P&IDs show the actual sequences of the valves, reducers, branches, instrumentation, and equipment in a piping system. As defined by the Institute of Instrumentation and Control a P&ID is:

• A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on the International Society of Automation (ISA) Standard S5.

• The primary schematic drawing used for laying out a process control installation (Piping & instrumentation diagram).