ML20215B804
| ML20215B804 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 06/30/1987 |
| From: | Bostrom T, Chern C BECHTEL POWER CORP. |
| To: | |
| Shared Package | |
| ML20215B794 | List: |
| References | |
| TAC-65472, NUDOCS 8706170458 | |
| Download: ML20215B804 (16) | |
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l EVALUATION REPORT OF TROJAN NUCLEAR PLANT MAIN STEAM PIPE WALL THICKNESS June,1987 BECHTEL WESTERN POWER CORPORATION 50 Beale Street San Francisco, California Originator c___L A d Check
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TABLE OF CONTENTS SECTION PAGE NO.
l.
SUMARY 1
2.
BACKGROUND 1
3.
INPUT DATA 2
4.
STRESS CRITERIA 3
5.
STRESS ANALYSIS 4
s 6.
CONCLUSIONS 6
7.
METHODOLOGY JUSTIFICATION 7
FIGURE 1 FLOW CHART OF STRESS CRITERIA 8
FIGURE 2 2-D FINITE ELEMENT MODEL
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FIGURE 3 3-D FINITE ELEMENT MODEL 10 FIGURE 4 LOAD HISTOGRAM 11 REFERENCES 12 1
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3 1.
SUMMARY
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The purpose of this evaluation tiss to assess the acceptability of thin wall regions on Trojan Nuclear Plant main steam lines inside containment adjacent to the flow elements. The assessment of the m
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existing wall thickness for the minimen wall thickness requirement was perfomed bued on a more complete and rigorous analysis than the
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ori inal desian is permitted by JSA Standard B31.7 code. This P
ana ysis was done in accordance with ASME B&PV Section III Appendix XIII. Two and t.hree dimensional finite element analyses were performed to qualify the piping for pressure effects. The piping 1
stress analysis for thermal, weight and seismic effects and fatigue e
' evaluatitm were performed based on ASME Section III HB3600.
r Ir; per fonning the assessment, two dimensional i,2-D) and three dimensicnal (3-D) finite element models Were established based on the F
I wo,rst use - Line B using the ANSYS conkiter program. The results of i
msre contarvative 2-D model demonstrate.that the maximum calculated
[
local membrane stress intensity is acceptable. The results of a more detailed 3-D model demmtrated that all local membrane stress i
intensities are acceptable. The piping stress and fatigue calculations were performed using three sets of stress indices to account for the effects of structural di'scentinuity due to wall thinning (namely, tapered transition, sear welded and girth butt weld, and elbow). The restits of these ca7culatiens show that the calculated stress intensities are acceptable and the cumulative usage factors are acceptable and quite low.
Therefore, the thin wall regions of the main steam piping inside y
containment adjacent to t'ie flow elements are acceptable and are safe P
for operation.
23 CACKGROUND p
o
'The thin will regions are on four 28" main steam lines downstream of k
the flow element of all 4 lines.
PGE has noted in ultrasonic testing measurementy that pipe wall thicknesses in some areas are less than the minimuni wd b thickness (0.858") required by design pressure ( ANSI 831.1, Section 104.1.2).
Trojan Nuclear Plant was designed per USA btandard B31.7 code.
It is stated in the introduction of the code
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that the specific design requirements of the code usually reelve
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around a simplified engineering approach to a subject.
It is intended that a designer capable of applying more complete and rigorous analysis to special or unusual problems shall have latitude in the development of such designs and the evaluation of complex or combined ctre nes.
=
E Since regions of the main steam lincs do not meet the minimum wall thickness requirement per pera. 2-704 of B31.7 code (a simplified approach), a more complete and rigorous analysis as specified in Appendix XIII of ASME Section III is used to qualify these regions.
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021F2-1 Page 1
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3.
INPUT DATA PGE has provided the ultrasonic testing measurements data for the four main steam lines downstreem of the flow elements. The measurements on lines B and D are given in file No. T027993 which includes file No. T027928 for line B and file No. T027967 for line 0.
The measurements on Line A and C are given in file No. T028007.
%e minimum wall thickness required by design pressure is 0.858".
The re:ults of our review on those measurement data are sumarized as follows:
Line A - Only one location was found to be below 0.858".
