Drawings and sketches play an important role in developing, solving and communicating system models. Drawings and sketches typically start simple in character (simple block diagrams) through to increasingly complex process flow diagrams (PFDs) and piping & instrumentation drawings (P&ID). Block, PFD and P&IDs are all symbolic representations of the setting. Real (or realistic) 3D representations of the physical setting play an important role as the system model complexity ventures into computational fluid dynamics. Symbolic and real drawings can both start simplistic and can grow to capture more details as the work progresses.
Block diagrams are simple boxes and they are not intended to convey the nature of the system within the box. In some cases, a single block (or box) is used to represent the whole system and can be valuable to convey the NET result of the system. Meaning what are the overall inputs to a system and the resulting overall outputs. Block diagrams with multiple blocks provided what parts of the system are connected to other parts. Meaning what are the flow paths (mass and energy) through the system.
Process Flow Diagrams (PFDs) replace the boxes in Block Diagrams with symbols that communicate the nature of the unit being represented. In some cases, the symbols are generic (“a pump”) and in other cases the symbol is representative of a quite specific type of unit (“a centrifugal pump”). Typically PFDs have greater detail than provided in a Block Diagram. There are some broadly accepted and used conventions around symbol usage; however, it is necessary to recognize that these are always known or followed and there are differences from sector to sector or from company to company.
An important element of Block diagrams and PFDs is a supporting data table. The data table has a column element of the drawing (both units and lines between units) and a row for each possible descriptor of interest. Common descriptors include temperature, pressure, state, concentrations of elements and compounds, mass or molar flows of elements and compounds, energy or thermodynamic values (enthalpy, entropy etc.). The data table aids model development (organizing known and unknowns etc.) and effectively communicates the final system description.
Piping & Instrumentation Diagrams (P&IDs) include another level of detail beyond the PFD. The details include all of the sensors and the corresponding control connection representations. The symbols for different sensors and control functions also follow conventions but also have the same variability in usage that is true for PFD symbols.
Simple Block Diagram
Simple Process Flow Diagram