Integrated Approach for Life Assessment of Reformer Tubes

Transcription

1 Integrated Approach for Life Assessment of Reformer Tubes M. C. Pérez (Tecnatom, Spain),R. Martínez-Oña(Tecnatom, Spain), D. R. Humphrey (ERA Technology, UK), J. M. Brear (ERA Technology, UK),M. Vrhovac (ISQ, Portugal), C. Vianna (ISQ, Portugal), G. Verdier (Manoir, France), R. Lopes (Air Liquide, Portugal) Abstract Reformer units are critical to many processes in refining and chemical plants, being used in the production of syngas, ammonia, methanol, acetic acid and various other chemicals. Due to the complexity of factors influencing the life of reformer tubes, present life prediction models are very limited and provide conservative estimations. Equally, current inspection systems are not able to detect and quantify reliably the present damage in the tubes. Therefore, several tools have been developed to produce reliable inspections and realistic life prediction. New developments are based on an inspection system, an analytical model, and a knowledge-based system. The aim of the inspection system is to detect and quantify creep damage at the earlier stages. The developed inspection system includes an intelligent scanner able to perform continuous ultrasonic testing and diameter measurement along the tube. The examination is performed with several transducers that allow acquiring a massive amount of information thus contributing to the reliability of the testing. The evaluation of ultrasonic data is based on the analysis of its relevant features such as amplitude, energy, RF echo shape and echo trends. A model, analysis route, has been developed enabling temperature, heat-flux and pressure distributions to be determined from the commonly recording operating parameters. The relationship between tube state and behaviour, derived from metallographic observations and mechanical test data, has been linked to calculate strain, damage and degradation levels. The knowledge-based system developed allows having a software platform that addresses databases, guidelines and life assessment routes. Main databases include material properties, historical data, relevant standards, inspection results, condition reports and previous assessments. Guidelines include plant operation, metallographic degradation, correct weld repair procedures, use and interpretation of ultrasonic results, inspection planning. The different developments have been tested at laboratory level with specimens removed from service. At a recent validation, in a plant and in a full tube mock-up, from the results achieved, the inspection system has demonstrated a correct performance and the capability for properly detecting material and diameter changes in reformer tubes. 1 Introduction Steam reformer units are critical to many processes in refining and chemical plants, being used in the production of syngas, ammonia, methanol, acetic acid and various other chemicals. Life management of reformer units is ordinarily dominated by the service capability of the radiant catalyst tubes. These tubes are commonly manufactured from high strength, creep and corrosion resistant alloys. Their lives are limited by creep, driven by a combination of internal pressure stress and through-wall thermal stresses generated by operational transients.

2 Whilst tube design is sufficient for achieving the design life, it does not provide a basis for remaining life assessment. In fact, experience shows actual life is much shorter than the designed one and the unpredictability of reformer tubes rupture produces high economical losses. Creep life exhaustion may be accompanied by progressive grain boundary cavitation depending on the microstructure of the material and may be exacerbated by microstructural degradation processes, such as sigmatization. The typical composition for alloys for reforming service was originally 25/20 Cr/Ni with the generic reference HK40. Alloy development with the introduction of several deliberate additions (Nb, W, Ti, Co, Zr), for the purpose of increasing creep strength, has led to the HP series and proprietary micro alloy tubes. As a result, thinner wall designs are possible with concomitant decrease in thermal through wall stresses. Consequently an increased service condition capability or service life is available with the upgraded metallurgy. The more recently developed alloys are less prone to creep cavitation and sigma phase precipitation. Usually, the means of carrying out inspections of these degradation mechanisms are dimensional measurements, dye penetrants, radiography, ultrasounds and eddy currents. As the creep damage accumulates initially near the inner wall of the tubes, outer surface standard inspection techniques are not appropriate until very late in life. In addition to that, the specific characteristics of the tubes material structure represent another difficulty to confront. To overcome these limitations, actual assessments are very conservative and therefore reformer tubes are removed from service in a conservative manner. There is a strong need of a reliable and quantitative inspection system and a life prediction programme in order to predict the state and residual life of tubes. On the one hand, realistic and quantitative prediction models directly contribute to enhance maintenance programmes. In the other, reliable inspections have a strong influence on the operationability and efficiency of reforming furnaces and plant production. Furthermore, due to the vast amount of data involved in the reformer tubes assessment, a programme to manage the (existing and new) relevant information and advise on assessment actions to take could be of great benefit. Based on these tasks, a number of activities have been developed in the framework of EC NART project. In this paper, these developments which intend to solve existing limitations are described, the results obtained and the validation carried out are also presented. 2 Inspection system For the purpose of detecting creep damage, two complementary examinations are performed: ultrasounds testing and diameter measurements. By means of ultrasounds testing the material structure, and the presence of voids eventually, can be assessed. By means of diameter measurements, the deformation in the tubes related to the creep damage can be introduced into the model. Ultrasonic parameters are dependent on the characteristics of the ultrasonic pulse emitted and the properties and state of the medium through it passes. When the material exhibits creep damage, due to the presence of cavities, the ultrasound parameters change from their standards values. Studying these changes the damage can be characterised. Nevertheless, due to the high anisotropy of the centrifugally cast austenitic material, there are changes of the ultrasonic response from point to point and high attenuation and scattering. Damage starts at the centre of the tube thickness and, due to operating conditions, develops to the inner surface. Then, a volumetric method such ultrasonic is required to detect voids at an early

