PVP Creep-Fatigue Crack Initiation Assessments of an Instrument Guide Tube within a Superheater Header

Transcription

1 Proeedings of the ASME 2014 Pressure Vesss & Piping Conferene PVP2014 July 20-24, 2014, Anaheim, California, USA PVP Creep-Fatigue Crak Initiation Assessments of an Instrument Guide Tube within a Superheater Header Jinhua Shi AMEC Rutherford House Olympus Park, Quedgey Glouester GL2 4NF United Kingdom T: Fax: jinhua.shi@ame.om Hassam Dodia AMEC Walton House Birhwood Park Warrington WA3 6GA United Kingdom T: hassam.dodia@ame.om ABSTRACT In order to extend the boiler lives at Advaned Gas-Cooled Reator (AGR) nulear power stations in the UK, new temperature measuring instrumentation to monitor reator gas temperature has been proposed to install on the bore of an intat boiler tube to provide additional boiler operating data to support the station lifetime extension. This paper details a reep-fatigue rak initiation assessment of the proposed installation of an instrument guide tube within the superheater header using the latest R5 high temperature assessment proedures based on detailed finite ement thermal transient stress analysis values for a bounding start-up and shutdown yle. The fatigue damage at wds has been alulated based on both wd and parent material properties. The new approah for assessing wdments has been used in this paper. This new approah involves splitting the existing Fatigue Strength Redution Fator (FSRF) into a Wdment Endurane Redution (WER), whih aounts for redued fatigue endurane due to wd imperfetions, and a Wdment Strain Enhaement Fator (WSEF), whih aounts for material mismath and loal geometry. The reep assessments of the wd material loations have been arried out on both parent and wd material properties inluding the wding residual stress. The total reep-fatigue damage is then obtained as the sum of fatigue damage, D f, and reep damage, D. 1. INTRODUCTION The boilers at Advaned Gas-Cooled Reator (AGR) nulear power stations in the UK are potentially life-limiting omponents. Enhaning the apability to monitor the boilers has been identified as having the potential to provide key evidene in justifying the extension of the generating lifetime of the stations. It has been proposed to install new temperature measuring instrumentation to monitor reator gas temperature and to provide additional data regarding boiler operating onditions. The modifiation will be to introdue thermoouples to the bore of an intat boiler tube to failitate temperature measurement at or near to the loations of interest. A typial boiler is shown in Figure 1. The modified boiler tube will be sealed at the feed header upstand. Between the upper surfae of the superheater header tubeplate, as shown in Figure 2, and the wall of the superheater header, the thermoouple bundle and sheath will be ontained within a rigid stainless ste guide tube. The guide tube will be attahed at both ends by wds, eah forming a pressure boundary. At the tubeplate, a wd will separate the bore of the sealed guide tube from the steam spae within the superheater header; a wd between the guide tube and the superheater header will separate the steam spae within the superheater header from atmosphere outside the header. This paper presents an assessment of the proposed pressure boundary omponents against the defet-free reep-fatigue rak initiation requirements of R5, Volume 2/3 [1] based on detailed 3-dimentional finite ement analysis results obtained using ABAQUS [2]. The assessment onditions are desribed in Setion 2. The stress values used in the assessment are detailed in Setion 3. The material properties are presented in Setion 4. The detailed reep-fatigue rak initiation assessment is desribed in Setion 5 and onlusions are drawn in Setion 6. 1 Copyright 2014 by ASME

