PFDs vs. P&IDs

Process Flow Diagrams (PFDs) defines the major elements of the process schematically i.e. the interconnections of process vessels, pipes and flow paths of process fluids. On the other hand, Piping and Instrument Diagrams (P&IDs) shows the layout of all relevant process vessels, pipes and machinery but with instruments superimposed on the diagram showing what gets measured and what gets controlled. P&IDs enable the instrument professional to view the flow of the process as well as the flow of information between instruments measuring and controlling the process. PFD generally represents the big picture of a process because the focus of the diagram is the process itself but the P&ID is made to show more details i.e. the equipment, the piping and all the control systems components.

Process Flow Diagrams (PFDs)

 A PFD is a basic representation of a process that schematically depicts the conversion of raw materials into finished products without delving into the details of how that conversion occurs. It defines the flow of materials and utilities like water and electric power. The PFD defines the basic relationships between key pieces of equipment. Additionally, the PFD defines the capability of a system by listing minimum, normal and maximum conditions. These may include ranges of flow, pressure, temperature and so forth.

The objective of the PFD is to define the design of the process. An example of a Process Flow Diagram is shown below:

Process & Instrument Diagrams (P&IDs)

Process & Instrument Diagrams or also referred to as Piping and Instrument Diagrams (P&IDs) show the interconnection of process equipment and the instrumentation used to control the process.

P&IDs use sets of common symbols and connections are shown between the symbols to represent the process elements and piping. The symbols represent the mechanical equipment, piping, piping components, valves, equipment drivers and instrumentation. These symbols are assembled on the drawing in a way that schematically and clearly defines the process in the correct process order.

An example of Process and Instrument Diagram (P&ID) is shown below.

In reference to the P&ID illustration above, we can notice several instruments included in the diagram in addition to the process equipment (evaporator, compressor and knockout drum); we have Flow Transmitter (FT), the differential pressure transmitter (PDT), a Flow Indicating Controller (FIC) and a Flow Control Valve (FV). We also have a pair of temperature transmitters. The Flow Transmitter, Flow Indicating Controller, Differential Pressure Transmitter and Flow Control Valve all have a common number 24. This common “loop number” indicates these four instruments form part of the same control system. An instrument with any other loop number is part of a separate control system, measuring and/or controlling some other function in the process, for instance the two temperature transmitters (TT) with their respective recorders (TIR), having the loop numbers 23 and 25.

You may also read:

• P&IDs and Loop Diagrams
• The Instrumentation for Furnace Control
• Instrumentation and Controls for a Steam Desuperheater
• Principles of Control Systems