A PFD is a schematic representation of the sequence of all relevant operations occurring during a process and includes information considered desirable for analysis.
The Purpose of a PFD:
Process Flow Diagrams are required for the following reasons:
Plant Design Basis:
A PFD shows the plant design basis indicating feedstock, product and main streams flow rates and operating conditions.
Scope of Process:
A PFD serves to identify the scope of the process.
A PFD shows graphically the arrangement of major equipment, process lines and main control loops.
A PFD shows utilities which are used continuously in the process
A PFD typically contains the following information: This list is by no means exhaustive.
1. All process lines, utilities and operating conditions essential for material balance
2. Type and utility flow lines which are used continuously within the battery limits
3. Equipment diagrams to be arranged according to process flow, designation,
and equipment number
4. Simplified control instrumentation pertaining to control valves and the likes to
be involved in process flows.
5. Major process analyzers
6. Operating conditions around major equipment
7. Heat duty for all heat transfer equipment
8. Changing process conditions along individual process flow lines, such as flow rates,
operating pressure & temperature, etc
9. All alternate operating conditions
10. Material balance table
There are also items that are not part of a PFD except in special cases or circumstances. These items include:
1. Minor process lines which are not usually used in normal operation and minor
equipment, such as block valves, safety/relief valves, etc.
2. Elevation of equipment.
3. All spare equipment.
4. Heat transfer equipment, pumps, compressor, etc., to be operated in parallel
or in series shall be shown as one unit.
5. Piping information such as size, orifice plates, strainers, and classification into
hot or cold insulated of jacket piping.
6. Instrumentation not related to automatic control.
7. Instrumentation of trip system, (because it cannot be decided at the PFD
8. Drivers of rotating machinery except where they are important for control line
of the process conditions.
9. Any dimensional information on equipment, such as internal diameter, height,
length, and volume. Internals of equipment shall be shown only if required for
a clear understanding of the working of the equipment.
This is essentially the basics of what a process flow diagram is and how they are used in the process and other industries depict a process.