calculation code for erosion corrosion induced wall thinning.pdf
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Nuclear Engineering and Design 119 (1990) 431-438 431 North-Holland
C A L C U L A T I O N C O D E F O R E R O S I O N C O R R O S I O N I N D U C E D W A L L T H I N N I N G IN P I P I N G S Y S T E M S
W. K A S T N E R , M. ERVE, N. H E N Z E L and B. S T E L L W A G
Siemens AG/KWU Group, Erlangen, Fed Rep. Germany
Received: first version October 1988, revised version 21 July 1989
Extensive experimental and theoretical investigations have been performed to develop a calculation code for wail thinning due to erosion corrosion in power plant piping systems. The so-called WATHEC code can be applied to single-pha~ water flow as well as to two-phase water/steam flow. Only input data which are available to the design engineer or the operator of a plant are taken into consideration. Together with a continuously updated erosion corrosion data base containing results from experimental investigations and aetuai damage in power plants the calculation exxle forms one element of a weak point analysis for power plant piping systems which can be applied to - minimize material loss due to erosion corrosion,
- reduce non-destructive testing and curtail monitoring programs for piping systems, - recommend life-extending measures.
1. Introduction
The severe damage at the Surry 2 PWR nuclear power plant in December 1986 and the Trojan PWR in June 1987 has attracted broad attention - including that of the licensing authorities - to the phenomenon of erosion corrosion in water systems, that is in single-phase flow. As is known, the incident at Surry 2 was a rupture in a section of a carbon steel feedwater pump suction line containing a T-junction/elbow combination due to a high degree of material loss not expected by the operator (fig. 1 a-c). At Trojan, non-destructive ex- aminations showed evidence of a reduction in wall thickness in carbon steel piping in the feedwater system. These results led to the prophylactic replacement of the components (but not to a change of material). At Trojan, erosion corrosion occurred not only at pipe elbows but, on the face of it surprisingly, in straight sections of the piping as well.
It will be demonstrated in the following that a plau- sible explanation for the occurrence of erosion corro- sion in both of these cases can be found by detailed analyses and consideration of the boundary conditions. Due to the large number of factors influencing the degree of material loss due to erosion corrosion, the analysis of such damage is possible only with the aid of a computer code. The main application of the WATHEC PC code developed by Siemens/KWU, however lies not
in obtaining confirmation of damage which has already occurred but in performing diagnostic analyses aimed at the development of measures to prevent further damage to the piping systems.
I 24" WCPO.Zl.301 I Flow dimetio~ I ~
Fig. la. Erosion corrosion damage in 18"-feed pump suction pipe at Surry-2 - Prineipai schematic of damage location.
0029-5493/90 /$03 .50 © 1990 - Elsevier Science Publishers B.V. (Nor th -Hol l and)
432 W. Kastner et al. / Calculation code for erosion corrosion induced wall thinning
Fig. lb. Erosion corrosion damage in 18"-feed pump suction pipe at Surry-2 - Damage configuration in 90 o elbow (top
view).
/ ...... \
Fig. 2. Mechanism of erosion corrosion.
l
The present report describes the development of the WATHEC computer code and its application to weak point analyses for power plant piping systems.
2. Description of the erosion corrosion phenomenon
abrasive corrosion (caused by particles in water) and droplet impingement erosion (caused by water droplets in steam) on the one hand and cavitation (caused by imploding gas bubbles) on the other. It also indicates that erosion corrosion is a mass transport phenomenon
The term erosion corrosion is defined in [1] as fol- lows:
"Erosion corrosion is a flow-induced process of material degradation. This phenomenon can affect metallic materials which owe their corrosion resistance to the formation of oxide films. Wearing away of the oxide films by turbulent water or wet steam flow is followed by dissolution corrosion of the unprotected metal." The metal surface is kept in a permanent state of
enhanced reactivity. This clearly distinguishes erosion corrosion from purely mechanical processes such as
Fig. lc. Erosion corrosion damage in 18"-feed pump suction Fig. 3. Appearance of erosion corrosion in a steam extraction pipe at Surry-2 - Ruptured pipe section (top view), fine with two-phase steam/water flow.
