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| document type = GENERAL EXTERNAL TECHNICAL REPORTS, TEXT-SAFETY REPORT | | document type = GENERAL EXTERNAL TECHNICAL REPORTS, TEXT-SAFETY REPORT | ||
| page count = 20 | | page count = 20 | ||
| project = TAC:60085 | |||
| stage = Other | |||
}} | }} | ||
Latest revision as of 03:16, 13 December 2021
| ML20138B898 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 03/31/1985 |
| From: | UTILITY POWER CORP. |
| To: | |
| Shared Package | |
| ML20138B837 | List: |
| References | |
| ER-8503, TAC-60085, NUDOCS 8512120407 | |
| Download: ML20138B898 (20) | |
Text
.
ff7TACHMEdT E r
Utility INnverCorporations ENGINEERING REPORT ER-8503 PROBABILrrY OF DISK CRACKING DUE TO STRESS CORROSION GRAND GULF UNfr I MARCH 1985 PROPRIETARY INFORMATION OF UTILITY POWER CORPOR ATION Not to be reproduced, cooled or dieseminated w6thout the empreet prior written consent of Utility Power Corporation.
0512120407 851210 PDR ADOCK 05000416 P PDR J
PRINTEDIN U S A.
ATTAci/M&rr D;T '
s Probabuity of Disk Cracking Due to Stress Corrosion Introduction The probabRity of turbine missiles from our 1800 r/ min nuclear steam turbine-generators with 44-inch last stage blades is documented in the Engineering Report No.
ER-504 of October 1975 to be 1.6 x 10-7 per unit year for a 6-flow turbine.
This number actually defines the probability of the occurrence of a 2120% speed or 2 20% overspeed event.
Engineering Report No. ER-503 on turbine missile analysis describes our LP turbine design with the inner casings featuring crash rings at the circumference of the last stage blade rows. With this design, the threshold speed for producing an external LP turbine disk missile is:
Disk No. Threshold Speed for External Missiles
- 1 2900 r/ min 161% of rated
- 2 3220 r/ min 179% of rated
- 3 2900 r/ min 161% of rated
- 4 2960 r/ min 164% of rated
- 5 3042 r/ min 169% of rated The following evaluation of disk eracking due to stress corrosion assumes speed operation below 120% speed and is, therefore, a probability of an LP turbine internal missile only.
Operating Speed Evaluating the probability of stress corrosion cracking for a speed 2120% of rated speed would lead to an insignificantly low. probability because the probability of the occurance of a 20% overspeed event being so small. Even an overspeed of 210%
presumes a failure of our highly redundant control system whleh could, however, occur 1
L
Arrmwry -
N with a probabuity of about 2 x 10-3 per unit year.
Since this again would lower the probabuity for a disk crack due to stress corrosion by about three magnitudes, we have, in the following report, conservatively evaluated the probabuity of a disk faRure at 110% rated speed. This will cover any small overspeed event which can occur during speed operation when the unit is not synchronized and which can also be the result of load rejections.
Crack Initiation Corrosion crack initiation has occurred on LP turbine disks in U.S. nuclear power plants. To-date, about 300 LP turbine disks of KWU design have been in operation in PWR and BWR power plants, accumulating approximately 1200 LP turbine disk service years. However, only four LP rotors were inspected and no crack initiation was found even after 83,000 service hours. Since we do not expect stress corrosion cracking of the KWU-designed rotom which operate in KWU-designed power plants, and because cracking has been found in U.S. power plants, we have chosen as a probability base of disk crack initiation the U.S. nuclear plant experience as described below.
Based on EPRI Report 2429 LD Vol.1 of June 1982, and EPRI Steam Turbine Disk Integrity Seminar of December 1.- 2,1983 (General Electric Nuclear SCC Experience),
the following probability of disk cracking can be established:
From 2429 LD Vol.1 Report Number of Number of Disks % of Disks Probability of l Inspected with with Cracks with 90%
Disks Indications Indications Confidence Level Disk #1 121 5 4.1 0.078 Disk #2 121 17 14.0 0.198 Disk #3 121 11 9.1 0.138 ,
Disk #4 121 1 0.8 0.032 ;
Disk #5 121 1 0.8 0.032 Disk #6 118 0 0 0.019 l l
l l
2 1
- . ArrncHsrer M
~
From EPRISeminar of December 1-2, 1983 Number of Number of Disks % of Disks Probability of Inspected with with Cracks with 90%
Disks Indications Indications Confidence Level 0.2 in.
