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DEVELOPMENT OF FLOW DIAGRAMS
The flowsheet is the key document in process design. It shows the arrangement of the
equipment selected to carry out the process, the stream connections, stream flow rates and
compositions, and the operating conditions. It is a diagrammatic model of the process. The
flowsheet is used by specialist design groups as the basis for their designs. These include
piping, instrumentation, and equipment design and plant layout. It is also used by operating
personnel for the preparation of operating manuals and operator training. During plant startup
and subsequent operation, the flowsheet forms a basis for comparison of operating
performance with design.
The flowsheet is drawn up from material balances made over the complete process
and each individual unit operation. Energy balances are also made to determine the energy
flows and the utility requirements.
The next step in process design after the flowsheet is the preparation of Piping and
Instrument diagrams (abbreviated to P & I diagrams or PIDs), often also called the
engineering flowsheet or mechanical flowsheet. The P&I diagrams, as the name implies,
show the engineering details of the process, and are based on the process flowsheet.
As the process flowsheet is the definitive document on the process, the presentation must be
clear, comprehensive, accurate, and complete. The various types of flowsheets are discussed
in the following sections.
A block diagram is the simplest form of presentation. Each block can represent a single piece
of equipment or a complete stage in the process. Block diagrams are useful for showing
simple processes. With complex processes, their use is limited to showing the overall process,
broken down into its principal stages, as in fig :3 (vinyl chloride). In that example, each block
represented the equipment for a complete reaction stage: the reactor, separators, and
distillation columns. Block diagrams are useful for representing a process in a simplified
form in reports, textbooks, and presentations, but have only limited use as engineering
documents. The stream flow rates and compositions can be shown on the diagram adjacent to
the stream lines, when only a small amount of information is to be shown, or tabulated