2 2. ASSESSMENT CONDITIONS It is assumed a design life of 15 years after modifiation and the superheater header is operated for hours at the modifiation. Normal operating temperature and pressure are assumed to be o C and MPa. The bounding start-up and shutdown transient temperatures are presented in Figure 3 and the total number of yles is ASSESSMENT STRESSES Based on the final onept design, a 3-dimentional finite ement (FE) mod, Figure 4, was generated using ABAQUS. The FE analyses have been arried out for the following bounding load ases: 1) Unit steam pressure, 1MPa, applied on the steam side inner surfaes and the outer surfae of the guide tube. Von Mises stresses are sown in Figure 5. 2) Unit gas pressure, 1MPa, applied on the gas side inner surfaes. Von Mises stresses are given in Figure 6. 3) Unit pressure, 1MPa, applied on the inner surfae of the boiler tube and the guide tube. Von Mises stresses are illustrated in Figure 7. 4) Reator start-up transient thermal stresses are presented in Figure 8 for the 25 seted setions shown in Figures 10a to 10d. 5) Boiler trip transient thermal stresses are presented in Figure 9 for the 25 seted setions shown in Figures 10a to 10d. In the R5 Vol. 2/3 assessments, the FE asti stresses obtained for the 1 MPa pressure load have been fatored and ombined as appropriate for use in this assessment. It has been shown from the FE analysis that the steady state thermal stresses are negligible. Stresses were obtained aross 25 setions in the struture, as shown in Figures 10a to 10d. These setions inlude representative ones through the header, header wd, guide tube, guide tube wd and the tubeplate wd. As the wd will not undertake a post wd heat treatment, wding residual stresses have been simply taken to be equal to the best estimate wd material 1% proof stress at operating temperature based on the advie given in Setion IV.4 Wding Residual Stress Distributions of R6 [3]. The residual stresses are only used when alulating the reep damage in the struture. 4. MATERIALS DATA The existing superheater outlet header and the proposed thermoouple guide tube are onstruted from Type 316L stainless ste. The tubeplate is manufatured from 316H stainless ste. All wds will use Type 316 filler and the wd will not be stress rief heat treated. The following setions desribe the materials properties required for the R5 Volume 2/3 assessment. 4.1 Physial and Tensile Properties Young s modulus, E (GPa), is taken as a funtion of temperature T ( o C): 316L & 316H E = T 316 wd E = T A Poisson s ratio value of 0.29 is assumed and shakedown fators, Ks, were taken from R5, thus: 20 o C, K s = o C to 500 o C, K s = o C, K s = 1.21 It has been reommends in [4] that the upper temperature limit for K S = 1.35 an be extended to 550 o C. The 0.2% proof stresses for header material are taken from test ertifiate data, giving values of 229MPa at room temperature and 132MPa at 550 o C. The 1% proof stresses for header material as 243MPa at room temperature and 168MPa at 550 o C. The Ramberg-Osgood equation, rating asti plus plasti strain,, to applied stress, σ, is as follows (monotoni urve): σ σ E K' = + 1 n (1) where K and n are material onstants, whih have been derived from the 0.2% and 1% proof stress values for 316 wd as follows: K = 525MPa n = Cyli Stress-Strain Data Cyli stress-strain properties rating total strain range, T, to stress range, σ, are shown bow: 1 β σ σ = + E A T (2) where A and β are material onstants. 4.3 Fatigue Endurane Data Lower bound ontinuous yling fatigue endurane data for 316L and 316H were obtained from [1]. These data apply to failure (N f ); the orresponding numbers of fatigue yles for rak initiation were pessimistially taken as the numbers of yles for rak nuleation, N i, and were alulated using R5 Volume 2/3, Equation 8.2, thus: 2 Copyright 2014 by ASME