W. Kastner et al. / Calculation code for erosion corrosion induced wall thinning 433
(fig. 2), which can occur under both single-phase (water) flow and two-phase (steam/water) flow conditions; in the latter case erosion corrosion and droplet impinge- ment erosion often occur in conjunction. The character- istic surface appearance of erosion corrosion caused by wet steam has long been known as the tiger skin or stripe pattern (fig. 3). Erosion corrosion under single- phase flow conditions had been experienced in the past as well, but it is only since the damage at Surry 2 that major interest has been shown in the effects of this phenomenon on water pipes.
3. Important factors affecting erosion corrosion
The parameters affecting material loss from steel surfaces due to erosion corrosion were established in extensive laboratory experiments carried out by Sie- m e n s / K W U over the past several years (since 1973) as part of a research program [2]. Comparable investiga- tions were performed by EdF [12,13], CEGB [14,15], MIT [16,17], etc. As fig. 4 shows other important factors besides the geometry conditions, which will be dealt with more closely in the following, are the material composition - principally the chromium a~a.d molybde- num, and, according to literature, also the copper con- tent - and the flow velocity of the water.
Of major impact are the temperature, pH and oxygen content of the water (fig. 5). The strong dependency of material loss on pH emphasizes that the AVT chemistry characterized by pH values above 9.8 in feedwater, as practiced in almost all PWR plants built by Siemens/KWU, has a favourable impact on erosion corrosion. In BWR plants, highly pure de-ionized water is used as the fluid of the steam-water cycle. It contains oxygen and has a pH close to neutral. The presence of an adequate amount of oxygen in the feedwater (opti- mum around 50 ppb) results in good water chemistry protection against erosion corrosion.
In most cases the fluid temperature is given. The water chemistry can be adjusted only within a limited range as dictated by other parameters. For this reason the following items are of great practical importance: - Ma te r ia l selection: here it should be mentioned that
• austeuitic stainless steels are not susceptible to erosion corrosion,
• the resistance of carbon and low-alloy steels in- creases with chromium, molybdenum and copper content,
• coatings rich in chromium, applied by flamespray- hag, may be highly advantageous, especially for local repairs [3,4].
- Flow velocity: this can be limited for a given mass flow by selecting suitable line cross sections, and
Wall thinning
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W. Kastner et aL / Calculation code for erosion corrosion induced wall thinning
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Cadam a a l , 0.3% Mo Cadam atad. 0.3 % Mo Cmboa aI~L 0.3% Mo
Fig. 5. Parameters of influence on erosion corrosion (2).
- P ipe geometry: this can be optimized to improve flow conditions by selecting sufficiently large elbow radii, replacing T-fittings with laterals, etc.
Type of exposure Ref lmmce vMoci ty w k c
-~ . .~ at pipes 1.00
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' i ' In leaky jMots ~ c<Mro4gondMg o.ae In ladlydnthl to I o a l u l tokM b u d 0.06
h - - - - - + I t Inllllllr Wl~l¢ I¢llpll IiI110~I~ O~ l~llp 0.20
Fig. 6. Geometry factors according to Keller.
Fig. 6 shows the effect of pipe geometry in detail. The figure includes the Keller factors kc [5] (here without units) for various flow configurations. These factors range from 0.04 for an ideal straight pipe unaf- fected by additional turbulence sources, to a maximum of 1.0 for stagnation point flow which represents the worst possible flow condition.
4. Calculation of material loss using the WATHEC pro- gram
Laboratory experiments were carried out to develop an empirical relationship for the calculation of material loss due to erosion corrosion, which links the factors presented in figs. 4 to 6 and also the parameter of the exposure time of the components involved, (fig. 7).
