Hi,

I have a process of 3 closed-loop pressure loops that slowly (30 min or so) oscillate 4% around setpoint 100% of the time. I have been trying to tune these loops for the past week using ExperTune software but since I lack experience with the software and tuning in general, I have found it to be less than helpful. I have been reading about the different types of loops and was hoping to get some thoughts from the group.

The series of loops in order are: Boiler->Header->Steambox. The boiler and header loops are always running and have fixed set points of 125 and 30 PSIG respectively. The header feeds multiple processes but only the steambox valve is oscillating with the header. The steambox setpoint changes depending on the product we run but stays fixed throughout the run (for reference, it's around 15 PSIG).

At this point, I have confused myself on how I should be tuning these loops. If I've interpreted what I've read correctly, I need to detune the loops so that they are not controlling at the same period. I believe I should be tuning the boiler loop to be the fastest and the steambox to be slowest but my intuition is telling me that it should be the other way around. That is to say, I'd want the steambox to be the fastest because that is what I'm controlling for my end process and therein lies my confusion.

I had actually gone the way of making the header and steambox feedforward loops and feeding the upstream of each into the downstream but I think I misunderstood the purpose and I havent been able to make it work like expected.

Any advice to the situation would be helpful. Thank you.

 

Out of curiosity:

1) How many other "multiple processes" are there and how do their steam loads compare with this process?

2) Have you compared the hardware of the other loops with the hardware of the "other multiple processes" to see if they are identical, roughly similar or considerably different?

3) Have you tried tuning the steambox process in the same manner as the "other multiple processes"?

 

My apologies for my delayed response. Apparently, I must subscribe to my own thread to get the notifications. Here are your answers.

> 1) How many other "multiple processes" are there and how do their steam loads compare with this process?

There are 5 parallel processes that come out of the headbox including the steambox. Three are temperature loops whos valves are open/closed all the way the vast majority of the time (not good control, I know) and the other has flow indication only and so is using the excess steam that is not used anywhere else. For the purposes of tuning, I should be able to treat this series of loops as the only process.

> 2) Have you compared the hardware of the other loops with the hardware of the "other multiple processes" to see if
> they are identical, roughly similar or considerably different?

I have not compared any of the hardware.

> 3) Have you tried tuning the steambox process in the same manner as the "other multiple processes"?

While tuned, the other processes are not really be controlled and are not pressure loops.

 

Let's go with an assumption that the pressure in the header is relatively fixed (not varying much).

Since the rate of oscillation is extremely slow (30 minutes), here is my method of approach:

1) Check the deadband of the process and reduce it to a minimum (+/- 0.5%).

2) Observe and record the output command to the valve throughout the oscillation. Is the valve responding to the command change? Is the line pressure for the pneumatic valve changing?

3) Do you have position feedback sensor that you can observe? What is it showing for the command changes? Without a sensor, observe the visual indicator on the valve during this time?

You said in your original posting that you de-tuned the steambox pressure loop. Can you respond and provide the tuning parameters for each of the three loop? Perhaps you have de-tuned the steambox pressure loop too much!