Research On Pulsed Eddy Current Testing Technique For Measuring Accumulated Height Of Exfoliated Oxide Scales In Austenitic Boiler Tubes

In recent years,in order to boost the power generation efficiency and develop a low-carbon economy,supercritical and ultra-supercritical units have become the main units of thermal power generation in China.As a key pressure component of power plant boilers,austenitic stainless steels which with high thermal strength and creep resistance are widely used in the boiler tubes business.Due to the poor resistance to steam oxidation of the austenitic stainless steel,boiler tubes prone to severe steam-side oxidation after long-term running at high temperature.Under the induction of the thermal stress,the oxide scale will fall off and gradually accumulates in the boiler tubes,causing tubes overheat and block,and eventually leads to unit accidents.Therefore,in order to find and replace the problem tubes in time,using non-destructive testing technology to regularly testing the thickness of the oxide scale accumulation is vital for the long-term safe operation of the unit.The magnetic testing method has poor sensitivity when there is a little amount of oxide scale accumulation,and the testing signal tends to be saturated when the amount of oxide scale accumulation is too large,so it is difficult to accurately quantify.And the conventional eddy current testing method is difficult to penetrate the tube wall and oxide scale accumulation when they are too thick,and neither solved the problem of misjudgment that causing by magnetic transformation of the austenite after long-term running at high temperature and high pressure.Exploratory research was carried out by using pulsed eddy current testing technology,and the main research works are as follows:(1)Based on ANSYS finite element simulation,aiming at quantitative testing of oxide scale accumulation in the boiler tubes,the relationship between the pulsed eddy current testing signal and the variation of the oxide scale accumulation thickness was studied.It is found that the peak value of the signal voltage increases linearly with the thickness of the accumulation,and this pattern can remains stable at any accumulation amount.(2)The influences of the wall thickness,sensor offset distance and oxide scale accumulation form on the pulse eddy current testing signal was analyzed.The simulation results showed that the peak value of the testing signal gradually decreases with the increase of the sensor offset distance,and it can be used to locating the oxide scale accumulation area in the boiler tubes;The peak value of the signal decreases linearly with the increase of the tube wall thickness,which indicates that the peak value of the sensor output signal needs to be linearly compensated when detecting boiler tubes with different specifications.A discrete element-hydrodynamic coupling simulation model for the oxide scale accumulation of the boiler tube elbows was established by EDEM and Fluent software.The location of oxide scale accumulation was determined.And the relationship between the location of oxide scale accumulation and steam flow rate,pressure and oxide scale parameters was studied.(3)A pulse eddy current testing platform was built in the laboratory,and an external sensor which consisting of a flat cable and a socket was developed.A series of experiments were carried out on the TP347 H boiler sample tube to verify the effectiveness of the testing method and its applicability under different testing conditions: The peak value of the sensor signal decreases linearly with the increases of the sensor lift-off distance,which can be use to lift-off compensation for the testing result;The unevenness of the oxide scale accumulation in the boiler tube can be distinguished by using sensor coil array,thereby can identifying those serious accumulation point which is likely to cause blockage.By pre-differentiating the testing signal,the influence of the magnetic transition of the tube wall after high-temperature services can be eliminated,and improving the testing accuracy.The research results expand the application range of the pulsed eddy current testing technology,and provide a new method for non-destructive testing of the oxide scale accumulation in boiler tubes,which preventing the boiler bursting accidents in Supercritical(ultra-supercritical)thermal power units.