Disk #1 180 0 0 0.013 Disk #2 180 0 0 0.013 Disk #3 180 3 1.7 0.037 Disk #4 180 17 9.4 0.133 Disk #5 180 25 13.9 0.200 Disk #6 180 1 0.6 0.022 Disk #7 180 0 0 0.013 Disk #8 146 0 0 0.016 Thesc :tati: tie: from ".S. pl:nt: revaal the highest probability for corrosion cracking is found in Disks #2 and #3 and Disks #4 and #5 respectively. Since these disks operate under similar conditions to our disks #2 and #3, we have chosen 0.2 as a conservative probability from these actual experiences. Disks which operate above the Wilson line have not shown any indication of stress corrosion. It must, therefore, be concluded that the probability of cracks in earlier disks as listed above must have been scrived at from units without reheaters. Our units are reheat turbines which would operate only for very few hours without reheaters. However, we assumed 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> operation without MSR per year which would reduce the probability of cracks for our disk #1 to:
0.2 30 Hours / Year = 0.001 6000 Service Hours / Year All other disks, #4 - 6 and #6 - 8 respectively, show probabilities in the range from 0.032 to 0.013. Because our Disks #4 and 5 operate under similar conditions, we have chosen to apply a probability of 0.03 as basis for all of our Disks #4 and 5.
Corrosion crack initiation is influenced by:
- Operating Steam Conditions
- Stress Level and Disk Material
- Purity of the Steam / Water Cycle
- Local Stagnation of Flow Crevices 3
- =. A7We&uar LE i .
Operating steam conditions fw all turbines in U.S. power plants are assumed to be the same. Stress levels and disk materials vary, but no significant differences have been -
found in regard to disk erack initiation. A major difference, however, seems to exist in the steam / water eyele cleanliness, but this fact has not been considered as part of this study.
Flow stagnation and crevices play a vital part in initiating corrosion eracks. These phenomena are design-related and do not exist with our disk-type rotw design. Our keyways are located where the metal temperature is higher than the temperature of the surrounding saturated steam which eliminates condensation (see Figue 1E88.808). The keyway areas fw all disks are stress relieved by a large circumferential groove in the disk as shown in Figure 1E84.154. The groove is open over the entire circumference to the outside by a 1 mm (40 mil) gap between the disk and shaft which allows breathing and therefwe, keeps the keyway from becoming a crevice trapping ewrosion products.
Crack initiation tating under various conditions revealed that disk material with 145 Ksi yield strength is not susceptible to stress corrosion under controlled environmental conditions, even when oxygen is present. However, crack initiation '
must be expected when carbon dioxide w other impuritiu are present from air-in leakages, as described in the American Power Conference Paper "Dmign, Operating and Inspection Considerations to Control Stress Corrosion of LP Turbine Disks" (1 ].
Further test roults shown in Figure 1884.073 indicate that not only tests with carbon dioxide, but also with air, led to similar crack initiation. An even more important finding is that tests with oxygen did not initiate cracks up to 30,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. However, it took only approximately 2,000 houm until crack initiation when the polishing of the test cycle with mixed bed ion filters was shut off and the low conductivity level was no longer maintained. This test closely simulates conditions such as flow stagnation and crevices because all these circumstances drastically increase the conductivity.
Such conditions do not exist with our keyway design. Therefore, it must be concluded that our advanced keyway design is a reason why stress corrosion cracking has not been found with our disk-type rotors.
