Affinity Law

[Orga78]

Hi,

I am interested to understand in depth on Pump's Affinity Law.

Is it possible to not change the pressure even if the pump's speed is reduced or increased? The main parameter that will change proportionally with pump's speed (RPM) is the flow rate (gpm).

However, the affinity law states that:
2. The pump head (pressure) varies with the square of the change in the speed. Two times the speed is four times (22) the head generated. Eighty percent speed is 64 percent (.802) the head generated.

What if I alter the pump RPM to be less than the square of the change in speed (as per affinity law)? Can I get constant discharge pressure from the pump?

 

[hydtools]

Pressure still changes as the square of the speed change.

Ted

 

[LittleInch]

From a centrifugal pump?

No. Not possible. Alter the speed you will alter the differential head.

Only way to get constant discharge peeing is to vary the inlet pressure if you're going to change the speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Orga78]

Thanks LittleInch.

Agreed on the affinity law where speed (RPM) of the pump will change flow rate, head/pressure and power (kW).

How about for positive displacement pump? In this case, to be specific, a triplex plunger reciprocating pump?

Will I be able to achieve (maintain) same pressure in the pipeline if I change the pump (motor) speed via VFD where the main objective is only to change flow rate?

 

[LittleInch]

Ah. Different pump, different story.

Yes a PD pump can maintain the same pressure at different speed/flow but the pressure is set by the downstream system.

If this pump is the only pump then as the flow rate drops in a long pipe so does the pressure needed to make it flow.

If e.g. you're pumping into a larger system at a more or less fixed pressure then yes the pd pump should be able to maintain a fixed pressure for a range of flow rates and speeds.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

The affinity laws do not apply to positive displacement pumps.
Look at the affinity laws as describing the capacity of the fan or centrifugal pump. The actual flow and pressure at the discharge will depend upon the system response to flow pushed through it and the pump capacity to meet that system response.

Ted

 

[LittleInch]

PD pumps are normally directly proportional speed to flow. But pressure and power is not the same.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Orga78]

Thank you very much, both LittleInch & Ted.

Good information indeed on the Affinity Law - for both centrifugal and PD pumps.

I now have the understanding of affinity law, that it applies to centrifugal pumps & fans, but not to PD pumps.

We are improving the efficiency of our tanks (5 units) cleaning (with different volume) via single PD pump by varying the speed with VFD, and the variables are flow rate and power (kW). While the parameter we want constant is Pressure.

Explanation from you both are on-point. Thank you.