Experimental and numerical investigations of flow-accelerated corrosion downstream orifices

Abstract

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 aging 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.