PID Tuning for Flow. Any suggestions please?

Hi there,

This is my first time tuning a PID for flow. I have some experience with level and temperature.

Anyway heres what I've experienced. A pump is running in Manual from SCADA. The operator enters a speed reference of about 93% for the delivery pump. Tjis will achive a flow rate of about 65MLD which is the desired flow.

When I switch to PID control as the setpoint is achieved the output from PID is 0%, and pump backs off and the flow drops drastically.

The PID then increases output and the flow does not increase under the pump is at about 85%, next BANG....the flow is back over 65MLD and pumps stops.

Essentially I have ON/OFF control. I appreciate you cant tell for sure but is the flowmeter causing problems? Its almost as if the flowmeter wont give a reading until the pipe is full.

Any advice would be great

I'm not sure what SCADA system you're using but here's a quick rundown of what I think is happening:

A) It sounds like the output of your controller is not utilising output tracking. This ensures that you don't have a sudden drop-off/spike in the MV when switching manual/auto/manual.

B) It's possible that the flowmeter is giving hassles, but impossible to say. However, if it's occurring at the same point every time, it seems rather that your process needs fine-tuning. I would suggest doubling your proportional band (or halving proportional gain) until the cycling (on/off control) has attained a steady offset. Once that has been acheived increase your integral to 50% of your proportional band setting (or twice the proportional gain) the insert slight disturbances into the loop, monitoring for overshoot/cycling. Derivative should generally be turned off for flow control, but since your description of the loop is so vague, it's impossible to say for sure.

This is just a very quick guide. There are multiple guides elsewhere on the net (and probably on this forum) that will give you a much better idea of PID tuning for your application.

Just saw slightly more insight into your question. What sort of flowmeter are you using?

MorphuisOGrady, It doesn't sound like the flowmeter is the problem from what I'm reading here. Do a search for "Bumpless Transfer" to get an idea on how to switch from manual to auto PID without CV going to zero in the transfer process. Also you didn't mention the fluid you are pumping or the pump type. Remember that centrifugal pumps won't put out much flow below about 40HZ in most cases. BD

MorphuisOGrady said:

Its almost as if the flowmeter wont give a reading until the pipe is full.

Click to expand...

It isn't clear whether that was a rhetorical statement or if the flowmeter might not actually be a full pipe.

FYI, there are VERY few flowmeters that make a claim of being able to read a partially full pipe. The assumption for 99%+ of the flowmeter world is that the validity of the flow reading assumes a full pipe.

The flowmeter's reaction to a partially full pipe can be an obviously bad reading, widely osciallating readings, no reading at all (fail mode), almost anything but an accurate flow reading.

If you don't have a stable, reliable input signal to the PID loop, all control bets are off.

Some useful information there. Thank you Gents.

I will attempt this when I return to site.

The pump is a centrifugal one. The Flowmeter is a MagFlow. Siemens I think???

I'll enable Output tracking and will look up Bumpless transfer for the PID instruction for ControlLogix now. Though that was just a feature of the PIDE.

Flow is usually the easiest to tune because the loop is so fast.
The loop will be non linear because at low speed the pump has little effect especially if it's pumping against a pressure e.g. at 50% speed there may not be enough pump head to overcome the static pressure so the loop just keeps winding up.
I would start with low gain 0.25 (400%) and quite a bit of reset e.g. 10s/repeat.
You need to get your tracking sorted out as Boswellw says.
Cheers
Roy