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FLOW-SHEETING
4.2.5. Layout
The sequence of the main equipment items shown symbolically on the flow-sheet follows
that of the proposed plant layout. Some licence must be exercised in the placing of
ancillary items, such as heat exchangers and pumps, or the layout will be too congested.
But the aim should be to show the flow of material from stage to stage as it will occur,
and to give a general impression of the layout of the actual process plant.
The equipment should be drawn approximately to scale. Again, some licence is allowed
for the sake of clarity, but the principal equipment items should be drawn roughly in the
correct proportion. Ancillary items can be drawn out of proportion. For a complex process,
with many process units, several sheets may be needed, and the continuation of the process
streams from one sheet to another must be clearly shown. One method of indicating a
line continuation is shown in Figure 4.2; those lines which are continued over to another
are indicated by a double concentric circle round the line number and the continuation
sheet number written below.
The table of stream flows and other data can be placed above or below the equipment
layout. Normal practice is to place it below. The components should be listed down the
left-hand side of the table, as in Figure 4.2. For a long table it is good practice to repeat
the list at the right-hand side, so the components can be traced across from either side.
The stream line numbers should follow consecutively from left to right of the layout,
as far as is practicable; so that when reading the flow-sheet it is easy to locate a particular
line and the associated column containing the data.
All the process stream lines shown on the flow-sheet should be numbered and the data
for the stream given. There is always a temptation to leave out the data on a process
stream if it is clearly just formed by the addition of two other streams, as at a junction,
or if the composition is unchanged when flowing through a process unit, such as a
heat exchanger; this should be avoided. What may be clear to the process designer is
not necessarily clear to the others who will use the flow-sheet. Complete, unambiguous
information on all streams should be given, even if this involves some repetition. The
purpose of the flow-sheet is to show the function of each process unit; even to show when
it has no function.

4.2.6. Precision of data

The total stream and individual component flows do not normally need to be shown
to a high precision on the process flow-sheet; at most one decimal place is all that is
usually justified by the accuracy of the flow-sheet calculations, and is sufficient. The
flows should, however, balance to within the precision shown. If a stream or component
flow is so small that it is less than the precision used for the larger flows, it can be
shown to a greater number of places, if its accuracy justifies this and the information
is required. Imprecise small flows are best shown as “TRACE”. If the composition of
a trace component is specified as a process constraint, as, say, for an effluent stream or
product quality specification, it can be shown in parts per million, ppm.
A trace quantity should not be shown as zero, or the space in the tabulation left blank,
unless the process designer is sure that it has no significance. Trace quantities can be
important. Only a trace of an impurity is needed to poison a catalyst, and trace quantities