What goes on a P&ID?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The seventeenth question is from Hector Torres in Mexico:

I was wondering if you would have some sort of guides to construct a P&ID that one can follow to design a new process and depict it in a P&ID. I found the InTech article P&ID: A roadmap for the rest of the trip – An ISA standard on the horizon for “faster, better, cheaper” P&IDs.

Answers from Hunter Vegas (Avid Solutions, Inc.):
I looked at the article about P&IDs and generally agree with the guy with the exception that I would always show fail position of the valves as the P&ID is the primary document that is used in Hazops here in the U.S. and the fail position is critical to the team’s understanding of what the system will do in emergency situations. Let me provide you with some examples of things I have seen that I consider both good and bad so you might learn from others.

First and foremost, the P&IDs should generally follow ISA symbols for the basic instruments. If you follow that symbology, most anyone can read and understand the P&IDs quickly. However, there are a lot of “gray areas” where the ISA rules don’t specifically say what you should do and this is where the engineer has some flexibility. Unfortunately, engineers tend to be very detail oriented so often the I/E engineer tends to want to show EVERYTHING in the interest of being thorough. While this is fine for a simple P&ID, it can become a NIGHTMARE for a P&ID that is showing a batch reactor. Suddenly you can’t even see the reactor for all the ISA symbols littering the page. Consider these examples:

VALVE_COMPLICATED shows how one client shows an on/off valve. I like the fact that the P&ID indicates the fail position (FC) as well as the valve type (it is a ball valve). However, going to the trouble of showing the solenoid as well as the air connections and both limit switches takes up a lot of room. It’s fine if there are only a few valves on the page but think what a batch reactor P&ID might look like with some 40 valves all over the page!







P&ID2_VALVES shows how another client did it. Here the client still shows the fail position but doesn’t indicate the valve type. The solenoid is implied but not shown and the limit switches are grouped as a single bubble (ZSC/O). This obviously takes up less room.







P&ID3_VALVE shows yet another on/off valve. This version doesn’t even mention the ZSC/ZSO, it just implies it with the “2DI” beside the bubble. What I do NOT like about this version is the XV-09 tag. The P&ID is in the 120 series so the implication is that the actual tag is 12009. I realize this method saves some room but it gets complicated and confusing when you are referencing valves from one page on another.







There are lots of ways to show interlocks but probably the best way is shown on SIS_ILOCK and SIS_ILOCK_PUMP. This particular client has an interlock table in the bottom corner of the P&ID with descriptions of the interlock and a number. SIS interlocks are indicated as such, DCS interlocks look the same but have no “SIS” designation, and hardwired interlocks are listed as such in the table and have no square around the interlock number. The use of a unique interlock number makes for easy referencing in the hazops and when interlocks span multiple pages. It is also very handy to have the interlock specifics listed right there during design reviews. Some clients do a similar thing but assign interlock numbers that are unique ONLY TO THAT P&ID. (So every P&ID has an “Interlock 1” that are different from P&ID to P&ID. See P&ID2_ILOCKS.) While this is better than nothing, it gets confusing when an interlock spans across multiple P&IDs. The unique number is a much better way to go.

















P&ID3_LT_VLV shows some interesting items that are worth mentioning. Look at the LI-08 instrument. It shows that the loop has 1 analog input, 1 digital output (that trips a pump elsewhere), and shows high, low, and lolo alarms (including a LAH setting of 90%). While this may be overkill, it is one way to indicate such detail if warranted on your project. Another item worth noting on this P&ID is that the line number includes the line specification in it. “S1B15” is a piping specification that the plant uses to indicate the details of the material of construction for that pipe. Some plants use large piping schedules on the drawing that indicate everything about the pipes (size, schedule, material of construction, pressure, temp, etc.) but this takes up a lot of room on the drawing. If the line number includes the spec number, you don’t have to have all of the other listed.









Hope this gives you some ideas moving forward.

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