ML20195G986
| ML20195G986 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 06/22/1988 |
| From: | Cockfield D PORTLAND GENERAL ELECTRIC CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| TAC-65727, NUDOCS 8806280192 | |
| Download: ML20195G986 (30) | |
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Portland General ElectricCompany David W. Cockfield Vice President, Nuclear June 22, I'vos Trojan Nuclear Plant Docket 50-344 Licenso NPF-1 U.S. Nuclear Dagulatory Commission ATTN: Docu c.c. Control Desk Washington DC 205S5
Dear Sir:
Metallurgical Report on Feedwater Pipinn Erosion / Corrosion In a July 15, 1987 meeting with your staff on feedwater piping erosion / corrosion, wo committed to provido the results of the metallurgical evaluation of sections of piping removed from the feedwater system. This evaluation has been completed, and the results are enclosed for your review. Sincerely, i At t ac!unent c: Mr. John B. Martin Regional Administrator, Region V U.S. Nuclear Regulatory Commission 1
- Mr. Bill Dixon State of Oregon Department of Energy Mr. R. C. Barr NRC Resident Inspector Trojan Nuclear Plant l
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y .. .. .. . . .. Trojan Nuclear Plant Document Control Desk Docket 50-344 June 10, 1988 Licenso NpF-1 pago 1 of 6 EXAMINATION OF REMOVED FEEDWATER PIPING FOR EVIDENCE OF EROSION-CORROSION DAMAGE
Background
Erosion-corrosion in piping and components of fluid systems is bottor thought of as flow-assisted corrosion. Where pure crosio.: or cavitation involves a direct mechanical attack on the metal surface, cros!on-corrosion increases the normal corrosion rate because of the flowing fluid. Whero the threshold velocity for mechanical orosion of carbon stool in water is about 100 fps, the threshold velocity for erosion-corrosion may be as low as 15 fps. Erosion-corrosion was first recognized in 1947 (Reference 1). Subsequent problems in Europo led to considerable interest and activity in both field and laboratory investigetions during the 1975-1985 timo period. Tha most comprehensivo summary document to date was issued by the Central Electricity Generating Board (Great Britain) in March 1987 (Reference 2). Typically, fluid systems are protected from corrosion by tho buildup of a protectivo coating of magnetito (Fe30 4) film. Under certain conditions, carbon and low alloy steols are unable to main-tain the protective magnetite film. Relatively rapid erosion-corrosion may occur. The variables of importance have boon observed to be:
- 1. Alloy composition.
- 2. Single-phaso pressurized water or two-phase wot steam conditions.
- 3. Temperature.
- 4. Steam / water velocity.
l 5. Steam / water chemistry. 1
- a. pH.
- b. Oxygen content.
The relative resistances to erosion-corrosion of the common power plant stools are: 18 wt% Chromium (Cr) - 8 wt% nickel austenitic stainless steels
>12 wt% Cr steels >2-1/4 wt% Cr - I wt% Molybdenum (Ho) low alloy stool
>l wt% Cr - 0.5 wt% Mo low alloy steel > copper bearing carbon steel >
plain carbon stool. l
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Trojan Nuclear' Plant Document Control Desk
. Docket 50-344 June 10, 1988 License NPF-1 Page 2 of 6 Trace levels of chromium and copper in plain carbon steels can substan-tially increase resistance.
