Chemical Engineering January 2018 - 36

Feature Report
Pump Sizing and
Selection Made Easy
Viscosity, power consumption, commercial availability and lifecyle cost analysis are all important
considerations in pump sizing. An automated spreadsheet method helps engineers take those
factors into account in centrifugal pump selection
Joseph
Sarver, Blake
Finkenauer and
Y.A. Liu
Virginia Polytechnic
Institute and State
University
IN BRIEF
PUMP SIZING OVERVIEW
CALCULATING FRICTION
LOSSES
NET POSITIVE SUCTION
HEAD
VISCOSITY AND PUMP
SIZING
PUMP CURVES
CALCULATING POWER
AND EFFICIENCY
SUCTION SPECIFIC
SPEED
AUTOMATED EXCEL
SPREADSHEET
M
any aspiring chemical engineers
enter industry after university
study without sufficient practical
knowledge about how to properly
size pumps. A number of recent articles
provide useful guidelines for sizing and selecting
pumps, but these articles focus on
certain specific aspects of proper pump sizing,
while leaving out others [1-4]. Chemical
engineering literature does not fully cover
other essential aspects of pump sizing and
selection - including the viscosity correction,
power consumption, commercial availability
and lifecycle cost analysis.
In industrial
operations, pumping alone
can account for between 25 and 50% of the
total energy usage of the process, depending
on the application [5]. The initial purchase
price of a pump is only a small fraction of
the total lifecycle cost. There are situations
in which purchasing a less expensive pump
actually leads to greater energy-usage costs.
This results in a higher lifecycle cost (see Example
1, p. 36).
Without a proper understanding of the
pump selection process, engineers cannot
effectively make both economic and
practical decisions. This article aims to fill
in some of the gaps in understanding and
provide a straightforward method for pump
sizing and selection. Along with this article,
we have created a useful Microsoft Excel
spreadsheet to assist with centrifugal pump
sizing. The automated Excel spreadsheet
assists in calculating the key parameters for
pump sizing and selection. Since the majority
of the pumps used in the chemical process
industries (CPI) are centrifugal pumps,
this article focuses on that equipment category,
rather than the other general classes
of pumps, such as rotary and positive displacement
pumps.
34
Suction side
Discharge side
PD
Ps
D
S
Vs, hs,h, hs
Vd, Hd,f, hd
FIGURE 1. The following components are needed to calculate
total dynamic head: suction and discharge elevation; fluid velocity;
friction loss and dynamic head; and tank pressure
Pump sizing overview
The concept of a pumping system is rather
simple. The suction side refers to everything
before the pump, while the discharge side
refers to everything after the pump. Figure
1 illustrates a simplified pumping system. A
key parameter in characterizing a pump is
the total dynamic head (TDH), which is the
difference between the dynamic pressure
of the discharge side and the suction side.
The dynamic pressure represents the energy
required to do the following: (1) to raise the
liquid level from the suction tank to the discharge
tank; (2) to provide liquid velocity inside
both suction and discharge piping; (3)
to overcome frictional losses in both suction
and discharge piping; and (4) to pump the
liquid against the pressure difference between
the suction and discharge tanks.
Six steps to pump sizing. In order to size a
pump, engineers need to estimate the temperature,
density, viscosity and vapor pressure
of the fluid being pumped. Pump sizing
can be accomplished in six steps, as follows:
1. Find the total dynamic head, which is a
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