Chemical Engineering January 2018 - 39

PUMP SIZING AND SELECTION EXAMPLE 1 (CONTINUED)
From looking at the TDH and Figure 3, the choice is between Option 1 and Option 2. Notice that most of the TDH comes from the significant
elevation difference between the suction and discharge side. Now that two pumps are feasible from the perspective of TDH requirements,
you can compare the economics. At first glance, it is tempting to choose Option 1, since the initial investment is significantly lower.
Although Option 2 has a higher initial cost, the lifetime cost over five years is dramatically lower. The problem shows that, in selecting a
pump, the costs associated with power consumption and maintenance are critical pieces of information for making an informed decision.
Year
Cost
1
2
3
4
5
Option 1
Option 2
Purchasing
20,000
Maintenance
2,000
2,000
2,000
2,000
2,000
Total cost
tion head loss of 25% of the total
calculated friction head loss on the
suction or discharge line where the
valve is located [4]. An illustration of
this solution can be observed in Example
2 on page 40. We also implement
the same heuristic within the
Excel spreadsheet.
The Darcy friction factor fD can
be calculated using the Churchill
equation, Equation (2), which is applicable
for all values of Reynolds
number (Re).
Electricity
52,862
52,862
52,862
52,862
52,862
$294,310
Total cost
less ratio of surface roughness to
pipe inner diameter. The equation for
the Reynolds number of a circular
pipe appears in Equation 3.
(3)
In the equation, µ is the fluid viscosity,
 is the fluid density, D is the
pipe inner diameter, and v is the average
fluid velocity.
A useful heuristic is to add a 15%
safety factor to reduce the chance of
underestimating the calculated frictional
head losses. Sample calculations
using these equations appear in
the examples within this article.
Calculating total dynamic head
To find the total dynamic head, the
difference between the discharge velocity
head (hD) and the suction velocity
head (hs) needs to be calculated.
(2)
In the equation, Re is the Reynolds
number and /D is the dimensionCHEMICAL
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(4)
(5)
Purchasing
40,000
Maintenance
3,200
3,200
3,200
3,200
3,200
Electricity
21,476
21,476
21,476
21,476
21,476
$163,380
(6)
(7)
The total dynamic head depends
on the elevation difference between
the discharge tank and suction tank
(Figure 1). In Equations (5) and (6),
P is the pressure of the suction or
discharge side converted to units of
length using the specific gravity of
the fluid as in Equation (7). The TDH
represents the difference between
Equations (5) and (6), in which users
actually add together the velocity
head and the frictional head loss
for both the suction and discharge
sides of the pump.
Net positive suction head
NPSH is used in the determination
of whether the liquid on the suction
side of the selected pump will vaporize
at the pumping temperature, thus
causing cavitation and rendering the
pump inoperable. NPSH varies with
impeller speed and flowrate.
37

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Chemical Engineering January 2018

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