Piping and instrumentation diagram

Balloon symbols

Balloon symbols or instrument bubles are circular P&IDs symbols representing a system's instruments or control functions.

[2] A circle symbol is comprised of 3 characters:

• The first top letter indicates the parameters being controlled. E.g., flow = 'F', temperature = 'T', level/pressure = 'L', etc

• The second top letter indicates the type of control device used. E.g., transmitter = 'T', valve = 'V', etc.

• The bottom number indicates the loop (tag) number, i.e., the control circuit the device is part of. It's called a loop number as circuits are physically/conceptually closed feedback loops where devices (sensors, controllers, controllers) reads and sent a data in loops.

• The line or the absence of a line indicates the device's location.