3 0.28 = ln( N f ) 8.06N f ln( N ) (3) i With referene to Figure 10a, seven bounding assessment loations are presented as follows: Fatigue endurane data for 316 wd are not required beause the fatigue endurane for wdments is based on best estimate parent data but inorporating a Wdment Endurane Redution (WER) and a Wdment Strain Enhanement Fator (WSEF). 4.4 Creep Deformation Referene [5] gives reep deformation data for the materials assessed in this paper. In addition, tertiary reep is modled by multiplying the reep strain rate by 1/(1-D 3 ). 4.4 Creep Dutility For Type 316L, the lower bound uniaxial reep dutility at temperatures between 500 o C and 550 o C is 2.8%. For the purposes of this assessment, it is assumed that the appropriate dutility is 50% of the uniaxial value, i.e. f = 1.4%. For Type 316H, the lower bound uniaxial reep dutility at temperatures between 500 o C and 550 o C is 2.6%. For the purposes of this assessment, it is assumed that the appropriate dutility is 50% of the uniaxial value, i.e. f = 1.3%. The reep dutility of 316 wd material is higher than that for 316L parent with a lower bound uniaxial reep dutility of 6.3% at 500 o C for as-wded 316 wd metal. 5. CREEP-FATIGUE CRACK INITIATION ASSESSMENT This setion presents the R5 Volume 2/3 assessment of the proposed thermoouple guide tube and its pressure boundary omponents. The following potential failure mehanisms are onsidered: I. Plasti ollapse of setions through the header, thermoouple guide tube and its pressure boundary omponents II. Creep rupture, evaluated aross setions through the header, thermoouple guide tube and its pressure boundary omponents III. Rathetting / Global shakedown aross setions IV. Loal damage due to reep and fatigue yling leading to rak initiation This paper onentrates on the last item, IV above. A simplified but pessimisti method is used for assessing the total damage and this is desribed in detail bow. This requires the separate alulation of fatigue damage aused by reator start-up/shutdown yles and boiler trip and reinstatement yles, and the reep damage due to raxation of the initial wding residual stress and operating dwls. Only the reator start-up/boiler trip yle has been assessed in this paper, as fatigue damage is negligible for other yles. (i) Bounding position in the bend Loation B2, with 316L parent material properties; (ii) Bounding position in the header Loation H2, with 316L parent material properties; (iii) Bounding position in the guide tube Loation T2, with both 316L parent and 316 wd properties; (iv) Bounding position in the guide tube wd Loation T3, with 316 wd properties; (v) Bounding position in the tubeplate wd Loation TP1, with both 316H parent and 316 wd properties; (vi) Bounding position in the tubeplate wd Loation TP2, with 316 wd properties; (vii) Bounding position in the header wd Loation W3, with both 316L and 316 wd properties. 5.1 Calulation of Fatigue Damage Bounding Parent Material Loations Based on the FE stresses, the signed asti von-mises stresses under the various onditions are alulated. When the von-mises stress range is alulated, omponent stress differenes are alulated before ombining them in a von- Mises manner. Two yles, whih are bounding in terms of the astiplasti strain, are defined from this, as follows: (i) Reator start-up, whih takes stress range of RT SU; (ii) Boiler trip, whih inludes the stress range during reator start-up and takes a range of BT SU. The total strain range tot during a yle is simply given by the sum of the asti, plasti, volumetri and reep omponents, thus: tot = pl vol (4) where the asti-plasti point, (, σ ) lies on the intersetion of the yli stress-strain urve σ σ = + E A 1/ β pl (5) r pl 3 Copyright 2014 by ASME

4 and the Neuber hyperbola from the asti equivalent surfae stress, σ, σ E 2 = σ ( ) In the above, E is the effetive Young s Modulus, given by E = 3E/2( 1+ν ). The volumetri strain range, vol, is alulated from the asti strain range using the following: ( ) pl (6) vol = kν 1 (7) where ν ( 1+ ν )( 1 ν ) ( 1+ ν )( 1 ν ) k = (8) Es Es ν = ν (9) E E E s σ = (10) pl The reep omponent of strain range for a single yle, r, under primary stress only is alulated for parent material assuming that the dwl stress equals the primary stress at operating pressure. In reality, the formation of a hysteresis loop would result in a lower total stress during the reep dwl so using the full pressure stress is pessimisti. The reep strain in one start-up/shut-down yle is alulated for a time period of 1/6 th of a year (1460hrs) starting at zero. The same reep strain is added to the total strain for the boiler trip yles again, this is pessimisti. This is a further pessimism in the assessment method, beause in effet this lev of reep strain is assumed to our for all yles, i.e. the primary reep strain is reset at the beginning of eah reep dwl. The upper bound fatigue damage, D f, for parent material is alulated from: n D + n 1 2 f = (11) N i,1 N i,2 where n 1 and n 2 are the numbers of reator startup/shutdown yles and boiler trip yles respetivy, and N i,1 and N i,2 are the lower bound values of fatigue endurane (for rak nuleation) for eah yle type Bounding Wd Material Loations The fatigue damage at the wds was alulated based on both wd and parent material properties. The new approah for assessing wdments as detailed in [6] has been used in this report. This new approah involves splitting the existing Fatigue Strength Redution Fator (FSRF) into a Wdment Endurane Redution (WER), whih aounts for redued fatigue endurane due to wd imperfetions, and a Wdment Strain Enhaement Fator (WSEF), whih aounts for material mismath and loal geometry. As desribed in [6], the WSEFs in the following table have been derived suh that the parent mean fatigue urve, fatored by the WSEF and WER, provides a mean fit to the wdment fatigue data assuming a log normal distribution of fatigue life [7]. WSEFs for Dressed and R5 Wdment Types Undressed (As-wded) The fillet wds are lassified as Type 3 and the full penetration tube butt wd is lassified as Type 1. For wdments, R5 Volume 2/3, Table A4.2 states that mean parent fatigue endurane data are revant. 5.2 Calulation of Creep Damage Bounding Parent Material Loations The reep damage at the bounding parent material position in the struture was alulated. The positions are suffiiently remote from the wd, suh as Loation B2, so as not to be affeted by wding residual stresses. Transient thermal stresses during reator trips and boiler trips are greatest in the same diretion as the pressure stresses, i.e. tensile. Any symmetrisation of the hysteresis urve would therefore tend to shift the urve downwards, setting up a ompressive residual stress fid loal to the assessment loation. During the reep dwl, the total stress would tend to inrease bak up to the initial operating value. This is similar to the situation illustrated in Figure A3.6(b) of R5, Volume 2/3. It was also heked and onfirmed that there are no loations where yli stresses would lead to an inrease in start-of-dwl stress to a lev higher than that assessed in this paper. The advie on effets of ompressive dwls is given in [6] and this has been implemented here. The total reep strain at this position needs to take aount of reep strains arued both before and after modifiation, and at temperatures appropriate to both periods of operation. Sine the applied stress is pury primary, no raxation of the stress 4 Copyright 2014 by ASME

