ML20210K853

Root Cause Evaluation of Fourth Stage Extraction Elbow at Ft Calhoun Station

ML20210K853
Person / Time
Site:	Fort Calhoun
Issue date:	04/30/1997
From:	Strom W AFFILIATION NOT ASSIGNED
To:
Shared Package
ML20210K845	List: LIC-97-0116, Forwards Altran Corp Technical Rept 97152-TR-01,Rev 0, Failure Investigation of Fourth Stage Extraction Steam Line Rupture, & Fpi Rept FPI-97-905, Root Cause Evaluation of Fourth Stage Extraction Elbow, in Response to 970421 Event ML20210K848 ML20210K853
References
FPI-97-905, NUDOCS 9708200139
Download: ML20210K853 (43)

Text

l Root Cause Evaluation of Fourth Stage Extraction Elbow at Ft. Calhoun Station FPI Report 97-905 April 1997 Principal Investigators Dr. Mostafa S. Mostafa Dr. Chong Chiu

)-

Prepared by: Approved by:

$af f N William

~

. Strom Dr.C[ngChiu President, FPI International This' report _was prepared by.FPI International for Ft.

Calhoun Nuclear Power Plant'.

Y 9708200139 9708 PDR ADOCK 0 5 8 P

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i FPI 97-905 Fourth Stage Extraction Elbow Rupture. Pt. Calhoun July 29, 1997 TABLE OF CONTENTS I. Executive Summary 1 II. Introduction 2 t

i III. Evidence Collection 3 A. On Site Visual Inspection 3 B. Metallurgical Examination of Failed Pipe 6

c. Metallurgical Examination of Elbow S32 7

c. Flow Modeling Results 9 D. Failure Mode Determination 10 l

IV. Root cause Determination 11 V. Recommendations 11 I

Figures i

l l Attachments 1.

i Pipe Isometric Drawing and Heat Balance l 2. Flow Modeling

3. Metallurgical Test Results of Failed and S32 Elbows i

. i IM

1' h

rPI-97 905 Fourth Stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997 EXECUTIVE

SUMMARY

At the request of Mr. Ralph Phelps at Ft. Calhoun Nuclear Station, FPI International performed a root cause analysis of the fourth stage extraction line failure that occurred in April 1997.

I i

With the reactor at full power, a 90 degree elbow failed in the extraction line from the high pressure turbine to the fourth

!* stage feedwater heater. A large steam release occurred and the lt reactor was tripped. There were no personnel injuries associated with this event,

[

i FPI International performed a visual inspection of the failed piping shortly after the rupture.

A section from the failed pipe bend that contained the ruptured portion was sent to PhotoMetrico Laboratory in Huntington Beach, California for metallurgical analysis.

Flow modeling was performed to determine flow conditions in the pipeline and how they could have contributed to wear of this i pipe.

Based on the data summarized above, we believe the direct cause of the event is that the piping elbow failed due to erosion /

corrosion (FAC) caused by high velocity water impingement by the wet steam, FPI believes that the root cause was a programmatic failure of 1

1 FPI.97 905 Fourth Stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997 the pipe inservice inspection program. Previous pipe wall

( thinning occurred in the same line earlier which required elbow replacement, f

Recommended corrective actions are:

1. Replace excessively worn elbows on the fourth stage extraction line with 2W% chrome-molybdenum steel.

(

l

2. Install additional steam traps in the line to remove

! condensed water; in particular, downstream of the first I

elbow.

3. Perform a review of the pipe inservice inspection program to determine if other pipe is in a similar degraded condition.

It is important to include engineering judgement along with the EPRI FAC code in the inspection program. Unique conditions such as flashing down stream of control valves, water impingement in wet steam, geometry changes due to pipe fitup anomalies, and malfunctioning components can establish flow conditions that ar.e not modeled by standard FAC codes.

(

INTRODUCTION r

In. April 1997, Pt. Calhoun Nuclear Station was operating at full power-when a 90 degree elbow failed in the extraction line from the high pressure turbine to the fourth stage feedwater heater.

