• Pump Theory

• Pump Terminology

• Reading Pump Curves

• Centrifugal pump
  Operating Range


• Pump Enquiry Information

• System Curves

• Pump Parallel / Series
  Operation


• Cavitation

• NPSH - Nett Positive
  Suction Head


• Affinity Laws

• Troubleshooting

• Disclaimer


Parallel & Series Operation


The use of two or more pumps to increase flowrate is called Parallel pumping. The use of two or more pumps to increase head (pressure) is called Series pumping. Operation of pumps under these circumstances may appear simple, but there are more complex issues to consider, ie:
In series applications: consider the pressure rating of pump, shaft seal, pipework and fittings. Placement is critical to ensure both pumps are operating within their recommended range and will have a constant supply of water.

Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the head each pump produces at the same flow. More curves can be added in the same way.


In parallel applications: confirm suitability of pumps by drawing a system curve (often 2 pumps will only deliver slightly more than one pump due to excessive friction loss. Also you can confirm that pump operation will be within its recommended range.). Non return valves are required especially if one pump operates alone at times. Dissimilar pumps or pumps placed at different heights requires special investigation.

Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the flows each pump delivers at the same head. More curves can be added in the same way.

Once the curve for two pumps has been drawn, add the system curve, the point where the system curve crosses the curve for two pumps , indicates the total flow from two pumps. Draw a horizontal line from this point back to the head axis. Where this horizontal line crosses the curves for a single pump indicates the amount of flow contributed by that pump to the total flow.


Unstable Operation

The following sketch shows a system curve crossing a pump curve twice. This is an example of unstable operation. Note that if the first pump is operating at point 'C' when the second pump is started, the second pump will operate at shut head, delivering no flow as it will never be able to open the non return valve (required to prevent one pump discharging through the other when only one pump is operating). If this was to occur, the pump could eventually explode!

In some cases it may be possible to change the order of starting the pumps, and the curves can be drawn to check this operation, however if there is any indication of unstable operation or possibility of one pump being 'over powered' by another, contact us for assistance.




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