Chemical Injection PID Control Help

[bfranklin]

Allen Bradley PLC 5

What operations wants to do is as the Oil Flow rate (0-3000 m3/d) increases more chemical is injected into the flow line, chemical pump can flow between 0-950 Liters/d

How could I do PID control using these 2 parameters?

-The oil flow rate would be the process variable
-The chemical injection pump would be the controlled variable.

What would be the set point if both flows are of different ranges? There is no dedicated set point to try and maintain. Just as the oil flow increases inject more chemical. Tuning will need to be done to get the correct ratio.

Thanks so much!

 

[widelto]

What you mean is called ratio control. This example is for PIDE on controllogix but can clarify what you need to do.
My file is too big even zipped. Please send me a PM with your mail, I'll send you file with explanation.

 

[Peter Nachtwey]

I don't see a problem unless you don't have a flow sensor for the oil and the chemical. It seems obvious to me that the set point for the chemical flow is some fraction of the oil flow so yes, the set point for the chemical will change as the oil flow changes.

 

[bfranklin]

I now get it.
I have never programmed PID ratio control.
Would you happen to have an example in PLC 5 I could go off of.

Thanks

 

[Ron Beaufort]

see if the example in this book will help ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1785-rm001_-en-p.pdf

page 14-38 [Adobe page 226 of 372] ...

 

[Peter Nachtwey]

branklin, listen to Ron about hot to configure the AB PID. However, the PID is secondary to understanding what is happening in your system.

Since this is a proportional system doesn't it make sense that you could simply use the flow signal and scale it and send it to the chemical injection system? No PID required!!! However this assumes that everything is linear and you have not provided that information. Is the injection system linear? Is the injection flow proportional to the control signal to the injection system. If so you should be able to get extremely close with a simple SCP block. It may not be perfect but it is a good place to start. A simple PI control can be added later if the feed back or injection system is not linear.

 

[bfranklin]

Thanks for the replies guys.
With my lack of programming experience I was definitely over thinking how to program this.

Thanks again

 

[Peter Nachtwey]

The goal is to estimate how much control signal is required and get close, say within 5%, using simple ratios. Then a simple PI control can adjust the control signal and correct the last 5%.

 

[kamenges]

Much of what you can do will be determined by the feedback you have available. Do you have independent flow feedback? Do you have continuous concentration feedback? If you have no feedback available it won't do you any good to implement a closed loop controller.

Keith