How does a pump trade head for flow rate?

[RobertGC]

A pump’s power is a product of its pressure rise times volume flow rate. Commonly, a pump is specified by giving the pressure rise, or head, it can provide and the flow rate, such as in gallons per minute(GPM).

But some pumps provide the user with a chart that shows how the pressure rise can be varied with a corresponding change in flow rate, inversely related.

This is what I need for my application. I need a higher head than what the specs say for the pump, allowing for the reduced flow rate. The specs don’t say whether or not these values can be varied. So is there some common method by which this is done for pumps with this capability?

I thought they just reduce the inlet size to change the flow rate, with an associated change in the size of the pressure rise. But then I thought this would just mean the pump would just suck harder on the water input source, making the flow rate stay the same.

So how do pumps with this variable capability do it, and can other pumps be adapted to also do it?

 

[Baluncore]

I assume you are writing about centrifugal flow pumps, where the motor runs at a set RPM, with an impeller of a particular diameter that transfers power to the flow.

The product of flow and pressure (or head) is the power transferred to the fluid. That is why the pump can automatically adjust the product of flow and head.

You should select a pump with a different power or impeller to optimise head and flow efficiency for your application. The water remaining in the pump when there is obstructed flow can boil.

Get the data sheets for a pump series to study the interrelationship between the variables.

https://en.wikipedia.org/wiki/Centrifugal_pump

 

[Dullard]

A pump will operate at some point ON the pump curve. That point is defined by the pressure/flow characteristics of your system. You will operate where your pump curve intersects with your system curve. If your target operating point isn't on that pump curve, you can't use that pump (at that speed).One common way to (dynamically) 'vary' a centrifugal pump is by varying the drive speed. The pump curve supplied by pump manufacturers is typically for 1 speed (60 Hz where I live). By changing the speed, you completely change the curve. The curve for a new (presumably lower) speed may be calculated by translating points on the original curve.

flow is reduced in proportion to the speed reduction
pressure is reduced in proportion to the square of the speed reduction
power is proportional to the cube of the speed reduction

As Baluncore already explained:
If you don't need 'dynamic' adjustment, most pumps can be had with different impellers and/or different motor speeds - essentially creating a different pump/curve.

 

[anorlunda]

If you need more pressure and less flow, then a positive displacement type pump may be more suitable. For example, pressure washers use positive displacement piston pumps.