AN EXPERIMENTAL AND NUMERICAL STUDY ON 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.
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