4
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=200*F T = 145'F 145 l
= 210* F T = 176'F )
210 f157 150 1 356 260 220 / i !
p 1 .g 2 g a 3 176 4 5 ,,, iS6 205 " 166 355 l 323 260 220 160 169 ;
37g 2.1 0 239 1
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1 A l i ; ;
l l' LP Turbine Rotor Half with Disks #1 through #5 tf l
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GRAND GULF Uglll y Ptmer ISOTHERMAL LINES OF Corgwwalkwi DISK-TYPE NUCLEAR LP ROTOR 1E85.086
il774cHmur .LE Circumferential Stress Relief Groove _
R10 = 0.39 in.
R22.5 = 0.89 in. ff pB l m 4 -..
0 208 = 0.79 in. -!i( .,3 i,
./ _ _ ."._ q . _ J w V
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l SHRINK-FIT AND C8IIIIYI M UF F l KEYWAY CONFIGURATION ONIMMU8I(M3 2 1E84.154
' l l !, 2 : '! . ' >, 1 t ;
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//774cNintar ))z In accordance with this finding, it must be conservatively concluded that the time ,
without crack initiation with Kh U-designed disks is at least 15 times longer:
30.000 Test Hours in Oxygen without Crack Initiation =15 2,000 Hours unta Crack Initiation without Polishing This quotient for crack init'ation time can directly be used for the reduction of the crack initiation probability in a given time:
Crack Initiation Probability of the KWU-designed LP turbine disks for Grand Gulf Units #1 and #2 are therefore:
LP Turbine Disks Crack Initiation Probability Disk #1 0.001 / 15 = 6.7 x 10-5 Disk #2 0.2 / 15 = 1.3 x 10-2 Disk #3 0.2 / 15 = 1.3 x 10-2 Disk #4 0.03 / 15 = 2.0 x 10-4 Disk #5 0.03 / 15 = 2.0 x 10-4 Crack Growth Crack growth in LP turbine disks can be defined as a function of the operating temperature of a disk and the yield strength of the disk material. The empirical equation for crack growth in LP turbine disks of nuclear units as given in the ASME paper 81-JPGC-PWR-31 (2) seems to reflect the findings in nuclear plants and has, therefore, been used in this study:
(-4.968 - + 0.0278
- R )
The yield strength value has been taken from Engineering Report ER-8102 Nov. 81 Rev.1, which is the actual yield strength in Ksi for each disk measured at 20*C (68'F) ambient temperature. The disk metal temperature T used in the equation for crack growth is the maximum temperature in the keyways when operating at fullload and derived from a calculation of the isothermic lines in the disk / shaft system (see Figure 1E85.006). The relationship of crack growth probability density over the crack depth of a corrosion crack in a disk i at the time t has been arrived from a normal distribution for the naturallogarithm of the crack growth rate assuming the standard deviation of s = 0.587. The distribution of f (at g)is n calculated up to 3s as the assumed maximum crack depth and is shown in Figure 1B85.087.
l 5
~ Artseamcor us Operating Temperature 220 F REVISION Disk Yield Strength 139 Ksl
/
0 b
C 1/in.1/mm g
e 0.10-2.5
.c 1 0.08:
2 2.0 0
$ I(Sol) t ,
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$ 0 10 20 30 40 50 mm d yj d!g % % % 1 1% 1% 1% 2 inch 2!
5
!{5 i
,n 3 a g Crack Depth 0[ iI$!
is,sii E5 5$5 l
GRAND GULF UNIT 1 U88118yINmer F CRACK GROWTH OF DISK #2 OWIMM~d8IO81 l TURBINE SIDE OF LP ROTOR #3 1E85.087 l . . - _ . _ . . _ _ . - - _ _ __. _ ._ _____, _ __
_ - _ ~ --.-- - - - _ - - - - .. . _ _ _ _ . _ . .
//Tucmur Y i
l Critical Crack Depth I
Crack Initiation has been assumed to occur at the most critical locations with the )
smallest critical crack size which are the keyways. Critical crack depth is calculated i from the calculated local stress at 10% overspeed and the fracture toughness derived from the Rp0.2 strength and the notch impact test results as measured from the individual disk material (see ER-8102). The remaining influence factors of the critical crack depth have been used as follows:
- Variation of crack configuration from eracks of unlimited length to half'eirele cracks through a uniformly distributed random variable from 0.77 to 2.2.
