Repair Procedure for High Temperature Boiler Piping

7/30/2019 Repair Procedure for High Temperature Boiler Piping

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Residual Life Evaluation and Boiler Piping Repair OMMI (Vol. 2, Issue 1) April2003

RESIDUAL LIFE EVALUATION AND REPAIR PROCEDURES FOR HIGHTEMPERATURE BOILER PIPING

Isamu NONAKA

Materials Department, Research Laboratory,

Ishikawajima-Harima Heavy Industries (IHI), Co., Ltd.

Dr Isamu Nonaka is a general manager in the materials department ofresearch laboratory at IHI Co., Ltd. He has around 26 years experience inthe fields of creep, high temperature fatigue and life assessment for FBRmaterials and power boiler materials.

[email protected]

ABSTRACT

First, Japanese guidelines for the residual life assessment techniques of power boiler materials aredescribed which are applied to extend the intervals of periodic inspection in the aged power plants.Secondly, damage morphologies which are important in the long-term operating components arementioned including creep-fatigue damage, type IV cracking and ligament cracking. Among thesedamages, the evaluation of crack growth should be allowed for creep-fatigue damage andligament cracking, while it shouldnt be allowed for type IV cracking. Thirdly, the concepts ofin-house maintenance procedures including repair welding are introduced for high temperaturesteam piping.

Key words: residual life, boiler, maintenance, repair weld, creep-fatigue, crack growth

1. INTRODUCTION

Some of the thermal power plants of Japanese utilities are in operation longer than their originaldesign life, and thus, certain degrees of degradation of some parts of the plants may be

progressing. The present economical requirements are that the plants shall be in operation as longas possible, however, safe operation of the plants must be ensured completely. This is the basicrequirement for establishing an accurate technology for assessing the residual life of these plants.

Figure 1 shows the difference between the life evaluation at the design stage of a boiler plant andthe residual life assessment of the plant after being in operation. The life assessment at the designstage is based on a design diagram which is obtained by multiplying the average life of materials

employed in the plant by safety factors. The remaining life assessment is to seek the time that thematerials reach their actual life in operation.

In Japan, the residual life assessment based on the guideline for extending intervals between theperiodic inspection was conducted from 1987 to 1995 until the recommendation was deregulated.Then, the guideline for extending intervals between the periodic inspection was resumed in effectin 1999. The residual life assessment technology issued on the new guideline is shown in Fig.2.Here, the objective of the assessment is limited to creep damage to be induced in header and hightemperature piping. Although appreciable damage has been rarely observed at headers and hightemperature piping, the creep damage in these components is considered to represent the agingdegradation of the boiler plant. The present paper reviews mainly the remaining life assessmenttechnology for the creep damage and the repair procedures that is to be induced in header and hightemperature piping.


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Residual LifeEvaluation and Boiler Piping Repair OMMI (Vol. 2, Issue 1) April 2003 2

Fig.3: Degradation mechanism and damage evaluation parameter for Cr-Mo steel

Fig.1 Difference between design life evaluation and residual life evaluation

location damagemode

waterwall header

superheater headerormain steam pipe

reheater headerorhot reheat pipe

creep

Fig.2 Location and damage mode of residual life evaluation in Japanese guideline


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2. STATE OF RESIDUAL LIFE EVALUATION TECHNOLOGY IN JAPAN

Since the time when the guideline was issued first in 1987 for extending intervals between theperiodic inspection, many efforts have been conducted actively among industries, academiccommunities, and government organizations for clarifying mechanisms that were responsible formaterial deterioration and for developing remaining life assessment technologies.

The degradation mechanism of aging of the major boiler steel containing chromium andmolybdenum has been clarified. Measured damage parameters of the chromium and molybdenumsteel based on the aging degradation mechanism with respect to creep damage is shown in Fig.3[1]. The parameters include the microstructure of the alloy, and the mechanical damage withrespect to deformation in steel matrices and welded joints. The microstructure is found to varyconsiderably in the first half of the damage, and it saturates gradually. The mechanical damage

becomes predominant in the second half of the damage as the deformation of the base metal andthe void formation in HAZ proceeds.

