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Performance Curves

Curve Corrections for Different Fluids

Pump curves are mostly established for water. Duty (H-Q) and efficiency (η-Q) curves become close to being valid for all Newtonian fluids, while the shaft power is proportional to the density of the pumped medium.
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

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Basic Concepts

Newtonian Fluid

A fluid is classified as being Newtonian if it conforms to NEWTON’s friction law, i.e. if viscosity remains constant with agitation or varying shear rate and its shear rate being proportional to the velocity gradient vertical to the direction of flow. That means, that the viscosity is only affected by temperature changes.

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