Accuracy requirements for life assessment

Int. J. Pres. lies. & Piping 39 (1989) 135-144

Accuracy Requirements for Life Assessment Pertti Auerkari VTT Metals Laboratory, Kemistintie 3, SF-02150 Espoo, Finland

& Jorma Salonen VTT Metallurgy Laboratory, Metallirniehenkuja 4, 02150 Espoo, Finland A BS TRA C T Several methods can be regarded as routine tools for creep life assessment of power plant components. The selection and extent of applying a technique, as well as interpretation qf the results, varies, but the main challenges in reliable predictions remain quite similar. In general, apart from small tubes inside the boiler, the final failure is rarely expected in components and locations where destructive sampling would be easy. Representative sampling, e.g. for creep testing of welded joints of heavy components, is particularly problematic, although miniature specimens can be helpful. However, the methods based on creep testing appear to have potential for long-term life prediction. Nondestructive metallographic inspection with replicas combined with other NDT is suitable for welds, but the present judgement on safe life is limited to short- or medium-term predictions. Improved monitoring of the service history in terms of mechanical, thermal and environmental loadings would deserve more attention. To fully profit from this requires more sophisticated instrumentation than the present average, and a systematic approach in dealing with the accumulating service records. This is likely to be justified in terms of the life cycle costs of most old plants.

1 INTRODUCTION The heavy components operating at high temperatures, like headers, hot live and reheat steam lines and H P / I P turbines, are usually assumed to be lifelimiting for the plant. In practice this is often the case, although any ageing 135 Int. J. Pres. Ves. & Piping 0308-0161/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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component in the plant can be either repaired or replaced by a new one, and therefore life extension can be regarded as a part of the maintenance operations. In any case the technical efforts in life assessments concentrate on high temperature components. The methods that are used for this purpose include similar calculations as in design, possibly supported with dimensional and material checks, and when necessary, with testing. The testing is typically some type of creep testing of specimens taken out of the components, or metallographic (replica) inspection combined with other NDT. Complete 100% coverage is not possible either in destructive or nondestructive testing. Any subsequent decisions are therefore based on selected information and in this sense only as good as the selection, or sampling. It is believed that practically all service-induced creep damage and cracking appears on the outer surfaces of the components, which makes nondestructive inspections practical 1 (Fig. 1). Certain component geometries and initial welding defects have been reported to cause internal creep cracking, 2 but doubts have been raised whether this could have much impact on typical life assessment in plant (Blum, R., 1988, pers. comm.). The emphasis in proper use of NDT, like the replica technique, lies in representative sampling and in successful interpretation of the results. The very scarce data on service history of old plants also limits possibilities for accurate life estimations by calculation, and therefore the need for direct monitoring of material strength and component integrity is increasing. The MATERIAL PROPERTIES T

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accumulated experience of the inspections and life assessments is expected to yield improved assessment rules and other new developments. This paper is an attempt to review in brief the Finnish experience from the past decade in these respects.

2 E X P E R I E N C E S OF P L A N T INSPECTIONS It is by no means obvious that an accurate creep life estimate can be given for a plant or component with typically limited records of the service history. However, creep damage including creep cavitation can be related to local creep strain, which is a classical measure used in design life calculations. Therefore cavitation should indicate creep strain and creep life, though the relation is not universal. 3 When it is normalized by time, strain and damage to failure, i.e. by corresponding critical levels leading to final failure, the relation becomes unique enough to indicate life fractions in creep tests 4 (Fig. 2). The most important feature of this relation is the remarkable insensitivity to stress and temperature. This means that if the type of loading is essentially unchanged during service, then the extent of creep cavitation is likely to indicate the consumed life fraction. From this an estimate of remanent life is obtained, when the time in service is known. In practice a more convincing proof of this is the so-called Neubauer scale for interpretation of metallographic, or replica inspection results. 5 These rules have been used with minor modifications in many countries, including Finland, where they are found in the supplementary part of the National Standard SFS 3280 (Fig. 3). 6 The rules originate from German inspection statistics and give recommended inspection intervals according to the degree of cavitation damage. To ensure fast classification of results, the rules define a scale from 1 (no indicated cavitation) to 5 (macroscopic creep cracking). The given inspection intervals correspond to the fastest damage accumulation in the original statistics, and remain therefore for most cases far on the safe side. When service-induced creep cavitation is found, the recommended inspection interval is not more than 3-5 years (or about 20 000 h). Although some 'common sense' exceptions are made to this at low levels of cavitation, at present the short maximum intervals limit the method to short- or mediumterm life assessments. This is not necessarily enough if the actual potential for life extension is much more, up to the order of the design life. On the other hand, the metallographic inspection with replicas is very practical for critical welds, where other methods can be difficult to apply. In welded joints the result automatically adjusts to the continuous change in materials properties and stress redistribution through the joint. The work concerning creep damage and replica inspections of power

