Process design begins typically with a process scheme. It’s a sequential depiction of all the unit operations and various components (equipment, lines, and control instrumentation). During plant start-up and subsequent operation, the flow sheet forms a basis for comparing operating performance with design. It’s also used by operating personnel for the preparation of operating manual and operator training.
Various drawings and sketches describe a process plant project, e.g. Block diagram, PFD, P&ID, Layout drawings, Piping isometrics, etc.
Block Flow Diagram
It’s the simplest form of the flow diagrams. A block flow diagram is simply a drawing of chemical processes but doesn’t describe how a given step will be achieved. It is used to understand the basic structure of a system.
Process Flow Diagram(PFD)
A PFD is a simplified flow diagram indicating only the major equipment, the main pipelines and the essential instruments, switches and control valves of a single process unit, a utility unit, and a complete process module. It’s not a PFD unless it has a mass balance, composition, pressure, temperature and energy balance. This type of diagram has its roots in the 1920s. A PFD can be used for better understanding, quality control, training of employees. A series of symbols and notations are used to depict a process. The most commonly used symbols come from agencies such as the International Organization for Standardization (ISO), the German Institute for Standardization (DIN) and the American National Standards Institute (ANSI).
Piping and Instrumentation Diagram (P & ID)
A piping and instrumentation diagram (P&ID) is a detailed diagram of the piping, process equipment, instrumentation and control devices. A P&ID contains information about the mechanical equipment (which include pressure vessels, compressor, heat exchanger, pump, cooling towers, etc.), Process piping, sizes and identification, Process control instrumentation and designation (names, numbers, unique tag identifiers) including sensors, valves, flanges. These drawings’ instrument symbols are generally based on the International Society of Automation (ISA) Standard S5.1.
P&IDs are essential in developing control and shutdown schemes, safety and regulatory requirements, start-up sequences, and operational understanding.
Isometric drawings are a pictorial representation of combined height-width-depth/length into a single view with 30 degrees from its horizontal plane. Piping isometrics are used for fabrication and construction services. It shows the best route from A to B, and sizes/valves, pipe stresses, expansion/contraction, pipe fabrication & welding, prefabricated pipe spools, etc. These drawings are very efficient to convey complex information. Earlier days these drawings were hand made, but nowadays isometrics are drawn by AutoCAD/Microstation software.
Apart from these significant drawings, there is Process Engineering Utility Flow diagram (PEUFD), Process Safeguarding Flow Diagram (PSFD) which are made only for complex installations such as offshore process platforms. For simple applications, the P&ID is usually sufficient to highlight safety devices and aspects.