Preliminary vibration analysis of the prestressed tie rods attaching the Paluel-4 main steam line No.1 stops to the concrete structures

This test report presents the results of the vibration analysis of the prestressed tie rods attaching the main steam system stops on Paluel-4 line no 1. The reasons for this feasibility study and main results are summarized below. During surface preparation for the routine ultrasonic examination of the tie rods, a nut was found to be completely unscrewed. A systematic tightening procedure, using hydraulic devices, was immediately implemented. For about one-third of the 80 tie rods on the four live steam clamps, the tension was below nominal. However, this checking procedure involves high costs. Until an explanation has been found, other plant units will have to be checked and it is consequently of particular importance to find a less costly way of checking the tension, so that the hydraulic tightening equipment would only be used in cases where prestressing was inadequate. The R and D Acoustics and Vibration Mechanics branch proposed to analyze the vibratory signature of the tie rods to assess the sensitivity of their dynamic response to prestressing variations. The purpose o the tests was to estimate the feasibility of the procedure, which could then be refined. If we consider theoretical results, we find that the first bending frequency for a beam in tension depends on the tension level. Unfortunately, with the hammer tests performed, we were unable to identify the first eigenfrequency of the tie rod. Other parameters measured would not appear to be directly related to the prestress level. Since checking the prestress condition had to be extended to other units, it seems appropriate to pursue these investigations with a view to determining the exact sensitivity of the vibratory and acoustic parameters to the level of prestressing applied. It would also seem important to improve test conditions so that the first eigenfrequencies of the tie rods can be identified. A sinusoidal sweeping test with an electrodynamic exciter could be used for any future tests which may be carried out. (authors). figs., tabs., 2 refs., 4 annexes