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P&ID Information

P&ID Information

SCADA/HMI systems are becoming very prevalent, and the ability to display any point in a PLC on a SCADA screen is easy and rapidly configurable. Also, it is easy to alarm every point, and many are configured to have multiple alarms. Should every physical point monitored in the SCADA system be indicated with an ISA shared display/control bubble on P&IDs? How about alarm bubbles?

I'm replacing a DCS with a PLC, and I have enough bubbles on my P&IDs from field devices, DCS points, etc. that are already in existence. I don't want to add bubbles for PLC points, then restructure the shared display/control bubbles to indicate every monitored/alarmed point. This would make my P&IDs very difficult to redline and redraw.

New projects pose the same problems: how much detail should be assigned to the bubbling of HMI/SCADA points? (I would bubble every PLC point for a new project.)

Anyone got any ideas or current practices they could share?

RE: P&ID Information

At my company, we draw a typical on side of the drawing and illustrate all the alarms. We then include text point out that all elements on the drawing have these same alarms unless other wise noted.

RE: P&ID Information

You're right - P&IDs can get very messy with all the bubbles and connection lines to depict the control system. It can also get very time consuming (and therefore costly) inputting and keeping all that information up to date.

The question to ask is what level of detail is useful. At our company, we show all the instrument valve and drive prime elements on the P&ID, indicating the process alarm conditions (Level, Flow and Pressure H, L, HH, LL, etc). We've taken the decision not to show the control interconnections and the PLC I/O.  I think they're better described in a Functional Specification, with I/O, alarm, inhibit and interlock schedules - in practice the control systems installers and maintainers will refer to these rather than a web of control links on a P&I Diagram.

Hope that's of some help.

RE: P&ID Information

I agree with senglish, especially with the use of a Functional Specification. You can sometimes show details in an attached I/O list or logic diagrams.

However, some companies I have worked for insist on showing every input and output within the control system - including addresses. It usually means double or triple the normal number of P&IDs. Tag-numbering plays an important role, too. You can easily run out of numbers with some P&ID schemes.

Some companies use one bubble [type] and differentiate PLC/DCS/SCADA/Computer with text labels next to the bubble. In other words, you might have one bubble (or would that be a square or diamond or ...?) with a small "PLC" and/or "DCS" next to it. Those don't take a lot of room or time, and they at least tell the observer that the signal is available in that system.

Alarms often don't appear in separate bubbles. Rather, a small "LAL" or "PAH", etc. is placed next to the controller or indicator bubble (usually upper right).

You just want to make sure the P&IDs are useable and don't become headaches to keep up to date.

RE: P&ID Information

Many companies advocate abreviated P&IDs.  I tend to agree.  For an example with a valve transducer mounted on the valve actuator, show a controller ouput from the flow indicating controller symbol to the actuator on the control valve.  Consider avoiding a separate bubble to identify the current to pneumatic transducer.  However, track the additional devices in the index and on the loop diagrams, etc.  Definetly minimize the assorted display data if you are doing the work.  The alarm or other aritmatic character outside the controller box is normal.  However, you may have to otherwise define the requirements with specification rules if the P&IDs and cause and effects lack such detail.

New projects differ from plant modifications.  On existing projects try to stay consistent with the existing practices.

On new projects consider several purposes of the detail.  For example, you could show a separate TE with signal to a TT if they are separate devices and if the electrical designers actually read the P&ID.  I recommend integral TE with TT.  In the integral transmitter installation I would avoid any reference to the TE.  Piping would locate the tap for the thermowell and an instrument or electrical designer would use this for any conduit, junction box, etc.  However, consider showing both devices if someone must accommodate conduit and cable between a temperature element and transmitter.


RE: P&ID Information

Thanks for all your posts. I've developed a schema for my own use from your suggestions.


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: P&ID Information

I work for an EPC company that has several end-user companies as clients (ie: Imperial Oil, Shell, Petro Canada, Chevron, Husky, ect).  Each client has different prefrences of what they want to see on the P&ID's.  Some client's prefer to only show the bare-minimum of what is necessary.

ie:  They won't have a call-out bubble for a Thermowell in a line since it's implied in one way or another (usually a design spec or job instruction).  

Same would apply to I/P converters - and positioners - on control valves.  They won't show those call-out bubbles unless it is necessary (say you have a solenoid in between the positioner and actuator).

Other end-users prefer to show everything, and I do mean everything on the P&ID.  Some examples would be: showing the instrument manifolds, showing the connection type (flanged vs threaded vs welded), showing all alarm points (engineered alarms and operator alarms - more on this in a bit).  These P&ID's tend to become cluttered and, in my opinion, lose their value as being the focal point of tying process, piping and instrumentation.

