A NEW CREEP-STRAIN MEASURING METHOD FOR EVALUATINGTHE REMAINING LIFE TIME OF POWER PLANT COMPONENTS

Dipl.-Ing. H.D. Joas, TÜV Süddeutschland, Germany

1. Introduction

In most cases bodies that write standard specification have directed their attentionprimarily to the rules relating to design manufacture and testing to the hand-over-stage.The rules for the operation and inspection/maintenance has largely been left togovernment and survey bodies regulation in the field of offshore plants, the refining andthe petrochemical industry but not for steam boilers, pressure vessels and pipeworks.Increasing profitability also in this sections is the reason that work on engineering lifeassessment, in-service-inspection, fit-for-service-inspection justified mainly on safetyand reliability grounds - is becoming increasingly justified by economic factors. Forexample possible points to be considered are materials deterioration due to creep andfatigue damage and its monitoring.

In the following, a new method for measuring the creep-strain will be presented whichgives a chance to be used as a measure for the material damage but also for earlydetection of a possible failure. It gives a chance for an risk informed assessment todefine a monitoring and maintenance strategy in view of an optimised plant availability.To begin with, there is a short overview of the API 579-code where monitoring already isincluded in the philosophy of Fitness-for-Service, but a final assessment for creepdamage has still to be developed.

2. The Fitness-For-Service-Method in the API 579 Code

− Basic Information

• The ASME and API design codes do not address the fact that equipmentdegrades while in-service and that deficiencies due to degradation or from originalfabrication may to be found during subsequent inspections.

• Fitness-For-Service (FFS) assessments are quantitative engineering evaluationswhich are performed to demonstrate the structural integrity of an in-servicecomponent containing a flaw or damage.

• API 579 provides guidance for conducting FFS assessments usingmethodologies specifically prepared for equipment in the refining andpetrochemical industry.

• The guidelines provided in API 579 can be used to make run-repair-replacedecisions to help ensure that pressurized equipment containing flaws which havebeen identified by inspection can continue to operate safely.

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• Methods in API 579 can be used for assessments based on „generally recognizedgood engineering practices“ until references from inspection codes are provided/1/

− API 579 Development Background

• An FFS assessment is a multi-disciplinary engineering analysis of equipment todetermine whether it is fit for continued service, typically until next shutdown

• The product of a FFS assessment is a decision to run as is, alter, repair, monitor,or replace; guidance on an inspection interval is also provided

• Maintaining safe, reliable operations with an increase in run-lengths and decreasein shut-down periods

− Overview of API RP 579

• Applicable to pressurized components in pressure vessels, piping, and tankage(principles can also be applied to rotating equipment)

• Multi-level assessment - higher levels are less conservative but require moredetailed analysis/data− Level 1 - Inspector/Plant Engineer− Level 2 - Plant Engineer− Level 3 - Expert Engineer

• Provides recommendations for in-service monitoring and/or remediation fordifficult situations

• Section - Remaining life

• Section - In-service monitoring

• Section 10 - Assessment of Equipment Operating in the Creep Regime

− Future Directions of the API In-Service Inspection Codes

• Use RBI (API 580) to set interval, scope, method for inspection, supplemented byappropriate monitoring

• If flaws or damage are found, use FFS (API 579) to assess acceptability fortypically one operating period, can be reendorsed for future operating periods.

• Technology Development for Assessment of creep damage (MPC ProjectOmega)

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3. Measuring method

The safe operating of creep loaded components of thermal power plants or chemicalplants is mostly depending on the local material properties and discontinuities due todesign, welding or cracks. Local crack initiation starts at those areas with stressconcentration, which crack growth should be assessed either by calculation or byexperiment. It is the aim to gain a better usage of the material not reducing the safeoperation and even extending life and service-cycles. The material damage isdepending on the geometry and size of the component and also of the crack-, notch- ordefect-geometry and in addition of amount, type and time of mechanical and thermalloading, and also of local material properties and its possible change because of long-term-loading [/2/ to /5/].

The procedure of the method is as follows:

• Defining of the measuring point at the most interesting spots or areas at a component

• Grinding and cleaning the metal surface

• Spotwelding two austenitic metal-sheets (gauges), where the distance of thespotwelding is the measuring length lº(5 mm or more) on the surface of thecomponent At room-temperature the two halves of the gauge form a gap /6/.

• Producing of replicas of the gauge surface and its gap in shut-down condition of theplant. For the replicas either the material for metallographic replicas is used or athermoplastic material is melted on the gap (Fig. 1).

Fig. 1: Measuring of the creep-strain using replica-technique (TCR-method) at spotweldedgauges at a component

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• Evaluation of the replicas in the scanning electron microscope (SEM) to estimate thegap distance at the beginning of the measuring sequence (zero-measuring).

• After zero measuring, the component is put into operation. For example after oneyear of operation there is a shut-down for inspection. Now a new replica is producedand measured as described before. If the gap has enlarged after this period ofoperation, related to the measuring length, lº, a certain amount of creepstrain has occured.

• The gauges can form either single measuring points or a measuring-chain of multiplegauges (Fig. 2).