2

h-FPI.97 905 Fourth Stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997

'i A large steam release occurred and the reactor was tripped, j There were no personnel injuries associated with this event, i

1.

Attacht..nt 1 is an isometric drawing of the fourth stage extraction piping showing the location of the failed pipe a

fitting, and a heat balance diagram.

h h-EVIDENCE COLLECTION li n

4 t,

FPI International was requested to provide an independent root cause analysis of the pipe rupture. The following information was gathered.

On-site Visual Inspection

( FPI International performed a visual inspection of the fourth stage extraction-line piping.--The pipe inside surface exhibited wear that appeared to be typical of Erosion-/ Corrosion (E/C),

g also referred to as Flow Accelerated Corrosion (FAC), caused by 5I wet steam impingement. Based on a review of the piping isometric y drawings and steam quality, high wear rates would also be ld expected in all the bends and elbows in this line h

I i

Pailed Elbow Wall Thickness Measurements

?

Figure 1 shows the ruptured pipe elbow. Wall thickness was measured at-the grid lines drawn along the failure surface.

I i

These ultrasonic measurements were later compared to physical 3

i s

FPI 97 905 Fourth Stage Extraction Elbow Ruptura, Ft. Calhoun June 24, 1997 micrometer measurements. The micrometer measurements were slightly lower than the UT data shown below, t

Failed Pipe Wall Thickness (inches) at Grid Locations Location A B C D E F G Thickness 0.170 0.167 0.141 0.125 0.138 0.137 0.099 A

( Location 11 I J K L M N i Thickness 0.104 0.076 0.082 0.077 0.077 0.079 0.066 l

Location O P Q R S T U Thickness 0.046 0.040 0.049 0.040 0.034 0.028 0.016 Location V W X Y Z AA AB Thickness 0.023 0.034 0.039 0.037 0.034 0.029 0.035 Location AC AD AE AF AG All AI Thickness 0.045 0.055 0.063 0.071 0.071 0.074 0.088 e

Location AJ AK AL AM AN AO AP

, Thickness 0.125 0.147 0.154 0.171 0.186 0.194 0.215

, For comparison, the wall thickness at the opposite wall was 0.364.

Downstream Elbow S32 Wall Thickness Measurements 4

i

i j

f FPI 97 905 Fourth Stage Extraction Elbow Rupture Ft. calhoun June 24, 1997 The wall thickness was also measured for elbow S32 which is

located down stream of the failed elbow. Attachment 1 shows the i, location of this elbow. Wall thickness was measured at grid 4

1 lines drawn at approximately the same location as measured in the a failure surface. This data is listed below,

?

i R- 532 Pipe Wall Thickness (inches) at Grid Locations b

l Location G H I J K L M h- .

l g Thickness 0.371 0.367 0.214 0.127 0.043 0.035 0.043  !

L L Location N O P Q R S T Thickness 0.070 0.042 0.066 0.087 0.123 0.134 0.157 Location U V W X Y Z AA Thickness 0.173 0.164 0.138 0.157 0.174 0.172 0.162

, Location AB AC AD AE AF AG AH s Thickness 0.136 0.145 0.112 0.094 0.111 0.126 0.114 Location AI AJ AK AL AM AN AO Thickness 0.095 0.094 0.108 0.235 0.346 0.334 0.350 Location- AP AQ c Thickness 0.375 0.339 l'or comparison, the wall thickness at the opposite wall was 5

)

3

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l I FPI-97-905 Fourth litage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997 0.396.

I Metallurgical Examination of Failed Pipe L

Samples from the failed elbow were sent out to Atlas Testing Laboratories to confirm the chemical composition and mechanical properties. The analysis identified the material as 1022 carbon steel with a combined 0.11% chromium and molybdenum, typical for i carbon steel. Tensile strength was 68.3 Kai, yield occurred at 36.6 kni.

Test documentation is included as Attachment 3.

The ruptured elbow was examined by FPI at PhotoMetrics Laboratory in Huntington Beach, CA. Figure i shows the elbow as received.

The elbow was cut into two halves by extending the rupture line and making a corresponding cut on the inside of the elbow.

