VSDs and Flat Curve Pumps

[Marke]

Recently, I have come across a number of applications of water pumps pressurising a system with a VSD and pressure transducer operating with a PID control loop with a "flat curve pump". The norm is to use the PID control to adjust the speed of the pump such that the pressure is maintained at a constant set point.
On most pump systems, this works well and the speed of the pump is modulated according to the flow.
I have come across a number of systems where the output pressure of the pump does not increase when the flow is reduced. This results in the pump operating at close to full speed all the time, negating the variable speed operation.
Not being an expert on pumps, I have some questions.
1. Is this "flat curve" due to the pump being incorrectly selected, i.e. operating at the left hand end of the curve, or is this peculiar to a particular type of pump design?
2. Is there any advantage in applying a variable speed controller to this type of pump?
3. Is there an improved algorithm to use other than constant pressure?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[quark]

1. Flatness of the curve is to do with geometry of the impeller. It depends upon the number of wanes and wane width etc. These pumps are of good help where the static resistance of the system is predominant over the dynamic component (in simple terms, static head or head loss across equipment is more than that of frictional losses). When you have variable flow requirement in such cases, choosing a flat curve pump ensures that water flows to the highest point at all flowrates and you will also get the reduction in power consumption in low flow conditions.

2. Advantage of a variable speed system depends upon the system curve and not the pump curve. Variable speed system is almost useless for systems with high static head component. Suppose if you erratically selected a flat curved pump for a system with high dynamic losses, you can, still, optimize the power consumption by applying variable speed system, if needed.

3. Location of the pressure sensor makes a lot of difference. If you locate a pressure sensor that controls the speed of the pump just after the pump discharge, you gain nothing with variable speed system. But the same pressure controller which is placed nearer to the user point can optimize the power consumption at all flow rates.

Regards,

 

[monfu]

Is the pressure an essential parameter for the process?. If it is not, then the pressure control could be switched to flow rate control.