Various nondestructive inspection techniques have been developed for measuring damage inmaterials due to aging during the development of the remaining life assessment technology.Figure 4 shows the nondestructive inspection techniques that appear on the 1999 edition of theguideline for extending intervals between the periodic inspection. These are selected during the

development. The surviving techniques are that in principle they can measure the damageparameters in Fig.3 directly. Many of indirect measurement methods of the damage parametersneed to be validated with regard to the relations between the measured values and the damage

parameters. The main nondestructive inspection methods utilize the replication method toestimate creep damage by measuring deformation in base metal and void area ratios in HAZ, andconverting these values to the damage based on calibration curves. Ultrasonic methods areemployed for measuring the damage under surface.

3. EXAMPLES OF PRECISION IMPROVEMENTS AND LABOR SAVING IN IHI

3.1 Precision Improvement by Creep Rate MeasurementsThe destructive test method specifies creep rupture tests to be conducted for the duration of about

several thousand hours to assess the remaining life. Thus, test temperatures and stresses arechanged for accelerating the tests. If the remaining life can be assessed based on creep rates thatare measured during the creep tests, no rupture test is required. Thus, no appreciable changes ofthe creep temperature and stresses are needed in a given creep test time, and the precision of thetest will be improved simultaneously.Figure 5 shows an example of this situation with the Omega procedure [2] for remaining lifeassessment. The omega value and the strain rate in an actual operation condition are required forobtaining the remaining life according to the method. The omega value depends on the testtemperature and stress, and these values have been stored in a database on certain alloys. Thestrain rate under actual operating conditions is required to be extrapolated based on the valuesobtained by acceleration tests. The remaining life is assessed with higher accuracy by theassessment based on laboratory test data [3].

3.2 Labor Saving in Site Measurement SystemThe replication method can replicate relatively easily the fine microstructure of actual plantmaterials. Thus, this method is the main stream among the applicable methods. However, thismethod requires a scanning electron microscope to observe the microstructure including voids,and thus, the replicas shall be brought to a laboratory from the site. Therefore, the time requiredfor calculating the residual life of the component is inevitably long, and thus, the direct responseof the assessment results to the site is impossible. The author and others have developed a creepdamage diagnosis system based on an laser microscope jointly with Chugoku Electric PowerCompany [4] for the site assessment of the remaining life to be practical. Figure 6 shows thesystem configuration in Fig.6(a), and the appearance in Fig.6(b). The time required for conductingthe residual life assessment by the system is reduced to 40% of the original time required.Furthermore, another characteristic is that an automatic image recognition system is installed in

the system for distinguishing voids present in the inspected field. The automatic system has madethe recognition to be practical by an untrained inspector.


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Fig.5 Residual life evaluation based on the Omega method

Fig. 4 Nondestructive evaluation procedures of creep damage


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(b) system appearance

Fig.6 Creep damage diagnosis system on site using laser microscope

(a) system construction


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4. DAMAGE MORPHOLOGIES TO BE CONSIDERED IN FUTURE

4.1 Creep-Fatigue Damage at Stress Concentrated Parts of Header and Piping WeldsThe header joints, piping joints, and Y-piece welded joints develop stress concentration at HAZ.Stresses originated during start-ups of a plant are usually relieved during steady operations .However, with increasing number of start-ups and shutdowns of the plant, the plant will besubjected to larger stresses each time. The fatigue damage thus must be considered and cannot beneglected, and the so-called creep fatigue damage becomes a problem. This is the possible reasonfor the recent detection of coexisting voids as creep damage and micro cracks as fatigue damagein these joints. Figure 7 shows an example of the damage in welded parts of a Y-piece of a mainsteam piping [5] . The optical micrograph shown in Fig.7(a) indicates voids and micro cracks to

be present in HAZ. The scanning electron microscope image in Fig.7(b) shows micro cracks to beinitiated probably at the voids. Similarly, Fig.8 shows a damage example of a stub weld of piping[5] . Voids are observed in the whole area of HAZ, as wellas micro cracks of about 1 mm length to

be in HAZ near the weld metal.

