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An Overview Of Chemical Process Diagrams Part 2: Process Flow Diagram

This resource is part of a series covering Chemical Process Diagrams:

What Is A Process Flow Diagram?

A Process Flow Diagram  (PFD) (figure 1) is a sequence of process units connected by process streams and shows the flow of both energy and materials. The PFD shows all the chemical engineering information and is drawn on large paper and is normally broken down into smaller diagrams and data tables to make it easier to read and analyse.

Figure 1: PFD of an oil refining process (Smartdraw, 2020)

The PFD contains all major pieces of equipment which are given unique names, and all process flow streams are shown and numbered, all the conditions such as temperature and pressure are stated, and the compositions of the streams are shown. Furthermore, all utility streams (required services on the plant such as a water line) that service major equipment are shown and a basic control strategy for normal operations.

For a PFD consistency is vital, so make sure the process equipment and utilities have drawn are consistent and not having different symbols for the same things. It always good (sometimes required) to put a legend in to give any reader a clear identification of what they are looking at.

PFD Drawing Conventions

When drawing a PFD there are a few conventions that need to be followed. Number the streams from left to right and horizontal lines are dominant (figure 2) and add arrows for a change in direction or inlet or outlet of process units.

Figure 2: PFD horizontal lines are dominant over vertical lines

To identify process equipment, they must be labelled in a specific format: XX-YZZ A/B

  • XX – identifies the equipment such as E – heat exchanger or V – vessel.
  • Y – the area of the plant the equipment is located, such as building 2 on the site.
  • ZZ – a designation for each item in an equipment class, such as the first vessel being 01 and the second vessel being 02.
  • A/B – identifies parallel or backup units for an equipment class
  • Any extra information will usually put in a box at the side or bottom of the PFD

(equipment class – this refers to the same type of equipment being present on the site such as having multiple heat exchangers.)

Stream And Equipment Information

The stream information can contain a lot of information and normally there is not enough space on the PFD, so flags are used such as stream numbers which can then be looked up in a table (Figure 3).

Stream Number 1 2 3
Temperature (°C) 25 56 79
Pressure (bar) 1.9 6.8 7.5
Vapour fraction 0 1 1
Mass flow (tonne/hr) 19 31 47

Figure 3: Flow summary table example

Similar to stream information equipment information, there is too much equipment information to put on the PFD and thus a table is used which is located at the bottom or side of the PFD, this can change so don’t be alarmed if you find it at the top it just the drawers personal preference (figure 4).

Tank T-101 T-102
Temperature (°C) 56 78
Pressure (bar) 2 12
Orientation horizontal vertical
Size
Height (m) 6 m 8 m
Diameter (m) 2.5 1.5

Figure 4: Equipment summary table

References

Smart draw. (2020). Oil Refining – Extraction Process Diagram. Retrieved from Smart draw: https://www.smartdraw.com/process-flow-diagram/examples/oil-refining-extraction-process-diagram/

What do you think?

Written by Jennifer Fuentes

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An Overview Of Chemical Process Diagrams Part 1: Introduction And Block Flow Diagrams

An Overview of Chemical Process Diagrams Part 3: Piping & Instrumentation Diagrams