Reduction effect of stress corrosion crack configuration on stress intensity through a normal distributed random variable with a mean value ofA= 0.65 and a .
standard deviation of s = 0.175 which covers the presently available data on crack ,
configuration, as shown in Figure 1E84.075.
l The resulting density function for the critical crack depth g(a g) e has been cut off at the smallest possible critical crack depth which is the theoretical corrosion crack of {
infinitive length without branching (see Figure 1B85.088.)
i The following Linear Elastic Fracture Mechanics (LEFM) equation is used:
2 A "
Q IC d g
1.21 7 ,k/K .
c/~2 i
ae = critical crack depth ,
Q = crack form parameter with random variation from 0.77 to 2.2 K IC = fracture toughness from tests for each disk (yield strength and notch impact tests) a = calculated keyway stress for 10% overspeed k/K = reduction of stress intensity due to branched stress corrosion cracking (mean valuep = 0.65, standard deviation s = 0.175) d/2 = radius of keyway in disk hub bore 6
~
Arrdcener 17 REVISION
/
0 b theoreticas calculations experiments C muNele bent branched mnlure cracks d - (a) 4 (b) (c) (d) (a) e {g
--g A
JJ - -g-.4 4
- 1,0' -V v
> \
?g* k/M C
$ 0,8- #A 5 . 4 e
~[ DA,_ _
[ ~-- A- T -~~ Mean Value sa 0,8- \\ #M
'A
-n-H4 y = o.es /
Qs '
fx , \ 4 ..
eg s .g Standard o g o,4 @ 06 tests Deviation gg -(a i '
/A s = 0.175 cu < ;, --
e$ QB D E 0,2-o5 , stress corroseon 20 45'A 30*A
--,e g crack models 60*A 1o .h e Wiem/Hodulek1978 AKitagawa1975 Ospeidel1971/1983 eMWU1984 j;!
g
,g C leid a 19 7 s <1Theocaris1972 OWilsorwCherepto1983 AKWU1983
, j8 QLo1978 (> Vitek 1977 BClark et al1982 1 8 al 1
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$8
- 53!3 Iii sII Tie a it
$i!af5 a z sa i
INFLUENCE OF CRACA CIIII8Y O*I Ii*Cf Ig l N 8IO" CONFIGURATION ON GRESS J INTENSITY FACTOR 1E84.075
Artscamsar 2 Fracture Toughness 178 Ksl 6 REVISION Tangential Stress at 110% of Rated Speed 84 Ksi
^/
0 b
1/in. 1/mm C
id
.e 0.25 0.010-8 is e
O 0.20 0.008:
3 g (aci)
!o 0.15 0.006=
\
a 8 0.10 0.004=
3 E
.c
.E o
E 0.05 0.002s
!l!
0 1O 20 3'O 4'O 5 mm j$
e-6 a &
% % % 1 1% 1% 1% 2 inch l 1
Es!I'1
>.l'o Crack Depth 48 I*$
4I I! E: t sit E5 5$! i i t i GRAND GULF UNIT 1 UtilityINMer F . CRITICAL CRACK DEPTH OF Cor1 5 rathwi kpJ ' DISK #2 TURBINE SIDE OF LP ROTOR #3 1E85.088 1
. . . _ . ___ _ _ _ _ _ , . . _ _ _ - ....__._.___.e
grMcMMar 17 Probability of Disk Rupture The probabnity of a disk I with an initiated crack to fail in the time period t is equal to the probability of a crack having grown to the critical crack size in the same time t . P (t) = Probability [ang(t)2 ac1 The failure probability of a disk can be evaluated through the density functions f t(ao) and da t) c with the convolution integral: oo act f ( Pg(t)= f t(agg)
- da cg)
- d a og*d a eg J .)
