Chemical Engineering December 2022 - 36

TABLE 1. The costs of the sized piping system according to the different design goals
are compared here
cost of the system. If we try to minimize
the initial cost of the system,
then smaller pipe diameters might
be used, which would require larger
pumps and higher power usage. In
the long term, this will lead to higher
energy costs. When attempting to
minimize the overall lifecycle cost,
then larger
pipe diameters might
be used to allow smaller pumps
with lower power requirements from
smaller pressure losses. This would
lead to higher initial costs, but can
save a sizable amount of money in
the long run.
During the sizing process, many
pipe size combinations will be used
to try to meet requirements and decrease
costs. Carrying this out manually
is very difficult, but flow-analysis
software with automated pipe-network
sizing capabilities dramatically
streamlines the process.
Imagine a situation where you
were given ten different pipe-network
size configuration options. You
might start by trying a couple of the
design options to see if they meet
the design requirements. If one option
did meet the requirements, how
many more design options would
you analyze to try to further minimize
the costs? Most likely, many would
be hard-pressed to analyze all ten
options to find which one gave the
lowest cost.
The reality is, there are not just ten
different design options that exist,
but potentially millions and billions.
Flow-analysis software tools have
sophisticated mathematical algorithms
to quickly analyze a large
number of different options. Engineers
will have much greater ability
to further investigate multiple
options, leading to pathways that
dramatically reduce costs while still
meeting requirements.
In the simple system from Figure
36
1, there are
a total of
21 pipes in
the model.
Pipe size
is the independent
variable
that
is being
changed
during the
automated
sizing process.
Without grouping pipes together,
there would be 21 independent
variables and hence, there
potential exists for 21 different pipe
sizes. Since this number is not very
practical, grouping various pipes together
will help reduce the number
of variables. This situation illustrates
how there really are many different
sizing combinations that can exist
for a given pump-system design.
Cost comparison
After assuming basic material, installation
and energy costs for the
system, the model will result in two
different pipe size designs. In one design,
the goal is to minimize the initial
cost for the system. Since energy
costs can still easily be calculated,
that information will be included for
comparison. But this design will not
consider energy costs for the sizing
process. For the other design,
minimizing the overall lifecycle cost
(which does consider energy costs)
will be analyzed.
Table 1 provides a comparison for
the cost results for the sized piping
system. The option to minimize
the initial cost results in a total initial
cost of $750,802. As seen in Figure
2, smaller pipe sizes are needed to
meet the design requirements. However,
calculating the energy costs of
about $3.3 million will lead to a larger
total lifecycle cost of just under $4.1
million over 20 years.
Considering pipe-size designs
with the primary goal of minimizing
the overall lifecycle cost will have a
dramatic impact on long-term energy
costs. This is because larger
pipe sizes will be used and that reduces
system pressure drop as well
as pump-power requirements. Although
the system shown in Figure
3 for the design goal of minimizing
lifecycle costs results in about 42%
higher initial costs, savings of 17% in
total cost is realized in the long run.
Notice that the energy costs for the
case of minimizing lifecycle cost is
about 31% lower than simply minimizing
initial cost.
Lastly, now that the piping has
been sized, the pump operating
points are simultaneously determined.
This allows the selection of
pumps that will be a much better fit
for the system demands, since the
pump will not be unnecessarily oversized.
They can also be selected to
operate at close to their BEPs (or as
close as possible) at the pump operating
points.
Simultaneous pump- and pipesystem
sizing will help establish a
design that will meet the required
flow, pressure, temperature and
composition demands needed for
various process operation. This allows
engineers to have much greater
certainty behind input values for their
process simulation - now they are
known instead of only being assumed.
And if the pump-and-piping
system is designed with the goal of
minimizing overall lifecycle cost, this
will have a dramatic impact in the
long term. The approach will realize
great monetary savings with reduced
energy costs when compared
to the approach of simply minimizing
the initial cost of the system. Flow
analysis can be accomplished with
great efficiency using flow-analysis
software with automated pipe-network
sizing capabilities.
n
Edited by Scott Jenkins
References:
1. Kelly, M., " Pump Sizing 101, " Pumps & Systems Magazine,
May 28, 2021, www.pumpsandsystems.com/
pump-sizing-101.
2. Keiser, B., A System Approach with Flow Analysis,, Chem.
Eng., October 2021, pp. 34-38.
Author
Ben Keiser is technical sales
manager for Applied Flow Technology
(AFT; 2955 Professional Place,
Suite 301 Colorado Springs, CO
80904; Phone: 719-686-1000;
Email: info@aft.com; Website:
www.aft.com), a leading dynamic
fluid-flow analysis and waterhammer
mitigation software company.
Keiser helps engineers understand
how to design safe and efficient piping and ducting systems.
His passion and expertise can be accessed around
the world as he trains engineering teams how to efficiently
perform flow-analysis simulations to find optimized
solutions. Prior to joining AFT, he worked for Eaton
Corp. and WellbornYX Corp. Keiser holds a B.S.Ch.E.
degree from the Colorado School of Mines.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
DECEMBER 2022
http://www.pumpsandsystems.com/pump-sizing-101 http://www.pumpsandsystems.com/pump-sizing-101 http://www.aft.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering December 2022

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