Chemical Engineering December 2022 - 35

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FIGURE 2. The pipe system shown here results when the design goal is minimizing initial system costs
for material and installation
FIGURE 3. When the design goal is to minimize the lifecycle cost of the pipe system, the diagram here is
the result. About 17% in cost savings can be realized compared to minimizing initial costs
tablishes the pump operating point,
which can be used to select a pump.
At the same time, different pipe
sizes throughout the system will be
used to meet the design requirements.
If any of the design requirements
are not met, then those sizes
would not yield a feasible design. As
the pipe sizes change, the pump requirements
for the desired flow will
also continually change. Therefore,
this is a highly iterative process.
Simply meeting the design requirements
is not the only thing that
should be considered in establishing
a " good design. " How can engineers
know if it is good, or if the
design could be better, or if it is the
best possible design available? To
answer this question, cost must be
considered. Monetary cost for the
system is certainly helpful in establishing
a high-quality design. But at
the very beginning of the design process,
detailed costs may not yet be
known. At that point, the overall piping
weight can be a good initial start
at the design.
Reducing piping weight typically
corresponds well with reducing the
initial material and installation costs
for the system. Designing the pipe
sizes with a maximum velocity requirement
in mind can also help reduce
energy costs in the long run.
Overall, the best design in theory
would be the one with the lowest
cost or lowest piping weight. It is
worth noting that even when sizing
the system based upon monetary
costs, it is not necessary to have incredibly
accurate cost data. Simple
estimates, such as cost per length
of piping or cost per power unit for
pumps, is sufficient. Even if these
costs are referenced from another
project, industrial cost tables, old
cost data, or even a guess, the " absolute "
value of the cost is not what
is important. What is important is the
cost savings that is generated during
the process of the system sizing.
Flow analysis
For the system being sized in Figure
1, the goal is to minimize cost for
the appropriate sizes that will work.
There are two different cost considerations
to minimize: either the initial
cost of the system or the lifecycle
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Chemical Engineering December 2022

Table of Contents for the Digital Edition of Chemical Engineering December 2022