure using actual material properties and actual service conditions (internal pressure, metal temperatures). In most cases this works well; however, in some cases also axial stresses have to be taken into account. For outlet components such as manifolds a deterministic approach is suitable. For many similar components, such as pigtails, a probabilistic approach can be used [ref.10]. 6. Inspection stategies Any inspection procedure should take the relevant damage mechanisms into account Reformer tube assemblies The damage mechanisms indicate diameter changing and creep damage (starting at the inner diameter). It is important that the inspection techniques used detect these diameter changes and creep damage. Several inspection methods are commercially available for reformer tubes assemblies. These include ultra-sonic, eddy-current and dimensional measurements on a crawler unit traveling up and down the reformer tube. The ultra-sonic measurements are performed from the outer tube diameter and are based on sound attenuation. The ultra-sound travels through the most damaged area, which are the burner sides. The signal is transmitted into the tube by a transmitter and received by a receiver. Creep damage in the tube results in sound attenuation, that can be measured quantitatively. The angles of the in-coming and out-going sound are critical to the success of this method. A TOFD-based ultra-sonic back-scattering technique has been developed as well, and it is claimed that this TOFD-based technique suffers less from false-calls due to the influence of a bad surface condition giving the impression of creep damage

12 Eddy-current measurements can be performed inside and outside. The method is based on the principle that an electro-magnetic field is induced by the test coil. If, for instance creep damage is present, the magnetic induction changes and generates a counter electro-magnetic field, which is detected by the measurement coil. EC is less sensitive to creep damage located towards the inner tube diameter, because the primary magnetic field has a limited penetration depth. Also, EC-measurements suffer from changes in the magnetic properties of the alloy, such as the oxide layer, the de-carburised zone at the outer diameter, the presence of σ-phase, and carburisation. The EC-technique can be used more reliably from the inside. The magnetic field is then more close the most damaged area. Dimensional measurements can be performed from inside and outside. As a general guideline, HK40 is thought to be at the end of it s service life when 1-3% diameter increase has occurred; for HP-materials this is about 5-7%. Automated measurements of the outside diameter along the tube length are offered by various companies. Chiyoda argues that the outer diameter is not suitable to determine diameter increase by creep [ref.13], because of oxidation of the flue gases. Consequently, Chiyoda recommends that the inner diameter should be used to determine diameter increase. Another explanation for decreasing outer diameters has been provided by ERA and DSM [ref.12]. However, also here it was recommended that diameter increase could be monitored easier using the inner diameter. Diameter measurements from the inner diameter can be performed by using laser profilometry. Apart from diameter measurements, defects such as pits, cracks and manufacturing defects can be detected. Internal diameter measurements can also be performed by capacitative displacement measurements. In summary, there are many inspection techniques available from various inspection companies that can detect and quantify the effects of the damage mechanisms (diameter increase and creep damage). These are available on crawler units that are able to travel up and down the reformer tube. Some inspection techniques are available from the outside, some from the inside. Each inspection technique has it's specific advantages and disadvantages, also on removal of the catalyst. These advantages and disadvantages are well known by S+C and GMS, therefore they can advise the end-user to choose the best inspection technique(s) for his situation. A complementary approach where inspection results can be implemented in a life assessment model, can be of great benefit to the end-user. S+C and GMS can help the end-user in such a complementary approach and in the decision making of the relevant inspection strategy, including method, frequency and moment of inspection

13 6.2. Outlet component assemblies The relevant damage mechanisms for outlet assemblies indicate diameter changes (by creep) and cracking by hindered thermal expansion. It is important that the inspection techniques used detect these. The diameter of the manifold can be measured by strapping or with a calliper rule along the tube length. By doing these measurements at regular time intervals (e.g. every turnaround) a trend can be established. As an end-of-life criterion a diameter increase of 5-10% can be taken for low-carbon alloys such as wrought Alloy 800H and cast 32/20+Nb. Thermal expansion damage occurs mainly at the welded joints. These welds should be inspected by liquid penetrant testing (LPT); however, special care should be taken by proper surface preparation. Since each reformer design has another outlet system design, no general guidelines for LPT-inspection can be given. An example is given for a typical hot manifold. The welded joints between manifold and T-piece and between T-piece and cone are critical and should be inspected, see figure 4. Figure 4 Outlet component assembly and advised inspection items (example for a "hot" manifold) diameter measurement LPT (liquid penetrant testing) of all the manifold and Tee- butt welds LPT (liquid penetrant testing) of 10% of the sockolet welds

