Research on Flow Accelerated Corrosion in Carbon Steel Piping

Yoichi UTANOHARA*, Michio MURASE* and Akira NAKAMURA* * Institute of Nuclear Safety System, Inc. 64 Sata, Mihama-cho, Mikata-gun, Fukui, 919-1205, Japan E-mail: utanohara[at]inss.co.jp

Abstract

  Flow accelerated corrosion (FAC) is an important issue for aging fossil and nuclear power plants. FAC causes thinning of pipe walls which occasionally leads to a piping rupture accident. Generally, influencing factors are temperature, pH, dissolved oxygen, material composition and fluid dynamics factor⁽¹⁾. The process of FAC is as follows: iron ions of carbon steel dissolve into the concentration boundary layer near the wall and some of them are transferred to bulk fluid. Turbulent flow accelerates the transport of iron ion to the bulk fluid and then the dissolution of carbon steel is also accelerated. FAC occurs near the pipe geometry where flow is strongly disturbed such as downstream from an orifice and elbow. It is said that the direct fluid dynamics factor is the mass transfer coefficient near the wall and recently the relation between FAC and local flow field has been focused^(2)-(5). The key point of the FAC study from the viewpoint of fluid dynamics factor is how to treat the local mass transfer coefficient near the wall.

  The authors also have studied the influence of local flow field on FAC downstream from an orifice experimentally and numerically⁽⁶⁾⁽⁷⁾. The evaluation parameter employed in the study is the wall shear stress (WSS) based on the analogy between momentum and mass transfer on the wall surface.

  First, the FAC thinning rate downstream from an orifice was measured using a high-temperature water test loop as shown in Fig. 1. The inner diameter of the test section D was 50 mm and the orifice diameter was 24.3 mm. The FAC rate was measured by the electric resistance method using corrosion sensors made of carbon steel plate (STPT 42: Ni, 0.02 wt. %; Cr, 0.04 wt. %; Mo, 0.01 wt. %). Figure 2 shows the distribution of the FAC thinning rate downstream from the orifice. The qualitative tendency of the distribution was the same for different flow velocities; the maximum FAC rate appeared at 1D or 2D, and gradually decreased downstream at 3D and 4D. FAC rate increased as flow velocity increased, particularly from 1D to 3D.

  Flow field downstream from the orifice was also measured by laser Doppler velocimetry (LDV). Figures 3 shows axial velocity profiles obtained at 1 mm from the wall along the axial length x from the orifice. The profiles did not depend on flow velocity when normalized by mean cross sectional velocity. Flow was reversed in the separation region and the reattachment point was presumed to be around 2.5D downstream. Flow velocity reached the maximum at around 1D.

  Large eddy simulation (LES) of orifice flow was carried out and the predicted velocity profiles are shown in the Fig. 3. LES predicted well the flow field downstream near the wall. Hence, wall shear stress was also expected to be predicted well by LES. The vortex structure downstream from the orifice is shown in Fig. 4. The vortex rings were shed from the edge of the orifice periodically and broke into complicated vortices over 0.5D.

  Figure 5 shows the relationship between FAC thinning rate and the wall shear stress. Here the wall shear stress τ_w,RMS was derived by the root mean square (RMS) of the instantaneous value τ_w, not the RMS of the fluctuation value. This value involves both the time-averaged and fluctuated values and has nature of the cumulative value worked on the wall surface. As shown in the figure, there is a clear relationship between FAC thinning rate and the RMS of wall shear stress τ_w,RMS. This result indicated that FAC thinning rate can be described as a function of the wall shear stress.

 

Keywords

Flow accelerated corrosion, Pipe wall thinning, Pipe flow, Orifice, Wall shear stress, Mass transfer coefficient

Figures

Fig. 1 FAC test loop

Fig. 2 Distribution of FAC thinning rate downstream from the orifice for each flow velocity⁽⁷⁾

Fig. 3 Axial velocity distribution near the wall measured by LDV⁽⁶⁾

Fig. 4 Vortex structure downstream from the orifice simulated by LES

Fig. 5 Relationship between FAC rate and wall shear stress downstream from the orifice⁽⁷⁾

 

References

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