PSI Explained: Block Flow and Process Flow Diagrams

In order to adequately explain the technology of a chemical process a Block Flow Diagram (BFD) or simplified Process Flow Diagram (PFD) must be prepared. Ideally the BFD/PFD will fit on a single sheet of paper and clearly outline the “flow” of the chemical throughout the process. For a closed-loop process such as an ammonia refrigeration system, the BFD/PFD will be a simplified version of the Piping and Instrumentation Diagram (P&ID) and will include all major components which are utilized in the process, but will lack the valve/instrument detail contained within a P&ID.

For a transient chemical process in which the chemical is being consumed or sold, the flow diagram must succinctly depict the process by which chemicals are received, stored, and used.

Emphasis is often put on the importance of P&ID for employee training, PHAs, and maintenance. However, the BFD/PFD is equally or more useful to someone desiring to get a “big picture” of a chemical process.