Identification Of Structural Parameters In Valve Piping System And Its Dynamic Behavior Analysis During Earthquake

NPP contain plenty of valve piping systems(VPS's)which are categorized into high anti-seismic grades.At present,the research on the real dynamic behavior of VPS under seismic excitation of different frequency components and the research on the hazard of high frequency seismic excitation to valves are largely unexplored fields.Based on this,the finite element model updating method which can directly use the seismic response to identify the unknown structural parameters in the VPS is derived.Then the VPS with the DN80 gate valve installed was built and the seismic simulation tests were carried out and the finite element models which can accurately reflect the dynamic characteristics of the test structures are established.Combined with the numerical model and test results,the dynamic behavior of VPS under seismic excitation of different frequency components is discussed,the hazard of high-frequency seismic excitation to nuclear safety valves is studied,and the error of the seismic qualification results of response spectrum method and equivalent static method were quantitatively analyzed.The main conclusions of this paper are as follows:1.A two-stage finite element model updating method which can directly use seismic response to identify unknown structural parameters in nuclear power system and equipment is proposed.The finite element models which can accurately reflect the dynamic characteristics of the test structures are established by using this method,and the practical feasibility of this method is proved.It is also proved that compared with the finite element model updating method based on frequency response function residue,which has been widely used in aerospace field,the method proposed in this paper has lower dependence on the initial estimation of unknown parameters,and is more suitable to use acceleration response to identify the parameters in the structure.The identification of structural parameters by using the finite element model updating method based on vibration control equation error can avoid the measurement of modal or frequency response function.The proposal of this method provides a simple way for the application of finite element model updating method in NPP,which is of great significance to the safe operation of NPP.2.Due to the randomness in the manufacture and installation of VPS,a large number of unknown structural parameters inevitably exist in the finite element model.In the service and test process of VPS,its response under basic excitation should be collected at as many characteristic points as possible,so as to improve the utilization rate of finite element model updating method in the modeling process of VPS to improve the reliability of analysis results.At the same time,the database of the updated model of typical structure should be established to be directly used in the modeling process of similar structures.3.The lumped mass finite element model can accurately predict the seismic response of pipeline and VPS,but it can only accurately predict the response of pipeline under special excitation such as white noise and sine wave.Due to the existence of valves,the damping distribution of VPS presents an obvious non-proportional characteristic.Rayleigh damping coefficient of pipeline and VPS increases with the increase of seismic excitation amplitude.4.Under seismic excitation of different frequency components,VPS will show different seismic vulnerabilities.It is feasible to deduce the potential seismic vulnerabilities of VPS from the theoretical model under different frequencies of seismic excitation under the premise of reliable model.The VPS is more sensitive to the medium and high frequency components in the seismic than the low frequency components.The medium frequency components in the seismic has the possibility of being amplified by the valve in the horizontal direction,while the pipe has the ability to amplify the high frequency components of the excitation in the horizontal and vertical directions.The maximum response of the valve and the pipe appears in the horizontal and vertical directions,respectively.The response of the VPS under high frequency seismic excitation exhibits sudden increase in local response.Different from the valve vibrating with the pipe under low frequency seismic excitation,the hazard of high frequency seismic excitation to the nuclear safety valve lies in the fact that the valve resonates with its own natural mode at a certain natural frequency of the whole system.At this point,the top of the valve replaces the position where the valve is connected with the pipe to become the most responsive position in the valve,which will cause the drive mechanism installed on the top of the valve to suffer from severe seismic conditions.Increasing the pipe damping and valve stiffness can effectively reduce the hazard of high frequency excitation to the valve,but increasing the valve stiffness will increase the response of other parts of the system.5.When response spectrum method is used to calculate the response of VPS during earthquake,the selected damping ratio and the actual damping distribution characteristics of VPS determine the margin of response spectrum method.The calculated results of response spectrum method may underestimate the response of the low damping part of the system and overestimate the response of the high damping part of the system.When the equivalent static method is used for seismic qualification of the valve,the internal force estimation of the joint parts of the valve and pipe are insufficient,and the internal force estimation of the upper part of the valve,such as the valve neck,the vertical part of the valve body and the valve cover,has a very large margin.Considering the coupling effect of valve and pipe in the calculation process,the result of equivalent static method is closer to the actual situation.