A Piping and Instrumentation Diagram (P&ID) is an engineering drawing/document that depicts how each of the process equipment are interconnected/related to each other. This drawing is genarted during the design stage and provides the basis for development of syatme control schemes such as HAZOP (Hazard And Operability Study) and SDK (Shutdown Keys).
Below is a sample of what a P&ID drawing looks like (source, P&ID & Engineering Drawings Interpretation, Calgary)
The purpose of this article is to illustrate how the P&ID instrumentation leter combination can be used. The table below is a typical lettering convension according to ANSI/ISA S5.1-1984 (R 1992) “Instrumentation symbols and identification”. Image is from “ANSI/ISA-S5.1-1984 (R 1992)”, page 21.
As an example, below are some typipcal leter combination for an instrument. Image is from “ANSI/ISA-S5.1-1984 (R 1992)”, page 21.