Tuning with a Honeywell PID controller

I'm being called in as a consultant on a job with a PID loop. The other integrator is in a situation dealing with an end-user who does not quite understand the correct setup/theory of a control loop. The system heats a "tank" with a heat exchanger and control valve. The kettle is just downstream of the exchanger and has its own temperature sensor. However, this is not their PV. Instead, they have chosen another sensor at the entirely other end of the loop, basically just UPSTREAM of the exchanger, for their PV. Because of this, the loop ends up in constant oscillation, no matter how aggressive or calm the settings are. It's been some time since I've tuned a loop outside of a PLC, but I was told this Honeywell unit is using formula PID A.

Ideally, we all know the PV input should be coming from the object we are trying to control, but we're getting some kickback with that idea.

Just looking if anyone has any thoughts, period.

If I'm understanding right, that will never work, because the whole idea is that you get feedback from the PV. Can't really get feedback from a temperature sensor that's upstream of where it gets heated. Upstream temp sensor isn't going to see your change in heat input, unless flow is basically stopped and heat conducts upstream.

See post #7 on this thread: http://www.plctalk.net/qanda/showthread.php?t=105263

Tom, I was regretting not being able to give your post a star for your reply earlier. It's one to bookmark. How TRUE.

Dan

I was hoping if I have enough people saying the same thing, I can prove to them their current setup will never work.

Tuning PID maybe take few days but Tuning people MIND and EGO take few decade and require high skill psychology.

Love the comments here.

To add to what I said earlier, it sounds like someone (end-user) has the idea in their head that if they know the temperature of the heated fluid (and you have a steady supply), you can simply open/close the control valve as needed to regulate the heat transfer to the kettle. Saying this because it sounds like an engineer or otherwise that has this idea in their head. If this is the case, I'd politely remind them that heat is going to dissipate from basically everywhere and that makes things too complex to use that idea. Best way (like you said and we all know) is to use the most direct source of feedback.

P.S. not trying to be captain obvious, just trying to understand the problem myself to learn problems that other people face. I only get a taste of oil & gas measurement related CSE here.

If the goal is to regulate the temperature of the contents of the kettle but there is no temperature sensor in the kettle to provide a PV for a control loop, then there is nothing to tune.

Using the upstream temperature of the inlet to the heat exchanger as a PV, means, by definition that one is attempting to regulate/maintain that upstream temperature. Why anyone would want to do that? Maybe they do, but I suspect that regulated kettle contents temperature is what they really want.

PID A for a Honeywell controller is conventional PID with the expectation of thermal responses in the mid to slow range. Its accutune algorithm can not handle higher speed flow rates with filtering/samping the input to make it appear slower than reality.

But no single loop controller or PLC PID loop will hold the temperature of a vessel with only HX inlet temperature.

Tomorrow is the day of convincing. If I fail, maybe I will drop them a link to this thread.

Sometimes its just hard to convince someone who doesn't "get it".
Thanks guys.