5 would be appropriate so the reep strains before and after modifiation were alulated from: & dt ( D ) = 3 1 (12) with strain hardening used to define an effetive reep time at the start of the post-modifiation period of operation. The reep damage, D, was alulated from: D = (13) r It should be noted that during the normal operation, the arrier tube and its wd at the tubeplate are under ompressive stresses. Aording to [6], the reep damage an be disounted. alulation. The total reep strain is therefore alulated as the sum of the reep strains under primary stress and the reep raxation of the seondary residual stress. Creep from primary stress The reep strains from primary stress were alulated using the revant pressure and temperature for both periods, based on strain hardening. Tertiary reep was not inluded for 316L beause the reep strains were too small for this to have any signifiant effet. Creep raxation of residual stress The initial wding residual stress is taken to be equal to the mean 1% proof stress for 316L wd material at operating temperature, following the Lev 1 reommendation for set on nozzles in R6, Setion IV.4.10 [4]. This gives a residual stress of 346MPa, whih is assumed to at in both the hoop diretion (out-of-plane w.r.t.) and the radial diretion (w.r.t. header axis). The total reep strain arising from raxation of this residual stress over a period of 15 years was alulated from the following stress raxation equation: Bounding Wd Material Loations The assessment of the bounding wd material loations differs from the parent material assessment in that it inludes the wding residual stress and that both parent and wd material properties are revant. Firstly, it was heked whether yli loading is insignifiant and reep behaviour is perturbed by yli loading, using the following tests from R5, Vol. 2/3 [1]:, max ( K ss y ) + ( K ss y ) n σ (14) dσ E d = dt Z dt (17) where an asti follow-up fator, Z, of 3.0 is used, based on advie given in R5, Volume 2/3. The reep strains were alulated for both 316L parent, 316H parent and 316 wd materials, at the operating temperature. When appropriate, the orretion for tertiary reep was inluded. The reep damage, D, was alulated from: where subsripts and n refer to reeping and nonreeping parts of the yle, respetivy; D = (18) r D 0.05 (15) f and Test 2 from R5, Vol. 2/3, Appendix A3: The most severe load yle should not ause yid at the non-reep extreme. This is taken to be equivalent to: σ ss + ( K ss y ) n σ (16) Elasti stress ranges on the boiler trip transient are obtained from the FE analysis. The steady state reeping stress, σ ss is assumed to be equal to the pressure stresses for premodifiation and after modifiation. It has been demonstrated that the onditions given in (15) to (16) are met. Therefore, all of the neessary onditions for reep behaviour not being perturbed by yli loading are met and stresses do not need to be reset during the reep strain 5.3 Calulation of Total Damage The total reep-fatigue damage is the sum of D f and D. Prior to the modifiation, the bounding reep-fatigue damage values are: Setion H2: Setion TP1: The bounding reep-fatigue damage values obtained for the post-modifiation period are: Setion H2: 0.23 Setion TP1: The bounding total damage values are: Setion H2: Setion TP1: Copyright 2014 by ASME

