Chemical Engineering January 2018 - 38

EXAMPLE 1. PUMP SIZING AND SELECTION
he following is a pump sizing problem to illustrate the calculations in this article. You are told to purchase a pump for your manufacturing
facility that will carry water to the top of a tower at your facility. The pump is a centrifugal pump that will need to pump 800 gal/min when
in normal operation. Assume BHP is 32 and 16 horsepower for the 3,500-rpm and 2,850-rpm pumps, respectively, for all pump choices
in the composite curve. The pump operates for 8,000 h/yr. Assume all of the pumps are viable for your required flowrate. The suction-side pipe
and discharge-side pipe diameters are 4 and 3 in., respectively. The suction tank elevation (S) is 12 ft, and the discharge tank elevation (D) is
150 ft. Pressure on the suction side is atmospheric pressure (1 atm = 14.696 psi) and the pressure on the discharge side is 1.1 atm. Assume
that both hd,f and hs,f are roughly 10 ft.
Based on a five-year life, the objective of the problem is to calculate the lifecycle cost to operate each pump (that is, the costs of installation,
maintenance and electricity, which is $0.18/kW), and to choose the pump with the lowest lifecycle cost (depreciation is assumed to be
negligible for this example). The pump curves in Figure 3 illustrate the following pump options to choose.
Option 1: 4 X 3 - 13 3,500 rpm
Installed cost of pump and motor: $20,000 for 3,500 rpm
Maintenance cost: 10% of installed cost per year
Motor efficiency: 65% (assumed)
Option 2: 4 X 3 - 13 2,850 rpm
Installed cost of pump and motor: $40,000 for 2,850 rpm
Maintenance cost: 8% of installed cost per year
Motor efficiency: 80% (assumed)
Solution:
Convert volumetric flow to velocity:
Option 3: 4 X 3 - 10 3,500 rpm
Installed cost of pump and motor: $10,000 for 3,500 rpm
Maintenance cost: 10% of installed cost per year
Motor efficiency: 65% (assumed)
Option 4: 4 X 3 - 10 2,850 rpm
Installed cost of pump and motor: $20,000 for 2,850 rpm
Maintenance Cost: 8% of installed cost per year
Motor Efficiency: 80% (assumed)
T
Continued on page 37
correlating to the discharge side of
the pump.
(1)
In the equation, fD is the Darcy
friction factor, L is the pipe length in
feet, I.D. is the inner pipe diameter in
36
inches, v is the average fluid velocity
in ft/s, g is the acceleration due
to gravity in ft/s2, ni is the i-th valve,
fitting, pipe contraction and enlargement
and so on, and ki is the resistance
coefficient.
The first term in Equation (1) represents
the frictional losses from the
fluid flowing through a straight piece
of pipe. The second term represents
the frictional losses due to valves, fittings,
pipe contractions and enlargements.
We have provided the values
for the typical resistance coefficients
and pipe surface roughness from
the chemical engineering literature in
the Excel spreadsheet discussed in
this article.
A control valve follows the widely
accepted heuristic of having a fricCHEMICAL
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