Flow-accelerated corrosion rate and residual life time estimation for the components of pipeline systems at nuclear power plants based on control data*

As of today, large volumes of data related to non-destructive operational control are accumulated on NPPs. For ensuring safe operation of power units, optimization of scope and scheduling operational control it is necessary to continue development of guidance documents, software products, methodological guidance and operational documentation (Baranenko et al. 1998, Gulina et al. 2013, Recommendation (NSAC-202L-R4) 2013). Approaches are examined to assessment of the rate of erosion-corrosion wear (flow-accelerated corrosion FAC) according to the data of operational control. The present study was performed based on the data of thickness gauging of different elements of pipelines of NPPs with different types of reactor. Further development of ideas exposed in (Baranenko et al. 2016) allowed revealing specific features of ECW processes on straight sections, bends and in the zones adjacent to weld joints of pipelines of NPPs equipped with VVER and RBMK reactors. Presence of the process of deposition of corrosion products on internal surfaces of pipeline walls results in the fact that residual lifetime of elements nominally increases due to deposition. However, real wall thickness under the layer of deposits is unknown just as the initial wall thickness is unknown as well. Investigation implemented in the present study is aimed at the substantiation of the methodology of calculation of FAC rate according to the data of operational control for the purpose of drawing calculation results closer to the reality keeping conservatism. Uniform approach to the assessment of FAC rate in the examined elements of pipelines was developed. Methodologies for evaluation of correction coefficients taking into account dimensional technological tolerances, special features of geometry of the element, as well as effect of deposits on the results of thickness measurements were suggested based on the data of operational control and industry standards. The implemented studies demonstrated efficiency of the developed procedures for pipeline welding zones. Analysis of known and newly developed procedures was performed for bends and ranking of these procedures according to the criterion of “conservatism of evaluation of residual lifetime” was executed. Introduction of correction coefficients allows enhancing conservatism of calculations of lifetime characteristics as compared with calculations performed on the basis of nominal values of thicknesses; the result depends on the type and dimensions of the element, its geometry, as well as on the type of reactor. Copyright Valery I. Baranenko et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Nuclear Energy and Technology 4(1): 35–42 DOI 10.3897/nucet.4.29850


Introduction
Substantiation of the methodology for calculation of FAC rate for NPP piping elements manufactured of carbon steel requires high-quality analysis of data of operational control and taking into account significant number of factors (Baranenko et al. 2010.Introduction of additional correction coefficients taking into account the technology used in manufacturing the pipelines and effects of deposits on the values of initial and minimal permissible thicknesses (Baranenko et al. 2016) results in the significant change of estimated FAC rate and the residual lifetime which ensures certain conservatism of calculations and provides certain time margin for implementation of the next control.
Methodology for estimation of FAC rate presented in (Baranenko et al. 2016) is applicable for calculating straight pipeline sections: where S nom is the nominal wall thickness; K 11 is the coefficient taking into account positive tolerance for wall thickness during manufacturing the pipeline; K 12 is the coefficient taking into account contribution of corrosion products depositions in the nominal wall thickness; K 2 is the coefficient taking into account corrosion products depositions to value of measured minimal wall thickness (S min ); K safe is the safety factor; Δτ 0 is the duration of operation of the element before the date of control.Geometrical features of such elements as bends and welding zones (Baranenko et al. 2010, Baranenko et al. 2009) will probably require correction of the methodology in question and accounting of real thicknesses of these elements.
The purpose of the present study is the development of methodologies for evaluation of FAC rate taking into account geometrical features of such element as bends and welding zones.

