Thursday, 6 March 2014

Process (or is it Piping) and Instrument Diagrams. And PFD's

With the ongoing efforts to produce a new standard for P&ID's I thought I would post a bit about my history with these diagrams and where they stand.

I first saw a P&ID in 1976 (only it was called an Engineering Line Diagram or ELD,) this huge sheet of paper covered with equipment connected by pipes, alphanumeric codes all over the place, circles (balloons) with or without lines inside them, strange little symbols - and Notes.
I had started a new career working for an EPC company having previously worked in electronics. I had some understanding of electronic circuit diagrams, but this was new to me.
The P&ID’s were on stick files in the Instrument department which I had joined as a novice Instrument Engineer.
Among the things I had to do were
List the instruments (as identified by the balloons) on paper, assigning them unique numbers in the paper based instrument index
Find all the back of panel instruments and complete specifications for them
Draw the diagrams for wiring them up
Specify the Zener barriers for intrinsic safety

This was about the time that the first DCS's were coming into use and before long I found myself working to specify the DCS, which required a lot more than just those tasks, not least understanding something about the process depicted on the P&ID's.

Soon I came to understand that our copies were always out of date, the only up to date ones being the marked up ones in the Process Engineers office. And even then there was information missing and a surprising amount that was actually wrong.

As my understanding and experience with these documents grew I discovered much more about how they develop and why they are so error prone. Of course these days these diagrams are CAD  drafted but otherwise the same problems are still apparent -  the CAD systems, even these days, being mostly little more than a computer based version of paper and pencil. So called Smart P&ID’s are helpful, but on the other hand they actually propagate these errors faster.

I now believe that P&ID's are based on an ancient paradigm that should have had it’s day.
For example, a process is a mass of data, mass and energy balances, pressures and temperatures, flow rates and so on. But there is absolutely nothing about a P&ID that really tells you these things.

Of course much of this is established on the PFD’s (Process Flow Diagrams) but not all, isolation between differing parts of the equipment for example. And there is no relationship between the PFD’s and the P&ID’s

Perhaps ISA5.7 will address this a little bit, there is at the time of posting some discussion going on!