P&ID Symbol Diagram Basics 3/3 Functional Identification and Naming Conventions (2024)

Typical p&id letter combinations are shown in the following table, the table is based on ISA-5.1-1984 (R1992):

NOTES:

• 4. A first letter used with a modifier is treated as a first-letter entity. Example: TDI for differential temperature.
• 5. To cover all analysis not described by a users choice letter. The type of analysis must be defined outside the tagging bubble.
• 6. To be used in lieu of a combination of first letters. Generally used for multipoint record- ers/indicators.
• 7. Using these modifiers is optional. Example: The letters H and L may be omitted in the undefined case.
• 8. To cover only emergency protective primary elements, such as a rupture disk (PSE), and emergency protective final control elements, such as a pressure safety valve (PSV).
• 9. Applies to instruments that provide an uncalibrated view, such as a sight-glass level gage (LG) and television monitors.
• 10. Normally applies to analog or digital readout.
• 11. Used for pilot lights. Example:A running light for a motor maybe identified as EL or YL, depending on whether the measured variable is voltage or operating status, respectively. Used also for process indicating light. Example: A high-level light (LLH).
• 12. Used instead of a combination of other functional letters.
• 13. Used for hand-actuated switches or on-off controllers.It is incorrect to use the succeeding letters CV for anything other than a self-actuated control valve.
• 14. Used generally for solenoid devices and relays. For other uses, the meaning needs to be defined outside the tagging bubble.
• 15. These modifying terms correspond to values of the measured variable, not to values of the signal. Example: A high level from a reverse-acting level transmitter should be LAH.
• 16. The terms high and low when applied to positions of valves denote open and closed positions, respectively.
• 17. Applies to any form of permanent storage of information.
• 18. Used for the term transmitter.
• 19. Used to perform machine analysis(where as the letter A performs more general analyses). Except for vibration, the meaning must be defined outside the tagging bubble.
• 20. Not to be used when control or monitoring responses are timed rive nor time/schedule driven.
• 21. To signify a time rate of change of the measured variable.Example:WKIC means a rate- of-weight-loss indicating controller.
• 22. Used to designate an operators control station, such as a manual loading station(HIK), or the operator interface of a distributed control system.
• 23. Used also to designate a restriction orifice(FO).
• 24. Used also to designate a temperature-scanning recorder(TJR).
• 25. Used also to designate a hand momentary switch(HMS).
• 26. For example, an analysis test point is identified as AP.

EXTRA RULES:

• There are several letters - C, D, G, M, N, O, which can be specified by the user.
• The second column, marked Modifier, adds additional information about the first letter, the process variable. For example, if an instrument is used to measure the difference between two pressures, perhaps the upstream and downstream pressure of a filter press, a P for pressure is used as the first letter and a D for differential as a second letter modifier. When instantaneous flow is being measured and a totalizer is added to provide total flow over time, the device identification is FQ. The first letter of the tag name is F for flow and the second letter is Q from the second column, signifying integrate or totalize.
• The next three columns further define the device. The first of these delineates a readout or passive function.
• The meanings need be defined only once. A users choice letter is intended to cover unlisted meanings that will be used repetitively in a particular project. If used, the letter may have one meaning as a first-letter and another meaning as a succeeding-letter. The meanings need to be defined only once in a legend, or other place, for that project.
• An S as a second letter can be a modifier for the first letter, or it can be classified as a succeeding letter. This can be a bit confusing. If S is used as a succeeding letter, it applies to emergency protective primary elements. In this case, a device normally labeled PCV could also be labeled PSV if it is used as a safety device. The term xCV implies a self-actuated control valve, such as a pressure regulator. The succeeding letter combination CV should not be used in cases where the valve is not self-actuating. If the letter S is used as a succeeding letter, such as in LSH, it designates a switch. Heres how to tell the difference: if the device in question is generating a discrete (on/off) signal, then the S in the second position indicates the device is a switch; if the device is reacting to a variable process condition, then the S signifies a safety function.
• Using X for the first letter is a special case. From ISA-5.1, The unclassified letter X is intended to cover unlisted meanings that will be used only once or used to a limited extent. If used, the letter may have any number of meanings. When properly applied, the letter X does not appear frequently only once, or to a limited extent. Instead, the user-defined letters should be used for devices that appear regularly, even if infrequently. Thus, in many modern industrial facilities, X may not be needed, since most devices appear with some regularity. For those of you that have an entire facility filled with XT transmitters or XY transducers, dont worry, this provision of ISA-5.1 is frequently ignored. The unclassified letter X is intended to cover unlisted meanings that will be used only once or used to a limited extent. If used, the letter may have any number of meanings as a first-letter and any number of meanings as a succeeding-letter. Except for its use with distinctive symbols, it is expected that the meanings will be defined outside a tagging bubble on a flow diagram.
• The grammatical form may be modified as required. Example: Indicate may mean indicator or indicating.
• Many sites will use ISA-5.1 as the starting point.
• Of course, the user must clearly document the specified meanings on the site P&ID legend sheet, and those meanings should be maintained, without ambiguity or change, for the entire facility or, ideally, the entire company. The legend sheet table can then be modified to incorporate assigned letter designations, or even specifically define acceptable or standard letter combinations for the facility.
• Usage will depend upon context. Depending on the circ*mstance, the third letters Control, Transmit, and Compute can also be used as a verb or noun, in which case they will appear in text or speech as Controller, Transmitter, and Computer, respectively.