Graduate Thesis Or Dissertation
 

An investigative approach to study the RETRAN modeling concepts in the analysis of a safety valve discharge transient

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  • This study focuses on investigation of the RETRAN modeling concepts in the analysis of a safety valve discharge transient. Three safety valve tests conducted at the Combustion Engineering test facility were chosen for evaluation with RETRAN. It was essential to apply the critical flow boundary condition at the downstream nodes of the valve to compute the test results. The results of the RETRAN calculation were compared with the test measurements and the RELAP5/MOD1 computations. Generally, the results were in good agreement qualitatively with the test data and the RELAP5 calculatios. The RETRAN and RELAP5 codes predicted very similar behavior for the transient pressure at various locations in the downstream piping. In the RETRAN evaluations, for steady state initialization of the problems incorporating the heat transfer effect, a small flow rate was maintained in the downstream piping. This was necessary because the test system was initially stagnant, and it was not possible to initialize the steady state solution of the equations of conservation without maintaining a small initial flow prior to the valve opening. The surface temperature of the downstream piping was initially set at 212 F. This was higher than the test condition temperature (80 F) and it limited condensation in the system, which led to overestimation of the RETRAN-calculated pressure in downstream of the piping compared to the test data and RELAP5 evaluations. Also, RETRAN predicted that changes in the system were faster-acting than what the test data showed or the RELAP5 computations predicted. This was due to the application of choked flow boundary conditions at the downstream junctions. In this case, the friction and local losses effects were absent from the calculations, making the system of equations underdamped. To examine the applicability of the critical flow boundary condition in the computations and the general behavior of the RETRAN homogeneous flow model for this special problem, the basic models pertinent to the RETRAN calculations were examined. Approximations were made to simplify the models which were then verified against the RETRAN results. A computer program (PIERAN) was developed utilizing the approximated models in an effort to determine the nature of the instabilities in the calculation attributed to the homogeneous flow model. PIERAN was used to investigate the flow model behavior for the problem of a valve discharge into a straight pipe. Several runs were made with PIERAN to investigate the effect of the time step size and the volume length along the downstream piping. The results showed instability in the calculation when larger number of volumes were used in downstream of the valve. The instability, however, did not occur for a single volume in the downstream segment of the pipe throughout the calculation.
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file details YoussefniaMohammadH1985.pdf 2017-08-10 Pubblico Scaricare