Thin layer activation technique for on line monitoring of the flow accelerated corrosion in a bend pipe and comparison with CFD modelling

摘要

Flow Accelerated Corrosion [FAC] of carbon steel in water has been a concern in Nuclear Power Plants for over 40 years.The degradation of carbon steel piping material due to flow accelerated corrosion(FAC)is one of the major problems in thermal and nuclear power plants.FAC impacts plant operation and maintenance significantly.The important feature of FAC are its linear variation in wall thinning Conventional methods such as gravimetry,ultrasonic methods are difficult to use for measuring the wall thinning as FAC is a very slow process.Hence,alternative method of measuring wall thinning was attempted.Pipe geometry is one of the important parameters which plays a key role in FAC.Thin layer activation(TLA)is a novel measurement technique,that can be employed for monitoring the thickness loss.Hence,the use of TLA technique was attempted for assessing the role of geometry on FAC.An experiment was conducted in simulation(SIM)loop with the thin layer activated specimen,in single phase at 7 m/s velocity and under neutral pH condition.The 15 NB carbon steel(ASTM A 106 Grade ?B')pipe bend with 4D bend radius and 58o bend angle,was exposed to neutral water in the SIMLOOP which was operated at 120℃.The variation in activity was monitored as a function of time and the rate of FAC measured.The specimen activity was measured using NaI(Tl)detector.The material loss from pipe inner surface due to FAC is proportional to the loss of cobalt-56 activities from the surface.The observed rate of FAC obtained by TLA method was compared by the rate obtained by measuring the initial and final wall thickness using ultrasonic thickness gauge.In addition,the FAC rate was obtained by carrying out computer modeling of the flow dynamics prevailing in the same bend pipe.The turbulence patterns in the bend were mapped by CFD(Computational Fluid Dynamics)at various locations and the rate of FAC was theoretically calculated and then compared with the corresponding corrosion rate obtained experimentally using the TLA technique.Good correlation was observed between thickness loss obtained by TLA and that of the ultrasonic thickness measurement and by CFD computation.This demonstrated the reliability of TLA in the measurement of rate of FAC.