Experimental investigation of flow accelerated corrosion under two-phase flow conditions

The main objective of this paper is to experimentally study the effect of two-phase flow on flow-accelerated corrosion (FAC) downstream an orifice. FAC is a major safety and reliability issue affecting carbon-steel piping in nuclear and fossil power plants. This is because of its pipe wall wearing and thinning effects that could lead to sudden and sometimes catastrophic failures, as well as a huge economic loss. In the present study, FAC wear of carbon-steel piping was simulated experimentally by circulating air-water mixtures through hydrocal (CaSO₄·1/2H₂O) test sections at liquid superficial Reynolds number, Re = 20,000, and different air mass flow rates. Experiments were performed for a test section with different orifice to pipe diameter ratios (d_o/D = 0.25, 0.5 and 0.74). The observed flow patterns were compared with the available flow pattern maps. Surface wear patterns downstream the orifices were also analyzed. The maximum FAC wear was found to occur at approximately 2-5 pipe diameters downstream of the orifice. The obtained results were found to be consistent with those from a single-phase flow study reported earlier. Moreover, FAC was found to depend on the relative values of the mixture mass quality and the volumetric void fraction. Lower values of FAC wear rate were obtained for higher values of mass quality. A modified correlation is developed in order to predict FAC wear rate downstream of the pipe-restricting orifice with an average RMS accuracy of ±10%. However, the location of maximum wear rate is well predicted. The current study is considered as an integrated effort to develop guidelines to FAC engineers in power plants in order to prepare more reliable plant inspection scope.