Now to answer your question:

On your P&ID's, show what is absolutley necessary and nothing more.

It appears you don't have any problems with the tagging of field devices, but you have problems with what detail you should show your control philosophy and how it is all tied together.

Let me ask you this question, are you switching everything from a DCS-type system to a PLC or SCADA system?  Does the company you work for have a P&ID legend sheet?  Do they make a distinction between DCS controlled instruments vs PLC?  

If so, then I suggest that you modify your P&ID's to however your P&ID legend sheet advises when it comes to showing what instruments are tied where and how.  This is the simplest answer, although it appears that you don't really feel like changing your P&ID to show this.  I suggest you do so for the following:  Your P&ID should be your focal design document.  It needs to be your scoping document as well.  If it isn't, how are you going to know which instruments are going to tie to the PLC and which ones won't?  Are you tying all your instruments or just a few?  How will other people know of your intentions?  Before making any changes to your DCS, PLC, even changing an instrument (one technology for another in this case:  Such as changing an orifice flow loop to a vortex), you should do them on the P&ID and have all responsible check and approve your design.  Even if it is something simple as changing an alarm value.  

This actually brings me to another point:  Alarms.
It seems to me that you are showing too many alarm points on your P&ID's.  It may be prudent to do an alarm rationalization.  Just because you can monitor, and have an alarm for every single point doesn't mean you should.  Moreover, too many alarms can be detrimental to safety as operators (or anyone monitoring a process) tend to start ignoring alarms and may ignore an important one.  Also, too many alarms brings to question alarm priority.  What if you have multiple alarms?  Which alarm needs to be addressed first?

Based on what you said, I think you should do an alarm rationalization first to determine which are engineered alarms.

Engineered alarms are alarms that come from Hazop, process, and the control systems personal for safe operation and shut-down.  In my opinion these alarms are the only alarms that need to appear on the P&ID.  If you want, you should allow for operators or others to create pre-alarms for their puroposes in the HMI.  These need not be shown.

After you've done an alarm rationalization, what should be shown on the P&ID's (if I were doing this) is as follows:

- Engineered Alarms
- Interlocks w/ Interlock numbers (some people don't like showing interlock numbers but in my experience it's easier to look at a P&ID to see what's going wrong rather than trying to figure out which narrative I need to read and interlock numbers on P&ID's help me quickly determine what's tied to what)
- Show safety and shutdown with different symbology than process (Refer to P&ID legend sheet)
- Only show on the P&ID what makes sense to be shown, not what you think should be shown.  Remember that the P&ID is a document used by everyone and as such should be kept to minimal detail to avoid clutter.
- Rationalize your HMI.  Multiple thermocouples in a vessel or compressor need not be shown as individual monitoring points on a HMI screen.  You can use one display and have the PLC only show which temperature is highest (or lowest, whichever one is most critical).
- Show enough of the control philosophy such that one knows which loops are interacting with what.  Show which loops are tied to each other and what systems are tied to each other and how (A physical wire or a "soft" or network link).  This is critical in troubleshooting.

Hopefully I answered your question and hopefully this helps!!

RE: P&ID Information


Your response was very helpful. I would like to see the use of Interlock numbers you reference in your reply. I have not seen such a thing on P&IDs before. If possible to send a P&ID illustrating this concept? I would be very grateful.

Bob Cook

RE: P&ID Information

RGCook, I can't send a copy of a P&ID due to licensing agreements with End-User companies.

However, if you give me you're email address I'll send some PDF "sketches" showing these examples.

My email is:



RE: P&ID Information

Also consider using several typical P&ID details.  These would have a graphical representation of the abreviated P&ID with another representation that shows much more detail.  Examples might include shutdown valves with a different detail to reflect where position switches and solenoids wire and whether or not the valve has a manual reset based upon the detail number.  Similar notes could cover thermowells, etc.


RE: P&ID Information

jsummerfield, in my opinion I wouldn't go into that much detail on a P&ID.  I would show the limit switches and solenoid valves, and even the manual resets but I wouldn't show where they are wired from.  As far as the P&ID goes, in my opinion, that is information that isn't required.  Moreover, I would only show the interlock (with interlock number of course) ;) that is tied to the solenoid and limit switches.

Even showing soleniods, limit switches and manual resets tend to sometimes create allot of clutter on P&ID's.  To get away from that, we usually identify just the valve and interlock number on the P&ID with a note refering to a typical P&ID that shows the valve and all it's accesorries (I can't spell).


RE: P&ID Information

I agree with cornejoi.  The detail that I describe is on a P&ID legend sheet so that the individual P&IDs can be very clean.  Sometimes the legend is associated with the PFDs.  Assure that the P&ID notes reference the PFD legend.


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