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I've also been working on some shapes for P&ID drawings, besides the basic shapes Visio shipes with, I made loads of shapes specific to a company I did the work for. As P&ID's are works in progress, you need to be able to change shapes without going true the hassle of reconnecting everything, so I made a template to easily add new shapes using CAD drawings.
I made variation to the shapes possible by for example having the top level group make the isolation group its normal size or 0*0 (hidden).
More basic shapes like valves have all the options like normaly open and normaly closed, 2-way, 3-way and angle valve etcetc.
All the shapes have shapedata that makes them linkable with PDM software. Visio is able to create (basic) listings from the visio file.
I then created a Excel template that can be then edited to look the way they like, and then be used to fill in data (instrument listing) then the Excel can be linked with Visio...
All in all I think Visio is an amazing tool for P&ID drawings, they become far smarter then the plain drawings you would end up with using CAD software.
I won't be able to upload my stencils though, as I did all this for a company...
- Lars