The process flow diagram shows the arrangement of the major pieces of equipment and their interconnection. It is more a description of the nature of the process than a design document.
The PFD is used by specialist design groups as the basis for their designs. These include piping, instrumentation and plant layout. During plant start-up and subsequent operation, the flow sheet forms a basis for comparison of operating performance with design. It contains the flow rates, temperature, enthalpy and pressure of all streams in the process.
The PFD must show equipment identification, and include a list of the principal assumptions used in the calculations. It is normally reviewed before the release to detail design, to ensure that there is enough information to support development of the piping and instrumentation diagrams (P&IDs) .
The utility connections required on each piece of equipment should be shown and labelled. Utility requirements should be tabulated in the flow sheet.
The process flow diagram completes the initial documentation of the process technology and automation tasks. As such, it becomes necessary to define in detail the measuring and control points. This is done by entering the points on the process flow diagram, and the process flow diagram becomes the P&ID.