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|3. Influence of pump rpm|
|4. Influence of fluid viscosity|
The pump characteristic is an essential tool in selection and
troubleshooting of pumps. It gives the performance of the pump by
plotting, on a same graph :
- The pump Head H (m)
- The pump efficiency η (%)
- The NPSH (m)
- The power P (kW)
As a function of the flow rate Q (m3/h)
The graphs can be presented 1 above the others, or together (with different axis).
The example below is representing a typical characteristic curve of a centrifugal pump at radial flow. The curve is valid at a constant speed (the supplier may give different characteristics at different speeds).
Figure 1 : Pump characteristic curve
The pump characteristic curve is determined at by the supplier using water.
The pump characteristic curve is interesting when combined with the system characteristic curve. The system characteristic curve is the response in head H (m) of the installation to a given liquid flowrate (Q m3/h). The system characteristic is to be calculated, knowing the geometry of the system, and calculating Friction factors then Pressure Drop (system curve = differential static heads + frictino losses). The higher the flow, the higher will be the pressure required.
Figure 2 : System characteristic curve
To understand which flowrate and head (pressure) a pump will deliver in a given system, it is necessary to plot both pump chracteristic and system characteristic on the same graph, the intersection of the system curve with the pump (head) curve is called the operating point.
Figure 3 : Operating point
During design, this exercise must be carried out to chose the best
pump for a given system. By testing different pumps characteristic,
one should reach :
- The desired flow Q (and/or the desired head)
- A pump not too sensitive to system characteristic curve changes
- An operating point close to the optimum pump efficiency (to reduce operating costs of the system)
- The pump requiring as less power as possible to operate
- A NPSH required by the pump that can be delivered by the process
Pump characteristics are plotted at constant rpm, but a pump can be run at different speed (either chosen at design, or because a VFD is installed). The effect of the pump speed is the following : at higher rpm, the curve H(Q) will go up, at lower rpm, the curve H(Q) will go down the graph.
Generally, pump suppliers are plotting the characteristic of the pump at several speed to ease the choice.
Pump characteristics are based on tests made with water. If the viscosity of the fluid actually handled changes significantly, the pump performance will be affected.
If the viscosity of the fluid is less than 10 cSt (this value can go up to 50 cSt for high capacity pumps), the performance will remain constant. With higher viscosity fluids, it will drop.
The actual pump capacity can be calculated a viscosity correction chart.
Figure 4 :
Viscosity correction chart (please refer to Standard of the
Hydraulic Institute, New York, for actual graph)