- However, the area is confined in approximately 1" x 3" areas (J-8, l
K-8, L-8 and M-9) and the minimum wall is not less than 0.855".
The slope is 1ers than 10%.
Line B - The thin wall areas were found in 2 locations. The first area is about 6" x 8" with the minimum wall of 0.846" and is located between the pipe whip restraint and the pipe-to-elbow weld. This zone is located from about I
1 o' clock to about 4 o' clock (looking upstream). The second area is about 5" wide and extends from 2 o' clock to 7 o' clock. The seam weld is located at 6 o' clock. Most of the thin wall is above 0.825" exce)t that in one area of about 1" x 14" (Region F-23 throug1 F-36), the thin wall is about 0.820" with the minimum wall thickness of 0.807".
In addition, there is an 1-1/2" x 3" area with a.780"
.790" area at about 6 o' clock adjacent to the upstream side of the pipe whi.n restraint. This area is identified in file No. T028006. The worst slopes am 13.8%, 14.2% and are located at F-33 and F-30, respectively. The slopes of other areas are less than 10%.
Line C - The thin wall areas were found in 5 locations. Three locations (B-11, F-4 and N-12) are only about 1" x 1" size with the minimum wall of 0.854".
One area is 1" x 3" with the minimum wall of 0.850".
The other area is 1" x 4" also with the minimum wall of 0.850".
The slope is very smooth and is less than 2%.
Line D - The thin wall areas were found in two locations.
Onc-location is about 23" x 13" with an average wall thickness of about 0.830" to 0.840", and only in spect fic areas the wall is below 0.830" but not less than 0.814".
The other area is about 6" x 6" with the minimum thickness being about 0.843".
In general the slope is smooth and is less than 10%.
Among these four lines, Line 8 has the thinnest wall and the wall thickness also changes more rapidly than the other three lines.
Therefore, Line B was selected for analysis.
021F2-2 Page 2
4.
STRESS CRITERIA a) Minimum Wall Thickness Evaluation The minimum wall requirements are acceptable, if the requirements of Appendix XIII of ASME Section III and NB3213.10/NC3217 (C) of ASME Section III are met.
Therefore, the minimum wall thickness will be considered as acceptable if the following conditions are met:
i) Primary General Membrane Stress intensity (P )< S - per lable XIII - 1130-1 and Fig. XIII-1141-1 of IppenIix XIII.
- 11) Primary local membrane stress intensity (P ) < 1.5S.
If L
m the stress region over which the local membrane stress intensity exceeds 1.14 does not extend in the meridional direction more than JRt. Where R is the minimum midsurfa e radius of curvature and t is the minimum thickness in the region considered - per NB3213.10/NC3217 (C) and Fig. XIII
- 1141-1 of ASME Section III.
This criteria allows the local membrane stress intensity above 1.1Sm (or the wall thickness is below the minimum wall t,_higkness) provided that the stress region is smaller than /Rt.
iii) Primary local membrane stress intensity (P )<l l S.
If L
the stress region which includes the effect of struEtural discontinuity extends more than [Rt. - Per NB3213.10/
i NC3217(C) of ASME Section III.
b) Stress and Fatigue Evaluations Primary stres2 intensity, primary plus secondary stress intensity and peak stress intensity due to pressure, other moment loads and thermal gradients were calculated per ASME Section III NB3600.
)
Since the pipe is seam welded and the thin wall is near the pipe-to-elbow weld, the piping stresses were calculated by combining the stress indices for a longitudinal buttweld and a girth buttweld. Two other calculations were also performed; (1) using elbow stress indices to account for the effect of the elbow and (2) using tapered transition stress indices to account for structural discontinuity. The stress criteria is as follows:
Primary Stress Intensity Nonnal & Upset
< 1.5 Sm Emergency
< 2.25 Sm Faulted
< 3.0 Sm Cumuhtive Usage Factor
< 1.0
.021F2-3 Page S
As a parallel evaluation, the piping stresses (sustained loadings, occasional loadings and thermal expansion) were also calculated per ANSI B31.1 Code (1983), Section 104.8. This evaluation was performed to demonstrate that the piping meets B31.1 equations 11,12, and 13 with the reduced wall thickness.
