Process flow diagram
A process flow diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment. The PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations. Another commonly-used term for a PFD is a flowsheet.
Typical content of a process flow diagram
Typically, process flow diagrams of a single unit process will include the following:
- Process piping
- Major bypass and recirculation lines
- Major equipment symbols, names and identification numbers
- Flow directions
- Control loops that affect operation of the system
- Interconnection with other systems
- System ratings and operational values as minimum, normal and maximum flow, temperature and pressure
- Composition of fluids
Process flow diagrams generally do not include:
- Pipe classes or piping line numbers
- Process control instrumentation (sensors and final elements)
- Minor bypass lines
- Isolation and shutoff valves
- Maintenance vents and drains
- Relief and safety valves
Process flow diagrams of multiple process units within a large industrial plant will usually contain less detail and may be called block flow diagrams or schematic flow diagrams.
 Process flow diagram examples
 Single process unit
The process flow diagram below depicts a single chemical engineering unit process known as an amine treating plant:
 Multiple process units within an industrial plant
The process flow diagram below is an example of a schematic or block flow diagram and depicts the various unit processes within a typical oil refinery:
 Other items of interest
A PFD can be computer generated from process simulators (see List of Chemical Process Simulators), CAD packages, or flow chart software using a library of chemical engineering symbols. Rules and symbols are available from standardization organizations such as DIN, ISO or ANSI. Often PFDs are produced on large sheets of paper.
PFDs of many commercial processes can be found in the literature, specifically in encyclopedias of chemical technology, although some might be outdated. To find recent ones, patent databases such as those available from the United States Patent and Trademark Office can be useful.
- ISO 10628: Flow Diagrams For Process Plants – General Rules
- ANSI Y32.11: Graphical Symbols For Process Flow Diagrams (withdrawn 2003)
- SAA AS 1109: Graphical Symbols For Process Flow Diagrams For The Food Industry