Curve Corrections for Different Fluids

In practice this will however become not applicable for fluids of kinematic viscosities above 20mm²/s ( 20 cst ) due to the influence of a higher Reynolds Number with increasing viscosity. Empirical conversion methods have been developed for curve corrections for medium- and high-viscous fluids, which in practical application in older versions meant the time-consuming evaluation of diagrams, but in the current versions were prepared by corresponding formula sets. The most widespread method worldwide is the method from the Hydraulic Institute (USA), which has been standardised as ANSI/HI 9.6.7 and ISO/TR 17766. In practice, the conversion is usually carried out by computer programmes such as the Spaix PumpSelector. The computer implementation of this method enables the conversion of performance curves, whereby the user only has to define the desired delivery data and the pumped medium. The best efficiency point of the pump is of vital importance with all the known curve correction methods. The following conditions can be stated for the validity of this method:
  • Centrifugal pumps with closed or semi-open impellers
  • Kinematic viscosity in the range between 1 and 3000 mm²/s
  • Flow rate at best efficiency point between 3 and 410 m³/h
  • Head per stage between 6 and 130 m
  • Fluid displacement by means of a centrifugal pump under normal operating conditions
  • Handling of Newtonian Fluids

Operating Point of a Centrifugal Pump

It indicates the values of Flow Q and Head H which will be obtained at stationary operation with the respective speed-related pump H-Q curve. The specified duty point is defined to be that point on the system H-Q curve for which a pump is to be selected in line with the calculated hydraulic design criteria. The objective of the selection is (apart from other criteria, such as maximum efficiency) to minimise the deviation between the specified duty point and the actual operating point. The operating point is always located at the intersection of pump H-Q curve and the actual system H-Q curve. At constant speed it moves up the pump H-Q curve with increasing friction losses towards a lower flow rate. The duty point should be chosen as close as possible to the best efficiency point.