Fig. 2: View of 7 creep-strain-gauges forming a measuring length of 7 x 10 mm

• Assessment of creep-damage for the operating time using the possible measuredcreep-strain with the two spotwelded gauges.

• Using the data of additional metallographic (Fig. 3) and non-destructive examinationsan additional proof of the creep-damage-stage can be derived for the measuredTCR-creep-strain /7/ and /8/.

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Fig. 3: Classification of creep damage into four levels of severity because of microstructuralchanges according to TRD 508 (6)

Long term-measurings starting in 1989 have shown that the described method has amax. scatter of ±0,2 % creep-strain. To answer engineering questions this isacceptable.

In the moment work is done to perform the replicas also during operation of thecomponent, this means under temperature condition (non-intrusive).

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4. Advantages of the Creep-Replica-Method

To realise a safe and economic operation of older components in power plants orchemical plants it is necessary to have a strategy for maintenance which gives thechance to operate a component reasonably longer than 200.000 hours if - for example -the operating conditions have changed in relation to the original design [/9/ and /10/].The experimental findings of the life-consumption can be compared with the codes ofTRD 301 and TRD 508 or KTA 3201-2, BS 5500, AFCEN and ASME N-47-28, but alsowith metallurgical and further non-destructive investigations.

Because of the uncertainties in calculation due to the scatter of boundary condition like:

• missing or insufficient operating data (start-up/shut-down, number of cycles ...) • incomplete material property-data (Young’s Modulus E, Poissons ratio ν, coefficient

of thermal expansion α, real creep strength data in the heat affected zone of aweldment, multiaxial stress-state ...)

• difference between design and operating data (pressure, temperature, gradient ...) • deviation in fabrication (wall-thickness, oval shape of a pipe-cross-section ...) • unknown additional operating loads (bending-moments, friction ...)

the theoretical (mathematical) life consumption can only be a basic result which shouldbe confirmed by additional NDT-tests.

To reduce these mentioned uncertainties in the calculation of residual life-time thefollowing inspections and their data are performed: Visual checks, real wall-thickness,diameter-widening, deviation of circle-shape, metallurgical and NDT-methods. With thenew TCR (TÜV-Creep-Replica)-Method from the engineering and economic point ofview an optimal additional proof is now possible. If the most highly stressed measuringpoints are found by this gauges, additional one or two capacitive gages for on-line-measuring can be installed (Fig. 4).

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Fig. 4: High-temperature capacitive strain gage(Max. temp. ∼540°C, gage length lo ∼25 mm, HITEC-Products Inc. U.S.A.)

5. Advantages for utilities

Because the real creep-strain can be measured at any component, questions toreliability, life consumption, schedules for maintenance outages, times for repair-replacecan be answered or up-dating for the applied calculation methods for residual life ispossible.

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The main advantages can be listed as follows:

• Most economic and cost-reducing measuring of creep-strain if it is required by codesor standards (e.g. in Germany TRD 508, 2 % creep-strain is allowed, Fig.5).

Fig. 5: Allowable operating hours due to TRD 508 if a component would have an uniaxialstress-state (see /10/ and Siempelkamp, B. Melzer)

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• Preventing unplanned shut-downs. • Increasing the reliability of components like boiler drums, vessels, pipe-lines. • Questions to realistic schedules for condition oriented maintenance outages,

economic times for repair-replace and possible life-extension can be answered (Fig.6), because the local stress-state is considered (Fig. 7).

Fig. 6: Start of creep-strain measurings after 11.500 operating hours

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Fig. 7: Comparison of measured and predicted creep-strain at a vessel model tested to failureat 593°C under multi-axial-stress-state (10)

• Mailing of the replicas, their pictures or world-wide transmission (fax, e-mail) of the

SEM-pictures (Fig. 8) from a plant or a laboratory to any place of an inspector orExpert Engineer /1/ for assessment is possible.

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Fig. 8: Transmitting SEM-pictures of replicas from Thailand to Germany by e-mail • Reliable reducing of damage-probability.

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• Improving the applied methods for calculating the residual life-time of creep-loadedcomponents (because of uncertain boundary conditions, Fig. 9) by measuring the realcreep-strain during operation (Fig. 7).

Fig. 9: Scattering of the calculated life-time of a component due to the scatter of boundaryconditions

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• Reliable chance of assessing the life-time of a component of new developed high-

temperature-steels when the historic proof and experience of the metallurgical creep-damage processes (Fig. 3) are not yet available.

6. Conclusion

With the TCR (TÜV-Creep-Replica)-Method from the engineering and the economicpoint of view an optimal new inspection method for creep loaded components [/7/ and/12/] and their Fitness-for-Service assessment (API 579) is available now. It is possibleto measure the real creep-strain due to service conditions, which allows a check and up-date of the boundary conditions in applied calculation methods or a comparing withother methods for predicting the life-time of components which are operated at higherstresses and temperatures. It is a tool to increase their reliability, availability andeconomic operation.

Literature

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/9/ Granacher, J., Kostenko, J., Maile, K, Fischer, R.: Kriechrißeinleitung bei warm-festen Stählen. DVS, S.92 - 102

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