Chemical Engineering December 2022 - 34

Cover Story
Part 2
Simultaneous Pump
and System Sizing
Flow analysis can aid in simultaneously sizing pumps and associated piping systems.
The concurrent sizing can reduce wasted energy and improve pump reliability
Ben Keiser
Applied Flow
Technology Inc.
IN BRIEF
SIZING EXAMPLE
FLOW ANALYSIS
COST COMPARISON
n the chemical process industries (CPI),
a variety of incredible process-simulation
tools are available to provide greater efficiency
and understanding to numerous
chemical processes. Key process-stream
parameters include flowrate, pressure, temperature
and process media composition.
From these inputs, an entire operation can
be modeled. However, these important process
inputs can be easily taken for granted.
Piping networks are the lifeline to plant
process units. Their operation is just as critical
as the process units themselves to ensure
that the required flows, pressures, temperatures
and compositions are delivered.
Pumps, piping systems, and other components
must be sized and selected properly.
Like process simulation, flow-analysis software
can greatly aid the pump- and pipesizing
process.
There is a vast amount of literature available
that discusses the procedure for sizing
a pump [1]. The procedure is simple for systems
that involve one, two, or maybe three
flow paths, and many engineers share the
experience of running through these calculations
at one point or another. However, when
dealing with much more complicated systems
that involve several flow splits, loops,
control features, multiple pumps in parallel,
and so on, it is not an easy task. This
is where flow-analysis software offers great
utility - not only to size the pump itself, but
to also to provide much more insight into
system operation, where you can model difI
ferent
scenarios, as well as system operation
and configuration changes.
Ref. 2 discusses how to improve pump
system reliability through determining how
to operate pumps closer to their best efficiency
point (BEP). The focus of that article
was on existing piping systems to improve
system reliability.
This article examines the benefits of concurrently
sizing the pumps and piping system
together, instead of sizing pumps and piping
individually. Sizing the piping system with the
pumps will allow the selected pumps to fit
the system application better, and thus prevent
dramatically oversized pumps. When
pump oversizing occurs, it is typically dealt
with by using throttling valves, an approach
that not only wastes a significant amount
of energy (leading to high electricity costs),
but also diminishes pump reliability. Sizing
the pipes and pump together establishes a
functional design that also saves a significant
amount of money by finding a lower-cost solution
for the system. This is especially true
when sizing the system with consideration of
the overall lifecycle cost.
Sizing example
FIGURE 1. In the initial piping system layout shown here, pipe sizes
and pump operating points have not yet been determined
34
Consider the system shown in Figure 1,
where the pumps, piping, fittings and control
valves need to be sized. For this system,
there are four requirements that will dictate
the piping size design. Fluid velocities inside
the pipes must be below a certain threshold.
There is a minimum pressure throughout the
network that must be maintained. The
coolers require a minimum flowrate for
proper cooling. The pumps need to have
plenty of margin between the net positive
suction head available (NPSHA) for the
pumps and the net positive suction head
required (NPSHR) by the pumps.
To size the pumps, a desired flowrate
will typically be specified in the model
and the increase in pump head (or pressure)
that is required to deliver the flow to
overcome system resistance and elevation
change will be determined. That esCHEMICAL
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