instrument legend "Mo3" question in the P&ID

[koba78]

Dear eng-tips member

I found a "Mo3" (express as middle of three values is used for alarms and downstream function ) located outside of the instrument circle in our project P&ID, but I don't get it why this should be
there, and it seems unnecessary. Could you please give me some
good explanation for this ?

Thank you very much.

 

[ash9144]

Its a lazy way to determine an average. This is fairly common to use with interlocks it is intended to prevent false shutdowns (or alarms) based on an erroneous data point.

 

[lacajun]

I've not put stuff like that on P&IDs because it can easily change then you have the hassle of MOC. The older guys trained us to not put stuff that easily changes on P&IDs such as set points, alarm points, etc. Just acknowledge the alarms through instrument bubbles and call it good. The younger generation wants to put all kinds of just on P&IDs because they have no desire to look beyond those. Well, at some point you're going to need loops, elementaries, control narratives, logic diagrams, etc.

I am not a fan of it but if they have the clout to put logic, alarm points, etc. on them, they can knock themselves out. I fought that battle and lost. They didn't even want to reference other drawings on P&IDs. But, I digress into minutia.