This relationship applies in the first instance to single-phase flow only. It can, however, fotlowing slight modification, be applied to two-phase flow, but only provided the metal wall is wetted by a continuously moving, coherent film of water (see fig. 7). This condi- tion exists, for example, in the case of annular flow of a steam and water mixture with a continuous film of water along the wall and the steam in the center of the flow. The flow velocity in such a case is taken to be the mean velocity of the water film along the pipe wall, which is dependent on the mass flow rate, on the
W. Kastner et al. / Calculation code for erosion corrosion induced wall thinning 435
[ ~ (u~) now
1 • Ivbtabl let, Mo, etc.) • Flow v.ioaty • Water temperature • Wmr cbemtry (pH, 02) • Opuh~ t~e
I Emp~k~ r~k~L 1 bmmd on parmele~l known to the d~ge ~ or opwator
I Two.plum (wutm'/Mmm) flow
~// ,A % / / / /
W,a _ Z
M~m velo~, of the we~w ~m at the wall of the compo~ cak:ulat~l to Rouhmi/11/ leom flow rata. stum quality and void fraction
Empidcal model to calculate: w a ~ u s ~ U v
_~,m crone in wator and wnw/guan Ilow
WATHEC
Fig. 7. Development of the model to calculate wall thinning due to erosion corrosion.
ation. These data are stored in a data bank (containing approximately 2000 sets of data), set up and continu- ously updated by KWU.
One particular problem recognlsed during the exten- sive testing of the WATHEC computer code is de- scribed in more detail in the following:
In piping systems, components which disrupt normal flow and thus create turbulence are often located only short distances apart. The piping geometry at the point of rupture at Surry 2, where an elbow followed a T-fitting, is an example of this. In such cases, the turbulence created by an upstream component A with geometry factor k~,A has not yet completely subsided when the water flowing in the pipe reaches component B with geometry factor kc, B. For reliable and conserva- tive prediction of erosion corrosion material loss a portion of geometry factor k~,A must thus be taken into account in determining the geometry factor for compo- nent B.
A relation for the declining effects of the geometry factor downstream of a component which disrupts flow was developed on the basis of analyses of the subsiding of turbulences carried out by Albring [7]. The portion of the geometry factor from component A remaining can be described by means of an exponential function [8]:
density of the water under saturation conditions, on the steam quality and the void fraction. At the same time, the pH and oxygen content parameters input into the calculation must take into account the temperature- and pressure-dependent distribution coefficients for alkaliz- ing agents and for oxygen.
The calculation procedure described here (see also [6]) now exists in the form of a computer code, the PC version of which is named WATHEC ( = WAll THin- ping due to Erosion Corrosion). Similar computer pro- grams for erosion corrosion analysis of s team-water cycles in power plants were developed e.g. by EPRI, see [18]. Common to these codes is that they are based on functional relationships established in laboratory tests under well defined flow and water chemistry conditions. Prediction accuracy depends on the quality of the input data. During development of these codes they had to be tested extensively and calibrated to plant data in order to ensure conservative prediction on the one hand and to avoid overestimation of in-plant erosion corrosion material losses on the other. These requirements were taken into account during development of the WATHEC code by comparing the calculated material losses with measured values taken from other laboratory experi- ments and with data gathered during power plant oper-
Akc, A = kc, A exp( - C z / D ) , (1)
where: D = pipe diameter z = distance from component A, C ffi constant = 0.231, see [8].
The validity of this relation is demonstrated by the following example:
Material loss due to erosion corrosion on a test setup consisting of a sharp elbow followed by straight piping of the same diameter was investigated in an experiment conducted in our laboratories [2]. Wall thinning due to erosion corrosion enlarges the pipe diameter. Fig. 8 plots this enlargement along the length of the test setup. The decrease in material loss due to erosion corrosion in the region 0 < z / D < 10 shows essentially the same pattern as the curve obtained with eq. (1) for the respective geometry factors. The geometry factor for a component B a short distance downstream of compo- nent A is now calculated using the equation:
(k~.a)to t --- kc. B + Akc. a. (2)
It should be borne in mind that the geometry factor for a combination of components cannot exceed the value
436 W. Kostner et al. / Calculation code for erosion corrosion induced wall thinning
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~ 16mm~
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7 O ~ ~ 0.85 pp~ 0.6
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-- 0.4
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• z/D t t ~ - ' - - - - ' - - - -i
4 8 0 m -"I
Fig. 8. Wall thinning due to erosion corrosion measured as pipe diameter increase and calculated decrease of the geometry factor in a pipe behind a sharp bend.
for stagnation point flow, that is to say
(k¢,B)to , < 1.0. (3)
The considerations presented here related to single- phase flow in which, according to [9] the extent of
turbulent effects is limited to z = 10 D. In two-phase (s team/water) flow this extent might be much larger, depending on the steam quality (see [10]).