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Single-phase pressurized water conditions are more severe- than two-phase wet steam conditions if the other contributing factors are held constant (eg, for the same velocity, single-phase water is more severe than two-phase). One empirical relationship predicts that wet steam with 10 percent moisture is about 32 percent as damaging as pressurized water. Super-heated dry. steam does not cause erosion-corrosion. Erosion-corrosion rates for carbon steel may become measurable at as low as 122*F. The erosion-corrosion rates then rise to a steep peak in the neighborhood of 265'F-365'F, and decline back to stagnant rates by the time 530*F has been reached. The initial increase is due to the increase in reaction rate with increasing temperature while the later decline is due to a change in the chemistry of the protective film formation. The rate of metal loss increases as the steam or water velocity increases. For a plain carbon steel, the relationship has been observed to have an-exponential form in single-phase laboratory studies, while empirical field data predicts a linear relationship. Erosion-corrosion of carbon steel is most pronounced in the pH range of 7.0-9.0, and Where the oxygen content is less than S ppb. As the pH is increased to 9.5,-the metal loss rate drops sharply. Similar effects are noted when the oxygen content becomes greater than 10 ppb. However, it should be noted that coolant oxygen levels sufficient to preclude erosion-corrosion of. carbon steel pose a substantial threat to the integrity of austenitic stainless steels and nickel base alloys. Erosion-corrosion incidents at Trojan have included the rupture of an 18-inch extraction steam line in January 1982, the perforation of several factory-installed steam generator feed ring vent tubes (J-tubes) dis-covered in March 1983, and the rupture of a 14-inch heater drain line in March 1985. The resulting monitoring and pipe replacement program is summarized in an attachment to Reference 3. Part of this response was to examine selected components removed during the replacement program. Summary Five removed components were examined for erosion-corrosion. These com-ponents were selected based on ultrasonic thickness measurement results that suggested wall thinning had occurred. The components examined were:
- 1. A 21-inch-long straight section from the B feedwater loop at the 63-foot elevation of the Turbine Building.
Trojan Nuclear Plant Document Control Desk
-Docket 50-344 June 10, 1988 License NPF-1 Pago 3 of 6
- 2. An 18-inch-long straight section from the A foodwater loop in Containment.
- 3. A straight section from EBB-3-1 at the 71-foot olevation of the B feedwater loop.
- 4. The dischargo elbow from the north main foodwater pump, DBD-3-5.
- 5. Feedwater elbow from EBB-3-1-1 at the 105-foot levol of the Containment.
Items 1 and 2 were found to be free of erosion-corrosion. Instead, the inside and outside diameters for both items were found not to be con-centric. While the averago wall thickness for Item I was found to be 0.594 inch (which is the nominal wall thickness), the actual wall thick-ness varied from 0.542 inch to 0.643 inch. The variation for Item 2 was from 0.519 inch to 0.636 inch, with an averago wall thickness of 0.5833 inch. Items 3 and 5 did show minor erosion-corrosion, but the loss in wall thickness was minimal. Item 4 exhibited severe erosion-corrosion, the inlet end weld region having been reduced from over 1 inch to 0.228 inch in spots. Although most of this was due to internal erosion-corrosion, the construction weld lacked the last 0.125 inch of cover pass, and was depressed to that extent. Results The tensilo and chemical requirements of the applicable material specif1-cations are provided in Attachment A. The chemical analyses of the various sections are given in Attachment B, with the numbers correspond-ing to the item number. Figure 1 shows the typical internal appearanco of both Items 1 and 2. No evidence of erosion-corrosion was found. Instead, the variation in wall thickness plotted in Figures 2 and 3 was observed. The chemistries given in Attachment B are both acceptable for American Society of Mechanical Engincors (ASME) SA 106 Crado B material. The hardness of the 21-inch section (Item 1) was found to be Rb 73.4. The estimated tensile strength of 64 kips /sq. in, would meet specification. The hardness of the 18-inch section (Item 2) was virtually identical (Rb 73.5), and the estimated tensilo strength would be the same. Figures 4-8 show the ferrite plus pearlite microstructure of the two pipo sections. The appearance is normal for this material. Item 3 is shown after initial sectioning (Figuro 9). Note the small eroded area in the foreground. After cleaning (Branson Ultrasonic 0xido Remover), the eroded area and associated copper / copper oxido deposit are seen in Figures 10 and 11. The scalloped structure typical of