3 stage. The method consists on a pair of probes arranged in pitch and catch fixture. The emitter sends a pulse, which travels following a chord and is picked up by the receiver (see figure 1). The beam incidence angle and the separation of the probes are designed to allow the central beam passes by the tube mid-wall chord. Longitudinal waves are selected to interrogate the tube material because they are less affected by austenitic anisotropy grain structure. To avoid water spilling within the furnace, contact technique is applied being the probe deployed by means of pressure. Based on this, Tecnatom and the Instituto de Automática Industrial have designed a robot for climbing the tube and performing the UT measurements and diameter measurements, adaptable to different tube dimensions (see figure 2). (1) Emitter (2) Receiver (3) (1) Probe tilt angle (2) Incidence angle (3) Probes separation Figure 1 Scheme of ultrasonic method for detection of creep damage Figure 2 Robot developed for automatic reformer tube inspection (Air Liquide, Estarreja-Portugal) 3 Assessment model 3.1 Predicting the Time for Tube Replacement Typically, the design of these components is based on pressure stresses, outside wall temperatures and factored lower-bound materials rupture data. However, this does not provide a realistic basis for remaining life assessment of steam reformer catalyst tubes. This is because the influence of start/stop-cycles (which have a significant and negative influence on tube life) is not taken into account in this procedure. The best alternative is a predictive approach using a computer based life model, confirming afterwards the predicted levels of strain & damage from the model against measured NDE. This is component specific and unlike alternative historical methods, can extract useful life consumption data from non-failed components as well as the likely number of failed components. This is an important consideration as the non-failed components form the largest population of the group under investigation - indeed, there may be no failed components to date. ERA s general approach to component life modelling and hence definition of probability of failure relies on a few fundamental components. These are:

4 1. Understand the process and define all the loading and damage mechanisms for the component in its service environment. 2. Ensure a complete understanding of the relevant materials characteristics, behaviour and properties are available. 3. Quantify data ranges representative of historic, current and future operation, both in space and time, for the foregoing life controlling parameters. 4. Develop and validate a dedicated mechanistic component model that realistically reflects the contribution to life consumption, individually and synergistically where appropriate, of each of the parameters. 5. Validation necessarily means that historic failures and current component condition should both be reported accurately by the model for the components/plant under assessment and for similar components in other plants. 6. Deploy the predictive capability of the model for process optimisation and component/unit life management. ERA's approach is centred on a creep life consumption algorithm, validated by critical experimentation, and accurately models the materials response to steady and transient conditions. It takes historical operating data and standard materials properties as inputs, augmented by specific test data and condition assessment information where appropriate. Variability in all input parameters is fully accounted for by standard probabilistic techniques. Output is provided in terms of cumulative failure probability or hazard rate as a function of future service time or of location within the unit. This approach thus provides operators with a capability to plan inspection and maintenance actions as well as tube replacement. The approach also allows operators to evaluate the cost benefit of upgrading to newer tube materials prior to ordering replacement tubes. 3.2 Refining & adding confidence to the assessment For each area of the unit considered, the crack initiation time, crack growth behaviour and final failure time (tube leakage) are recorded, together with times to pre-selected diametral strain levels. Cumulative probability curves for times to crack initiation, failure and each chosen strain level are produced. Profiles of the strain and damage levels along the length of the tube are calculated as well. Unfortunately, it is not generally possible to provide probability of failure data on a tube by tube basis, only on a row by row basis. This is not due to shortcomings in the approach, but because the requisite input data is not available on a tube by tube basis. For example tube process outlet temperature is generally only available as a mixed process outlet temperature from thermowells in the outlet manifold from a row of several tubes. In certain critical situations the life predicted by the above method is insufficient to meet the operational requirement, or is put in some doubt by the uncertainties associated with known instances of abnormal plant history. The calculations may then potentially be refined by identifying the most sensitive input parameters and improving their accuracy. It must be stressed

