An In-Depth Guide To Chemical Engineering Drawings | BFD, PFD & P&ID

The terms flow sheet and flow diagrams are often used in engineering and design applications. But this is not an accurate way to describe process drawings. This post will give you a glimpse into the types of process drawings.

There are three types of process diagrams in a chemical engineering context:

1. BFD – Block Flow Diagrams.
2. PFD – Process Flow Diagrams.
3. P&ID – Piping and Instrumentation Diagrams.

In brief a BFD represents an entire process in a single sheet, where as in a PFD you can find more detailed information like plant operating conditions and process flow. It uses symbols to represent the equipment. P&IDs provide detailed information when compared the other two drawings. They use standard nomenclature, symbols, and tag numbers to fully describe the process.

These drawings are very useful as they convey the right amount of process information as needed during various stages of bidding, engineering design, procurement, construction, operating & decommissioning phases of a process.

BFD-Block Flow Diagram

A BFD has the ability to represent the complete process on a little more than a single sheet. It contains mainly text enclosed by boxes, interconnecting lines with flow arrows. In some BFDs mass flow rates may be mentioned so as to describe the material balance over the entire plant.

PFD-Process Flow Diagram

PFDs contain more information than the block flow diagrams from which they are derived. They show more details regarding major equipment, subsystems and the flow of product between them. A typical PFD contains equipment symbols connected by process lines where temperature and pressure information are shown on the line itself. You can easily refer the properties of a particular stream through stream number. For designing a P&ID, equipment layout PFD is mandatory.

P&ID-Piping and Instrumentation Diagram

P&IDs carry a lot of information to define the process. Derived from PFDs it is the best way of accurately documenting the operation of a process. P&IDs takes the conceptual aspects of a PFD and expand them by adding

1. Detailed symbols.
2. Detailed equipment information.
3. Equipment order and process sequence.
4. Process and utility piping.
5. Process flow direction.
6. Major and minor bypass lines.
7. Line numbers, Pipe spec, Pipe size.
8. Isolation and shut off valves.
9. Instrumentation controls.
10. Types of process component connections.

A P&ID can be best understood with the help of a legend sheet given by the P&ID developer. Legend sheets contain information regarding the nomenclature, tag philosophy, symbols etc.