PROCESS FLOW DIAGRAM

What is PFD?

Process Flow Diagram (PFD) is a type of flowchart which provides the basic information about the process flow in processing industries.

PFD is a common diagram in chemical and process engineering that shows the basic operation of plant machinery and processes.

Process flow diagram shows the graphical representation of major equipment used, control valves and other critical instruments which are used to monitor and control the process.

The below figure shows the PFD of conventional combined cycle power plant

What are the contents of PFD?

Typical components of a single unit process flow diagram are as follows:

1. Process piping.
2. Flow direction of the process
3. Major equipments  in the process
4. Control valves and other  critical valves
5. Connections to other process systems
6. Significant by-pass and recirculation (recycle)System
7. Operational data like temperature, pressure, mass flow rate, density, etc., frequently referenced to a mass balance by stream references.
8. Names of the process stream
9. chemical composition of product/fluid

PFD does not provide the information about the whole plant. It mainly used to represent the specific operational area in a plant.

The process flow diagrams does not contain minor components used in the system, details of the instruments, type of pipes and sizes, instrumentation controllers used in the process, safety, relief and isolation valves.

What are not included in PFD?

Details not covered in the PFD:

• No line specification specified.
• Insulation details.
• Instrumentation information.
• Kind of vales
• Rooting of pipes.
• Fitting and supporting particulars

What is the significance of PFD?

The Main purpose and benefits of PFD is

1. To record a procedure for improved comprehension, quality assurance, and employee training
2. To standardize a process in order to achieve maximum efficiency and repeatability
3. To investigate a process in order to improve its efficiency. It helps to identify unnecessary steps, bottlenecks, and other inefficiencies.
4. To create a new process or model an improved process.
5. To communicate and collaborate using diagrams that represent various roles within and outside of the organization

Who uses the PFD?

The most important use of process flow diagram is giving the information about entire process for working personnel in that work area, trainers and visitors.

PFD is used for the process, engineering & maintenance team members to discuss and make needful changes and improvements wherever it is possible to reduce the production cost and down time in the process.

How to read a process flow diagram?

Process flow diagram shows the graphical representation of major equipment used, control valves and other critical instruments which are used to monitor and control the process. The PFD used to understand the sequence of the process, process model, process documentation, etc. It is used to state the process in a visual way with sequential process steps. The complete process of plant is expressed by using some interconnected process flow diagrams.

The most important information that a PFD gives is one of the following:

Topology of the process:

Process Topology: Skeleton diagrams depict the locations of equipment and the linkages between equipment made by process streams.

Process topology refers to the positioning of equipment and process streams and their interactions. The topology of the process, which shows how all the streams and equipment are connected.

Equipment is shown by “Icons” that show how each unit works.

The American Society of Mechanical Engineers (ASME) produces a set of symbols to be utilized when creating flow sheets.

The process stream numbers are used to identify streams on the PFD as well as the information provided for each stream.

Flow information:

Process streams are designated by a number positioned in a diamond-shaped box on the stream. The stream’s direction is indicated by one or more arrowheads. The process stream numbers identify the streams on the PFD. The Example stream flow information is below.

Equipment information:

This summary gives you the information you need to figure out how much equipment will cost and to start designing it in detail. The example equipment information shown below

Equipment is identified on the PFD by number and a label (name) placed above the equipment. Separate table contains basic information such as size and important data.

Methods for Identifying Process Equipment:

Format XX-YZZ A/B in General

The letters XX are provided as the equipment classification’s identifier.

C – Compressor or Turbine

E  – Heat Exchanger

H – Fired Heater

P – Pump

R – Reactor

T – Tower

TK – Storage Tank

V – Vessel

Y indicates a location inside the factory.

ZZ is the numerical identification for each item in an equipment class.

A/B indicates backup or parallel units that are not displayed on a PFD.

On top of the PFD, further equipment details are provided.

For Example: XX-YZZ A/B/… means

XX denotes a one- or two-letter equipment identification (P = pump).

The one- or two-digit unit number is Y. (1-99)

ZZ stands for the unit’s equipment number (1-99)

A/B/… denotes the presence of spare parts.

PFD symbols:

The most common PFD symbols come from the International Organization for Standardization (ISO 10628: Flow Diagrams for Process Plants, General Rules), the German Institute for Standardization (DIN), and the American National Standards Institute (ANSI.)

Some basic symbols used in PFD is shown below:

The below picture shows the example of a control loop symbol in PFD:

The PFD only shows the main control loops.

Instrumentation and other control loops are not displayed.