Examination of the inside pipe wall near the rupture showed the surface was shiny without a buildup of magnetite. This zone coincided with the highest flow rate in the pipe as depicted by flow modeling. Figures 2 and 3 show the inside of these two i elbow halves.

1 Figures 4 and 5 show the area near the rupture which had wear lines in the direction of flow. Fir,ure 6 shows the pipe surface away from the rupture where the flow lines became a string of craters (with comet tails).

Several samples were cut from the pipe and examined under a 6

FPI-97 905 Fourth Stage Extraction Elbow Rupture, Ft. Calhoun June 24, 1997 scanning electron microscope (SEM). Figure 8 shows a typical sample. The region near the rupture was labeled 5; the region away from the flow lines at the rupture was labeled A. Figure 9 shows a close up SEM image of the 5 region which is characterized l by a scalloped wear pattern typical of Flow Assisted Corrosion (FAC). Figures 10 and 11 are close up views of these scallops. ,

l i

l Figure 12 shows the craters in the a region away from the rupture.

Here the flow lines become a series of discrete craters which contain smaller craters on their downstream edges.

The general conclusion to be drawn from this analysis is that the pipe contained turbulent high velocity steam with high moisture I

content. The high flow areas kept the surface clean of any magnetite buildup and eroded the pipe surface due to water impingement. The rupture occurred where the wall thickness had been reduced to the thinnest, otherwise, nothing unusual was found in the pipe material composition or strength, i

Metallurgical Examination of Elbow S32 Samples from the elbow S32 were sent out to Atlas Testing Laboratories to determine the chemical composition. The analysis showed that the combined chromium and molybdenum weight percent was 0.03%. This was lower than the 0.11% measured for the failed elbow. Which would make elbow S32 more susceptible to FAC.

7

FPI-97 905 Fourth Stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997 Review of the piping isometric indicated that all the elbows in this extraction line should be subject to the same environment

}, conducive to FAC and erosive wear. To confirm this down stream elbow S32 was removed and examined at PhotoMetrics. The location of the elbow is shown on the isometric drawing in Attachment 1.

b Figure 13 shows the inside surface after the elbow had been cut

. in two. The general condition is very similar to the failed i

elbow, shiny region at outer radius which has been thinned and f!

H has the appearance of flow assisted corrosion and water l

impingement.

Figure 14 shows sample area #5 which extends from the outer radius to a region near the inner radius which has a magnetite buildup. Figure 15 is a closeup of the removed sample area which has been further divided into regions 5A, 5B, and SC for further analysis. Figure 16 is a SEM image showing the scalloped wear pattern typical of FAC in region SA. Region 5B has a similar wear pattern as shown as shown in Figure 17. The magnetite layer ever region SC is shown in Figure 18. The results of Energy persive Spectromotery (EDS) analysis of the magnetite are i saown in Figure 19. No contaminants or unusual material was i found. Th< magnetite layer simply consisted of iron oxide.

4 The conclusion to be drawn from the analysis of elbow S32 is that the same wear was occurring as in the failed elbow.

4

. 8 e

t FPI.97-905 Fourth Stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997

( Flow Modeling Results 1

f'

, The flow conditions in the fourth stage feedwater heater extraction line were modeled using the CFD 2000 code. The

, results are included as Attachment 2.

I' Pressure and velocity contours are shown for curve #1, which is i

the first elbow downstream of the nigh pressure turbine. The i

pressure contours show a 20 psia difference between the highest and lowest pressure regions of the elbow. This large difference will result in increased moisture condensation in the steam. The steam velocity near the inside radius of the elbow is calculated to be 180 ft/sec, and 135 ft/sec near the outside radius as a result of a vortex creation due to sharp geometry of the first elbow.

The second elbow is an out of plane 90 degrJe turn. The pressure contours s'now a pressure difference as much as 5 psia, which will result in increased moisture content. The velocity contours show a large difference in velocity between the top and bottom. Steam i velocity is 80 ft/sec at the top, and 175 ft/sec at the bottom.

