What is a process flow diagram?

The process flow diagram shows the relation between the components in an industrial plant, basically, it is a documentation of an industrial process. This diagram uses many symbols to display a process. The process flow diagram consists of major equipment, main piping, the direction of flow, operating pressure, and temperature. These diagrams are mostly used in petroleum refineries, natural gas processing plant, petrochemical, and chemical plants and it is also used in many other industries to indicate the flow of plant process stream and equipment. The process flow diagram will display the relationship between the equipment, piping material which is used for the construction, and also the piping size. This is a one-line diagram and it displays the layout of processes and equipment. The process flow diagram is an illustration of a piping system in which conventional symbols are used to represent the equipment and instrument.

What are the details that we can see in a process flow diagram?

• Symbols of equipment, names, and identification number
• Flow direction and main piping lines
• We can see major instrumentation which is valves and control valves which is an important factor for the operation
• Interconnections with other systems can be seen in this
• We can also see bypass and recirculation lines
• We would also get details such as system ratings and operational values like temperature and pressure
• Fluid composition

The process flow diagram would show the flow of the process fluid and the equipment that is involved in the process will be displayed too. It would show the temperature, pressure, and fluid density of the process fluid. So basically the PFD can give the details about the complete plant process. All the equipment in the process will be shown in the diagram with the equipment description. All the equipment in the diagram would have a number and a descriptive name. So a PFD would provide information such as process topology, stream information, and equipment information. The control strategy which is used by the control loops to operate the process will be shown in the diagram.

The process flow stream will be shown and it will have a number, process condition description, and also the chemical composition of the stream will be shown in the PFD. All these data will be either shown in the diagram or can be seen in a flow summary table.

What are the details that can’t be seen in a process flow diagram?

• Pipe class
• Pipeline numbers
• Minor bypass lines
• Isolation and shutoff valves
• Maintenance vents and drains
• Safety and relief valves
• Code class information
• Seismic class information

What are the uses of a process flow diagram?

• We can determine the process topology
• It will be useful to convey the heat and material balances
• Major equipment will be shown
• Processing conditions can be determined
• We can also get the info about the conveying utilities
• Heat and material balance
• Size of the equipment

What is the purpose of a process flow diagram?

• It documents a process for good understanding, control, and training of the employees
• The efficiency and repeatability of the process can be improved
• We could get a description of the process for better efficiency and improvement, this diagram would also show unnecessary steps and other inefficiencies
• We can create a better process or model a process with the help of this diagram

How is a PFD different from a PID?

The PFD stands for process flow diagram and PID stands for piping and instrumentation diagram. The PID is a drawing of a system in a section of the industrial plant, the PID shows the component which needs to run, monitor, and also to control specific process. Unlike the PFD the PID diagram won’t show the chemical reaction in a process or the procedure. The PFD and PID would use symbols in their diagrams, the PFD won’t show all the piping connections or the other details like the PID.

How to read a process flow diagram properly?

The process flow diagram can be divided into five parts and they are

1. Title block
2. Main drawing
3. Equipment description
4. Issue description
5. Explanation notes.

Title block

The title block of the process flow diagram has the following information, consist of project title, facility name, drawing name/title, drawing number, and also the information such as who has drawn it. So we must check the title of the drawing and it would describe which process we are looking at.

Main drawing

The main drawing of the PFD diagram shows the equipment, connection of the piping to the equipment, instruments, line that connects the instrument, instrument control loops, and line numbers, value codes, etc.  

Equipment description

The PFD drawing would also show the equipment which is in the process and it would have information related to the equipment type. We could get equipment information such as capacity, physical size, pressure, and temperature information, the horsepower of pumps, differential head of the pump, and it also shows the equipment number which can be seen in all the equipment.

Issue description

There could be changes in the plant process so according to that there should be changes in the PFD. So if a certain area of the plant is changed then the PFD will be revised and reissued all the time of these changes. There will also be the revision description which would be mostly above the title block and by looking at it we could understand what changes are made. There would be revision clouds and triangle to determine the changes which are made from the previous one.