5 that such a process improves the quality of the life prediction; it does not necessarily lead to a longer predicted life! Advanced UT & diametrical inspection data can be correlated with model predictions to enable progressive monitoring of the tubes against predicted behaviour and thus allow revision of the life assessment if differences emerge. To date, only diametral (or circumferential) strain has proved rigorously compatible with the model. As a strain based creep model is used, the strain accumulation in the tubes with time can be determined and compared with the observed values. Careful measurements are required with this technique, particularly for low ductility and low failure strain materials. As part of the NART project ERA is developing correlations with the new UT system being developed by Tecnatom, aiming to provide a stronger link between predicted and measured levels of sub-surface damage. It is thus recommended that the tubes be inspected regularly, using NDE methods capable of detecting inner surface cracking, once the predicted crack growth regime has been entered. 3.3 Advantages of using Life Assessment Service By using a validated model approach and probabilistic techniques provides quantitative life assessment data in a risk of failure format. This, together with the consequence analysis, enables a risk based life management strategy to be adopted. A great advantage of a predictive model is that consideration can be given to hypothetical changes in future operation. Frequently it is necessary to investigate the effect of an improvement in catalyst efficiency, feedstock or firing pattern on tube life, or to evaluate the cost benefit of different choices of replacement tube material or geometry. Provided a reasonable estimate of the potential new parameters can be made, then the model can be run several times using the actual history with each candidate future scenario. This type of assessment also allows operational changes to maximise production in response to market demand to be planned and balanced against the price to be paid in terms of the life of the component. Alternatively it allows operational changes to be made to optimise both product yield and component/plant damage accumulation rates. Risk of failure data can be used for definition of inspection frequency, maintenance timing and ultimately refurbishment or replacement timing of individual components and or complete units. These activities all have a direct effect on profitability. Thus in today s increasingly competitive environment the timely availability of fully quantitative data is a major benefit. 4 Intelligent Modularised Advisory System (KBS) The objective of developing a Knowledge Base System is to allow end-users as well as research companies or materials engineers working in the field of reforming tubes to have a Software platform that addresses safety analysis in operating plants in a predictive maintenance or the scheduling of inspection point of view or performing failure or damage analysis. The architecture of the KBS is shown in figure 3.

6 Figure 3 - KBS architecture 5 Results 5.1 Inspection system To allow implementation of the ultrasonic method for in-service inspection, the ultrasonic probe holder is mounted on a mechanical scanner, which moves vertically up & down along the tube generatrices [3-4]. Up to four probe holders can be installed in the device, which makes possible to examine the four corresponding axial zones. Increment of tube diameter is an indicator of creep relaxation presence. Then, as the scanner is inspecting and moving vertically, two perpendicular tube diameters are measured along the tube. The scanner has two pairs of clamps, which allow it to fix around the tube and, properly synchronised, move it vertically. The principle of diameter measurement is based on the measurement of the opening/closing of each pair of clamps around the tube. An accurate displacement gauge is associated to the clamp opening from which the reading is taken. The accuracy of the diameter gauge measurement is better than 0.1%. The inspection is performed automatically as well as the data acquisition of both ultrasonic and diameter measurements making use of Midas data acquisition and analysis system [5-6], which shows up acquired signals in real time. Subsequent processing computes C-scan maps for displaying the variables of interest either ultrasonic amplitude, energy or tube diameter along the tube surface inspected (see figure 4). Repeatability of results is clear when robot is upwards and downwards and in several repetitions of that movements. Moreover, C-scan plots can be represented codified by damage level. Correlation between ultrasonic results and diameter measurements is also clear (see figure 5), as well as the detection of interferences like welds, changes in the material, etc.

7 c2mhzb1 (dry contact) SMR1, Tube 11, Channel 2, 2 MHz, W, Max. amplitude 30 Tube generatrix (x10º) Amplitude (V) Degradation indicators Tube length (x10mm) Tube length (mm) 30 C-scan: mean values. 2 Mhz (dry contact) (s0:b,s1:r,b0:g,b1:k) SMR1, Tube 11, D1 (E-W), Up(b), Down(r) + 3% 25 Attenuation (a.u.) Tube Diameter (mm) Nominal Diameter - 3% Tube length (x10mm) Tube length (mm) Figure 4 C-scan of attenuations and repeatability test. Figure 5 Strip chart of UT amplitudes and diameter measurements along the tube 5.2 Assessment mode A preliminary stage in tube life prediction is the pre-processing of the operating condition data. A module has been written for calculating the axial distributions of temperature and heat-flux, for a variety of reformer designs (see figure 6).. Figure 6 Heat-flux model calculation spreadsheet The life-assessment program calculates the probabilities of crack initiation and tube failure, together with those for various intermediate levels of strain and damage accumulation. In this example (see figure 7), cumulative probabilities of crack initiation (10% of wall) and final failure are shown, with future years of service indicated to aid in determining the optimal replacement time. The predicted damage and strain levels may be compared with observation, as a validation

