Akira Maekawa

Abstract This paper proposes an improved contactless measurement method for vibration stress of piping systems, by which the measurement time is shorter and the measuring works are more simple. The proposed method includes two processes, in which the bending mode shape of piping vibration is identified by a transmission-type light emitting diode (LED) displacement sensor and the vibration stress is calculated based on beam theory using the approximated curve of the bending mode shape. The proposed method uses one advanced LED displacement sensor to measure the vibration stress in contrast to multiple conventional LED sensors, which must be used in the previous method developed by the authors. Therefore, the measuring system could be reduced in size and a light-weight and portable measurement instrument was developed. The measurement accuracy and reliability of the new method were validated by the vibration experiment using a mockup piping system.

This study presented a contactless method to measure vibration-induced stress (vibration stress) using multiple laser displacement sensors and the applicability of the method was investigated using beam theory and vibration test. First, the measurement errors were discussed based on the beam theory when the method was applied to the measurement of piping vibration. The classification of the errors caused in the measurement was clarified and the measurement accuracy was estimated by taking these errors into account. Next, the vibration test by the random excitation using the beam-plate was conducted and the applicability and measurement accuracy of the presented method was examined. By comparing with the vibration stress values measured by the conventional method using strain gauges, the presented method was demonstrated to be applicable to the vibration stress measurement. Additionally the awaiting solution for the practical use was extracted.

Accurate evaluation of occurrence location and amplitude of pressure pulsations in a piping system can lead to efficient plant maintenance to prevent fatigue failure of piping and components because the pulsations could be main causes of vibration fatigue and acoustic noise in piping. To measure pressure pulsations easily and directly, a method to combine strain measurement on outer surface of pipe with the formula for thick-walled cylinders was proposed. The experimental validation demonstrated the proposed method could measure amplitudes and behavior of pressure pulsations with a practical accuracy. Furthermore, the applicability of the formula for thin-walled cylinders was examined for various shaped pipes.

Residual stress caused by welding process affects characteristics of strength and fracture of equipment and piping in nuclear power plants. Numerical analysis is powerful tool to evaluate weld residual stress in the actual plants. However, the three-dimensional precise analysis requires enormous computation time. In this study, the finite element analysis code based on iterative substructure method developed to speed up welding simulation was proposed to simulate welding process of the plant equipment and piping such as multi-pass welding, machining and post weld heat treatment. Furthermore, the proposed analysis code was validated by measurements and other analysis results.

Stress-free lattice spacing d₀ has the most influence on reliability of neutron stress measurements made using an angle dispersive method. However, it is hard to evaluate the lattice spacing of welded structures and ductile materials such as stainless steel accurately. In this study, suitable measurement conditions for d₀ of welded pipe joints of austenitic stainless steel were discussed. The d₀ values derived from {311} and {111} reflections, which are often used in austenitic stainless steel for residual stress measurement, were examined. Comparison of the residual strains and stresses evaluated using the obtained d₀ and the finite element analysis showed that the way the d₀ values were chosen affected the measurement accuracy significantly. The stress measurement accuracy was remarkably improved when the {311} reflection was used and the proper d₀ value was chosen in the respective neutron diffraction measurements. For instance, for the axial diffraction measurements using the {311} reflection, it was recommended that only the axial d₀ value of the {311} reflection be used; the measurements using the {111} reflection were less accurate due to the large Young's modulus. Additionally, a lower diffraction angle was judged to be one of the factors leading to a decrease of the strain measurement accuracy.

In thermal and nuclear power plants where many piping are connected to vibratory sources like pumps, it is important to estimate vibrational stress efficiently to keep the plant operating safety. This study proposed a method using multiple laser displacement sensors to measure vibrational stress directly in contactless. Furthermore, the applicability of the method was verified by vibration tests using sinusoidal and random waves. The vibrational stresses measured by the proposed method were compared with the result obtained from conventional method using strain gauges for the vibration tests. These results demonstrated that the proposed method could have sufficient measurement accuracy for practical use.

Stress-free lattice spacing d₀ has the most influence on reliability of neutron stress measurements made using an angle dispersive method. However, it is hard to evaluate the lattice spacing of welded structures and ductile materials such as stainless steel accurately. In this study, suitable measurement conditions for d₀ of welded pipe joints of austenitic stainless steel were discussed. The d₀ values derived from {311} and {111} reflections, which are often used in austenitic stainless steel for residual stress measurement, were examined. Comparison of the residual strains and stresses evaluated using the obtained d₀ and the finite element analysis showed that the way the d₀ values were chosen affected the measurement accuracy significantly. The stress measurement accuracy was remarkably improved when the {311} reflection was used and the proper d₀ value was chosen in the respective neutron diffraction measurements. For instance, for the axial diffraction measurements using the {311} reflection, it was recommended that only the axial d₀ value of the {311} reflection be used; the measurements using the {111} reflection were less accurate due to the large Young's modulus. Additionally, a lower diffraction angle was judged to be one of the factors leading to a decrease of the strain measurement accuracy.

In this study, the influence of valve closing and opening operations to change inner pressure during pump operations on the pulsation boundary condition was investigated to improve the prediction accuracy of pressure pulsation. Vibration experiments were conducted by using an actual size mock-up piping system with 40 m long as found in nuclear power facilities. A drastic change in the boundary condition of the acoustic resonance by a slightly different valve opening ratio to set a different inner pressure was seen in the experimental results. The simulation results by using the method of characteristics showed that the boundary condition of the acoustic resonance changed when the valve opening ratio was changed slightly from 10 % to 15 %. This indicated that the boundary condition of the acoustic resonance had a pulsed change and changed even at the small valve opening ratio, which was approximately 15% though the design threshold value was 25 %. It was concluded that the boundary condition of the acoustic resonance was sensitive to a slight change of the valve opening ratio and this was one of the reasons why it is difficult to predict the pressure pulsation behavior including acoustic resonance accurately in actual plants.

This paper describes a nonlinear behavior of oval-type vibration in a cylindrical liquid storage tank which is high order vibration mode in the circumferential direction of the wall (axial half wave number m ≥ 1 and circumferential wave number n ≥ 2). When the vibration experiment of a cylindrical liquid storage tank was conducted by exciting its base, a nonlinear behavior of oval-type vibration occurred. At large excitation, oval-type vibration with sub-harmonics of order one-half occurred as well as that with the fundamental harmonics. Frequency analysis showed the oval-type vibration with sub-harmonics was dominant. The result demonstrated the nonlinearity of oval-type vibration. The nonlinear behavior of oval-type vibration was dynamically simulated by nonlinear explicit finite element analysis method considering fluid-structure interaction as well as large deformation. The numerical analysis results showed the same oval-type vibrations with fundamental harmonics and sub-harmonics of order one-half occurred as those in the experiment, that is, analytical vibration modes coincided with the experimental modes. It can be concluded the proposed numerical analysis method can simulate the nonlinear behavior of oval-type vibration accurately.

This study describes residual stress distribution of large-bore and heavy-walled stainless steel pipe with butt-welded joint as measured by inherent strain method with distribution function in which the inherent strain distribution is represented as function. The deviation of the most probable residual stress value obtained by the method was enough small to indicate accurate measurement. The detail feature and 3D-distribution of the residual stress generated within the heavy-walled pipe with welded joint were discussed on two variously-sized pipe joints. It was found that the residual stress distribution had more complicated shape as the bore and thickness of the pipe was larger.