4.2 Type IV Cracking in High Temperature Piping WeldsType IV cracking is the damage occurring near the boundary between HAZ and the base metal asshown in Fig.9. This is often observed in girth welds of high temperature piping made of

European-made Cr-Mo-V steel when they are subjected to system stresses. The damage isdifficult to detect because it tends to initiate under the surface, and the time for the damage toreach the total fracture after being detected is short. The sensitivity of the joints to type IVcracking depends on the alloys used, and alloys like Cr-Mo-V steel and 1.25Cr-0.5Mo steel aresaid to be sensitive to the type IV damage. The Japanese-made 2.25Cr-1Mo steel that is one of themain alloys for high temperature piping has been considered to be less sensitive to the type IVcracking due to sufficient ductility. However, the alloy should not be considered to be free fromtype IV cracking, but should be considered to have a longer time period before the damage

becomes appreciable.

The dynamic factor for inducing the type IV damage is the excessive level of the system stress.The system stress may vary according to the structure that support the piping, and thus, no

simplified FEM analysis can predict the system stress level accurately. Figure 10 is an example ofthe system stress calculated by elasto-plastic creep FEM analysis for hot reheat piping. Theelement No.85 is the circumferential joint between the Y-piece and the piping, and the residualstress in the element after stress relaxation is as high as 60 MPa.

4.3 Ligament Cracks in HeadersIn Europe and USA relatively large cracks have been reported to exist at superheater outletheaders although very limited reports exist in Japan. Figure 11 shows the illustration and a

photograph of an actual ligament crack in a superheater outlet header. The crack initiates radiantlyat the ligament area, and propagates to the outer surface of the piping after adjacent cracks tocoalesce each other. The interconnected cracks may propagate to the stub welds and may causesteam to leak. The driving mechanism for the ligament cracking is the repeating excessive thermaltransient stresses that are caused by start-ups and shutdowns of the plant, namely, by thermalfatigue. However, stress analyses being conducted on an actual plant that has experienced thistype of cracking fail to detect any stress that is induced to be large enough for explaining the crackformation mechanism. These analyses indicate that the number of the start-ups and shutdowns ofthe plant that experienced this type of cracking is not directly related to this type of damage.Figure 12 shows an example of the stress analysis results in the superheater outlet header. Thefigure shows that when the temperature gradient in the thickness direction at a plant start-up islarge, relatively large stress is induced there.


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Main steampipe

Fig.8 Creep- fatigue damage at HAZ in weld stub of piping

Fig.7 Creep-fatigue damage at HAZ in Y-piece of main steam pipe


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Fig.10 System stress calculated from FEM analysis in hot reheat piping

Fig.9 Type IV cracking in high temperature piping


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Fig. 12 Stress due to the temperature gradient at a plant start-up

Fig.11 Ligament cracking in superheater outlet header


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5. MAINTENANCE AND REPAIR PROCEDURES FOR PIPING IN IHI

Figure 13 shows the flow diagram of life evaluation for high temperature piping in IHI. First, thedamage on the surface of component is measured as the creep void area ratio by replica and alsothe damage under the surface is measured by ultrasonic noise method. The measured damagevalues are compared with the threshold values and are divided into four cases. Furthermore thecracks are detected by TOFD method and the crack growth rates are evaluated. As a result,damage levels are classified into four levels i.e. Level I , II, III and IV and repair procedures whichcorrespond to each level are selected.

Figure 14 shows the principle in the repair procedures for high temperature piping in IHI. Mostconcerned components are the seam weld in hot reheat straight pipe, the seam weld in hot reheatelbow and the girth welds in main steam pipe and hot reheat pipe. If the residual thickness afterremoval of defect is larger than the required value which is defined based on the design allowablestress, only the removal of defect on the surface by grinding is performed in site. If the residualthickness is less than the required design value, the repair welding after the removal of defect is

performed in site for the girth weld and in factory for the seam weld. Pre-heating and post-heatingare conducted in the repair welding both in site and in factory.