14 7. Conclusions The state-of-the-art regarding materials for reformer tube assemblies and outlet components has been presented. There is a trend towards the use of micro-alloys for both reformer tube assemblies (HK-MA and HP-MA) and outlet components (HP-LC-MA). Compared to HK40 much smaller wall thicknesses can be obtained by using HP micro-alloy. Therefore, HP-MA is the primary choice for new reformer units. This enables higher catalyst volumes (up to 50% more) and lower tube weights (up to 40% less). HK-MA can be low cost alternative to HP-Nb for re-tubing of existing reformer furnaces. The main damage mechanism for reformer tubes is the combination of thermal stresses across the tube wall and internal pressure stresses. This combination causes that creep damage typically develops at the inner diameter or just below the ID surface. The damage occurs at the location with the highest thermal loading and final rupture occurs in axially. The main damage mechanism for outlet components is hindered thermal expansion. The outlet system cannot expand (or shrink) freely and causes Low- Cycle Fatigue (LCF-) problems during start-up and shut-down. The damage concentrates near the welds and final rupture occurs in circumferential direction. Besides the above mentioned main damage mechanisms many other damage mechanisms may limit the life of reformer tube assemblies and outlet components. Any life assessment and inspection procedure should take the relevant damage mechanisms into account. For reformer tubes the REFORM life assessment model is capable of predicting tube life. Inspection techniques are available on crawler units to detect diameter increase and creep damage along the tube length. Such inspections can be performed both from the outside and from the inside of a reformer tube. For outlet components inverse design procedures can be used by using actual material properties, actual service conditions (internal pressure, metal temperatures) and eventually axial stresses. The diameter of outlet components can be measured easily. Hindered thermal expansion damage occurs mainly at the welded joints. This can be inspected by liquid penetrant testing (LPT); special care should be taken by proper surface preparation. A complementary approach where inspection results can be implemented in a life assessment model, can be of great benefit to the end-user. GMS and S+C can help the end-user in such a complementary approach and in the decision making of the relevant life assessment and inspection strategy (including method, frequency and moment of inspection)

15 Acknowledgements The authors would like to thank the inspection companies and ERA Technology who provided information about their respective inspection techniques and their life assessment model. Both S+C authors would also like to thank their management for permission to publish this paper. Rob Gommans is working as an independent metallurgical consultant under the name of Gommans Metallurgical Services (GMS) in Stevensweert, NL. References [1]. M.B.El-Din Zaghloul, On the strengthening of centrifugally cast austenitic heat resistant HK40 steel, Ph.D.-Thesis (1976) [2]. R.Gommans, J.Sundermann, H.Schrijen, W.Steinkusch, W.Hering, Gefüge und Eigenschaftsänderungen der Stahlgußsorten GX10 NiCrNb und GX10 NiCrNb nach langzeitiger Auslagerung bei C, WG on Heat- and Creep-resistant steels, VDEh symposium 1. December 2000 [3]. European Creep Collaborative Committee, ECCC Data Sheets 1999, BRITE EURAM Thematic Network BET2-0509, doc.0509/mc/47-issue 1 [4]. F.Simonen, User s manual for the computer programme TUBE for creep analysis of thick wall tubes, Battelle project Materials for steam reformers II, Battelle OH (15jan1976) [5]. T.Kawai, T.Mohri, K.Takemura, T.Shibasaki, Stress analysis for prolonging tube life, AIChE symposium Denver Ammonia Plant Safety, 24 (1983) p [6]. O.Saugerud, S.Angelsen, Probabilistic calculation of remaining life time of steam reformer tubes, ASME PVP-conference Damage assessment, reliability and life prediction of power plant components (1990) p [7]. C.Thomas, A.Tack, Life prediction of reformer tubing based on assessment of thermal strains, AIChE symposium Ammonia Plant Safety, 38 (1998) p [8]. J.Brear, J.Williamson, Life assessment of fired heater tubes in the refinery and petrochemical industries", NACE Int Conf 'Corrosion Asia', Singapore, Sept [9]. J.Brear, P.Aplin, R.Timmins, The effect of primary creep on the Kachanov - Rabotnov model - results on ½CrMoV, 1CrMo and Type 316 steels, ESIS/SIRIUS Int Conf Behaviour of Defects at High Temperatures Sheffield, April ESIS Publication 15, ed. R. A. Ainsworth and R. P. Skelton, Mechanical Engineering Press, London, 1993, pp [10]. J.Williamson, J.Brear,"Risk based life management of catalyst tubes and pigtails", Fourth Annual Ammonia & Urea Conference 'Asia 2000', Singapore, June 2000 [11]. J.Brear, M.Church, D.Humphrey, M.Zanjani, Life Assessment of Steam Reformer Radiant Catalyst Tubes - the use of damage front propagation methods", Second HIDA Conf 'Advances in Defect Assessment in High Temperature Plant' MPA, Stuttgart, Germany, October Paper S6-3 [12]. R.Gommans, JA.Schelling, G.Kamphuis, J.Brear, M.Church, Life assessment of steam reformer catalyst tubes diameter decrease is not negative creep!, AIChE symposium Ammonia Plant Safety Montreal 2001 [13]. T.Shibasaki, T.Mohri, K.Takemura, Remaining tube life assessment by dimensional check, AIChE symposium Ammonia Plant Safety, 39 (1999)