o o This equation, with the data for crack growth and critical crack size for each specific disk allows the evaluation of the probability of disk rupture by numerical integration. A distribution density from such evaluation is shown in Figure IE85.089. The probability P(t) of a disk failure within a turbine-generator with n disks that would occur in the time t , is with small val 2s of P g(t) equal the sum of the failure probabilities of all disks. With a probability of the crack initiation of g the following equation is applied: n P(t) n., Pg(t)
- gg i=1 7
Ar74curur 1z
. l 1
I l l l 50000 Service Hours ' REVISION Probability Pi(t)= 2.35 x 10-3 O b C 1/in. 1/mm d e 1.0-25 T E LD 20- 0.80:: 3 I e
$ 15 0.60m Q
o
?
E 10 - 0.40- pi(t) 8
=
}} 5 0.20=
88 1 e UiQ z * *! 0 1O 20 30 4'O 5 60 mm gI ;'l j % % % 1 1% 1% 1% 2 2% inch
-8 s i- ! f! Crack Depth 52 e> j!! .2 $5 $li 25 115 i
GRAND GULF UNIT 1 UtilityINMer F RUPTURE OF DISK #2 Corixwation J TURBINE SIDE LP ROTOR #3 1E85.089
.~
! jfrMcHmtar tg
/ ~
Conclusion The probability of a postulated disk falling within an LP turbine of the Grand Gulf Unit I within a time span of 50,000 service hours has been evaluated and listed in Figures IE85.090, IE85.091 and IE85.092. The total probability for the turbine-l generator from the three LP turbines combined amounts to: ht) =_101 *_l[- - This result covers any overspeed event with S 110% speed, since the probability of a 10% overspeed has been assumed to be equal 1. The probability of reaching an overspeed of more than 10% assumes a failure of our control system. The probabuity of such a control system (audre is about 2
- 10-3 which reduces the total probabuity of disk cracks at higher than 10% overspeed to be an insignificantly small value.
Tbc disk rupture probabuity for the six-flow Grand Gulf Unit I is compared to the NRC reliabuity criteria in Figure IE85.093. For an unfavorable turbine orientation, the NRC criteria wpid require an LP turbine disk inspection between 40,000 and 50,000 service hours,, L References , (1) , EgetNe, W., Schleithoff, K., Jestrich, H.-A., Termuehlen, H., " Design, Operating and Insp%ction Consideratform to Control Stress Corrosion of LP Turbine Disks," Proceedings American ower Conference, Vol 45, pp.196-206 (1983).
,. /
4 (2) Clark / N. G. Jr., Seth, B.B., Shaffer, D.H., " Procedures for Estimating the Probability'of Steam Turbine Dise Rupture from Stress Corrosion Cracking" ASME 81-JPGC-Pwr-31. i e
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Artscan u r g REVlSION
/
0 b Turbine Disk # and d Turbinescen. side 1Ts 10s 2Ts 20s 3Ts 3Gs 4Ts 4Gs STs Sos e i Tangential stress at 110% Speed Ksi 80 80 84 84 75 75 83 83 81 81 Fracture To hness Ksl 199 204 191 212 201 190 210 221 179 171 Operating Temp.
*F 260 260 220 220 180 180 157 157 156 156 Yield strength Ksl 143 133 143 132 129 115 125 127 136 134 Probabilities 7.3x 2.0x 1.6x 6.9x 1.7x pi(n 0 0 0 0 0 10-s 10-3 10-3 10-s 10-7 6.7x 6.7x 1.3x 1.3x 1.3x 1.3x 2x 2x 2x 2x 91 10-s 10-s 10-2 10-2 10-2 10-2 10-3 10-3 10-3 10-3
$3 4.9x 1.3x 2.1 x 8.9x 3.4x pi(D , 9 1 0 0 0 0 0
!l 10-7 10-7 10-8 10-7 10-58 Ei z
' 20 "0s :
5 15 Probability P(t) for Rotor #1: 18 si e 2.28 x 10-s l
== .ag -
$8
!!! i 115 l 58 E.$ !
