P&ID Letter and Symbols Reference
Common letter used in Piping and Instrumentation Diagrams (P&ID)
Table is based on ISA-5.1-1984 (R1992)
| Symbol | First Letter | Succeeding Letters | |||
|---|---|---|---|---|---|
| Letter | Measured or Initiating Variable | Modifier | Readout or Passive Function | Output Function | Modifier |
| A | Analysis (5, 19) | Alarm | |||
| B | Burner, Combustion | User Choice (1) | User Choice (1) | User Choice (1) | |
| C | Conductivity | Control (13) | Close | ||
| D | Density/Sp. Gravity | Differential (4) | Deviation | ||
| E | Voltage | Sensor (Primary Element) | |||
| F | Flow Rate | Ratio (Fraction) (4) | |||
| G | Gauge | Sight Glass, Viewing Device (9) | Ready | ||
| H | Hand (Manual) | High (7, 15,16) | |||
| HH | High High | ||||
| I | Current (Electrical) | Indicate (10) | |||
| J | Power | Scan (7, 24) | |||
| K | Time, Time Schedule | Time Rate of Change (4, 21) | Control Station (22) | ||
| L | Level | Light Pilot (11) | Low (7,15, 16) | ||
| LL | Low Low | ||||
| M | Motor or Moisture | Momentary (4, 25) | Middle, Intermediate (7, 15) | ||
| N | User Choice (1) | User Choice (1) | User Choice (1) | User Choice (1) | |
| O | User Choice (1) | Orifice, Restriction (23) | Open | ||
| P | Pressure, Vacuum | Test Point connection (26) | |||
| Q | Quantity/event | Integration, Totalize (4) | |||
| R | Radiation | Ratio | Record or Print (17) | ||
| S | Speed, Frequency | Safety (8) | Switch (13) | ||
| T | Temperature | Transmit (18) | |||
| U | Multivariable (6) | Multifunction (12) | Multifunction (12) | Multifunction (12) | |
| V | Viscosity, Vibration, Mechanical Analysis (19) | Valve, Damper,Louver (13) | |||
| W | Weight, Force | Well or Pocket | |||
| X | Unclassified (2) | X-axis | Unclassified (2) | Unclassified (2) | Unclassified (2) |
| Y | Event, State or Presence (20) | Y-axis | Relay, Compute, Convert(13, 14 18) | ||
| Z | Position, Dimension | Z-axis | Driver, Actuator, Unclassified Final Control Element | ||
NOTES:
- A first letter used with a modifier is treated as a first-letter entity. Example: TDI for differential temperature.
- To cover all analysis not described by a users choice letter. The type of analysis must be defined outside the tagging bubble.
- To be used in lieu of a combination of first letters. Generally used for multipoint recorders/indicators.
- Using these modifiers is optional. Example: The letters H and L may be omitted in the undefined case.
- 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).
- Applies to instruments that provide an uncalibrated view, such as a sight-glass level gage (LG) and television monitors.
- Normally applies to analog or digital readout.
- 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).
- Used instead of a combination of other functional letters.
- 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.
- Used generally for solenoid devices and relays. For other uses, the meaning needs to be defined outside the tagging bubble.
- 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.
- The terms high and low when applied to positions of valves denote open and closed positions, respectively.
- Applies to any form of permanent storage of information.
- Used for the term transmitter.
- 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.
- Not to be used when control or monitoring responses are timed rive nor time/schedule driven.
- To signify a time rate of change of the measured variable. Example: WKIC means a rate-of-weight-loss indicating controller.
- Used to designate an operators control station, such as a manual loading station (HIK), or the operator interface of a distributed control system.
- Used also to designate a restriction orifice (FO).
- Used also to designate a temperature-scanning recorder (TJR).
- Used also to designate a hand momentary switch (HMS).
- For example, an analysis test point is identified as AP.