Curve for sliding vane pump

[heinasirkka]

Dear all,

I don't know a lot about pumps and I'm having difficulties in understanding the pump curve provided by the manufacturer. I tried to contact them several times but they are not very helpful. It is a rotary positive displacement pump (sliding vane). These curves are for 1450 RPM.

- Why does the required power decrease as the flow rate increase? I thought the flow rate increases with the increasing RPM.
- What do they mean by the pressure? Is it the Net Required Inlet Pressure or the pressure difference to be overcome by the pump?
- I control the speed with a variable frequency drive. I'd like to go to higher RPMs for short periods as far as the power limitation allows. Then I suppose the flow rate and the required power will increase linearly as well. But will that "pressure" increase in squares, am I right?

I'd be very thankful if you could help me

 

[electricpete]

- Why does the required power decrease as the flow rate increase?

I assume your curve labeled pressure is showing pump dp.
As we go from left to right on the curves:
The dp is decreasing dramatically for a relatively small increase in flow.
So the fluid power (product of dp times flow) would be expected to decrease almost as much as the dp decreased (since flow is almost constant)
It does not seem that the pump input power decreases when fluid power decreases (unless you have some insight into the not-shown efficiency that contradicts this)



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(2B)+(2B)' ?

 

[heinasirkka]

Thanks a lot for your answer. I think I'll just run the pump and observe how it behaves when I change the speed.
Have a nice day

 

[LittleInch]

I'm sure I've seen this question before recently or am I losing my memory?

- Why does the required power decrease as the flow rate increase? I thought the flow rate increases with the increasing RPM.
This is a PD pump remember. Power decreases because although flow increases, the differential head/pressure is going down much faster. Hence the work done by the pump is reduced.
PD pumps, flow will increase with increased RPM, but these curves / lines are for a fixed speed.
Basically a PD pump at a fixed RPM pumps a fixed volume per revolution or per minute. In theory the pumps seal 100% in which case the top line would be vertical. In reality there is some slip / losses as the flow decreases when the pressure increases. If you look the actual spread of flow is quite low (310 to 360 l/h) If you plotted this on a 0 -400l/h scale it would look nearly vertical


- What do they mean by the pressure? Is it the Net Required Inlet Pressure or the pressure difference to be overcome by the pump?
Pump curves know nothing about the system they will be installed in so the pressure (the vendor would need to know the fluid density) or head is always the differential across the pump.

- I control the speed with a variable frequency drive. I'd like to go to higher RPMs for short periods as far as the power limitation allows. Then I suppose the flow rate and the required power will increase linearly as well. But will that "pressure" increase in squares, am I right?
No. The flowrate will increase linearly with RPM. The pressure and hence power is dependant on the "back pressure" or system curve of your pump outlet.
Say you're pumping into a vessel which has a pressure of 5 barg at flowrate X with an inlet pressure of 0 barg into the pump. That will give you a differential pressure of 5 bar at your flowrate ( set by your motor speed) and generate a power requirement.
Same thing but the pressure has now gone to 10 bar in your vessel. Now you're still pumping at the same flowrate X, but at a higher differential pressure so you will need twice the power.
I hope this demonstrates that for this type of pump, the power required is not set by the pump, but by the system it is pumping into.

"I think I'll just run the pump and observe how it behaves when I change the speed.".
Good way to understand, but take care to monitor & control inlet and outlet pressure.




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

 

[heinasirkka]

Thanks a lot for your helpful answer. The back pressure and the effect of slip were unclear to me, but now I think I get it more. Moreover, I have pressure measurements before and after the pump. Have a nice day!