How to

Deatailed explanation of “How to read a Piping and Instrument Diagram(P&ID)?”

This topic is already(Piping and instrumentation diagram) discussed in instrumentpedia. But now I feel that more explanation is required on this topic.That is why I decided to write a detail post about it.
For reading a piping and instrumentation diagram someone should be very much familiar with the symbols.We can classify the instruments in to three large families.

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:

Process lines
Pneumatic line
Capillary line
Electric line


Control valves are representation 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

Related post:
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asrar February 2, 2012 at 9:17 am

i thank the man who has shared this information and started this blogging i am myself an instrumentation engineer and i wanted to know the piping and instrumentation ddrawing as it will be required by me in futute . i wud b very gr8ful if anybody helps me in sending some type of model p and i diagram with ex i wud do it as my homework

Ankit Chugh November 22, 2012 at 7:41 am

Dear Denizen, even i have written a similar article as per your topic. Here it is- Making and Importance of Process P & ID (Piping & Instrumentation Diagram). Would you mind reading it once and let me know some feedback about it. Thanks!!

inam ul haq February 17, 2013 at 11:16 am

Hi, These symbol are very helpful for information, if I get some more information about the interpreting of P AND ID Drawing,example staring, ending of piping connections,north,south details and piping legends etc.

Jefferson Besa March 2, 2014 at 9:56 pm

You're already an instrument engineer and yet you still don't know how to read a P&ID? How could you,calling your self an engineer

Karren Barlow March 25, 2014 at 10:14 pm

Thanks for the help! I have also found P&ID Diagram Examples through lucidchart that are very easy to use!

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