For the purpose of this analysis, the wall thickness was conservatively assumed as a uniform thickness of.780".
The stress criteria for the above evaluations are summarized in Figure 1 - Flow Chart of Stress Criteria.
5.
STRESS ANALYSES a) Two finite element models were established based on the ultrasonic measurements of the pipe dimensions of Line B.
The finite element analyses were used to verify the primary n.embrane stress intensity due to internal design pressure 1100 psi per the stress criteria as specified in 4. a). The computer program used for this analysis was ANSYS.
i
- 1) The first model was a 2-D axisymmetric model using STIF 25 (axisymmetric harmonic solid element). The model is shown in Fig. 2.
Thit model was utilized to investigate the worst case minimum wall.
The midsection of the pipe was 18.7" in length and the wall thickness was modelled as t = 0.797" (which is the measured wall less 10 mils.). The lengths of both ends of the model were 10" each with a wall thickness of t = 0.955".
The tapered transition between the thin wall to the end sections were modelled with a 4 to 1 slope. The internal pressure was applied in the radial direction and longitudinal direction with the magntiude equal to the design pressure of 1100 psi, ii) The second model was a 3-D finite element model using ANSYS STIF 45 (3-D isoparametric solid element). The actual measured wall thickness data of Line B thin wall areas minur 0.01" (to provide margin), were modelled in this analysis.
In the thin wall area where the actual measured wall thicknesses are not available, the wall thickness was modelled as 0.858".
Between the two thin areas the wall thickness was modelled as 0.858".
The wall thickness of the remainder of the model was 0.955".
The thinnest spot on Line B was identified in file No. T028006 near location C-8 of the
,s Nortec 133 data taken on May 15,1987 (file No. T027928).
The wall thickness is down to 0.780" to 0.790" in an area 1-1/2" x 3".
Since this area is smaller than the(Rt, it was not modelled in the finite element analysis. This area is considered as acceptable provided that adjacent areas meet the acceptance criteria.
The model is shown in Fig. 3.
The internal pressure was equal to the design pressure of 1100 psi.
021F2-4 Page 4
b) Stress and fatigue evaluations were performed based on ASME Section III NB3600.
In this analysis, to calculate the pipe stresses due to internal pressure and moment loads, the section modulus of the pipe was conservatively calculated based on the wall thickness equal to 0.800".
The thermal gradient data was calculated based on a wall thickness of 1.000".
The average temperature difference at the structural discontinuity was calculated based on the wall thicknesses of 1.000" and 0.800".
Bechtel standard computer programs ME912 and ME913 were used.
ME912 was used to calculate thermal grcdient data and average temperature differences based on the operating condition provided in the load histogram. ME913 was used to calculate the primary stress intensities and the cumulative usage factors based on three different set; of stress indices, i.e.:
- 1) The combination of longitudinal butt weld and girth butt weld stress indices to account for the weld joint effect.
ii) Elbow stress indices to account for the effect of the elbow.
iii) Tapered transition joint stress indices to account for structural discontinuity effect due to the thin wall problem.
The computer outputs of these calcultions are given in Appendix 3.
Since the surface of the pipe wall free of abrupt changes and the minimum wall areas are uniformly distributed, the effect of discontinuities and concentrations were minimal. Therefore, it is appropriate and justifiable to use a stress index approach in lieu of.'inite element analysis for the stress and fatigue evaluation.
The piping with an assumed reduced section war also shown to meet the requirements of B31.1 Section 104.8 equations 11,12, and 13.
t 021F2-5 Page 5
6.
CONCLUSIONS
~
a) Minimum Wall Thickrass Evaluation
- 1) The results of the 2-D finite element analysis show that the calculated maximum local membrane stress intensity is 19.478 ksi and is located at element 4 shown in the Fig. 2.
This is the location which includes the effect of structural discontinuity.
S at 558F is 19.456 ksi. Therefore, the m
calculated maximum local membrane stress intensity is slighty above Sm (by 22 psi) and below the local membrane stress intensity allowable 1.1 Sm = 21.402 hsi.
