Process Flow Diagrams

Instrumentation has its own standardized way of making descriptive diagrams. In this article, we are going to look at the types of diagrams commonly used in industrial instrumentation like the Process Flow Diagrams (PFDs). Essentially we have the following types of instrumentation diagrams:

• Process Flow Diagrams (PFDs)
• Process and Instrument Diagrams (P & IDs)
• Loop Diagrams (Loop Sheets)
• Functional Diagrams

At the highest level, an instrument technician is interested in the interconnections of process vessels, pipes and flow paths of process fluids therefore he/she would be more likely go for the Process Flow Diagram (PFD), that represent the big picture of the entire process.

At the lowest level, the instrument technician will be more interested in the interconnections of individual instruments including all the wire numbers, terminal numbers, cable types, instrument calibration ranges etc. The proper form of diagram for this level of fine detail is a loop diagram.

Process and Instrument diagrams (P & IDs) lie somewhere in the middle between process diagrams and loop diagrams. A P & ID 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. You are able to view the flow of the process as well as the flow of information between instruments measuring and controlling the process.

Functional Diagrams are used to document the strategy of a control system. In a functional diagram, emphasis is placed on the algorithms used to control a process, as opposed to piping, wiring, or instrument connections.

An instrument technician has the responsibility of reading the different diagrams when troubleshooting a complex control system. First you begin with a PFD or P&ID to get an overview of the process to see how the major components interact. After identifying which instrument or loop you need to investigate, you go to the appropriate loop diagram to see the interconnection details of that instrument system so that you know where to connect your test equipment and what signals you expect to find when you do so.

You can also read: 

• Introduction to Industrial Automation
• The Basics of Industrial Instrumentation Systems

Process Flow Diagrams

To help understand better process flow diagrams, we are going to examine the diagrams of a compressor control system. In this process, we assume that water is being evaporated from a process solution under partial vacuum that is being provided by the compressor. The compressor then transports the vapors to a knockout drum where some of them condense into liquid form. As a typical PFD, this diagram shows the major interconnections of process vessels and equipment. But it omits details such as instrument signal lines and auxiliary instruments.

From the diagram above you might find it hard to determine which control system if any, controls the compressor. All that the PFD shows relating directly to the compressor is a flow transmitter (FT) on the suction line. This level of uncertainty is acceptable for a PFD, because its purpose is merely to show the general flow of the process itself and very little details on Control Instrumentation.

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