Process Flow Diagram (PFD)

What is Process Flow Diagram (PFD) ?

Process Flow Diagram (PFD) is a drawing which essentially captures the process flow for a processing plant. PFD is used to capture the main process equipment’s, main process stream, process/design conditions in these equipments and the basic process control scheme in a single drawing. Entire process of a plant can be described using a few interconnected PFDs.

History

This type of diagram has its roots in the 1920s. In 1921, industrial engineer and efficiency expert Frank Gilbreth, Sr. introduced the “flow process chart” to the American Society of Mechanical Engineers (ASME). Over the next several decades, the concept spread throughout industrial engineering, manufacturing and even business, in the form of Business Process Diagrams, and information processing, in the form of Data Flow Diagrams and other chart types.

Frank Gilbreth Sr

Typical content of a process flow diagram

Some typical elements from process flow diagrams, as provided by the open source program, Dia.

Click for image legend.

Typically, process flow diagrams of a single unit process will include the following:

Process piping
Major equipment items
Control valves and other major valves
Connections with other systems
Major bypass and recirculation (recycle) streams
Operational data (temperature, pressure, mass flow rate, density, etc.), often by stream references to a mass balance.
Process stream names

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
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 should normally contain the following information about the plant process.

Main process equipment’s with reference tag numbers, name. Process operating and design conditions are also usually provided.
Main process streams are normally provided with reference stream numbers. It should be noted that streams are different than lines and stream numbers are not related to line numbers in any way. The streams that normally do not have any flow are indicated with an abbreviation NNF (Normally No Flow).
Basic process data for each stream is sometimes given in the PFD against each stream number. This includes data such as operating temperature, pressure, flowrates, compositions etc. for each process stream. Sometimes, this process data is represented for each stream number, in a separate drawing known as heat and mass balance. Design conditions for a stream are not normally indicated in a PFD.
Important isolation valves are also indicated in the PFD. Not all manual valves appear in the P&ID, only a few which can improve the understanding of a process from the PFD. Some valves are indicated as normally closed or locked closed depending on requirement.
Automatic valves – motor operated valves / emergency shutdown valves / control valves appear on the PFD without the associated tag numbers. The purpose is better description of process. Associated control elements are also represented very briefly on the PFD.
Notes are added wherever required to improve the understanding of the process from PFD.
Legend is a list of symbols used on the PFD with brief explanation. This ‘legend’ can appear on each sheet of PFDs or can appear on a single sheet with other sheets referring to a ‘Legend Sheet’.
Interconnections from one PFD sheet to another are used for process streams and instrument control signals to maintain continuity between different drawings.

Some items which appear on the P&ID but may not appear on the PFD are – safety valves, detailed instruments, lines, fittings, drains vents and tag numbers for all of them.

A variation of the process flow diagram is a utility flow diagram (UFD) which captures the essence of utilities required for the process such as, steam, nitrogen, water etc.

Process flow diagram example

The process flow diagram below depicts a single chemical engineering unit process known as an amine treating plant:

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.