r. Trojan Nuclear Plant Document Control Desk Docket 50-344 June 10, 1988 License NPF-1 Pago 4 of 6 erosion-corrosion is evident in Figure 11. The scalloped structure is also shown in Figure 12 at a magnification of about 25X. The cleanor was only partially effectivo against the copper / copper oxide. Howevor, that copper which was dissolved had a tendency to plate out from solution, and produced some unusual coloration on the cleaned sections. The hardness of Item 3 was Rb 74.7. The estimated tensile strength of 66 kips /sq in, meote specification requirements, as does the chemistry given in Attachment B. The microstructure delineated in Figures 13 and 14 was, again, a normal forrito plus pearlite structure. Typical thick-ness in the area of interest was 0.571-0.580 inch. The discharge elbow from the north main foodwater pump (Item 4) is shown shortly after removal in Figures 15-17. The worst erosion-corrosion was located at the inlet end. The minimum wall thickness of 0.228 inch was found to be at the wold to the recirculation teo. This interface may be seen in Figures 18'and 19. The section with the thinnost wold region also contained the worst local-ized damago (Figure 20). The scalloping was evident here, as was the adjacent heavy coppor/coppor oxido deposits. Minimum wall thickness at the bottom of the worst scallop was 0.453 inch. The most heavily scal-loped region is shown after derusting in Figuro 21, with an approximately 2X closeup in Figuro 22. A 25X magnification view of this scalloped region is found in Figure 23. Metallographic sections were also taken. Figures 24 and 25 are longitud-inal sections which show the scalloped surface in profilo, and also demonstrate the lack of subsurface deformation typical of crosion-corrosion. Longitudinal and transverso views of the typical micro-structure are found in Figures 26-28. The hardness of the dischargo olbow was found to be Rb 72. This hardness corresponds to an estimated tensile strength of 63 kips /sq in., which meets specification requirements. The combination of a normal micro-structure, acceptable hardness, and wppropriato chemistry (Attachment B) indicate that this component was within normal expectations. It is of interest to note that this elbow does have a very low level of trace impurities such as chromium, copper, or molybdenum that havo boon observed to enhanco erosion-corrosion resistance. Item 5 and the minor inlet end crosion-corrosion observed are seen in Figuros 29 and 30. After cleaning (with the usual copper platoout from adjacent deposits), a longitudinal center section from the affected area is shown in Figuroc 31 and 32. Note that there was no erosion-corrosion prior to or on the weld, and that the light scalloping in the counterbore region was not doop enough to remove the tool marks from the counterbore
Trojan Nuclear Plant' Document Control Desk Docket 50-344 June 10, 1980 License NPF-1 Page 5 of 6 machining (Figure 31). Figure 33-is a 25X view of the typical' erosion-corrosion scallops. Metallographic sections in_both the longitudinal (Figures 34-35) and the transverse (Figure 36) directions revealed the typical lack of. subsurface damage and the normal ferrite plus pearlite microstructure. The hardness of Rb 77.1'and the estimated tensile strength of 69 kips /sq in, were as expected. The wall thickness in the area of interest was about l 0.580 inch, increasing to-over 1-inch beyond the counter bore.
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l Discussion and Conclusions The examination of these components produced evidence of erosion-corrosion on Items 3-5. . Items 1 and 2 were free of erosion-corrosion.- However, because of the wall thickness variation in these straight sections (Items 1 and 2), it was impossible to establish this fact without removal for examination. The results of the examination indicate that these two pipe sections did meet ASME specification requirements at the time of th ir removal. Items 3 and 5 exhibited minor erosion-corrosion accompanied by copper / copper oxide deposits. The component materials appeared to be normal and acceptable in-all respects. This was also true for Item 4, the north main feed pump discharge elbow. -At this elbow, the erosion-corrosion was quite severe and the copper / copper oxide deposition quite heavy on adjacent areas. It was apparent that fluid flow in'the elbow was rapid and with a complex turbulence. The presence of deposited copper has been noted on many components that have been damaged by erosion-corrosion at the Trojan plant. There is a strong possibility thal-the plating out of copper was acting as an addi-tional cathodic (reduction) reaction, and was accelerating the adjacent erosion-corrosion. A mechanism discussed in Reference 4 (Pages 228-229) suggests that copper in this type of circuit can repeatedly dissolve and l reprecipitate with concurrent corrosion of adjacent steel components. l This points out another reason for utilizing chemistry control and purification systems to remove the copper inventory from the secondary l system as rapidly as possible, as is being done at Trojan. l l
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l l Trojan Nuclear Plant Document Control Desk ! Docket 50-344 June 10, 1988 Licenso NPF-1 Page 6 of 6 I REFERENCES
- 1. Wagner H. A.; Decker, J. M.; and Marsh,-J. C., 19747, "Corrosion-Erosion of-Boller Feed Pumps and Regulating Valves", Trans. ASME, 69, 389.
