181 Process Flow Diagram (PFD) Symbols for Engineers

A process flow diagram is a flowchart that depicts the relationships between major components in a process or circuit. The concept originated in 1921 – it was designed by industrial engineer Frank Gilbreth. Today the concept is often used in industrial plants for chemical and process engineering but the concepts can also be applied to a number of other applications.

Process flow diagrams consist of a series of flowchart symbols and notations to illustrate a process. The different types of flowcharts can vary hugely from hand-drawn flowcharts to complex software flowcharts.

For a single unit process, the flowchart diagram includes:

• Process piping
• Major equipment items
• Operational data (pressure, temperature, etc.)
• Major bypass
• Process stream names

• Pipe classes and line numbers
• Minor bypass lines
• Instrumental details
• Instrumentation
• Isolation and shut off valves
• Controllers (such as a flow controller)
• Maintenance vents and drains
• Flanges
• Relief and safety valves

Process flow diagrams can be used to document a predefined process, improve an existing process, or model a new process. Process flow diagrams can also be known as:

• Process Flow Chart
• Flowsheet
• Plumbing & Instrumental Diagram
• Macro Flowchart
• Top-Down Flowchart
• Block Flow Diagram
• Schematic Flow Diagram
• System Flow Diagram
• System Diagram

Process symbols represent an action, process, or function. They are also referred to as an ‘action symbol’ and are the most commonly used symbols in flowcharting. These types of symbols are often used in software.

Here are 6 steps to create a process flow diagram:

1. Define the scope of the system, what will it achieve?
2. Determine the process boundaries, what causes it to start? Where does it end? What are the boundaries of the process?
3. Identify the process outputs, what is the final goal?
4. Identify the inputs, are there any manual inputs? What is required at each step?
5. Use this data to create a model, add necessary detail such as control points and measurements. Think methodically how do components interact with one another.
6. Add process symbols to describe the component/ equipment.
7. Create a data dictionary, outlining data omitted from the flowchart.
8. Review, revise, and test when necessary.

There are a number of standards related to process flow diagrams in industry, to ensure adequate representation of engineering systems. The most common standards are:

• ISO 15519-1:2010(en): Specification for diagrams for the process industry — Part 1: General rules
• ISO 15519-2:2015(en): Specifications for diagrams for the process industry — Part 2: Measurement and control
• ISO 10628-1:2014(en): Diagrams for the chemical and petrochemical industry — Part 1: Specification of diagrams
• ISO 10628-2:2012(en): Diagrams for the chemical and petrochemical industry — Part 2: Graphical symbols

For software-based process flow diagrams, there are a number of basic symbols to identify. These include:

• Oval – input/ output of the system
• Rectangle – a step in the process
• Arrows/ lines – indicate directional flow
• Diamond – represents a decision

These symbols are not commonly used in engineering diagrams such as mechanical, chemical, and electrical process flow diagrams. These symbols, however, can be found in computer software.