PID not cutting it.

rdischar

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Join Date
Dec 2018
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Walton
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16
Greetings Everyone,

I am currently writing a program that controls a chemical reactioin chamber. At a certain point I need automatically control the temperature of the fluid inside the tank. The system is comprised of a hot oil skid and a chiller skid. I send this skid a setpoint and it either diverts the hot oil to the chiller or heater as needed to match my setpoint. There is an RTD in the tank that shows the actual fluid of the temperature. I tried using a PID loop to send the output to the hot oil system based on the RTD signal from the vessel but this resulted in huge over and undershoots. The oil runs around the outer jacket of the tank. I think the problem is that I am using a pid loop to control another pid loop that is built into the hot oil system. Does anyone have any ideas of how I could get better automatic control of this system? I am using a panelview 7 plus 10" and a control logix 5571 controller. Thanks in advance.

Russell
 
I think the problem is that I am using a pid loop to control another pid loop that is built into the hot oil system.

Hi Russell and welcome

Have you tried removing your PID and letting the 'built in' PID run and how good does it control? maybe just sending a PV to the built in would be enough?
 
The only control I have over the hotoil system is sending it a 4-20mA signal which rellates to 70-450 degrees F. If I just send it a setpoint it controls the hot oil temp well but but it does not take into consideration the reactor temp. I tried the PID on the first batch we made but it was constantly cycling between heating and cooling. We ended up just manually monitoring the temperature and adjusting the setpoint in manual mode to get the desired reactor temp. adding chemicals drops the reactor temp considerably and at certain points there is an exothermic reaction that occurs where we need to lower the hot oil supply temp to keep a steady reactor temp.
 
Without knowing about your system sounds like you need a second control or a second PID, can you break your system into two parts?
 
The system is already in place and I do not know how I could break it into two parts. The controller on the hot oil skid is a TIC-2101. How would I incorporate a second PID? I am fairly new to PID loops so any knowledge gained would be beneficial.
 
.... so any knowledge gained would be beneficial.

Nothing like putting the pressure on me :D

Has it ever worked or is this a new system, do you have any rough sketches of your system? there are a few guys that are unbelievable on PIDs on this site so if its just a tuning factor they can help (Peter and/or Ron)

If its a hardware screw up then thats a different issue

The only time I have done a dual loop was in a Honeywell controller
 
Without having more info about the system, I'd probably lean toward one of these scenarios:
- Spend more time tuning the PID. It's possible that you can get better parameters into your instruction thus making it much more stable. You can easily find basic procedures online which should guide you in that process.
- It's possible that the size of your heater/cooler system simply isn't enough to keep it at temperature. In other words, if your heater is too small, it simply can't keep the temperature even when running at 100%. In this case, you'd need a bigger heater or better insulation.

To the above replies:
- What would a second PID accomplish exactly?

Cheers,
Vlad
 
Here is a drawing of the system. On scan 2 is where the reactor rtd (2601A) is located. It is midway down the baffle. I send a setpoint to the hot oil control system. This system either heats or cools the oil going to the jacket of the reactor.
 
Without having more info about the system, I'd probably lean toward one of these scenarios:
- Spend more time tuning the PID. It's possible that you can get better parameters into your instruction thus making it much more stable. You can easily find basic procedures online which should guide you in that process.
- It's possible that the size of your heater/cooler system simply isn't enough to keep it at temperature. In other words, if your heater is too small, it simply can't keep the temperature even when running at 100%. In this case, you'd need a bigger heater or better insulation.

To the above replies:
- What would a second PID accomplish exactly?

Cheers,
Vlad
I think the problem lies in the fact that it takes so long for the reactor to reach the temp i am calling for. It takes about an hour for the temp to get to say 300F. The hot oil is at max 450 degrees by this time. once I reach 300F reactor temp it throws the hot oil system into full cooldown mode turning on all the compressors in the chiller. before the hot oil can cool down enough my reactor temperature overshoots by around 30 degrees. once the reactor actually cools back down to my desired temp the hot oil is around 200 degrees so it then goes into ful;l heat mode and my reactor temp falls even more untill this whole process repeats.
 
And the other systems are the same exact design ? if yes then you know it 'should work'

Have you tried raising/lowering the primary temp? also do you have any flow adjustments you can make? can you increase your oil flow?

If you can run it manually 'good enough' you should be able to run it with a program to mimic the manual running
 
As a chem E, I have to ask, where's the process description?

I'm currently working on a similar project.

So, TYPICALLY, when you have a electrical heater it means long lag time. It means you have to use Feed-Forward somehow and just use PID as a trim to get to your desired setpoint. List out all your IOs. And like other asked, why are you doing this, did it work at one time?

There's way too many going on with the PID you attached. Can you just draw only the relevant control points?
 
It sounds to me like you're on the right track; just need to spend more time fine-tuning the parameters of the new system. Review the documentation to make sure everything is set up correctly, get some trends going on the setpoints, current values, etc & start fine-tuning the parameters of the PID routine.
 
And the other systems are the same exact design ? if yes then you know it 'should work'

Have you tried raising/lowering the primary temp? also do you have any flow adjustments you can make? can you increase your oil flow?

If you can run it manually 'good enough' you should be able to run it with a program to mimic the manual running
The other systems are much smaller. They have a 200 gallon reactor tank. This one is 2000. I can not control hot oil flow. The only thing I can do is control the hotoil setpoint.
 

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