No abstract

Mechanical loadings on pipe systems caused by water hammer (hydraulic transients) belong to the most important and most difficult to calculate design loadings in nuclear power plants. The most common procedure in Sweden is to calculate the water hammer loadings on pipe segments, according to the classical 1D theory of liquid transient flow in a pipeline, and then transfer the results to strength analyses of pipeline structure. This procedure assumes that there is quasi-steady respond of the pipeline structure to pressure surges — no dynamic interaction between the fluid and the pipeline construction. The hydraulic loadings are calculated with 1-D so-called “network” programs. Commonly used in Sweden are Relap5, Drako and Flowmaster2 — all using quasi-steady wall friction model. As a third party accredited inspection body INSPECTA NUCLEAR AB reviews calculations of water hammer loadings. The presented work shall be seen as an attempt to illustrate ability of Relap5, Flowmaster2 and Drako programs to calculate the water hammer loadings. A special attention was paid to using of Relap5 for calculation of water hammer pressure surges and forces (including some aspects of influence of Courant number on the calculation results) and also the importance of considering the dynamic (or unsteady) friction models. The calculations are compared with experimental results. The experiments have been conducted at a test rig designed and constructed at the Szewalski Institute of Fluid–Flow Machinery of the Polish Academy of Sciences (IMP PAN) in Gdansk, Poland. The analyses show quite small differences between pressures and forces calculated with Relap5, Flowmaster2 and Drako (the differences regard mainly damping of pressure waves). The comparison of calculated and measured pressures and also a force acting on a pre-defined pipe segment show significant differences. It is shown that the differences can be reduced by using unsteady friction models in calculations. Recently, such models have been subjects of works of several researches in the world.

The presented experimental investigations of static characteristics of check valves are a contribution to work leading to improve check valve models in 1D network programs. Two designs of check valves were tested: a swing disc and a tilting disc check valve. For each type of these valves static characteristics for the two directions of water flow were performed. Following physical quantities were measured during the experiments: the angular position of the valve closing element (disc), water discharge, pressure losses caused by the check valve, moment of the hydrodynamic forces acting on the valve disc and pressure at selected locations of the flow system of the test rig. The paper describes the test rig designed for developing the static characteristics of the check valves and presents the results of tests for the characteristics of the swing and tilting disc check valve under steady flow conditions. These characteristics are presented as time-averaged parameters measured during the tests. Following dimensionless quantities are calculated basing on these parameters: - the flow rate coefficient, - the coefficient of the moment of hydrodynamic forces acting on the valve disc, - the local loss coefficient of the valve. The above-mentioned coefficients were determined for disc angular position of selected values (opening rate) and interpolated within the full range of openings for both directions of flow through the valve. The static behaviors of the tested valves are discussed and compared with each other.

The present work describes an experimental investigation of the dynamic characteristics of check valves, which means experimental examination of closing function and ability to generate pressure transients under different flow decelerations in the pipeline. Two designs of check valves are tested: a swing disc and a tilted disc check valve. The valves are mounted on discharge pipe of a centrifugal pump. The fluid transient is generated by stopping the pump motor from actual velocity to completely stop in a prescribed time. Each of the check valves is subjected to tests covering different pressure levels in the upper reservoir, initial flow rates in the pipeline, several decelerations of pump rotation, three settings of torque acting on the valve disc, three values of the moment of friction forces acting on the valve axis and finally free fall of the disc in stagnant water and air. The test stand, the instrumentation and chosen valves as well as scope and conditions for performing the experiments are described. Selected measured results like angular velocities of the discs, pressures in the pipe at different conditions, and volumetric flow rates are presented and discussed. The dynamic behaviors of the tested valves were compared with each other.

The paper presents a Relap5 study of the influence of the centrifugal pump characteristics on the dynamic loads on piping system after power failure. Interpolated and experimental pump characteristics are used. The differences between the interpolated and measured pump curves, the general description of Relap5 model and results of calculations in form of selected time curves for rotational speed, volume flow, pressures and dynamic forces are presented and discussed. The analysis of the results shows that the maximal dynamic force on pipe section calculated with experimental pump curves can be up to 6 % higher than respective calculated using interpolated curves. However, it is not possible to determine to what extent the differences are caused by the interpolation itself or caused by the differences in the design of the centrifugal pumps. The latter since it differs more than 50 years between the pumps whose characteristics are used for interpolation and the pumps with corresponding experimental characteristics.