6 It is seen that the maximum damage is in the parent material at the tubeplate (setion TP1). Sine these damage fators are less than unity, there is no need to postulate the formation of a defet. 6. CONCLUSIONS In this paper, the proposed design for the thermoouple guide tube and its pressure boundary omponents in a superheater outlet header has been assessed against the defetfree requirements of R5, Volume 2/3. The following onlusions an be drawn: 1) A reep fatigue damage assessment has been performed for all areas of the struture. The maximum loal reep-fatigue damage fator is less than unity; therefore rak initiation is predited not to our. 2) The integrity of the proposed thermoouple guide tube and its pressure boundary omponents have therefore been demonstrated for operation of 15 years after modifiation. 3) The reep fatigue damage assessment has played an important role in the modifiation design. ACKNOWLEDGEMENT This paper is published by the permission of AMEC, Clean Energy - Europe. The authors would like to thank EDF Energy Boiler Lifetime Inspetion and Monitoring Programme (BLIMP) for the finanial support. REFERENCES 1. EDF Energy Nulear Generation Ltd., Assessment Proedure for the High Temperature Response of Strutures, R5 Issue 3, Revision 001, August ABAQUS - Finite Element Stress Analysis Program, Version , EDF Energy Nulear Generation Ltd., Assessment of the Integrity of Strutures Containing Defets, R6 Revision 4, inluding updates to Amendment 10, Marh Austin, C, Bate, S K, Higham, L A, Lynh, M A and Dean D W, Methods for the Determination of Shakedown Fators (Ks) for Type 316 and Type 321 Stainless Ste at High Temperature, Proeedings of ICPVT-13, May 2012, London. 5. RCC-MR Code. Design and Constrution Rules for Mehanial Components of FBR Nulear Islands and High Temperature Appliations, Appendix A16, Tome I, vol. Z. Paris: AFCEN; 2007 edition. 6. D Dean, M W Spindler, M Chevalier and N G Smith, Reent Devopments in the R5 Volume 2/3 Proedures for Assessing Creep-Fatigue Initiation in Defet-Free Components Operating at High Temperatures, Proeedings of PVP2013, PVP , July N G Smith, D W Dean and M P O Donnl, Strutural integrity assessment of wdments at high temperature: a proposed new approah for R5, Pro 2008 ASME PVP Conf, Chiago, Illinois, PVP (2008). FIGURE 1: A TYPICAL BOILER 6 Copyright 2014 by ASME

7 FIGURE 2: SUPERHEATER HEADER FIGURE 4: FINITE ELEMENT MODEL Temperature ( C) Reator Start-up, Transient Boiler Trip Transient Time (s) FIGURE 3: THERMAL TRANSIENT TEMPERATUERS FIGURE 5: UNIT STEAM PRESSURE 7 Copyright 2014 by ASME

8 300 Von Mises [MPa] B1 - N:161 B2 - N:163 B3 - N:1311 H1 - N:11 H2 - N:1 H3 - N:2 H4 - N:2549 T1 - N:159 T2 - N:150 T3 - N:134 T4 - N:129 T5 - N:152 T6 - N:122 TP1 - N:88 TP2 - N:87 TP3 - N:80 TP4 - N:77 TP5 - N:83 TP6 - N:81 W1/W2 - N:111 W3/W4 - N:108 W5/W6 - N Time [s] FIGURE 8: THERMAL TRANSIENT VON MISES STRESSES START-UP FIGURE 6: UNIT GAS PRESSURE Von Mises [MPa] B1 - N:161 B2 - N:163 B3 - N:1311 H1 - N:11 H2 - N:1 H3 - N:2 H4 - N:2549 T1 - N:159 T2 - N:150 T3 - N:134 T4 - N:129 T5 - N:152 T6 - N:122 TP1 - N:88 TP2 - N:87 TP3 - N:80 TP4 - N:77 TP5 - N:83 TP6 - N:81 W1/W2 - N:111 W3/W4 - N:108 W5/W6 - N Time [s] FIGURE 9: THERMAL TRANSIENT VON MISES STRESSES SHUTDOWN FIGURE 7: UNIT INTERNAL TUBE PRESSURE 8 Copyright 2014 by ASME

9 FIGURE 10A: ASSESSMENT LOCATIONS AT BEND AND HEADER FIGURE 10C: ASSESSMENT LOCATIONS AT TUBEPLATE FIGURE 10B: ASSESSMENT LOCATIONS AT TUBE AND HEADER FIGURE 10D: ASSESSMENT LOCATIONS AT HEADER WELDS 9 Copyright 2014 by ASME