VFD controlling a pump by GPM instead of RPM

[Tom Jenkins]

Hey Tom,
I have a pump performance issue I have been hunting for a long time (years) - is there a good online forum for discussing it with folks like yourself ?

I don't know of one but here are some sites that could be a resource:
https://www.pumpsandsystems.com/magazine
https://www.pumps.org/

I found this, but haven't used it.
https://www.eng-tips.com/threadminder.cfm?pid=407

If you want to PM me with your problem maybe I can help.

 

[PreLC]

Just a lookup table... X RPM = X GPM. It does require swallowing a lot of "IF" though. But yea, maybe that's OK here.
Such a table could be printed an just posted near the device.

If we can find a reliable relationship between RPM and GPM, why not just put this lookup table in the PLC, and allow operators to just enter GPM as they requested?

There should be alarms in the system to take care of failure modes to prevent deviation from the lookup table though.

 

[drbitboy]

If we can find a reliable relationship between RPM and GPM, why not just put this lookup table in the PLC, and allow operators to just enter GPM as they requested?

If you have 9 [X=GPM,Y=RPM] pairs of points, about five rungs are all that is needed to find, and interpolate between, the two bounding pairs.


There may even be a single instruction that can do it all, and if not it would not be difficult to put into a UDFB.

 

[JeremyM]

If we can find a reliable relationship between RPM and GPM, why not just put this lookup table in the PLC, and allow operators to just enter GPM as they requested?

There should be alarms in the system to take care of failure modes to prevent deviation from the lookup table though.

It's water, so I imagine that it's a linear relationship.

 

[cardosocea]

There may even be a single instruction that can do it all, and if not it would not be difficult to put into a UDFB.

Siemens does have one. Interpolate, I believe its called.

 

[Tom Jenkins]

It's water, so I imagine that it's a linear relationship.

True for positive displacement pumps. Not true for centrifugal pumps.

https://www.mswmag.com/editorial/2020/07/flow-pressure-and-pump-performance

 

[Robb B]

A lot will depend on the type of pump and the media you are pumping? Is the pump positive displacement or a centrifugal pump or something else?

Is the media stable in its temperature and viscosity?
Does the pressure remain constant or fluctuate?

How accurate does the actual flow have to match the entered number?

It's a centrifugal pump, moving a mix of bunker c and creosote, at anywhere from 160F to 210F.

The viscosity changes with temperature, but not hugely. The pressure remains pretty constant, and the flow accuracy is not as important as the presence of flow, by which I mean if the actual flow was half of the commanded it's not a big deal so long as there is actual flow.

We're trying to prevent cavitation primarily. Adding to the fun is that the medium is pumped in a vacuum at both inlet and outlet.

 

[drbitboy]

by which I mean if the actual flow was half of the commanded it's not a big deal so long as there is actual flow.

Then all that is needed is a simple linear model of GPM vs. RPM.

The operator enters GPM, the PLC converts to RPM = GPM * m + b, and sends that RPM value to the VFD.

 

[Tom Jenkins]

It's a centrifugal pump, moving a mix of bunker c and creosote, at anywhere from 160F to 210F.

The viscosity changes with temperature, but not hugely. The pressure remains pretty constant, and the flow accuracy is not as important as the presence of flow, by which I mean if the actual flow was half of the commanded it's not a big deal so long as there is actual flow.

We're trying to prevent cavitation primarily. Adding to the fun is that the medium is pumped in a vacuum at both inlet and outlet.

In that case, you can probably get by measuring the motor power and use the following equation:

gpm = (hp x 3960) / (feet head x specific gravity)

Or, you could create a table of rpm vs. gpm by running the pump at a few speeds and measuring the wet well level change over time.

 

[Chuck Woodbury]

What is the pipe size? Flow meters are relatively inexpensive for smaller diameter pipes. ifm has inline meters up to 2" for about $1000. Keyence has clamp on for about the same price.

 

[Robb B]

What is the pipe size? Flow meters are relatively inexpensive for smaller diameter pipes. ifm has inline meters up to 2" for about $1000. Keyence has clamp on for about the same price.

The pump inlet is 8" and the outlet is 3", the whole system is under vacuum during operation.