6. PLANT MAINTENANCE AND MANAGEMENT IN THE FUTURE

6.1 Risk Ranking of Components and Inspection Intervals based on RBMThus, the components and facilities to be subjected to remaining life assessment, remaining lifeassessment methods and the inspection intervals are conducted according to the experiences thatare accumulated in actual plant operations and at accidents. Attempts are made in the US

petrochemical industries to establish more quantitative and rational risk-based plant maintenanceprocedures. The move has activated similar activities for developing advanced maintenancestandards in European and US thermal power plants. Figure 15 shows the definition of the risk andthe qualitative ranking matrix. The risk is defined as follows: Risk = (likelihood of fracture) (safety severity). The risk is larger at the right hand corner in the figure. Each risk for a componentis calculated, and the risk ranking is attributed for planning and executing maintenance and

management plans. Figure 16 shows the method for deciding the maintenance interval based onthe risk. The shorter the maintenance interval is, the higher the inspection cost is. However, therisk mentioned above decreases because the likelihood of fracture decreases. A suitable inspectioninterval is found to be decided from the minimum expenditure of the inspection cost and risk cost.The present method is applied to manage the total life of a plant. However, the remaining lifeassessment decides finally the next inspection time of a component, and the remaining lifeassessment technology still exists at the center of the maintenance and management of a plant.

6.2 Maintenance and Management based on Maintenance StandardsThe ASME pressure vessel code includes the Section III on the design standards for nuclear

power plants, and the Section XI on the maintenance standards. The program to establishJapanese maintenance standards for nuclear power facilities is in progress at present. However, nomaintenance standards exist for non-nuclear facilities including thermal power plants and

petrochemical facilities in the world. Thus, a program for establishing the maintenance standard isin progress at US Petrochemical Society. The maintenance standard will be constituted with theinspection standard, defect assessment standard, and repairing technology standard. The defectassessment includes the assessment of propagation characteristics of defects, the crack

propagation assessment, and fracture limit assessment. Japanese thermal power plant facilities donot permit any crack to propagate, although it may be permissible in the future as depending onthe crack morphology. Figure 17 shows the damage morphology that may have permissible crack

propagation, and that have no possibility in permissible crack propagation. The damagepermissible crack propagation may be the ligament cracking in header with definite crack pathunder stabilized crack growth and the localized creep-fatigue damage at stress concentrated partse.g. pipe stub welds, header stub welds and Y-piece welds. However, the damage impermissiblecrack propagation may be the type IV cracking in girth welds of high temperature piping under

system stress and the damage in the seam welds of hot reheat piping subjected to internal pressurewhere the damage cant be detected easily and the fracture may be catastrophic as well.


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Fig.13 Flow diagram of life evaluation for high temperature piping in IHI

Component Defectsize Repair procedure Locationof repair

tr> td Removal of defect In siteSeam weld in hot

reheatstraightpipe tr< td Repair weld after removal of defect

with preheating andpostheating

In factory

tr> td Removal of defect In siteSeam weld in hot

reheatelbow tr< td Repair weld after removal of defect

withpreheating and postheating

In factory

tr> td Removal of defectGirth weld in mainsteam pipe and hot

reheatpipe

tr< td Repair weld after removal of defect

with preheating andpostheating

In site

Remark tr : residual thickness after removal of defecttd: required thickness in design

Fig.14 Principle of repair procedures for high temperature piping in IHI


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Fig. 16. Maintenance interval based on RBM

Fig. 15 Qualitative risk ranking matrix in RBM


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Fig. 17. Damages of permissible or impermissible crack propagation


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REFERENCES

[1] I. Nonaka et al; Recent Techniques of Residual Life Estimation for Fossil Power BoilerMaterilas, J. of High Pressure Technology Japan, Vol.34, No.3, 1996.

[2] M. Prager; Development of the MPC Omega Method for Life Assessment in the Creep Range,J.of Pressure Vessel Technology, Vol.117, 1995.

[3] I. Nonaka et al; Evaluation of Creep Residual Life for Modified 9Cr-1Mo Steel Based onOmega Method, J of the Society of Materials Science Japan, Vol.46, No.4, 1997.

[4] I. Nonaka et al; Development of Creep Damage Assessment System for Aged Thermal PowerPlant, Proc. of Baltica IV Conf., Vol.2, 1998

[5] I. Nonaka et al; Recent Techniques for Residual Life Assessment of Aged Fossil PowerMaterials, IHI Journal, Vol.36, No.4, 1996

Documents

Repair Procedure for High Temperature Boiler Piping