- 2. Woolsey, I.S., "Erosion-corrosion in PWR Secondary Circuits", Central Electricity Cenerating Board", TPRD/L/3114/R87, March 1987.
- 3. Letter from David W. Cockfield to USNRC Document Control Desk, July 31, 1987.
- 4. Pourbaix, Marcel, "Lectures on Electro-Chemical Corrosion", Plenum Press, New York - London, 1973.
Attachment A Page 1 of 1 Material Tensile and Chemical Requirements ITEMS 1, 2, 3 ASME SA 106 Crade B Carbon Manzanese _ Phosphorous sulfur Silicon 0.30 wt% max. 0.29-1.06 wt% 0.048 wt% max. 0.058 wt% max. 0.10 wt% min. Tensile Strength. Min. Yield Strength. Min. 60,000 psi 35,000 psi Elongation 22% Longitudinal 12% Transverso ITEMS 4 & 5 ASME SA 234 Grade WPB Carbon Manzanese Phosphorous sulfur Silicon 0.30 wt% max. 0.29-1.06 wt% 0.050 wt% max. 0.058 wt% max. 0.10 wt% min. Tensile Strength. Min. Yield Strength. Min. 60,000 psi 35,000 psi Elongation 22% Longitudinal 14% Transvorso JWC/36120
NORTHWEST TESTING LABORATORIES,INC, Attachment B co ernuenen meesevien 9# " nonio* vaheta vierme c u s.. 6 .... set . P.O. Box 17126 .sto,=. cent ,,c.,,o=
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$,'c I tee," Portland Oregon 97217 0126 ' ' ' ' , , ' , ',, " ' , " ,
- Pnone:(503) 2891778 May 12, 1988 Portland General Electric 121 S.W. Salmon Portland, Oregon 97204 f
Attention: Mr. Jeff Carter
Subject:
Chemical analysis performed on five (5) steel samples submitted on 5/10/88. REPORT: Item: Steel Piping Samples Chemical Composition: No. 1 No. 2 No. 3 No. 4 No. 5 Carbon (C)%....... 0.222 0.249 0.244 0.229 0.222 Manganese (Mn)%... 0.832 0.854 0.868 1.00 0.931 Silicon (Si)%..... 0.148 0.152 0.137 0.138 0.139 Chromium (Cr)t.... 0.035 0.032 0.035 <0.01 <0.01 Copper (Cu)%...... 0.040 0.040 0.041 0.013 0.015 Nickel (U1)%...... 0.041 0.042 0.041 <0.01 <0.01 Molyodenum (Mo)%.. 0.020 0.006 0.017 0.004 0.004 Vanadium (V)%..... 0.008 0.008 0.008 0.008 0.008 Aluminum (A1)% ... 0.023 0.029 0.019 0.015 0.026 Phosphorus (P)%... 0.006 0.006 0.007 0.009 0.009 Sulfur (S)%....... 0.020 0.020 0.025 0.020 0.022 Columbium ( Cb) %. . . 0. 0091 0.0087 0.0083 0.0081 0.0077 Respectfully, NORTHWEST TESTING LABORATORIES,INC.
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raul'It iW Tech 61 cal Di rector Report No. 314385
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