Forcing controller in Auto

[controlnovice]

Just ran across a request by operations where they want to force a flow controller in Auto, such that the operator can not place it in Manual.

Looking at the process, they couldn't think of any reason why an operator would need to place it in manual. They have in the past, only for them to make bad product, because they've forgotten to place back in Auto.

I'm not comfortable not allowing the operator to have control when needed. To me it should be handled with operational procedures. (Written: Don't touch this!)

What's the best practice? Is it okay to force the controller in Auto (done in a DCS)? (of course, we would allow it when unit/phase is not running).

______________________________________________________________________________
This is normally the space where people post something insightful.

 

[IRstuff]

Since the operator can't get proper product in Manual, the assumption must be that the only reason to put it into Manual is to Stop the process, and so it seems reasonable to have a STOP button or control.

TTFN

FAQ731-376

 

[FrancisL]

You really must ask Why the operator put the loop in manual before. They don't normally do that sort of thing with no reason, if they do re-train them.
Preventing operators from operating is not a good idea.

 

[JLSeagull]

A surge control or override could be an example of a PID application that excludes the manual selection. Excluding the manual selection is done but rarely.

 

[IRstuff]

Any anomalies detected by the operator would be way too late for the operator to do anything control-wise about it. Nor is it likely that an operator could control the process any better than the control system that's supposed to be doing it.

Moreover, if there are such anomalies, the control system should be designed to mitigate them or control through them.

TTFN

FAQ731-376

 

[roydm]

There are many reasons to hold a control loop in Auto.
For example:
A pressure control loop where you drop down from a high pressure to a lower pressure. If you put the loop in manual how would you prevent the output from over pressurizing the LP.
Sometimes a PID doesn't even need an operator faceplate.

 

[danw2]

What happens if the flow meter, presumably the input to the PID, fails?

Is that considered a "shut the line down" situation?

 

[JLSeagull]

Actions upon failure of the input are configurable. Few discussions occur regarding this matter outside the controls group. This is rarely discussed by process engineers even during the process hazard analysis. As operations know more about the DCS than most home office engineers, this is an area where the plant operators participating in the hazard analysis could be helpful.