The conservative and reliable application of the WATHEC computer code to the conditions existing in
Water velocity 18.6 m/s Pmuure 40 bar Water temperature 180 °C pH.V~ 7 Oxygen cone. 0.85 ppb Expo~uR time 513 h
Connecting • I piece 1.4550 I
L 480 ~]
K15115Mo3 (not machined Intemally) \
I, RIO/St37.2
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- R12/13CrMo44
Connecting piece 1.4550
K13/13CrM044 48c
Fig. 9. Arrangement of the 6 ID 16-elbows and 7 ID 16-piping sections within the KWU-piping-test-geometry.
W. Kastner et al. / Calculation code for erosion corrosion induced wall thinning 437
• Loud (wan th i ,~M~) - - - Avmgl (mm~ht)
10" • I
1000 2 ~ 3000 t e ~ h of p i p ~ ~ M in ~,u
Fig. 10. Comparison of wall thinning calculation and measure- ments for the KWU-piping-test-geometry (ID 16 mm= 5/8").
a complex piping system was to be demonstrated on a test setup consisting of straight pipes and elbows (see fig. 9) made of different materials. The experiment was carried out in the Siemens/KWU laboratories using water as the flow medium [2].
The results calculated with the WATHEC code can be output in either list or graphic form (see fig. 10). The presentation of material loss over the length of the piping system also includes the local (obtained by wall thickness measurement) and average (by weighing) material losses determined as part of the experiment. In addition to a quite good agreement between calculated and measured values, the comparison also shows that the computer code is adequately capable of calculating the flow disturbances caused by turbulence-inducing components.
The code's capabilities also become clear when the code is used to perform re-calculations of the Surry 2 and Trojan damage within the limits allowed by the design and operational data available on the two plants. The exact initial wall thickness of piping is often un- known. Residual wall thickness measurements can thus be correlated only with nominal wall thicknesses. As a result, the wall thinning data thus obtained also include the initial within-tolerance deviations from nominal wall thickness which, in the case of Trojan, are as much as + 12.5%. Nevertheless a comparison of the calculated and measured values is convincing: in Surry 2, a mea- sured material removal of max. 11.5 nun is to be com- pared with a calculated thinning of 7.7 mm; in the case of Trojan, thinning by 5 nun in 90 ° elbows was calcu- lated, as compared to measured thinning values which, in keeping with the nominal tolerances, may lie between 3.2 and 7.1 ram, while in straight piping 1.3 mm thin-
ning was calculated and between 0.6 and 4.4 mm thin- ning was measured.
5. Weak point analysis
As mentioned earlier, the WATHEC program was developed for the determination of the material removal to be expected in a power plant component or a piping system and /or to predict the remaining life expectancy of a component under the given loading conditions. This application is best pursued as part of a "weak point analysis", the sequence of which is as follows:
On the basis of the data available at the plant, those plant systems are first selected which lie within the crucial range of relevant parameters within which ero- sion corrosion on a significant scale is possible at all. Within these systems, the points comparatively most susceptible to damage are identified on the basis of the pertaining geometry and flow conditions. The material loss to be expected at these potential weak points is then calculated with the aid of the prediction model. The results of these calculations are subjected to a critical analysis including, for instance, an appraisal on the basis of the measurement data available in the data base.
L . . . . . . .
I . . . .
I Fig. 11. Erosion corrosion management program.
438 W. Kastner et al. / Calculation code for erosion corrosion reduced wall throning
The advantage of this procedure is clear: the scope of inspection and non-destructive examination, and of remedial work in response to the findings thereof, can be significantly reduced and concentrated on the plant components most susceptible to erosion corrosion.