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plant steam pipings was initiated in Finland in 1980. The accumulated inspection experience after that time seems to follow similar trends as reported, e.g. from German, Danish and Belgian plants. In short, some creep cavitation is almost always found in old plants exceeding the nominal design life, especially if the live steam temperatures exceed about 500°C (for lowalloy Cr-Mo steels). Only a small fraction (well below 10%) of the components, where cavitation damage was indicated, appears to require any action other than recommended reinspections within the maximum inspection interval of the Neubauer scale. On the other hand, less than half of all replicas indicate no cavitation, which presumably shows that cavitation is likely to be found if it occurs, and that it is relatively widely distributed. This together with the conservative nature of the Neubauer scale predicts a very good potential for considerable creep life extension of the relevant components in the old Finnish power plants. One particular feature in the replica inspections of the Finnish plants has been the extensive use of scanning electron microscopy (SEM) in routine work. This may be because a major part of the classifications, about 70% of the reinspection replicas, has been performed by one body (VTT) with these facilities, and because the work was started to test the concepts and needs for wider scale inspections, rather than to give extensive damage data of pipelines fight away. The emerging cavities are typically between 0.1 and 1 #m in size, and they are at first practically invisible under an optical microscope (Fig. 4). While the smallest cavities are not easily observed in SEM either, the definitions for the initial low-damage end of the Neubauer scale depends on the tool used for classification. It remains to be seen, how this could extend the recommended scale for early stages of creep cavitation. A natural goal would be to reach at least the turbine overhaul periods, i.e. six to eight years between inspections for the lowest consistently observable damage. There is a need for a standardization of the main concepts in the replica inspections, as well as of the quality requirements of the results. This has been demonstrated both by international round-robin exercises on replica inspections 7 and by the early attempts to establish a common inspection practice in Finland. It is expected that utilities show still widening interest in life extension, as a major part of the present thermal capacity will have exceeded the design life by the 1990s. This will motivate further work on the procedures to aid decisions of possible life extension. One expected, and so far most neglected, direction is the widening use of various types of routine creep tests in life assessment.S-1o Another direction could be the development of improved N D T methods for fast large scale detection of creep damage. Any one of such techniques requires validation, preferably in full scale. 11

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Fig. 4. Examples of creep damage as seen from replicas taken from components: (a) early damage with few small cavities in 10CrMo910 (2.25Cr-lMo) base material; (b) extensive cavitation in 13CrMo44 ( l C r ~ ' 5 M o ) steel base material.

Accuracy requirements for life assessment 3 POTENTIAL

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LIFE EXTENSION

The experience of plant inspections suggests that in spite of the considerable average potential for life extension, variations between plants are quite significant. Considering differences in design, manufacturing, control and maintenance practices as well as type of operation, the plants producing nominally identical power with nominally identical steam temperatures and pressures are far from identical in performance regarding life. As a consequence the actual life of a given component can be anything between perhaps 70% and 500% of the design life. Such a range is too wide to imply accuracy, and essential narrowing is needed for reasonable life assessment. Creep is very sensitive to service stress and temperature levels, and knowledge of these easily limits the accuracy that can be obtained in life assessment (Fig. 5). Excessive local service temperature is a common cause for early failures, but also possible to tackle with corrective action guided by appropriate temperature measurements. This often means increasing instrumentation in plants where optional measurements were not considered in design. The recent trend of additional temperature measurements in old plants probably reflects the common needs for more sophisticated temperature control. It is likely that even in an old plant the economic burden of some additional control is justified in terms of life cycle costs: not only can the costly unexpected outages be minimized but also the overall plant life can be extended by avoiding excessive thermal overloads. Stresses are much more difficult to measure or control directly than temperatures, apart from simple stresses from the internal pressure. The most important stresses arise from the weight, thermal displacements or hindered displacements of the structures. The stress-reducing measures can include strain measurements or material changes followed by structural modifications. Very often it is simply checked that thermal expansion can take place as assumed in design. Nevertheless considerable overloadings are

