One control variable controlling two loops

Simplified description of the system:
We have a VFD driven raw water pump puping water into a filtration unit. There is a flowmeter and modulating valve at the intake of the filtration unit.

Current control strategy in the specifications is such that the operator enters a flow setpoint and the PLC adjusts the speed of the VFD to meet the flow setpoint into the filter. However, the PLC also modulates the valve at the filter intake to maintain the same entered flow setpoint.

So now we have one flow setpoint that controls the VFD and also modulates the intake valve.
This is not practical in the real world as the VFD and valve will fight each to maintain the setpoint.

But my question is will this current setup work?

We want to avoid making changes in the code as the project has turned political and we want to make as less change in the control narrative as possible.

What are the possible "quick and easy" changes that we could implement in the logic to make this work?
1. Ones that I can think of is make the valve full open/close and allow the VFD to control flow.

OR
2. introduce some large deadband in the program. (dont know how to do it and some direction will be useful).

Looking forward to your help.

Thanks

sameer said:

So now we have one flow setpoint that controls the VFD and also modulates the intake valve.
This is not practical in the real world as the VFD and valve will fight each to maintain the setpoint.

Click to expand...

Why do you assume that the two control devices would "fight" one another? Is there a substantial process or transportation lag involved? Shouldn't the pump speed and the valve opening both modulate in the same direction for a given process error?

Option #1

I would also disable the valve manually, put it in the full open position.

I have seen this pump / valve combination before where a flood irrigation system required the system curve to be modified when irrigating some plots. This was to avoid the pump speed being ramped down below the minimum allowed by the manufacturer. As the VSD speed was approaching minimum, the valve was supposed to begin closing, also on a PID chasing the same flow setpoint.

( I didn't get to see how well it worked, I changed jobs before the irrigation system was ready for commissioning.)

But - this was all for a specific reason. If the range of flow-rates required can be achieved by using the VSD alone without going beyond the pump manufacturers limits, then using a valve to throttle the flow back is surely just inefficient? Extra restriction, extra power required to overcome that.

I'd check with the mechanical engineer. if there's no good reason, then set the valve fully open as Mickey said. Ramp it open before starting the pump and close it afterwards. It'll look like it's working great!

Fight (WWF) = Conflict

The FT will control the VFD/VSD and the modulating valve put in manual mode,

Maybe you need to make adjustment manually on the Valve due process problem.

Bit_Bucket_07 said:

Why do you assume that the two control devices would "fight" one another? Is there a substantial process or transportation lag involved? Shouldn't the pump speed and the valve opening both modulate in the same direction for a given process error?

Click to expand...

Bit_Bucket_07, thanks for your reply. There is hardly any process or transportation lag in the system. However, I have read and have been told by others that the valve and the VFD will fight each other when being controlled by the same flowmeter. In theory, they should both respond in the same way for a given error signal, however, may not work that way in practice. I have experienced that to an extent.

Mickey, Saffa, Osmanmom, I guess I'll have to go with changing the valve to open/close and controlling only from the VFD.

Read this

Sameer:

This is a last year article, it may help you.

We have a setup similar to this for some of our raw water pumps, although we use the VFD to control the flow rate, then modulate the valve to maintain a differential pressure across the pump using two pressure transmitters (suction side and discharge side). This way we can keep the pump working at its optimal pressure differential and running most efficiently. This was code that was designed and written 25 odd years ago.

Oceansoul said:

We have a setup similar to this for some of our raw water pumps, although we use the VFD to control the flow rate, then modulate the valve to maintain a differential pressure across the pump using two pressure transmitters (suction side and discharge side). This way we can keep the pump working at its optimal pressure differential and running most efficiently. This was code that was designed and written 25 odd years ago.

Click to expand...

In my opinion this strategy is flawed.

You shouldn't try to operate the pump at its Best Efficiency Point (BEP). You should try to operate the pump at its lowest power demand for the required flow.

A VFD saves energy compared to throttling because instead of dissipating head over a valve it operates more slowly and doesn't create the head in the first place. By throttling that valve you are artificially creating head with the additional restriction and then turning that head (energy) into heat by throttling. There are limits, of course, and if you run at either extreme of the pump curve you can get into internal recirculation and heating issues. However, creating artificial head just to operate closer to the pump BEP is counterproductive. Doing it that way for 25 years doesn't make it a good idea.

Some process application

Require the speed motor with flow and/or modulate valve.

It will request the motor to increase/decrease the speed/flow and open/close the valve graduating

Or

Increase/decrease the speed/flow and valve opening constant.

It provides 2 system selections.