Taking into account specific features of geometry of pipeline elements in the calculation of FAC rate
Determination of correction coefficient for bends.Value of thickness measured during pre-start control must be substituted in formula (1) in the calculations of FAC rates for extended elbow bends as the nominal thickness S nom , otherwise the following dependence (Nakhalov 1983, ОSТ 24.321.26-74 1975) must be applied for calculation of thickness of extended elbow bend sections S bend : where R is the radius of pipeline bend, mm; D is the pipeline outer diameter, mm; S bend is the thickness of extended part of the bend, mm.
The following coefficient is introduced using dependence (2): (3) Dependence (3) takes into account that maximum thinning can be found on the section located beyond the extended part of the bend.
Determination of correction coefficient for welding zones.Large numbers of welding zones -WZ -are present on NPP power units.Quantities of fittings installed on power units of NPPs with VVER-440, VVER-1000, RBMK-1000 and RBMK-1500 reactors are presented in Table 1.There are two weld joints (WJ) made on each element and, correspondingly, there are two zones adjacent to weld joints on each of them.Besides fitting elements significant number of WZ is found on pipeline systems.Cases of washing out of WZ of NPP pipelines because of FAC were registered in the practice of operation of NPP power units (Korhonen and Hietanen 1994).On straight sections of pipelines and on WZ with outer diameter less than 108 mm initial thickness is similar to nominal thickness.Technological boreholes creating conditions for high-quality mating of joints during welding are drilled on WZ with outer diameter over 108 mm in accordance with industry standard OST 24.125.31-89.Drilling bores is performed on the input and output WZ on the section with length equal to 50 mm (ОSТ 24.125.30-89 1989).
Thickness S k determining the wall thickness in the zone of weld joint along the length of 50 mm has on the average the value smaller than the thickness of the bore S r * by approximately 1.3 times.In the process of operational control thickness S k can be taken as the minimum permissible value.
Values of S k in the zone of boreholes for WZ of different types and dimensions, nominal thicknesses and boreholes S r * calculated according to diameters of boreholes (ОSТ 24.125.30-89 1989) are presented in Table 2. Thickness of the borehole S r * must be accepted as the initial value for WZ with outer diameters ≥ 108 mm.Measurements of wall thicknesses of WZ are performed on the cross-sections located at the distance of less than 50 mm from the weld joint.For pressure equal to 3.92, 5.89, 8.44, 11.77 MPa average value of the ratio S r /S nom is equal to 0.9.Thus, the value equal to 0.9S nom can be used as the nominal or initial thickness.For determining coefficients K 12 and K 2 it is necessary to know average values of wall thicknesses of pipelines which can be calculated on the basis of data of operational control.Calculations of average thicknesses are performed for pipelines minimum thicknesses of which are smaller than the initial values.Average wall thicknesses are determined according to the following dependence: where S i is the wall thickness in the i-th measurement point; n meas is the number of measurement points.The following is determined in the calculation of coefficients K 12 and K 2 : -Difference between the average and initial (nominal) thicknesses: (if S av < S init , difference between the values will be negative and value of K 12 will be larger than unity); Calculation of FAC rate and residual lifetime.Corrosion rate aggravated by the flow W FAC is calculated according to equation ( 1), nominal thicknesses are replaced with corresponding values of S bend for bends and 0.9S nom for welding zones.Residual lifetime of pipeline element is calculated according to the following equation: where S perm is the minimum permissible wall thickness of the element (RD EO 1. 1.2.11.0571-20152012, Case of ASME Requirements 2003).

