Development of method to measure vibrational stress of small-bore piping with multiple contactless displacement sensors. Examination of applicability by vibration test and analytical approach

In a nuclear power plant, where many pipings are connected to vibration sources like pumps, it is important to estimate vibrational stress efficiently to keep the plant operation safely. In this study, the applicability of the method to measure vibrational stress by multiple contactless displacement sensors was examined by vibration tests and analytical approach. (1) It was shown that the stress measured by the proposed method agreed well with that measured by strain gauges. The measurement error by the proposed method was about ±3MPa, when the measuring intervals of the displacement sensors were 76mm. When the measuring intervals were 38mm, the maximum measurement error was about ±15MPa and decreased to about ±7MPa with an increase in the vibration stress. (2) The proposed method could be applied to evaluate the stress in the joint between the small-bore piping and main piping with about ±10MPa errors. (3) Two characteristics of noise influence on the proposed method were identified. The first one is that measured stress by the proposed method seems to overestimate as the noise proportionally increased. The other is that the influence of noise was inversely proportional to second power of the measuring interval. (4) Based on the characteristics of noise influence on the proposed method, the correction method to improve the accuracy of the measurement was proposed and its validity was confirmed. (author)