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frequently caused, e.g. by old, ill-working piping supports and hangers. Accurate stress calculations in creeping components require considerable computing power, and therefore the calculations are usually simplified to the elastic range. This adds one more error source to the stress estimation, though in many cases perhaps not the major one. Finally, the scatter in materials properties is often the most significant error source in the life estimation. To reduce this error only material testing can help. It is easily shown (Fig. 6) that without taking into account the errors in assumed service history and materials properties it is unlikely that reliable safe life estimates beyond a few years could be obtained, even if the A(~ with

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actual remanent life were longer than the design life. Therefore the work to better define the actual service temperatures, stress levels and especially the true service-exposed materials properties in the critical components is likely to be well rewarded. Such work would also better demonstrate the predictive powers of the available creep life assessment methods, now largely untapped due to the inaccuracies in the input data.

4 CONCLUDING REMARKS The available destructive and nondestructive methods for obtaining creep and creep damage data of service-exposed materials of power plant components allow reasonable short- or medium-term life assessments. Accurate long-term life prediction, that would reliably indicate safe life to the order of the typical design life would probably be possible at least with methods based on appropriate creep testing. However, this possibility cannot be often utilized due to the uncertainties in the service history, and sometimes also due to sampling limitations. The need for improvements is evident, since the present failure and creep damage statistics from plant inspections suggest that there is considerable potential for life extension in the typical Finnish power plants that have already exceeded the design life.

REFERENCES 1. Arnswald, W., Blum, R., Neubauer, B. & Poulsen, K. E., Einsatz von Oberflachengefugeuntersuchungen fur die Priifung zeitstandbeanspruchter Kraftwerkbauteile. VGB Kraftwerktechnik, 59 (1979) 581-93. 2. Etienne, C. F. & Prij, J., Optimization of lifetime of creep loaded structures-results of projects of the Netherlands Institute of Welding. In Proceedings of the International Conference on Life Assessment and Extension. Netherlands Institute of Welding, The Hague, 13-15 June, Session 3, 1988, pp. 38-47. 3. Auerkari, P., Satoh, K. & Toyoda, M., On cavitation and residual creep life. Technical Reports of the Osaka University, 36 (1986) 259-65. 4. Shammas, M. S., Predicting the remanent life of 1Cr½Mo coarse-grained heat affected zone material by quantitative cavitation measurements. CEGB Report TPRD/L/3199/R87, 57 pp. + appendix, 1987. 5. Neubauer, B., Criteria for prolonging the safe operation of structures through the assessment of the onset of creep damage using nondestructive metallographic measurements. In Proceedings of the International Conference on Creep and Fracture of Engineering Materials and Structures, Swansea, 24-27 March 1981, ed. B. Wiishire & D. R. J. Owen, Pineridge Press, Swansea, pp. 617-28. 6. Finnish Standard Association, Inspection of pressure vessels. Assessment of creep damage. Finnish Standard Association, Helsinki, 1984, 4 pp. + app. (in Finnish).

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7. Blum, R., de Witte, M., Ludwigsen, P. B., Rintamaa, R., Steen, M., van Elewyck, D. & Vierros, P., Replica testing--summary report on a round robin programme. Eurotest, Brussels, 1987, 61 pp. 8. de Witte, M. & Steen, M., Impact of structural instability on the extrapolation of short term creep test results. Part II: Rupture behaviour. In Proceedings of the Third International Conference on Creep and Fracture of Engineering Materials and Structures, Swansea 5-10 April, ed. B. Wilshire & R. W. Evans. The Institute of Metals, London, 1987, pp. 789-802. 9. Steen, M. & de Witte, M., Impact of structural instability on the extrapolation of short term creep test results. Part I: Deformation behaviour. In Proceedings ~/" the Third International Conference on Creep and Fracture o.["Engineering Materials and Structures, Swansea, 5-10 April 1987, pp. 773-87. 10. Evans, R. W. & Wilshire, B., Creep of Metals andAIloys. The Institute of Metals, London, 1985, 314 pp. 11. Coleman, M. C., Parker, J. D. & Walters, D. J., The behaviour of ferritic weldments in thick section ~Cr~Mo~V ~ ~ 1 pipe at elevated temperature. Int. J. Pres. Ves. & Piping, 18 (1985) 277 310. 12. Auerkari, P. & Salonen, J., The accuracy in life assessment of old steam pipings. In Proceedings of the International Conference on Life Assessment and Extension. Netherlands Institute of Welding, The Hague 13-15 June 1988, Session 3, pp. 88-91.