A STUDY ON DETERMINATION OF VIBRATORY STRESSES AND MODAL STRESS VECTORS REQUIRED FOR QUALIFYING COMPLEX PIPING IN NUCLEAR POWER PLANTS.

摘要

A great deal of effort is put in qualifying piping for vibration in various systems of nuclear power plants. Like many critical components, piping is also required to perform specific function in shutting down a reactor to safe shut down condition, in maintaining the safe shutdown condition, or in mitigating the consequences of an accident. During the design stage of generation 3 reactors, ASME brought out guidelines to assess vibration levels of applicable piping system for pre-operational qualification. The guideline recommends test methods, test intervals, parameters to be measured and evaluated, acceptance criteria and corrective actions if required. The guidelines classify piping vibration into steady state and transient condition. Under each condition, the guide recommends rules for assessing piping vibration in three levels in increasing order of sophistication. The lowest level is based on visual evaluation backed with judgment and experience of similar type. The middle level is based on vibration measurement on given locations. The highest level is based on monitoring vibration with sophisticated devices to accurately determine vibratory stresses in the pipe. The third level is rigorous method to be carried out on the portion of piping which exhibits vibration response that cannot be characterized by simple piping modes. In nuclear plants, where one of the key objective is to maintain minimum fluid inventory in the system, piping are very densely laid and are designed to cater for multiple types of loads besides the conventional thermal hydraulic loads. Besides, there are number of locations in the plant where the laid piping is not accessible during the entire life time of the plant. All such lines need to be qualified by detail monitoring using sophisticated devices. The test carried for qualification should be sufficiently specific to the condition to which the piping shall be experiencing in operation. Such a test requires that the modal displacements and natural frequencies of the piping system be determined from the test. It also requires modal analysis of the piping system be performed yielding analytically determined natural frequencies and mode shapes and modal stress vectors corresponding to the mode shape vectors. The analysis and test results are correlated to obtain Modal Response Factor which can be used to determine actual state of stress in the piping due to the measured modal displacement. The correlation between the results should confirm the validity of the analysis and should indicate that the analytical results are conservative. The paper describes correlative analysis carried out between analytical and experimental results of a typical three dimensional piping section. The pipe was extensively instrumented with sensors to measure vibration, strain and pressure fluctuation in the flow through the pipe. By applying bending moments, the stress vectors of the representative analytical model are obtained which is then multiplied with the modal response factor, the ratio of test modal displacement and analytical modal displacement. This product then gives the actual stress vector developed in the test. The stress vector so obtained is compared with the experimental stress vector obtained from the strain measurement. Thus the procedure given for rigorous evaluation of piping vibration as per ASME O&M code has been verified.