3 At the outlet side of tne elbow, t. 1 velocity difference has been reduced to 165 / 115 ft/sec and the low velocity region has i l

rotated from the top to near the bottom of the elbow, I i

~

Curve 3 models the elbow that ruptured. The pressure contours a

show about a 3 psia difference. The velocity contours show the low velocity region rotates up 90 degrees from the side to the 9

9

ypt.97.pos rourth stage Extraction Elbow Rupture, Ft. Calhoun June 24, 1997 l

1 top of the elbow. This rotation is further illustrated in j_ streamlines. The steam velocity profiles are shown for five '

locations along the elbow. The 20 ft/sec velocity change from

!i- inner to outer radius at the inlet is smoothed out about a 10 3 ft/sec difference at the outlet.

s t In summary, flow modeling showed that the pressure differences in the elbows were sufficient to increase the moisture content in '

L the steam. This was sufficient to produce wet steam at very high velocities causing impingement FAC wear in the failed elbow. The velocity profiles showed large differences between the inner and outer elbow radii. Highest velocity on the ruptured elbow was at the area that failed. The high velocity and high moisture content were conducive to accelerated FAC.

Failure Mode Determination

' The failure mode was found to be Flow Assisted Corrosion (FAC) .

, Highwearrateswouldbeexpectedinallthebendsanbelbowsof I this line.

4 Possible reasons why_the forged elbow immediately downstream of the failed elbow did not show as significant wear _are that:

j.

i' l'. Pipe fittings and elbows have differences in construction.

4 Significantly greater initial wall thickness for fittings 4

could have led to under predicting wear rates.

3 10 e

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) FPI.97 905 Fourth stage Extraction Elbow Rupture, Pt. calhoun June 24, 1997

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2. Pipe fittings and elbows may have differences in impurities

, which may significantly affect FAC rates.

. )

a ROOT CAUSE DETERMINATION t

Sased on the evidence and findings assembled to date, FPI International believes the direct cause of the failed elbow was t

flow assisted corrosion (FAC) cauJed by high velocity Wet steam I

impingement. FPI believes that the root cause was a programmatic failure of the pipe inservice inspection program. Previous pipe wall thinning occurred in the same line earlier which required elbow replacement.

RECOMMENDATIONS Recommended corrective actions are:

1. Replace excessively worn elbows on the fourth stage extraction line with 2M% chrome-molybdenum steel.

i 3 2. Install additional steam traps in the line to remove .

i condensed water; in particular, downstream of the first elbow, d

3. Perform a review of the pipe inservice inspection program to determine if other pipe is in a similar degraded condition.

It is important to include engineering judgement along with

, 11 i

l' l

FPI 97-905 Fourth stage Extraction Elbow Rupture, Pt. Calhoun June 24, 1997

, the EPRI code in the inspection program. Unique conditions o such as flashing down stream of control valves, water impingement in wet steam, geometry changes due to pipe fitup c

anomalies, and malfunctioning components can establish flow i conditions that are not modeled by standard FAC codes.

b e

t 12 l

i FPI.97-905

). ( Fourth Stage Extraction Elbow Rupture. Pt. Calhoun , June 24, 1997 j

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FIGURES 1

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f FPI-97-905 Fourth Stage Extraction Elbow Rupture, Ft. Calhoun June 24, 1997 1

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Attachment 1 Pipo Isometric Drawing and Heat Balance

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AttachmeLc 3 Metallurgical Test Results of Failed and S32 Elbows I

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I; 6

L PATLAS h'

1 -

TESTING LABORATORIES, INC.