Explanation notes

In this part we could see many information related to the process

We can determine how the signals are transferred between the equipment and also the connection of instruments in a process with the help of PFD. The PFD also shows the route and direction of the incoming and outgoing material, it also shows the incoming and outgoing flow too.

What are the symbols which is used in a process flow diagram?

Symbols and Notation in Process Flow Diagrams

Standardized Symbols

Process flow diagrams (PFDs) use standardized symbols to represent various types of equipment and instruments, such as pumps, compressors, heat exchangers, reactors, distillation columns, and storage tanks. These symbols follow industry standards (e.g., ISA, ISO, or DIN), ensuring consistency and clarity. This standardized symbolic representation enables engineers, operators, and technicians to quickly identify components and interpret the process without ambiguity, making PFDs a universal language for industrial process flow.

Connections and Integration Points in PFDs

Utility System Integration

PFDs provide insights into the interconnection of primary process equipment with utility systems essential for operation. These include steam systems, cooling water, compressed air, electrical power, and fuel supply lines. Such visibility into utility connections helps in understanding resource usage, assists with troubleshooting, and ensures efficient process flows across interconnected systems.

Limitations of Process Flow Diagrams

Scope of Information

PFDs offer a broad view of the process but do not contain detailed design or operational control specifics. They lack details on control systems, such as instrumentation wiring, loop drawings, and control logic, which are instead displayed in P&IDs (Piping and Instrumentation Diagrams).

Exclusions and Operational Details

PFDs do not show minor bypasses, isolation points, or safety devices like relief valves and maintenance provisions (e.g., drains and vents), which are critical for safe and routine operation. This makes PFDs a high-level overview rather than a comprehensive operational diagram, leaving such details to be specified in P&IDs.

Types of Process Flow Diagrams

Conceptual PFDs

Conceptual PFDs are high-level diagrams used early in process development. They focus on showing major equipment and process flows, without detailed operational values. These are often used for project planning, initial design reviews, or feasibility studies.

Detailed PFDs

Detailed PFDs provide more specific data, including flow rates, temperatures, pressures, and compositions of process streams. These diagrams are essential for process design and optimization, offering engineers a more complete view of the process for fine-tuning operational conditions.

Key Components Displayed in Process Flow Diagrams

Major Equipment Information

Each piece of major equipment in a PFD is labeled with an identification number and name, and typically includes specifications like capacity, size, operating temperature, and pressure. This information helps engineers verify equipment compatibility with process requirements.

Process Stream Details

Process streams are displayed with information on temperature, pressure, and fluid composition. Stream numbers and descriptions are also shown, helping to trace the flow of materials through the system and understand phase changes or other transformations along the process.

Advantages of Using Process Flow Diagrams

Efficient Communication and Training

PFDs provide an easy-to-understand visual representation of the process, allowing for effective communication among project teams, operators, and trainees. They can serve as training tools for new personnel to understand the process flow without needing detailed technical knowledge.

Process Improvement and Optimization

By mapping out the entire process flow, PFDs highlight inefficiencies, unnecessary steps, or bottlenecks in the system. This helps in identifying areas for improvement, leading to increased process efficiency and reduced operational costs.

Differences Between Process Flow Diagrams (PFDs) and Block Flow Diagrams (BFDs)

Block Flow Diagrams

Block Flow Diagrams (BFDs) are simpler versions of PFDs, focusing on high-level process steps without specific details on equipment or stream data. BFDs use basic shapes to represent process units and show the main flow between them, primarily for conceptual and strategic discussions.

Comparison with PFDs

While PFDs include specific equipment, stream details, and operating conditions, BFDs are limited to major process steps, making them more useful for initial discussions and high-level planning than for detailed process engineering.

The process flow diagram of a company is not similar to the other company for the same process it will be different but there will be few similarities.

• Basics of P&ID (piping and instrumentation diagram)
• P&ID legends
• What is the need for line blinding in a pipeline?
• How to do the control valve inspection?
• How to do the control valve maintenance?