A process flow diagram is a diagram commonly used in 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.
Typically, process flow diagrams of a single unit process

Unit process

A Unit Process is a step in manufacturing in which chemical reaction takes place, e.g, the oxidation of paraxylene to terephthalic acid is a unit process, the hydrogenation of vegetable oil to ghee is a unit process. In 1930 P.H. Groggins introduced Unit processes in order to classify and...

 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
  Relief valve
  The relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire....
  
   and safety valve
  Safety valve
  A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits....
  
  s
• Flanges

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.
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.