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Characteristic curve

The characteristic curves of centrifugal pumps plot the course of the following parameters against flow rate (Q): head (H) (see H/Q curve), power input (P), pump efficiency (η) and NPSHr, i.e. the NPSH required by the pump. The characteristic curve's shape is primarily determined by the pump type (i.e. impeller, pump casing or specific speed. Secondary influences such as cavitation, manufacturing tolerances, size and physical properties of the fluid handled (e.g. viscosity, solids transport or pulp pumping are not taken into account in these diagrams.

For the normal operating range of centrifugal pumps (n, Q and H all positive), it is sufficient to plot the characteristic curve in the first quadrant of the H/Q coordinate system.
Figs. 1 to 4 Characteristic curve

Characteristic curve: Centrifugal pump curves for various specific speeds. Fig. 1 Characteristic curve: Centrifugal pump curves for various specific speeds. Curves plotted in percentage ratios referred to the best efficiency point (ns increasing from left to right)

 

Characteristic curve: Radial flow centrifugal pump, specific speed Fig. 2 Characteristic curve: Radial flow centrifugal pump, specific speed Characteristic curve: Mixed flow centrifugal pump, specific speed nS 80 rpm Fig. 3 Characteristic curve: Mixed flow centrifugal pump, specific speed nS 80 rpm

Characteristic curve: Axial flow centrifugal pump, specific speed nq ~ 200 rpm Fig. 4 Characteristic curve: Axial flow centrifugal pump, specific speed nq ~ 200 rpm

As the specific speed increases, the (negative) slope of the H/Q curve becomes steeper.

In the case of centrifugal pumps with a low specific speed, the efficiency curve is relatively flat in the vertex, whereas the efficiency curve of high specific speed pumps is more pointed (see High specific speed).

The power input curve of a low specific speed pump has its minimum value at Q = 0 (shut-off point), whereas the power input of a high specific speed pump reaches a maximum at Q = 0.

The presentation of parameters in a characteristic curve can provide

  • Qualitative information
    See Fig. 1 Characteristic curve
  • Quantitative information on existing centrifugal pumps of various specific speeds
    See Figs. 2 to 4 Characteristic curve

Evenat a given specific speed the characteristic curve's shape can still be influenced via the selection of an appropriate head coefficient. The higher the head coefficient for given operating data, the smaller the impeller diameter is, the flatter the H/Q curve and the steeper the P/Q curve. It is thus possible to match the pump's characteristic curve to the specific systems requirements. See Fig. 5 Characteristic curve


Fig. 5 Characteristic curve: Influence of head coefficient on characteristic curve's shape Fig. 5 Characteristic curve: Influence of head coefficient on characteristic curve's shape

For the normal operating range of centrifugal pumps (n, Q and H all positive), it is sufficient to plot the characteristic curve in the first quadrant of the H/Q coordinate system.
The operating points at which pumps are usually not operated, are situated in the other three quadrants. These include, for example, operation in turbine mode, the pump's behaviour following a drive failure or start-up (starting torque at reverse direction of rotation).

A centrifugal pump's complete characteristics chart (four-quadrant characteristic curves selection chart) is primarily established on the basis of experiments and depends on the pump type. Figure 6 shows an example of a double-suction centrifugal pump with ns = 35 rpm (according to Stepanoff).
See Fig. 6 Characteristic curve
Characteristic curve: Complete characteristics chart of a double-suction centrifugal pump with nq = 35 rpm Fig. 6 Characteristic curve: Complete characteristics chart of a double-suction centrifugal pump with nq = 35 rpm (according to Stepanoff)

The clearest overall presentation is obtained by plotting the following centrifugal pump's operating parameters in one diagram: the rotational speed as relative speed (n/nN) against the flow rate as relative flow rate (Q/Qopt) with the head (H) and the torque (T). All parameters are specified as percentages of their design values (including negative values) in order to facilitate translating the results into diagrams for similar pumps.