EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF FLOW-ACCELERATED CORROSION DOWNSTREAM ORIFICES
Flow-Accelerated Corrosion (FAC) is a form of corrosion that affects carbon steel or low-alloy steel piping and fittings in power plants. Piping degradation due to FAC, especially downstream of control valves and restricting orifices, is considered to be one of the major safety and reliability problems facing ageing power plants, where piping rupture occurs in high pressure systems. Accurate prediction of the highest FAC wear rate locations enables the mitigation of sudden and catastrophic failures, and the improvement of the plant capacity factor. The objective of the present study is to evaluate the effect of the local flow and mass transfer parameters on flow accelerated corrosion downstream of orifices. Orifice to pipe diameter ratios of 0.25, 0.5 and 0.74 were investigated numerically, under single phase flow conditions, by solving the continuity and momentum equations at Reynolds number of Re = 20,000. Laboratory experiments, using test sections made of hydrocal (CaSO4.½H2O) were carried out under both single and two phase flow conditions, in order to determine the surface wear pattern and validate the numerical results. The maximum mass transfer coefficient found to occur at approximately 1- 4 pipe diameters downstream of the orifice. This location was also found to correspond to the location of elevated turbulent kinetic energy generated within the flow separation vortices downstream of the orifice. The FAC wear rates were correlated with the turbulence kinetic energy and wall mass transfer in terms of Sherwood number. The current study provides FAC engineers in power plants with very useful information for better preparation of plant inspection scope.
Corrosion is the degradation of materials by means of chemical or electrochemical reactions, occurring at the material surface, with the environment. The corrosion pattern can be categorized into either uniform corrosion or localized corrosion, depending on the resulting surface morphology. The first type involves uniform material loss from the surface while the second occurs at local points or areas on the surface.
Many if not most cases of corrosion processes involve some relative motion between the corroding material surface and the environment. The rate of such motion and the type of corrosion mechanism depend on the exact nature of the environment (air, soil, water, etc.) in which the corrosion takes place. Corrosion due to electrochemical reaction involves mass transfer from the corroding material to the environment, which is usually a solution. An example of this type of relative motion is seen in the case of carbon steel corrosion in an aqueous environment, in which ferrous ions (Fe2+) are released into water at the (concentration) boundary layer where their concentrations become supersaturated.
The rate of release of the ferrous ions is increased if the aqueous environment is flowing.
This type of corrosion mechanism is known as Flow-Accelerated Corrosion (FAC).
1.2 Flow-Accelerated Corrosion
FAC is a form of corrosion that results in general reduction of wall thickness of carbon steel or low-alloy steel piping or fittings, by flowing water or wet-steam mixture over the surface. FAC frequently occurs over a limited area within a piping system where local high turbulence exists. The wall thinning occurs due to the mass transfer of dissolved corrosion products near the inner wall into the bulk of flowing fluid. This explains the two major processes responsible for FAC. The first process involves chemical reaction, which occurs at the metal surface, while the physical removal of dissolved corrosion products to the bulk fluid takes place through a flow dynamics process as explained by Uchida et al. . They also explained that the chemical process is initiated by the existence of electrochemical potential difference between the metal surface and the bulk fluid, which leads to the formation of the normally protective magnetite (Fe3O4) film over the inner surface of the pipe. Then, the existence of concentration gradient between the protective film and the bulk fluid initiates the dissolution process of the film into the bulk fluid. This dissolved magnetite is carried away by the flow dynamics process, which creates potential for more magnetite formation followed by more dissolution, until equilibrium film thickness is attained where the dissolution rate equals the convection rate.
This repeated process of magnetite dissolution/convection results in pipe wall thinning. Furthermore, the fluid hydrodynamics parameters, and whether the flow is single-phase or two-phase, have a great impact on the FAC process and consequently the wall thinning rate. Therefore, the work performed for FAC has been classified into single-phase FAC and FAC under two-phase flow conditions. Single-phase FAC occurs when water flows (the velocity is not zero) in carbon steel or low-alloy steel piping/fitting at a temperature greater than 95oC. On the other hand, the two-phase FAC additionally depends on the steam quality, (Okada et al., )
In the case of two-phase flow FAC, the flowing fluid is wet steam, and the flow-induced corrosion process is termed FAC when the steam quality is low. However, for high mass quality two-phase flow, the process is referred to as liquid droplet impingement (LDI) corrosion (an FAC process), only when the liquid droplets in the steam impinges the pipe wall at a velocity less than 100 m/s. For an impingement velocity greater than 200 m/s, the process is called liquid droplet impingement erosion (a mechanical process). This classification is based on the fact that liquid droplet impingement velocities less than 100 m/s have been observed to be incapable of removing the solid corrosion products, Okada et al. .
In both single and two-phase flows, FAC is found to depend on complex interaction of several parameters such as: material composition, water chemistry, and fluid hydrodynamics. Moreover, the repeated inspections at both fossil and nuclear power plants have shown that piping components located downstream of flow singularities, such as orifices, valves, tees, etc., are most susceptible to FAC damage. Hence, the piping geometry can also be considered as an important parameter that affects FAC process.
Therefore, the local flow hydrodynamics identify the local distribution of the wall thinning, while the water chemistry and material composition identify the overall tendency of FAC occurrence.
Since, FAC is identified as a major safety and reliability problem affecting piping systems in power plants, accurate prediction of FAC wear locations enables the mitigation of sudden pipe rupture, catastrophic failures, and plant shutdown. Therefore, the aim of the present study is to investigate FAC downstream of pipe restrictions, such as orifices, using experimental and numerical methods. Two approaches were used to simulate FAC wear pattern downstream of orifices. Experiments using test section fabricated from hydrocal (CaSO4.1/2H2O) is used to simulate similar wear pattern in a shorter period of time. In addition, CFD analysis of FAC downstream an orifice at different orifice-to-pipe diameter ratios is also performed. The choice of hydrocal is based on its property as non-reactive but sparingly soluble hemihydrates, so it can mimic FAC mechanism that occurs at the metal surface and flowing water in real applications.
MOTIVATION AND OBJECTIVES
The motivation for the study is the need to predict FAC in piping downstream of an orifice in order to avoid sudden pipe rupture, catastrophic failures, and unplanned shutdown of power generation plants.
The specific objective of the study is to experimentally and numerically investigate the effect of hydrodynamic parameters on FAC downstream orifices under both single and two-phase flow conditions. Therefore, the research program is divided into two parts with the following specific objectives:
Part I) FAC under single-phase flow conditions:
In this part, both numerical and experimental methods will be used to:
1) Characterize the flow downstream of orifices of different diameters, for constant Reynolds number, in order to clearly identify parameters affecting FAC.
2) Determine the local flow structures and their relation to FAC rate.
3) Study, both experimentally and numerically, the geometry effect on FAC.
4) Validate the computational model using experiments conducted for single-phase flow downstream of an orifice. Correlations available in the literatures are also used for validation.
Part II) FAC under two-phase flow conditions
This part of the study will be investigated only experimentally in order to:
5) Characterize the two-phase flow redistribution downstream of orifices for different inlet flow regimes.
6) Identify the effect of two-phase on FAC wear rates in comparison with the single-phase flow case.
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