What is a Process Flow Diagram?

A process flow diagram is a representation of various components, how they are arranged, and in what manner or sequence they work in correspondence to each other. It is a graphical way of representing a process.

The process flow diagram in chemical engineering

In chemical engineering, a process flow diagram is a graphical representation or flowchart of all the chemical engineering processes that happen in a plant. A process flow diagram is an important document of any plant and shows the flow of processes in a plant and its equipment. A process flow diagram is also known as a flowsheet. A flowsheet only shows the major equipment of a plant and not the minor ones. A flowsheet is usually prepared by the chemical engineers and it represents the typical interaction of various major equipment of a plant and in which manner or sequence they work to obtain the desired result.

As a process flow diagram or flowsheet shows the flow of all the processes in the form of a flowchart that occurs within a surrounding, the flowsheet is also used in other sectors along with the chemical engineering sector. For instance, the flowsheet is used in a business administration to represent the business process and in what order the company and its components work to achieve the desired objective and maximum efficiency in a business process. A process flow diagram has many applications in various other fields of engineering, science, and others such as pharmaceutical, physics, chemistry, environmental, mining, economics, and so on.

Contents of a Process Flow Diagram

The following are the contents of a process flow diagram-

• Representation of all the major equipment along with their description and equipment number.
• Representation of flow streams of processes and their names and numbers.
• Representation of chemical composition of each stream and description of process conditions.
• Representation of all the utility streams that are supplied to the major equipment.
• Representation of all the basic control loops.
• Representation of connections of various systems.
• Representation of major bypasses.
• Representation of operational data such as the pressure, temperature, density, and so on of the units.

The image below shows the process flow diagram of an amine-treating plant. The following flow diagram is an example of a single chemical engineering unit process.

Types and uses of Process Flow Diagrams

In brief, the process flow diagram can be categorized into two types, a single-unit process diagram, and a schematic flow diagram. The schematic flow diagram is commonly known as a block flow diagram. A single-unit process diagram is a process flow diagram that is used for small plants. In the single-unit process diagram, all the equipment and processes are represented in detail in the form of a flowchart. A block flow diagram is used for larger plants. In the block flow diagram, all the types of equipment in the process are represented in the form of a flowchart but the details given are very less. The flowsheet has various applications and is an important document, hence, the flowsheet development and process design need to be done properly by the chemical engineers. A flowsheet is necessary for preparing piping and instrumentation diagrams, often referred to as P&ID. The piping and instrumentation diagrams include various information such as the equipment in a plant, utility lines, pipes, vents, dents, bypasses, and so on, and hence it is a very complex diagram as it contains such minor details. The process flow diagram is also useful for obtaining an overview of all the processes taking place in a plant and making improvements as required to obtain maximum efficiency.

Process flow diagram symbols, signal, and process lines

Process flow diagram symbols or PFD symbols are the collections of diagrams that show different components of a chemical process and also represent the various instrumentation devices used in a process. The PFD symbols also represent the interconnection of various processes in the form of a flowchart. Signal lines are the lines that represent the type of signal provided in the process such as pneumatic signals, electronic signals, and so on. The process lines are thicker than the signal lines and are used to represent the path through which the substance flows in a flowsheet.

Process topology, stream information, and equipment information

All the basic information provided by the process flow diagram can be categorized into three categories - process topology, stream information, and equipment information.

Process topology

Process topology is the term used for the illustration of the location of various equipment and process streams as well as for the interaction between equipment and process streams. The equipment used in the system is represented in the form of a flowchart using the standard symbols as prescribed by the standard organizations. During renewal or modification of the equipment, the newly replaced equipment is designated with the older equipment's name and number in the process flow diagram. Whereas, if new equipment is added to the existing equipment, the newly added equipment is designated with a new name and number.

Stream information

The stream information is very important in the process flow diagram and hence needs to be prepared properly by the chemical engineers during the flowsheet development process. The direction of the stream is designated by using one or more arrowheads. Process streams and utility streams are the two types of streams in a process flow diagram. The streams are identified using the process stream numbers in a process flow diagram. The various utilities in the process flow diagram are identified using the utility streams. Utilities are the services available within a process flow diagram, for instance, in a chemical plant, the various utilities are electricity, steam, chemical sewers, wastewater sewers, and so on. The various other factors such as temperature, pressure, vapor fraction, mass flow, and so on are represented in the flow summary at the end of the process flow diagram.

Equipment information

The equipment information is the final element of a process flow diagram. The specifications, numbers, and other information of the various equipment used in the diagram are mentioned in detail in a tabular form in the process flow diagram.

Context and Applications

In chemical and process engineering, a process flow diagram is commonly used to indicate the general flow of plant processes and equipment. It displays only the relationship between major equipment of a plant facility. It does not show minor details like piping details and designations.

The process flow diagram is useful for the students undergoing the following graduate courses-

• Bachelors of Engineering or Technology (Chemical)
• Masters of Technology (Chemical, Petroleum, Oil)

Practice Problems

1. Which of the following is an alternate name for the process flow diagram?

1. Flowsheet
2. Flysheet
3. Toposheet
4. Stream sheet

Answer- a

Explanation– The alternate name for the process flow diagram is flowsheet.

2. Which of the following is a type of process flow diagram?

1. Storm-flow diagram
2. Equipment-flow diagram
3. Block-flow diagram
4. Chemical-flow diagram

Answer– c

Explanation– Block-flow diagram is a type of process flow diagram.

3. Which of the following information is not included in the piping and instrumentation diagram?

1. Pipes
2. Bypasses
3. Vents
4. Surrounding temperature

Answer- d

Explanation– Information about surrounding temperature is not included in the piping and instrumentation diagram.

4. Which of the following are the services available within a process flow diagram?

1. Utilities
2. Topology
3. Weather conditions
4. Directions

Answer– a

Explanation– Utilities are the services available within a process flow diagram.

5. Which of the following is the final element of a process flow diagram?

1. Process topology
2. Stream information
3. Equipment information
4. Owner information

Answer– c

Explanation– Equipment information is the final element of a process flow diagram.