6929 EAST SLAUSON AVENUE e LOS ANGELES. CA 90040 e 213-722-8810 FAX 213-888-1493 PAGE 1 OF 2 PHOTO METRICS FORM 500H2 ATTN PAUL 15151 SPRINGDALE STREET

, HUNTINGTON BEACH, CA.9264 DATE...: 05/10/97 MATERIAL.:

[~ LAB 70946 SPECIFIC.:

PO NBR. 3000 PART NBR.:

SHIPPER: IDENTIF'D: SECTIONS OF PART

[J , BC l NECHANICALTESTREPORT YltlD STRENSTH TENS!LE STRENGTH ELONGATION

.......... ....... REDUCTION

................... .............. op ARg3 ACTUAL ACTUAL BS PER ACTUAL LBS PER IX -- --

ID $1lE l AREA LOAD o In LOAD 50 IN 2.88'  ! DIN l ! HARDNESS

.2781.584 .1488 5,538 39,588 9,718 69,488 .67 33.5 .165 59.8 72.8/HRB

.2781.584 .1488 5,138 36,688 9,568 68,388 .68 34.8 .163 68.8 71.8/HR5 liAIIMUN REQUIREMENTS:

MINIMUM REGUIREMENTS:

YlELD STRENGTH AT .21 0FFSET NETH00 USED: ASTN-E8 / ASTM E18 CHEMICALANALYSIS -

CHt0MW N0LYBDENun 8.861 ( ' A o. ll o 8.85 (

NETHOD: NETlDCP STD$iNISTTA88139 FOR INFORMATION ONLY A THORIZED S NATURE rt L Norton President j

ATLAS TESTING LABORATORIES, INC. SUBMITS THIS REPORT AS THE CONFIDENTIAL PROPERTY OF OUR CLIENT, R

[ RESERVED PENDING OUR WRITTEN APPROVAL IAND THEN MAY ONLY BE REPRODUCED IN FULLI

L v~ PATLAS I

1 TESTING LABORATORIES, INC.

i 6929 EAST SLAUSON AVENUE e LOS ANGELES. CA 90040 e 213-722-8810 FAX 213-888-1493 t* PAGE 2 OF 2 PHOTO METRICS FORM STEL1

,3 ATTN PAUL I 15151 SPRINGDAL STREET

, HUNTINGTON BEAC , CA.9264 DATE...: 05/10/97 MATERIAL.:

LAB NBR: 709463 SPECIFIC.:

1 PO NBR.: 3690 PART NBR.

SHIPPER: IDENTIF'De SECTIONS OF PART I

CHEMICALANALYSIS RESULTS REPORTED IN ! BY MEIGHT CARBON 9.22 MANGANESE 9.86 PH0$PH020U3 8.907

$ULFUR B.823 SILICON l.21

- CHRON!UN 9.f5 q NICIEL 6.81 NOLYBDINUM 9.94 @

TITANIUM Nlt COLUN5IUM 8 TANTALUM Nlt COPPER 8.9) i ALUMIN'JN N/R d CARBON (BYCOMBUST!0XI 8.21

, SULFURIBYCOMBUST10NI f.825

IRON REN.

, TEST NETHOD: NETH00:0.E. STDS:NBS12635 NATERIALIDENTIFIEDAS:1822C/ STEEL i NIRELEMENTSNOTREPORTEDTHISSAMPLE FOR INFORMATION ONLY

{

AU HORIZED $16N URE /_

l. p #"

t. teo rton, President ATLAS TESTING LABORATORIES, INC. SUBNITS THIS REPORT AS THE CONFIDENTIAL PROPERTY OF CUR CLIENT, REPRCOUCTION RIGHTS A RESERVED PENDING OUR WRITTEM APPROVAL AS A NUTUAL PROTECTION TO 00R CLIENT, THE PutLit AND OURSELVES.

m, ~ ., m.m...,~._,_

L . . . . .

c: - PATLAS

[ TESTING LABORATORIES, INC.

6929 EAST SLAUSON AVENUE e LOS ANGELES. CA 90040 e

{ 213 722 8810 FAX 213 888-1493 PAGE 1 OF1,

[ PHgTOMETRICS FORM STEL1 N1N INGDALE STREET HUNTfNhTONBEACH, CA.9264:

{

DATE...: 05/50/97 MATERIAL.: STEEL LAn NBR: 711051 SPECIFIC.:

[ PO NBR.: 3707 SHIPPER PART NBR.:

IDENTIF'D:

.. .... ..--.....---.. ----...-----------------------------------~~~--------

CHEMICALAXAlV$l$

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