Operating two circulating pumps in parallel using one VSD

[MarineElect]

Hi,

I am looking at an application from an energy saving perspective, where there are three circulating pumps in a manufacturing cooling process working in parallel with one another to provide the required amount of flow to the cooling application. All pumps are on/off control, and in most cases the flow required is just above the pumping capacity of one pump, so two pumps need to be left on at all times.

I am looking at the idea of operating the second pump using a Variable speed drive to control the speed of it, and the VSD controlled by a differential pressure input transducer to cause the VSD to operate at the required speed to maintain the required differential pressure across the pumping operation, and thus the load application.
thus only one pump would be run flat out, with considerable energy savings on the other.

Has anyone here ever tried this out, or can anyone see a reason why it might not work. Looking at the pump power curves I would see this as giving a constant head pressure across both pumps and the relative power input determining the flow through each pump, with the first pump pumping close to max capacity all the time.

Am i missing something that might cause this to operate incorrectly.

Thanks for any assistance rendered.

Marineelect.

 

[jraef]

This is done all the time, in fact many VFDs have built-in application macros (canned programs) for this very thing which that will do most of the work for you, usually using just one transducer.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[CJCaldwell]

You need to check the pump performance curves to see how each pump will operate at reduced speed. There is a risk of running the reduced speed pump too far left on it's curve when it's running in parallel with a full speed pump. This could damage the reduced speed pump over time, and increase maintenance costs, eliminating any cost savings that you would get from reduced energy consumption.

 

[itsmoked]

Perhaps a larger VFD to run any two pumps simultaneously.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[suriae]

You need a CASCADE CONTROLLER, most VFD manufacturers have this as an option.

 

[Drivesrock]

Just a thought...if the flow is just above the capacity of one pump why not put the VSD on one motor and run it above base speed and leave the other one off? OK, I appreciate that there are limitations to this as we have seen before in other threads.

Drivesrock

 

[itsmoked]

As long as there is enough remaining HP that would be a good solution.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com