=
- 11) The results of 3-D finite element analysis show that the calculated maximum local membrane stress intensity is 17.649 ksi, at element 722. This calculated stress intensity has considered the effect of structural discontinuity and is below the general primary stress intensity allowable S -
m Therefore, the reduced thickness regions are acceptable.
b) Piping Stress and Fatigue Evaluation Conditions Longitudinal d oow Tapered Allowable and Girth Transition (KSI)
(KSI)
(KSI)
(KSI) i) Primary Stress Evaluation Design, Normal &
15.22 18.01 15.22 29.18 KSI Upset (1.5 S )
m Faulted 15.69 20.13 15.69 58.37 KSI (3 Sm)
- 11) Fatigue Evaulation Cumulative Usage 0.04 0.02 0.04 1.0 Factor The above vsults show that the calculated primary stress intensities using the minimum wall thickness of 0.800" are acceptable and that the cumulative usage factors for 40 years of plant life are insignificant 1y low. Therefore the effect of s
minimum wall thickness is acceptable from piping stress and fatigue points of view.
In addition, piping stresses calculated based on the original code requirements (i.e., ANSI B31.7 code) also show that using the reduced wall thickness of 0.780", the piping components still meet the code requirements.
(B31.1 Equations 11,12, and 13) 021F2-6 Page 6
7.
METHODOLOGY JUSTIFICATION a) USA St&ndard 831.7 code permits a designer to use a more complete and rigorous analysis in place of the simplified approach provided by the code in the design and evaluation of complex or combining stresses for special or unusual problems. A similar statement also is made in NC3211(C) of ASME Section III Code.
This section states that when complete rules are not provided or when the vessel designor chooses, a complete stress analysis per Appendix XIII of ASME Section III shall be performed. Appendix XIII is the same as Ap)endix 4 (Design Based on Stress Analysis) of ASME Section VIII w11ch is applicable for non-nuclear pressure vessels.
Both appendices are similar to NB3200 of ASME Section III which is required for Nuclear Class 1 pipe design.
Therefore, Appendix XIII of ASME Section III is considered as an acceptable rigorous analysis as permitted by the B31.7 code, b) The method of evaluation described in 4. was discussed with Mr. R. F. Reedy of Reedy Associates, on May 20,198711:30 a.m.
Mr. R. F. Reedy is a recognized consultant for ASME B&PV Code,Section III and is a part ct'irman of Section III as well as a member of the ASME B&PV diain committee. He concurred with Bechtel that using Appendix XIII of ASME Section III to qualify this component is acceptable. The introduction of B31.7 code discussed in background section of this write-up was also referenced by R. Reedy.
He further pointed out that if the local primary membrane stress of a piping component can be qualified per ASME Section III NB3213.10 or NC3217(C), the piping component should be considered as acceptable for minimum wall requirement and no further calculations are required.
Further, for this specific component, the effects of discontinuitics and concentrations are minimal, therefore other calculations, such as fatigue calculation performed per Appendix XIII would be more than adequate and may not be necessary.
However, in order to provide the additional assurance, a simplified evaulation based on ASME Section III NB3600 was used to perform the fatigue calculation for this component.
021F2-7 Page 7 1
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- 11) Thermal gradient data was calculated based on the wall thickness = 1.000" iiii) The average temperature difference at structural discontinuity was calculated based on the wall thickness of 0.800" and 1.000" l
- detailed analysis used updated code requirements for completeness 4
Figure 1 FLOW CHART OF STRESS CRITERIA 021F2-9 Page 8
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REFERENCES 1.
PGE ultrasonic testing measurement data file No. T027993, T027928, T027967. T028007 and T028006.
2.
Load histogram. File No. T027997 issued from R. W. Fosse to L. Memula dated May 22, 1987.
3.
Forces and moments tabulation for M.S. flow elements prepared by E. Yang, dated May 21, 1987.
4.
Outline and dimensional data of steam flow restrictors, Permutit Job No.
A17305359S dated June 29, 1971.
(Vendor Print No. 6478-M1P(1)-4-3) 5.
USA Standard 831.7 Code (1969 with addenda through summer 1971) and ANSI B31.1 Code (1973 and 1983).
6.
ASME Section III Code (1983).
021F2-8 Page 12
_ _ _ _ _