A Block Flow Diagram (BFD) is a drawing of a chemical process used to simplify and understand the basic structure of a system. A BFD is the simplest form of the flow diagrams used in industry. Blocks in a BFD can represent anything from a single piece of equipment to an entire plant. For a complex process, block flow diagrams can be used to break up a complicated system into more reasonable principal stages. BFDs provide an overall view of the process, generally on a single sheet of paper, with each major operating step represented by a block. BFDs are used to explain the general material flow throughout an entire plant. They are also useful for conceptual safety studies because they provide a good overview of the process.
Breakdown of Block Flow Diagrams
Major process streams are shown connecting the blocks. The flow of process streams is generally from left to right, with a gravity bias, where possible, which means that liquids will leave from the bottom of a block, gases from the top. A BFD may also show a few of the more important operating parameters, such as flow rates and temperature. Excluded from BFDs are single pieces of equipment and package numbers.
The figure above is an example of a very simple block diagram. Two feed streams enter the unit. Each is purified; they are then mixed with one another and sent to the reaction section. The material leaving the reaction section—a mixture of product and unreacted feed materials—goes to a separation unit. Product is sent to purification; unreacted feed returns to the reaction section.
Block Flow Diagram Conventions
There are several conventions regarding the construction and format of BFDs that are commonly used in the engineering community.
Some of the recommended conventions are:
- Operations/equipment are represented with blocks.
- Material flows are represented with straight lines with arrows giving the direction of flow.
- Lines are horizontal and/or vertical, with turns at 90-degree angles.
- Flows go from left to right whenever possible.
- If lines cross, the horizontal line is continuous and the vertical line is broken.
- Light streams (gases) are typically closer to the top of the bfd than the heavy streams (liquids or solids).
- Critical information unique to the process (such as a chemical reaction) is supplied.
- A simplified material balance should be provided.