Calculation of corrosion rate and residual lifetime of welding zones according to the data of operational control and taking into account correction coefficients
Values of wall thicknesses in circular points (Fig. 1) for 16 inlet and outlet WZ of pressure pipelines of feed electric pump and emergency feed pumps, minimum, average and maximum thicknesses, number of measurements with less than nominal thicknesses and thicknesses of boreholes of WZ on the pipeline with standard size equal to 426×24 mm are presented in Tables 3 and 4. Measurements of thicknesses were performed in eight points along the WZ circumference.
Measurements in axial direction were performed in one cross-section with step equal to 45° (corresponding to 12:00, 1:30, 3:00, 4:30, 6:00, 7:30, 9:00 и 10:30 "on a clock dial").In accordance with Fig. 1 measurements are performed in the zones with wall thicknesses determined by the diameter of the borehole.In the present case thickness of the borehole must be equal to 21.6 mm.Results of measurements on inlet sections of WZ are presented in Table 3, those for outlet sections are presented in Table 4.
It follows from Tables 3 and 4 that the number of measurements with values of thicknesses less than 24 mm amounts on the WZ inlet and outlet sections to 78.1 and 73.4%, respectively.Number of measurements (on the borehole section) with values of thicknesses less than 21.6 mm amounts for the WZ inlet and outlet sections to 18.8 and 17.2%, respectively.
Results of calculations performed using formulas ( 5) -( 8) are presented in Tables 5 and 6.Calculations of corrosion rates are given for welding zones for which the values of minimal thicknesses have values less than the initial value of the borehole thickness equal to 21.6 mm.Factor K 11 was taken in the calculations to be equal to 1.075, time of operation before measurements -24 years, minimum permissible wall thickness S perm = 18.5 mm.
Estimated value of residual lifetime obtained using the formula for the nominal thickness S nom (W FAC1 and Т 1 ) is by 1.8 times less than the value obtained taking into account the borehole (W FAC2 and Т 2 ).
Use of borehole thickness instead of S nom results in the estimated value of residual lifetime larger by approximately 1.9 times, i.e. similarly to the picture for WZ inlet.Values of FAC rate and residual lifetime for WZ inlet without correction coefficients are presented in Table 7; borehole thickness equal to 21.6 mm was taken as the value of initial thickness.Analysis of Table 7 demonstrates that accounting of correction coefficients reduces the difference between the values of residual lifetime due to the indirect taking into account of depositions on internal surface of pipeline.Even taking into account the borehole thickness calculation produces estimations of residual lifetime which are smaller by approximately eight times for WZ inlet and by 5.8 times for outlet.Therefore, calculation methodology with introduction of correction coefficients and using the borehole thickness as the initial thickness is certainly recommended for WZ.
Bends.For calculating FAC rate and residual lifetime of bends let us examine the data of measurements performed in 1995, 1996, 2000 and 2002 on bends 06-K and 16-K of feedwater pipelines 273×16 mm of the Dukovany NPP during implementation of operational control.Selection of the bends was predetermined by the large num-ber of measurements performed on each of the elements (from 276 to 394) (Rushchak et al. 1996).
The formula W 2 = [(S nom ×K 11 ×K 12 -S min ×K 2 )]×K safe / Dt 0 (Baranenko et al. 1998) was suggested for estimating the FAC rate for straight sections of pipelines.Corresponding values of FAC rate W 2 and residual lifetime Т 2 are given in Table 8.Average value of the ratio T 1 /T 2 = 1.5, i.e. application of correction coefficients results in the reduction of the value of estimated residual lifetime of the examined bends by approximately 1.5 times.
In accordance with formula (2) thickness of extended part of the bend is equal to where R is the bend radius.For the examined bends (90°) R = 1370 mm.Then S bend = 14.5 mm and the value of factor Average value of the ratio Т 3 /Т 2 = 0.5, i.e. calculation of FAC rate according to the formula presented in the new edition of the regulatory document (RD) (RD EO 1. 1.2.11.0571-2015 2012) results in the reduction of estimated value of residual lifetime by two times relative to the formula for straight sections with correction coefficients.
Formula for calculating FAC rate taking into account the bend geometry W 4 is similar to formula (1) with corresponding replacement of S nom with S bend : Estimated value of residual lifetime for the described case (Т 4 ) is by 3.5 times larger than that obtained using formula (1) for straight sections (Table 10).
Thus, residual lifetime calculated according to RD is by approximately tree times less than that calculated using the formula for bends.
Summary of results for bends are presented in Table 11.
As the final result the most conservative estimation was obtained using the formula taken from RD (W 3 ) and the most optimistic estimation was obtained with introduction of correction coefficients for bends (W 4 ).Reasonable conservatism was demonstrated in this case with introduction of correction coefficients for bends similar to those for straight sections (W 2 ), i.e. without taking the geometry into account.However, it is difficult enough to substantiate this result and, therefore, the whole set of results must be used in order to make decision on the implementation of the next control when already expired time gets close to the minimum among the calculated values of time.

