Controllogix PID in very slow reacting loop

Hey guys,
I have a Controllogix and I am tying to control a PID loop for Chlorine (CL2). The trouble is the mixing point is 15 minutes away from where the CL2 is injected, so there is 15 minutes time lag!!

What method do you think is best for control- straight PID or a cascaded PID?

Thanks for any help.

Chlorination is tough. I wouldn't use PID at all.

Residual transmitters used to be notoriously unreliable. Newer ones may be better - it has been some years since I worked with them.

I would start out with flow paced feed, where the feed rate is a ratio times your wastewater flow rate. Once that is getting you close (a little higher residual than necessary is better than under-dosing) then you can add a simple roll your own control. Use a time delay. Periodically add or subtract a small increment to you ratio if your residual is over or under setpoint. Use a slightly different increment on increase than decrease to avoid hunting.

Thanks Tom, but I am stuck using the PID due to contractual issues.

Tom, didn't you basically describe what a PID does? Just use the P and the I, D in this case would just make it harder to get to where you need to get.

Edit -
That isn't a statement meaning to offend. Just ribbing a little. Oh, I would say straight PID but then what are your two control variables going to be in the cascade arrangement?

Thanks Van,
Yes my latest solution is to put the PID in a periodic task (60seconds) and that sort of works, but slowly.

I guess with a 15 minutes delay, not much you can do to speed it up... In less you know what is coming before it gets there.

Nope, the CL2 reading only changes every 5 minutes.

terry56 said:

Hey guys,
I have a Controllogix and I am tying to control a PID loop for Chlorine (CL2). The trouble is the mixing point is 15 minutes away from where the CL2 is injected, so there is 15 minutes time lag!!

What method do you think is best for control- straight PID or a cascaded PID?

Thanks for any help.

Click to expand...

A cascade loop will do you no good unless you have two feed back devices.
Tom is right about using a PID. I have one of Tom's articles where he used a model. I have never done a chlorine system but I have researched enough to know that that free chlorine is not linear with the dosing and this is why Tom doesn't like a PID. However, a PID can be made to work with care and a lot of effort. A Smith Predictor like queue is required and some idea of how fast the reactions take place is also required. From what I have read this is not an easy application for a PLC and I am surprised that no one has made a dedicated controller for this application yet since it seems to be a common one.

By contractual issues I assume you have a set of plans and specs that call for PID control, undoubtedly written by a process/civil engineer who has never commissioned a control system in his life. Ah yes - the wonder of shelf specs. I have come to the conclusion that a great many programmers and engineers think that "PID" is synonymous with "analog control", even though that is not really the case.

You and Peter are correct - you can make PID work, although it can be somewhat painful. Refinements to the PID algorithm such as tolerance can improve your odds, and very low gain will also help. Nonetheless, even the textbooks will state that PID is not suitable for slow response systems.

One trick I have used in similar circumstnces is to provide two operator selectable algorithms. The specs probably do not say you can provide only PID. I basically provide two subroutines. One implements PID, one implements logic similar to my description above. The operator can switch to the one that works best. Set up both, demonstrate both at the acceptance test, and then let the operator decide. You comply with the contract documents, the system will work, the engineer will have his spec met, and everyone will be happy. Something to consider if you get too frustrated tuning the PID at commissioning.

Tom Jenkins said:

One trick I have used in similar circumstnces is to provide two operator selectable algorithms. The specs probably do not say you can provide only PID.

Click to expand...

Very clever ! !

Tom Jenkins said:

Nonetheless, even the textbooks will state that PID is not suitable for slow response systems.

Click to expand...

Do you mean systems with long dead times? PIDs will work fine on systems with long time constants. It is the dead time that is the killer. Even then you can use a Smith Predictor so the PID thinks there is no dead time.

Right, Peter - I meant systems with large process time lags. In the case of chlorination there are transport lags because the distance between the chlorine feed pump and the injection point is long, and involves a lot of equipment such as evaporators or dosage pumps. Even more significant response delays occur because the residence time in the chlorination tank from the point of injection to exit is a minimum of 15 minutes and more likely an hour or more. To make it more problematic the reaction isn't stoichiometric because suspended solids, ammonia, and nitrate concentrations vary and these absorb and react with the chlorine.

Thanks Tom, yes it is a clever solution. Going out to site tomorrow.

Tom Jenkins said:

Right, Peter - I meant systems with large process time lags.

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You aren't clear. There are dead times and there are time constants. Time lags is not a recognized term.

To be honest, Peter, I am not a sufficient scholar in this area to differentiate between transport time, process time constants, and dead time. All I know is that there is a time lag of many minutes between a change in control output and the response in the measured process variable. This plays heck with the control, regardless of which mechanism predominates.