Structural behaviour of a piping system at blowdown: Significant parameters for a comparison of measurement and calculation

Abstract

As part of the German Reactor Safety Program, blowdown tests were conducted on a feedwater line in the HDR superheated steam reactor, which had been shut down, under PWR conditions. This work included the pre- and postcalculations of the dynamic structural behaviour of piping systems on the basis of measured blowdown pressure/time histories. In the present paper it is shown that the degree of agreement of the calculated results with the measured structural response is determined primarily by the establishment of equivalent force/time functions from the measured pressures at blowdown and by the application of these forces to the structural model of the piping system. In the precalculation the fluid-dynamic forces were determined by means of time-related extrapolation of the pressure/time histories measured in the straight pipe sections to the elbow centre points. When the measured and computed structural responses were compared, deviations were revealed. The reason for this was that the pressure/time functions measured during the blowdown test included significant portions from the reflection of pressure waves. These portions are not correctly covered by the time-related extrapolation method. In a postcalculation we selected a geometrical interpolation method to determine the fluid-dynamic elbow forces. This method does not give an exact reproduction of the frequency content of the pressure/time functions but does cover the magnitude of impulse for both the incoming and the reflected pressure waves. The clear improvement in compound results shows that this simple method is acceptable for determining structural responses. In contrast to the sensitivity of the computed results when the fluid-dynamic forces are determined in different ways, it became evident that the piping codes in common use today (which are based on the beam theory) make it possible to give an adequate description of the dynamic structural response with pressure waves arising from blowdown.