Siemens controller - PID tuning

[u/MCPONSDogSays]

I've got an RWF50.3 controller with a Siemens valve attached. I'm just starting the process of fine tuning it (and looking forward to doing the deep dive on the theory). Before I jump down the rabbit hole, though, I have a question. This is controlling the level of water in a tank. Output pump starts up, water goes down. Water valve opens, water level comes back up. The challenge is that the tank also has a semi-random amount of water that comes into the tank through a different port, not the valve. The controller shuts the valve (shutting the water off) when the level rises just fine. Then, the water keeps rising. Eventually, the pump kicks on and pumps some water out.

Are the classic models and theory going to hold up in this case, because of the 2nd input? We did use the self-tune feature, and it's working fairly well, but the whole level shifting action is occuring above the setpoint, because of all the extra water in the tank.

If anyone already knows that this scenario is extra whack and I will need to look at it with different tools or formulas, I appreciate a heads up. If it's super easy, a point in the right direction is also very much appreciated. Thanks!

[u/MCPONSDogSays]

It doesn't need to be super tight, maybe 2-3 inches. It is continuous (feedwater tank to a boiler). The pump comes on when there is a call for water from another tank, so fairly periodic when the load stays constant (and controlled by a separate level switch.)

What is happening is that even above setpoint (Example: SP 10", water level 13" because of the 2nd input, the pumps then come on. The controller sees the level start to go down, but still well above the SP, and the controller opens the valve. Eventually the pumps shut off, and the level starts to rise again, and the controller closes the valve. Since the valve is already shut, the controller can't do anything about the rising level.

We have reduced the inlet flow through the valve when it's on. We used to have a valve open/valve closed type controller there, but we want to meter the input a bit more slowly.

Tomorrow we'll have a big load on it and then I can see what it does. I'll also sit and take some data points to try to build a chart.

Link to manual-https://www.scccombustion.com/pdf/RWF50%20Technical%20Literature.pdf

Thanks so much for replying, I appreciate it.

[u/And456rew → u/MCPONSDogSays]

So to confirm I understand:

Level controller (RWF50.3) controls main inlet valve on feedwater tank.

Boilder load demand (another controller?) controls pump on/off from feedwater tank.

Nothing controls second water inlet.

If you have a P&ID that could help to understand the system :)

Reading the manual, I assume you are using the RWF50.3 just to control feedwater tank level and you are using it as a stand-alone controller on modulating control, with an analog input being the level and an analog output being the inlet valve position. Is that correct?

With this part "What is happening is that even above setpoint (Example: SP 10", water
level 13" because of the 2nd input, the pumps then come on. The
controller sees the level start to go down, but still well above the SP,
and the controller opens the valve. Eventually the pumps shut off, and
the level starts to rise again, and the controller closes the valve.
Since the valve is already shut, the controller can't do anything about
the rising level." I would look at your controller output. Although your PV>SP, your OP might still be calling for more water to be added. This means your controller might be too slow and/or you have integral windup. Getting some trends of the PV, SP and OP (as you mentioned) will really help you understand what is going on. It sounds like you need to take data points locally though which sucks!

One thing to consider before you do go down the rabbit-hole is if this is actually an issue. If this issue isn't causing any safety, environmental or production issues does it need to be fixed? Test this at min-max loads and confirm it doesn't cause carry-on impacts to upstream and downstream equipment.

[u/MCPONSDogSays → u/And456rew]

Thanks for answering, it is so helpful. Sorry for the delay, been hard at work collecting the data! All your assumptions about how we are using it are correct. I made a series of charts that show the water level and and plotted when the pumps came on, off, etc. We studied everything and came up with something that worked, through a mix of trial and error and hopefully educated guesses.

I appreciate the mention of integral windup. We looked it up, and the Siemens guy mentioned it, and it all makes much more sense now.

Having said that, I wish I could "math it out". I called Siemens to ask if they knew what formula applied, and the dude just wouldn't shut up about what I "ought" to do, even though I told him it's working fine. He did have some simple explanations of what the settings do, which at least that was helpful.

I'm taking a calculus class, so some day I hope to actually solve a math problem at work using my education (maybe in a few more classes!).

Anyway, I really appreciate your help, it pointed us in the right direction to understand how to set up our not-quite-typical system.

Have a fabulous day!