Control Logix PID tuning help!

[chadhoot78]

I have a PID installed in ladder on a very slow process. My PV is an ORP analyzer. The CV is connected to a chlorinator which pumps chlorine into our clarifier (tank with water). It takes a couple of hours by the time the chlorine is pumped to see any change in the PV. It works great until it reaches the setpoint. The PID backs down the CV, but by the time it reaches my analyzer, it overshoots badly. What is the best way to tune a process this slow?

 

[Tom Jenkins]

You've got a tough assignment. Not only is your response time very slow because of the poor mixing and large volume of the clarifier, the ORP response to chlorine is going to be non-linear.

I would suggest that you consider abandoning PID control and go to a flow paced control - chlorine feed rate is wastewater flow times a constant. You can trim the constant based on the ORP error, clamp min and max rates, and implement the trimming loop on a time delay basis.

Just out of curiosity, why are you chlorinating in the clarifier instead of in a chlorine contact tank.

 

[chadhoot78]

Well, i didn't explain all of the logic properly. The output of the PID is added to a constant times the flow rate into the clarifier. The PID's output is scaled to either add to the flow x's constant or trim it back. It was working fine until the sun went down and it rained. The ORP shot up through the roof. This may not work at all. The operators can watch it and adjust it manually but automatic adjustment isn't working out. Wonder if it would be better to try and add a chlorine analyzer.

Flow x's constant (250 x.05)=12.5(varies based off flow) 12.5 + pid output 0-100% = -5 - 25
the chlorinators are oversized to that's why 20 is 100%

 

[jimfun71]

I don't know what kind of volumes your tank is but I used to have 20,000 gallon chiller units that chilled hot fill botteling lines and we used the Hach CL-17 chlorine analizer that worked very well even with constantly changing conditions. Can't remember the exact tolerences but thet were pertty tight.