What is a Process Flow Diagram

The most common PFD symbols in use today come from agencies such the International Organization for Standardization (ISO 10628 – Flow Diagrams for Process Plants, General Rules), the German Institute for Standardization (DIN) and the American National Standards Institute (ANSI.) However, many companies use their own symbols, which are often similar but vary as they become more detailed.

You can find a comprehensive list of standardized symbols with our PFD symbols guide. 

A typical PFD for a single unit process will include these elements:

• Major equipment: Including names and ID numbers. Examples include compressors, mixers, vessels, pumps, boilers and coolers.
• Process piping: Moves the product, usually fluids, between equipment pieces.
• Process flow direction
• Control valves and process-critical valves
• Major bypass and recirculation systems
• Operational data: Such as pressure, temperature, density, mass flow rate and mass-energy balance. Values often will include minimum, normal and maximum.
• Composition of fluids
• Process stream names
• Connections with other systems

What to exclude in a PFD

Typically, these more detailed items are omitted:

• Pipe classes and pipe line numbers
• Process control instruments
• Minor bypass values
• Isolation and shutoff valves
• Maintenance vents and drains
• Relief valves and safety valves
• Code class information

Other Types of PFDs

When the diagram needs to show multiple unit processes at a plant, it becomes more of an overview, containing less detail. These are also called Block Flow Diagrams and Schematic Flow Diagrams. Each block can depict a single piece of equipment or a stage in a process. A rectangle is usually used to show a piece of equipment and labels illustrate function.  The process flow is usually shown from left to right, and arrows show flow direction.

On the other hand, a Piping and Instrument Diagram (P&ID) is more technical, describing mechanical details for piping designers, electrical engineers, instrument engineers and other technical experts who need this detail more than they need process details. P&IDs take the conceptual aspects of a PFD and add detail about the equipment, process sequence, process and utility piping, bypass lines, instruments, valves, vents, drains and other items.