Identification and Reference Designation
Based on Standard ANSI/ISA S5.1 and ISO 14617-6, the P&ID is used for the identification of measurements within the process. The identifications consist of up to 5 letters. The first identification letter is for the measured value, the second is a modifier, 3rd indicates passive/readout function, 4th - active/output function, and the 5th is the function modifier.
| Letter | Column 1 (Measured value) | Column 2 (Modifier) | Column 3 (Readout/Passive function) | Column 4 (Output/active function) | Column 5 (Function modifier) |
| A | Analysis | Alarm | |||
| B | Burner, combustion | User choice | User choice | User choice | |
| C | User's choice (usually conductivity) | Control | Close | ||
| D | User's choice (usually density) | Difference | Deviation | ||
| E | Voltage | Sensor | |||
| F | Flow | Ratio | |||
| G | User's choice | Glass/gauge/viewing | |||
| H | Hand | High | |||
| I | Current | Indicate | |||
| J | Power | Scan | |||
| K | Time, time schedule | Time rate of change | Control station | ||
| L | Level | Light | Low | ||
| M | User's choice | Middle / intermediate | |||
| N | User's choice (usually torque) | User choice | User choice | User choice | |
| O | User's choice | Orifice | Open | ||
| P | Pressure | Point/test connection | |||
| Q | Quantity | Totalize/integrate | Totalize/integrate | ||
| R | Radiation | Record | Run | ||
| S | Speed, frequency | Safety | Switch | Stop | |
| T | Temperature | Transmit | |||
| U | User's choice (usually alarm output) | Multifunction | Multifunction | ||
| V | Vibration, mechanical analysis | Valve or damper | |||
| W | Weight, force | Well or probe | |||
| X | User's choice (usually on-off valve as XV) | X-axis | Accessory devices, unclassified | Unclassified | Unclassified |
| Y | Event, state, presence | Y-axis | Auxiliary devices | ||
| Z | Position, dimension | Z-axis | Actuator, driver or unclassified final control element |
For reference designation of any equipment in industrial systems the standard IEC 61346 (Industrial systems, installations and equipment and industrial products — Structuring principles and reference designations) can be applied. For the function Measurement the reference designator B is used, followed by the above listed letter for the measured variable.
For reference designation of any equipment in a power station the KKS Power Plant Classification System can be applied.
Read more about this topic: Piping And Instrumentation Diagram
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