Flow and pressure control

[Jeff Burrows]

Hello all,
I am new to pid loops and control. I was wondering how to control a system with two process variables. This is a water system with a setpoint of say 60 psi and we would want to maintain that in less flow exceeded "X" lets say 800gpm. Then we would need to back it down.
Plc is 1669-L33er.

Thanks in advance
Jeff

 

[Aardwizz]

We're going to need a lot more information (and some better grammar) before we can help you.

You have a pressure transmitter. Fine. Is it measuring the air pressure of a sealed vessel, or the pressure of the water in a tank (e.g., level) ?

You have a flow transmitter. OK. Is it measuring the infeed or outflow rate?

What control mechanisms are you trying to tie with (maybe) a PID loop:
- a flow control valve on the infeed side?
- a flow control valve on the effluent side?
- a gas inlet valve to increase pressure?
- a vent valve to relieve pressure?

You (we) can't control something unless you (we) know what it is you're trying to control.

How does the pressure affect the flow rate, and vice versa?

I assume also that you mean

... we would want to maintain that [setpoint] unless flow exceeded "X".... Then we would need to back it down.

What do you mean by "back it down" ? The control valve position (assuming that you only have one object of control), or the pressure setpoint?

Backing down the pressure setpoint might be the easiest to implement: a separate flow control loop (PID) would try to achieve X flow rate. If the flow is less than X, the loop's CV hit's 100%, which means the pressure loop is 100% of its setpoint ("60 PSI"). As the flow exceeds X, the CV backs down, and so the pressure setpoint is some percentage of "60 PSI".

Easy to implement; probably be a bear to tune. And without knowing all of the above details of your system, this seat-of-the-pants kludge is the best that I (at least) can come up with.

The real PID experts will have better ideas (and a lot more questions).

Cheers,

 

[goghie]

It looks like an autoclave. Am I right?
If yes, we need to know some data about type of valves that are used.

 

[GaryS]

That’s not that difficult to do. And there are many ways to do it.
I assume that you are using a VFD or Multiple VFD’s
One way could o use 2 PID control loops

The primary PID controls the Pressure
The output of the primary PID control I connected to control the VFD output frequency
The primary PID control will operates in forward that’s to say as the pressure decrease the output increases
This would be normal as expected
The secondary PID control will control the Flow
The secondary PID control will be setup to operate in forward as well that’s to say as the flow decreases the PID output increases.
The output of the secondary PID control will be the primary (Pressure)PID controls High Limit Value
So as the flow increases above the high flow set point the PID Controller will limit the output of the primary PIC Control (VFD output frequency )

The low limits on both PID controls should be set to Lowest output frequency of the pump VFD
You will have to tune both PID controls to get things working the way you want.

As a note I have used this method more than a few times

Another way would be to just bump the pressure PID high limit down until the pressure is where you want it then back up when the pressure drops
But I prefer the 2 PID controls better

 

[Bit_Bucket_07]

Hello all,
I am new to pid loops and control. I was wondering how to control a system with two process variables. This is a water system with a setpoint of say 60 psi and we would want to maintain that in less flow exceeded "X" lets say 800gpm. Then we would need to back it down.
Plc is 1669-L33er.

Thanks in advance
Jeff

This could be a candidate for cascade control, with the pressure loop adjusting the CV while the flow loop adjusts the pressure loop's setpoint.

https://www.controleng.com/articles/fundamentals-of-cascade-control/

 

[Peter Nachtwey]

There was a similar topic a couple of weeks ago.
You can't control two things with one output.
Why is it that the OP never gives all the facts.

 

[GaryS]

Peter
Before you condemn it you should set it up and try it. I have been using that for many years

What the control dose is provide a seamless change over from pressure control to flow control and back again.
Below flow set point you are controlling the pressure when the flow increases above the flow set point you are controlling the flow. You could achieve the same results by adjusting the pressure set point using the flow PID control output. When the flow is below the flow set point the flow PID control output will be 100% so the pressure PID high limit will be 100%
As the flow increases above the flow PID controller will reduce its output - the Pressure PID high limit
There are other ways to get the same results you could sent the output of both PID controls to a summing junction and the result of the summing junction to the VFD reference I think that is more complicated
As with any PID control you will need to spend the time to tune it.
And with the Logix processor you should set the PID program to run in continuous loop of 100ms so it can keep track of what’s going on

 

[Peter Nachtwey]

Peter
Before you condemn it you should set it up and try it. I have been using that for many years

You can only control flow or pressure with one output. It is possible to make a compromise if you allow the pressure or flow to deviate from their desired values or set points.

What I am complaining about is the lack of information. No diagram is provided.

I know very well how pressures and flows change.
Maybe I shouldn't waste my time until all the necessary data is provided.