Fig. 11 gives an impression of potential actions to combat erosion corrosion. The present work discusses only the use of ultrasonic techniques to measure compo- nent wall thickness. The results of these measurements are important to the verification of the calculation output generated by the W A T H E C program. In this context it should be mentioned that the accuracy of the input (plant operating) data is important to the reliabil- ity of the calculation results. Due to the large number of measurement data obtained from ultrasonic examina- tions, a " D a t a Management System" compatible with W A T H E C has recently been developed to store and process these data for comparison with the results of calculation.
6. Conclusions
In response to the events in Surry 2 and Trojan, analyses using the predic t ion mode l descr ibed (WATHEC) were performed in a number of nuclear power plants and the results thereof verified by means of non-destructive examination. The findings confirmed both the reliability of the computer program and also the low susceptibility of the piping systems in plants built by S i e m e n s / K W U to erosion corrosion. This low susceptibility can be attributed to the continuous feed- back gained from design, construction and operation: at no point were significant material losses detected.
The prediction model is of broader benefit for two reasons: particularly in plants in which the crucial aspects identified have not been fully taken into account, for instance in terms of flow conditions or of pH levels, the model can give an indication of the current compo- nent condition. Application of the model is also particu- larly interesting whether a thought is being given to reducing measurement and monitoring effort and to extending the life expectancy of components of whole piping systems.
References
[1] H.-G. Heitmann, Praxis der Kraftwerk-Chemie (Vulkan- Verlag, Essen, 1986).
[2] W. Kastner, K. Riedle, H. Tratz, Experimentelle Un-
tersuchungen zum Materialabtrag durch Erosionskorro- sion, VGB-Kraftwerkstechnik 64, 5 (1984) 452-465.
[3] G. Faber, A. H~usermann, R. Svoboda, Ten years of experience with erosion corrosion in a PWR and a BWR power plant, ASME Joint Power Generation Conference '82, 8 2 - J ~ - P w r - 4 6 (1982).
[4] J. Tavast, Flamespraying combats erosion-corrosion in wet steam, Nucl. Engrg. Int. (March 1988) 44.
[5] H. Keller, Erosionskorrosion an Nassdampfturbinen, VGB-Kraftwerkstechnik 54, 5 (1974) 292-295.
[6] W. Kastner, K. Riedle, Empirisches Modell zur Berechnung yon Materialabtr~gen durch Erosionskorro- sion, VGB-Kraftwerkstechnik 66, 12 (1986) 1171-1178.
[7] W. Albring, Elementarvorg~lnge fluider Wirbelbewegun- gen (Akademieverlag, Berlin, 1981).
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[12] J. Ducreux, The influence of flow velocity on the corro- sion-erosion of carbon steel in pressurized water, Proc. 3rd BNES, London, 1983, pp. 227-233.
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[14] G.J. Bignold, C.H. de Whalley, K. Garbett, I.S. Woolsey, Mechanistic aspects of erosion-corrosion under boiler feedwater conditions, Proc. 3rd BNES, London, 1983, pp. 219-226.
[15] I.S. Woolsey, G.J. Bignold, C.H. de Whalley, K. Garbett, The Influence of oxygen and hydrazine on the erosion- corrosion behaviour and electrochemical potentials of carbon steel under boiler feedwater conditions, Proc. 4th BNES, Vol. 1, London, 1986, pp. 337-346.
[16] L.E. Sanchez-Caldera, The mechanism of corrosion-ero- sion in steam extraction fines of power stations, Ph.D. Thesis, MIT (1984).
[17] R.G. Keek, Prediction and mitigation of erosive-corrosive wear in steam extraction piping systems, Ph.D. Thesis, MIT (1987).
[18] B. Cfiexal, J. Horowitz, Flow assisted corrosion in carbon steel piping - parameters and influences, presented at 4th Syrup. on Env. Deg. of Mat. in Nucl. Power Systems - Water Reactors, Jekyll Island, Georgia, USA, Aug. 1989.