E S' l f'S!$3)5 E5115 l f l GRAND GULF UNIT 1 Utility Power F PROBABILITY OF DISK Corixwation J CRACKING OF LP ROTOR #1 1E85.090 1
t - 1 477dt//ntrar 5 REVISION
- /
0 b C Turbine Disk # and ,d Turbin iGen. Side 1TS 1GS 2TS 2GS 3TS 3GS 4TS 4GS STS SGS e Tangential Stress at 110% Speed Ksi 80 80 84 84 75 75 83 83 81 81 Fracture Tou hness i Ksi in 201 193 196 186 197 184 251 265 184 200 Operating Temp.
/ 'F '
260 260 220 220 180 180 157 157 156 156 I Yield Strength Ksi 133 144 139 139 117 113 120 125 135 127 Probabilities 2.4x 1.1x 7.2x 1.3x 0 0 0 0 0 0 p1(g 10-8 10-8 10-4 10-8 6.7x 6.7x 1.3x 1.3x 1.3x 1.3x 2x 2x 2x 2x Al 10-s 10-s 10-2 10-2 10-2 10-2 10-3 10-3 10-3 10-2
}} pg , q 1.6x 7.2x 9.4x 10-8 1.7x 10-s 0 0/ 0 0 0 0 5i 10-7 10-7 18 m s"
$5 B!
$5 ii Probability P(t) for Rotor #2
$5
== !Ie y 2.75 x 10-8 28 Iii b li!
s' Et sit 5 !$!
.t l GRAND GULF UNIT 1 C8III8Y R N PROP ^BILITY OF DISK G ON 8IS I { QJl CRACK!AiG OF LP ROTOR #2 1E85.091
sf17densw w r W REVISION
/
0
-b C
Turbine Disk # and J~
- g .
, TurbinelGen. Side 1TS 1GS 2TS 2GS 3TS 3GS hYI* Pl3 STS SGS Tangential Stress at 110% Speed Ksi 80 80 84 84 75 75 83 83 81 81 Fracture Tou hness Ksi in 178 198 178 185 207 189 213 193 187 183 Operating Temp.
'F 260 260 220 220 180 180 157 157 156 156 Yield Strength Ksi 142 139 139 139 111 109 128 131 133 134 Probabilities 1.8x 5.2x 2.4x 1.5x 0 0 0 0 0 0 pI(g) 10-8 10-8 10-8 10-8 6.7x 6.7x 1.3x 1.3x 1.3x 1.3x 2x 2x 2x 2x 91 10-s 10-s 10-2 10-2 10-2 10-2 10-3 10-3 10-s 10-3 is i- 1.2x 3.5x 3.1 x 2.0x 0 0 0 0 0 0 P (t)
- q1 10-* 10-7 10-8 10-s ,,
Eiz[~s 20 95 11 jg ?;, Probability P(t) for Rotor #3 EI 5.21 x 10-f
=' !!!
28
!"a
$l W>
lI$;j 2 Probability P(t) for Rotor #1, #2 and #3: 1.02 x 10-4
- *5 38:
5 !!! 1 GRAND GULF UNIT 1 Utility Ptmer F i PROBABILITY OF DISK Corgiorathm k J CRACKING OF LP ROTOR #3 AND TOTAL 1E85.092
I' Arr4warar w l I l I REVISION / 0 b c d e 10 NRC Reliability Criteria (Probability, yr-1):
<10-s for Unfavorable < 10-4 for Favorable Turbine Orientation Turbine Orientation 2 10 '
ks _ E x /
/e8* #
e 5 / o 10-4. b j Minimum' Detectable
/p i j Indication of 0.1 in, ji o /
m ! ! No initial
$5 3! 10-s- ! Crack 95 Ri l
, $5 za
!!c -
/
!! 1:g5 a
g
;= 1.5:u /
eg 1 52 .!! 10-* y g> o . , , , , , , gh-j_gy 0 2x10' 5x104 8x104 ; EE EE5 i-Service Hours between inspections UtillavINmer rn1 GRAND GULF UNIT 1 ong h ggm ( WJ DISK RUPTURE PROBABILITY t 1E85.093 :
- - - _}}