Conclusion
1. Methodologies were developed for calculating the rates of flow-accelerated corrosion and residual lifetime for bends and welding zones with introduction of correction coefficients taking into account the effects on the corrosion process produced by the manufacturing technology, as well as influence of depositions of corrosion products.
2. Reasonable conservatism of the calculations is ensured by introduction of correction coefficients and geometry of the elements.It was demonstrated that calculated results obtained taking these correction factors into account are not overly pessimistic.
3. Values of correction coefficients introduced in the calculation dependences are determined on the basis of processing the data of operational control.Calculation of FAC rate and residual lifetime of the WZ are performed using nominal thickness and borehole thickness as the initial value of thickness.Use of borehole thickness instead of nominal thickness results in the estimation of residual lifetime which is by approximately 1.9 times larger both for WZ inlet and outlet of pressure pipelines of feed electric pump and emergency feed pumps with size type 426×24 mm.
4. Calculation without introduction of correction coefficients for the borehole thickness produce estimated values of residual lifetime which are smaller by approximately eight times for WZ inlet and by 5.8 times for outlet.Therefore, calculation methodology with introduction of correction coefficients and application of borehole thickness as the initial thickness is certainly recommended for welding zones.
5. Data of measurements performed on bends of feedwater pipelines 06-K and 16-K 273×16 mm of the Duko-  in 1995, 1996, 2000 and 2002 are examined for calculation of FAC rate and residual lifetime.The following four formulas for calculating FAC rate were examined: for straight sections without corrections, for straight sections with introduction of correction factors, with introduction of correction factors for bends and the formula recom-mended in RD (RD EO 1.1.2.11.0571-20152012).Ambiguousness of the conclusions thus drawn requires application of the whole set of available results for making the decision on the implementation of the next control when already expired time gets close to the minimum among the calculated values of time.

Figure 1 .
Figure 1.Layout of measurements of wall thicknesses of ZAWJ in eight points along the circumference: 1 -inlet zone; 2 -outlet zone.

Table 2 .
Thicknesses of boreholes and nominal thicknesses, ratios of thicknesses S r * /S nom and S k /S r * for control pressure and temperature.p = 3.92 MPa, Т = 200°C p = 5.89 MPa, Т = 275°C

Table 3 .
Wall thicknesses in circular points of WZ inlet , minimum, average and maximum thicknesses, number of measurements with values of thickness less than S nom and S r *.

Table 4 .
Wall thicknesses in circular points of WZ outlet, minimum, average and maximum thicknesses, number of measurements with values of thickness less than S nom and S r *.

Table 5 .
Values characterizing residual lifetime of WZ inlet of pressure pipelines of feed electric pump and emergency feed pumps.

Table 6 .
Values characterizing residual lifetime of WZ outlet of pressure pipelines of feed electric pump and emergency feed pumps 426×24 mm.

Table 7 .
FAC rate and residual lifetime for WZ inlet (W FAC11 ) and outlet (W FAC21 ) without correction coefficients.

Table 8 .
FAC rate and residual lifetime for bends without taking (9) into consideration and with correction coefficients (1).

Table 9 .
Estimated values of ECW rate and remaining lifespan for bends obtained according to formula (10).

Table 11 .
Residual lifetime for bends (summary of results).

Table 10 .
Estimated values of FAC rate and residual lifetime (W 4 and Т 4 ) for bends obtained according to formula (1) taking into account the bend geometry.