- This topic has 3 replies, 2 voices, and was last updated 4 years, 2 months ago by PLCGuru.
- November 7, 2018 at 12:58 pm #3526Jim ManleyModeratorKarma: 262Rank: Jedi
Looking for a quick sanity check on the following approach to shutting off steam to a couple of devices.
Background: I have two stills that are heated using steam. I also have a cooling tower that is used to provide chilled water to the condensers connected to the stills. In the event the pump that is moving water from the cooling tower to the condensers should stop running, I need to shutdown the steam to the stills. The pump is connected to an AB E300 overload relay.
Approach: My approach is to add logic to monitor the pump’s condition (pump fault bit, motor aux bit, etc.). In the event the pump stops running, I plan to set ProgOverrideReq input on the PIDE controlling the steam valve for each of the stills. The expected outcome is that the CV output of the PIDE would drive to the value of CVOverride, which I would have set to zero, and the steam control valves to the stills would shut off.
Once the issue with the pump has been cleared up, the ProgOverrideReq would be reset to zero, and control of the CV output would go back to the program.
Am I missing anything here?
JimNovember 7, 2018 at 1:36 pm #3527PLCGuruKeymaster
Hi Jim, there is certainly more than one way to approach this. I don’t see any problem doing what you’re suggesting, of course modelling off-line and testing it would be the ideal approach.
I’ve accomplished this too using the CVInitReq and CVInitValue as demonstrated in some of my PID videos on YouTube. You can have a “Process_Stopped” bit driven by all your fault logic and then have CV driven to the desired position (in your case 0 or off) by writing that value to CVInitValue.
When in Override mode the instruction does not compute the change in CV (which is what you want), instead, CV = CVOverride, regardless of the control mode. Since it sounds like you want to handle all of this programmatically your approach should be fine, my only concern is if the duration the system is “down” is too long, handling the restart programmatically could create some other problems? I don’t know the dynamics of the process but just wanted to point that out and maybe something to consider.
Have you tried to implement and it’s not working as expected?November 7, 2018 at 1:47 pm #3528Jim ManleyModeratorKarma: 262Rank: Jedi
The particular situation this is designed to deal with is an “oh crap” moment. Running the stills without the condensers being cooled will result in a pressure build up in the stills, not a good thing. The objective is to shut down the distilling process until the condenser pump is fixed. A by-product of shutting off the steam to the beer still is that all the mash in the still “drops” and we have to start the process over from the beginning.
I’m working on modeling the implementation now. Once I’ve tested and validated the approach, I’ll update my production configuration.
JimNovember 7, 2018 at 1:58 pm #3529PLCGuruKeymaster
Sounds great Jim. I’m sure you will, but be sure to update us too 🙂
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