Page 163 - Chemical engineering design

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CHEMICAL ENGINEERING
can determine the selection of the materials of construction; see Chapter 7. If the space in
the data table is left blank opposite a particular component the quantity may be assumed
to be zero by the specialist design groups who take their information from the flow-sheet.
4.2.7. Basis of the calculation
It is good practice to show on the flow-sheet the basis used for the flow-sheet calculations.
This would include: the operating hours per year; the reaction and physical yields; and
the datum temperature used for energy balances. It is also helpful to include a list of the
principal assumptions used in the calculations. This alerts the user to any limitations that
may have to be placed on the flow-sheet information.

4.2.8. Batch processes
Flow-sheets drawn up for batch processes normally show the quantities required to
produce one batch. If a batch process forms part of an otherwise continuous process,
it can be shown on the same flow-sheet, providing a clear break is made when tabulating
the data between the continuous and batch sections; the change from kg/h to kg/batch.
A continuous process may include batch make-up of minor reagents, such as the catalyst
for a polymerisation process.

4.2.9. Services (utilities)

To avoid cluttering up the flow-sheet, it is not normal practice to show the service headers
and lines on the process flow-sheet. The service connections required on each piece of
equipment should be shown and labelled. The service requirements for each piece of
equipment can be tabulated on the flow-sheet.

4.2.10. Equipment identification
Each piece of equipment shown on the flow-sheet must be identified with a code number
and name. The identification number (usually a letter and some digits) will normally be
that assigned to a particular piece of equipment as part of the general project control
procedures, and will be used to identify it in all the project documents.
If the flow-sheet is not part of the documentation for a project, then a simple, but
consistent, identification code should be devised. The easiest code is to use an initial letter
to identify the type of equipment, followed by digits to identify the particular piece. For
example, H heat exchangers, C columns, R reactors. The key to the code should
be shown on the flow-sheet.

4.2.11. Computer aided drafting

Most design offices now use computer aided drafting programs for the preparation of
flow-sheets and other process drawings. When used for drawing flow-sheets, and piping
and instrumentation diagrams (see Chapter 5), standard symbols representing the process
equipment, instruments and control systems are held in files and called up as required.