I have a new problem to solve and was interested how the solution I'm considering will (or won't) function.
First, I am thinking I can configure a "Splitter" block in the engineering units of the slave PID blocks (which is the same for both - GPM). If this can't be done - please comment.
Assuming that's not an issue, I am wondering how the loop will behave: I am maintaining a level with two different flow controllers. I very much prefer that the pristine and pleasant flow from LOOP1 is fully utilized and maximized before beginning to use the defiled and despicable flow from LOOP2. However LOOP1's maximum may not always be the same - under certain conditions, it may fall short of its setpoint by a large margin. So if my "Master" level controller output is spanned 0-600 GPM, and the first 0-300 is all directed to LOOP1 as its setpoint, after which 301-600 is directed to LOOP2 for the remainder, what happens should LOOP1 start to slump or become "limited"? Will the output of the MASTER increase to make up the difference?
What happens if one of the slaves - especially LOOP1 - isn't in CAS? If the other slave is in CAS, will the level control still function or will it go back to IMAN?
If you have any thoughts about a simple function block solution to this challenge I would welcome your advice. I know I could "roll my own" splitter in a CALC block, but I'm not as proficient at all the statuses for master-slave handshaking as I would like . . .
John, I think you could use a splitter block as you described. I would have to try it first to make sure. I'm pretty sure that as long as one of the downstream loops is in CAS, you would be OK (LIC would not go into IMAN). I don't want people to think I believe every problem requires MPC, but this would be a good example where you could use either Predict or PredictPro. Predict would actually be a good solution. Your MVs would be the two flow loops. Your 1st CV would be the level and your 2nd CV would be the tieback of the pristine and pleasant flow loop, as a maximizing shadow CV. The controller would preferentially control the level with the pristine and pleasant feed, but would use the defiled and despicable flow as required if the other flow were not in CAS or otherwise limited. This is not really too different from using Predict for split range control as described by Greg McMillan and others in the past. In PredictPro you would have a 2x1 controller and set the pristine and pleasant flow MV to MAX in the LP Optimizer. No tieback or shadow CV required, although you could easily add both loop outputs (actual valve positions would be best) as shadow constraints. If the defiled and despicable flow comes on too soon, you could also give it a MIN objective in the optimizer. Either MPC approach would require 2 MV license, but setting it up would likely take a day or less. You could spend that much time mucking around with the splitter and you wouldn't have to worry about the dynamic response when the pleasant and pristine loop saturates prematurely. The TSS for the level and flows would be very short and step testing both MVs could probably be done in half a day or less. If you already have decent level and flow loop dynamic models previously, you might be able to skip the step testing altogether.
In reply to DBacker:
In reply to Bruce Brandt:
In reply to Andre Dicaire:
In reply to John Rezabek:
In reply to Lou Heavner:
This is the official online community site of the Emerson Global Users Exchange, a forum for the free exchange of non-proprietary information among the global user community of all Emerson Automation Solution's products and services. Our goal is to improve the efficiency and use of automation systems and solutions employed at members’ facilities by sharing our knowledge, experiences, and application information.
User Groups |
World Areas |
Community Guidelines |
Legal Information |
Contact Community Manager
Website translation provided by
© 2015-2019 Emerson Global Users Exchange. All rights reserved.