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.

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.

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?


post-graduate in Electronics & communication.

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