Incertitudes à prendre en compte sur les courbes H (Q) et P (Q) à débit partiel des pompes et de pompes-turbines

Abstract

At partial flow, the characteristic curves of pumps and pump turbines can differ substantially from the forecast curves drawn up using a model (scale-model or other). The differences between these characteristics curves and those of the industrial machine are : - geometrical differences between the model and the industrial machine ; - uncertainties regarding the hydraulic scale effects ; - uncertainties relating to the measurement of the characteristics of the model and of the industrial machine. A large number of geometrical and physical parameters have an effect on partial flow curves. The present paper, using concrete examples as illustration, shows the geometrical differences of hydraulic parts (impeller, diffuser, pump case, etc.). The influence of differences on certain mechanical parameters such as the axial gap between the impeller and diffusor, diameter of balancing holes, is studied using the results of tests. A third set of examples gives : - the dispersion of identical pumps on characteristic curves ; - the influence on the curves of the NPSH and the behaviour of H (Q) and P (Q) in ambient air ; - the uncertainties due to pressure and torque value fluctuations of pump turbines ; - the analysis of the differences between the curve drawn from the model and the real curve of a pump turbine. The second section of the paper discusses the requirements and constraints entailed for the pump operator in PWR nuclear power stations. The functional circuit constraints, in particular in partial flow conditions, determine : - minimum and maximum permissible heads ; - the obligation to operate pumps in parallel on a same circuit. It is impossible indeed unrealistic, given the volume of corresponding documents and calculations (for purposes of safety and guaranteeing high standard), to re-use the same studies if the findings of theoretical analyses differ from the measurements of the actual performance of pumps. These considerations allow little lattitude vis-a-vis the differences between fore cast and reality. In addition, pumps must be interchangeable, i.e. have identical mechanical and hydraulic characteristics, which necessitates manufacturing, quality control and strict and specific testing. The mastery over the characteristic curves of partial flow pumps is a basic element of the operation of certain safety pumps in PWR nuclear power stations. A model allowing for these phenomena can be devised for the system as a whole and equations of movement can be written to take account of the inertia of ducts and tanks. It is shown that it is indispensable to take account of the relatively low inertia of sections in which the various pumps are installed to explain these phenomena of rapid switch over of the operating mode of pumps. The explanations are given in graphic form in the simplest of cases. The graph discloses at all times the forces which slow down or accelerate the rate of flow in any section, especially sections comprising pumps. It can be seen that for due rigour, parallel pump flows must never be added for a same head in dealing with these problems. By generalising, any change of operating conditions could be calculated for any system including pumps which can operate in parallel or in series with stable or unsteady characteristics at various locations. However, when the flow is characterised by high pressure values (case of feedwater pumps) or to be more precise, when CVO/gHQ is high, concentrated capacity cannot be substituted for the distributed capacity that is due to the elasticity of the fluid and pipings ; waves must be taken into account and hammering fully calculated.