Poll: PLC information on P&IDs

[ecampos]

The attached image shows 4 different ways of drawing a pressure transmitter input to a PLC as it might be shown on a P&ID. My question is which of these is correct or preferred? Or is there another, better way?

The only way I have ever seen this done is some variation of method #2. I don't feel that it should be correct to have the actual I/O channel information on a P&ID though, in my opinion that information belongs only on an electrical diagram.

I can't find many good examples of P&IDs on the internet, and none of the ISA documents I have looked at are very clear about how this should be done either. Anyone have an opinion on this?

 

[Tom Jenkins]

In my opinion, the P&ID is intended to show process relationships, functions, and interconnections. Trying to put too much info on the drawing tends to obscure that intent. Other documents, such as the I/O Point list and loop diagrams, are better suited to identifying analog input channels, PLC addressing, etc.

 

[jkerekes]

I agree with Tom. I wouldn't use any of the four shown.

 

[ecampos]

Interesting. I agree completely, and I would prefer not to have this information on the p&id at all. But this symbol (diamond inside of a square) is listed in ISA S5.1 as being the symbol for PLC connections. I guess my real question is what is this symbol for?

 

[Ken Moore]

I do a lot of work in the process industries, and #1 is very common. It let's engineers know there is a PT connected to the PLC. There are usually more boxes next to the top one to indicate HH or LL alarms, and then software connections to show what devices may be interlocked by these alarms.
On DCS's we replace the diamond with a Circle..... In systems having DCS's and SIS PLC's we would have both, so you can see where the signal is going. You can also show 2oo3 voting etc...