1.Locally mounted instruments.
2.Instrument present in the front end.
3.Instrument present in the back end(Not accessible by operator. This class of instrument is located in the controller.
Each instrument is also identified by means of letters that describes its function as well as progressive numbers to ensure proper identification.
The first letter stands for the variable being measured.
Instrument function is represented by the second letter
Alarm function is represented by the third letter.
Shutdown function is represented by the third and fourth letter.
H-Shut down due to a high value
L-Shut down due to a low value
HH-Shut down due to a high high value
LL- Shut down due to a low low value.
Letters used in Piping and instrumentation diagram
Some examples of instrument representation in piping and instrument diagram
Locally mounted temperature Indicator
Panel mounted temperature Indicator.
Temperature indicator in the DCS/PLC/Controller.
Locally mounted temperature transmitter.
Thermo element representation(Thermocouple/RTD)
Representation of pipes and signal in P&ID:
LCV – Level control valve
PCV – Pressure control valve
TCV- Temperature control valve
FCV- Flow control valve
PSV- Pressure safety valve
ROV- Remote operated valve
MOV- Manually operated valve
FIC/V- Anti surge valve
How to read a piping and instrument diagram?