8 and as a means of identifying deficiencies in the input data or previously unrecognised operational problems. The reconciliation module of the KBS provides advice on this. The example in figure 8 shows predicted and observed diametral strain distributions, at two successive inspections % Zone A (East Row) - Cumulative Probability Distribution for the Time to Initiation and Failure for the Reformer Tubes Initiation Time 95% Confidence Interval Cumulative Probability 10.00% 1.00% 0.10% Current Operational Hours Failure Time 95% Confidence Interval Subsequent Years Of Operation Probability of reaching given strain level % Oct 1998 Jul 1999 Oct 1998 July % 10, ,000 1,000,0 Time (Hours) Figure 7 - Cumulative Probability Distribution for the Time to Initiation and Failure Diametral strain, Figure 8 - Comparison between observed and predicted strain distributions 5.3 Intelligent Modularised Advisory System Databases containing historical data, inspection results, condition reports, previous assessments, material properties, relevant standards and guidelines have been produced. The models developed for tube life prediction, temperature distribution calculations and relating NDE data to tube condition have been incorporated into the KBS. Table I shows the main characteristics of the KBS. Table I- KBS characteristics and capabilities Reliable data storage: Technical events and circumstances. Design parameters. Operation parameters. Ultrasonic and other non-destructive test results. Creep strain and accumulated damage assessment results. Planning and cost details. Hyperlinks to pertinent documents. Automated data presentation: Spreadsheet tables and diagrams. Interactive component sketches. Direct access to relevant information: Interactive inspection procedures. Interactive reconciliation procedures for improvement of management efficiency. Complement and historical data analyses: Compiled data documents. Trend analysis. Management planning and evidence: Database reports.

9 Figure 9 shows some of the capabilities of the KBS. RECONCILIATION CONSULTING RESULTS LIFE ASSESSMENT AGAINST NDE RESULTS Does the Strain and/or Damage profile along the tube length differ from the NDE results at the specified time? No PREDICTED CRITICAL VALUE AGAINST THE NDE RESULT Does the Strain and/or Damage critical value differ from the NDE results at the specified time? No PREDICTED STRAIN AND DAMAGE RATE AGAINST THE NDE RESULTS Does the Strain and/or Damage rate differ from the change in NDE over a specified time period No Does the predicted profile and critical values still align with the NDE results? No Suspect a problem or change in operation earlier in life. Run analysis from current condition. Next End 6 Conclusions Figure 9 KBS modules: Database and Reconciliation module The first test of feasibility of the prototype for on-site inspection has been done. Several improvements have been identified and performed afterwards. The correct performance of the scanner (achieving a inspection speed of 9 minutes/tube) has been demonstrated. As a general conclusion, it can be affirmed that the fundamentals of the technique for creep characterisation are still valid. The inspection system developed in NART project is capable of detecting differences along the tubes and from tube to tube (position of welds, dimensional information, etc.). Anyway, the analysis of the information collated allows to perform the assessment of the tubes stating that tube state is good and there is nothing to remark. Ultrasonic and diameter measurements are representative enough. They could be used as a reference in future inspections. Detection of creep damage in reformer tubes is a difficult task due to the anisotropic structure of these materials. In this paper, an inspection method based on the measurement of several ultrasonic parameters and tube diameter has been described. Characterisation of ultrasonic attenuation and energy band, associated to tube diameter, allows determining the presence and level of creep damage in reformer tubes. Complementarily to the NDE results, a life prediction model has been developed to estimate the remaining lifetime if the reformer tubes in different conditions (furnace design, tube material, operating conditions,.). To make easier the maintenance management for reformer furnace owners, the KBS Software allows compiling reformer characteristics as well as the history of the different shutdowns and associated inspections, connecting directly this information with the life prediction Software.

10 7 Acknowledgements This work has been carried out in the framework of European Commission BE-5126 project. Special gratitude is owed to the Instituto de Automática Industrial (CSIC-Spain) for their collaboration in the design of the inspection robot and its manufacture. References [1] Metals Handbook, Vol 11. Non-destructive inspection and quality control. 8 th Edition. American Society for Metals (1976). Metals Park, Ohio [2] E. Schreiber, O. Anderson, H. Soga, Elastic constants and their measurements, McGraw Hill, New York, [3] A. Scheerder, Experiences at DSM with NDT and Evaluation of Creep Damage in Reformer Tubes. 7 th ECNDT. Copenhagen, [4] R.R. Dalton, Ultrasonic inspection of cast HK-40 tubes for creep fissures. Standard Oil Company of California, Richmond, California. Materials Evaluation, [5] M. Borrás, E. Cabrera, J. Aldea, Multitechnique data acquisition systems. Use and benefits in nuclear market. 7 th ECNDT. Copenhagen, [6] R. Martínez-Oña, Enhanced ISI of austenitic materials: experience and equipment. IAEA Regional Workshop on environmentally assisted cracking of NPP austenitic piping. Slavutych, 1998.