Process and Instrument Diagrams (P & IDs)

In the previous post, we looked at Process Flow Diagrams (PFDs),  where we indicated that a PFD represents a big picture of the entire process. In this post, we look at the Process and Instrument Diagrams (P & IDs), where we will try to get more details that weren’t shown in the PFD. Let's consider the compressor control system diagram below:

From the above, we can see that there is more instrumentation associated with the compressor than just a flow transmitter. We have the differential pressure transmitter (PDT), a flow indicating controller (FIC), and a recycle control valve (FV42), that allows some of the vapor coming out of the compressor discharge line to go back around the compressor suction line. Also, we have a pair of temperature transmitters (TT41 & TT43) reporting suction and discharge line temperatures to an indicating recorder.

Additional details emerge in the P & ID above, the flow transmitter, flow controller, pressure transmitter and flow valve all bear a common number 42. This common, ”loop number” indicates these four instruments are all part of the same control system. An instrument with any other loop number is part of a different control system, measuring and/or controlling some other function in the process like the two temperature transmitters and their respective recorders, bearing the loop numbers 41 and 43.

You can also read: Process Flow Diagrams

The other information we can derive from the P & ID above, are the different instrument ” bubbles” used. Some of the bubbles are just open circles, while others have lines going through the middle as shown below:

Each of these symbols have meaning according to the ISA ( Instrumentation, Systems and Automation Society).

The type of “bubble” used for each instrument tells us something about its location. The rectangular box enclosing box enclosing the temperature recorders (TIR 41 and TIR 43) shows they are part of the same physical instrument i.e. this indicates that there is really only one temperature recorder instrument, and that it plots both suction and discharge temperatures (most likely on the same trend graph). This suggests that each bubble may not necessarily represent a discrete, physical instrument, but rather an instrument function that may reside in a multi-functional device.

The P & ID shows more details than PFD, but we cannot see other details like the cable types, wire numbers, terminal blocks, junction boxes, instrument calibration ranges, failure modes, power sources etc. To examine this level of details, we need to look at the loop diagram.

Don't miss out on key updates, join our newsletter list here.