Chemical Engineering January 2018 - 42

PUMP SELECTION, EXAMPLE 2
A
n additional pump selection problem is shown Example 2. For this example, consider a discharge line that is 50 ft schedule 40, 4-in.
diameter, with two gate valves, 12 elbows, 1 expander (2-4 in.), a control valve, and a branched tee. The velocity is 12.84 ft/s, Reynolds
number is 1,601, and the Darcy friction factor is 0.119. The elevation difference on the discharge side is 17 ft, the total dynamic suction
head is 50 ft, and the pressure on the discharge side is 14.696 psi. The objectives in this example are to accomplish the following: 1) Calculate
the discharge frictional head loss and total dynamic head; 2) Correct for viscosity of the fluid, which is 300 cP at 125°C; and 3) select an appropriate
pump from Figure 3; and 4) Ensure that cavitation is not an issue with the selected pump given the vapor pressure is 13.93 mm Hg
and specific gravity is 1.20.
Solution: For choosing the appropriate pump, see Figures 3 and 4. Notice on the pump composite curve, the 4 x 3 - 10 section is very
close to the 4 x 3-8G. Both pumps should be analyzed by performing a lifecycle cost analysis using the pump efficiencies from the individual
pump performance curves.
Our NPSHa is much greater than the NPSHr and thus should avoid cavitation under normal operating conditions.
cosity-corrected TDH and pumping
capacity. The y-axis of the graph is
the equivalent water TDH. The x-axis
of the graph is the equivalent water
volumetric
flowrate.
Figure 3 has
multiple shaded sections, with each
corresponding to a different-sized
pump. In the individual sections, the
pumps are specified by the suction
pipe diameter, discharge pipe diameter,
and impeller size (4 × 3 - 8G
for our selected pump in Example 2).
Remember that the larger pipe diameter
is always the suction side. For
this pump composite curve, there
are two x-axes for different impeller
speeds. Notice that the two red
points both correspond to 570 gal/
min of flow and 110 ft of TDH for the
different impeller speeds (2,850 and
3,500 rpm). The point that corresponds
to this TDH and flowrate may
not be the pump that is ultimately
selected. For example, if the point
is close to the boundary, engineers
40
would need to move vertically up on
the composite curve and choose a
pump with a larger impeller size (4 ×
3 - 10 versus 4 × 3 - 8G). It is very
important to always compare the
lifecycle cost for the different pumps
(see Example 1 on p. 36).
After looking at the pump composite
curve and selecting potential
pumps, the next step is to look
at the individual pump performance
curves to obtain the pump efficiency,
NPSHr, and impeller size. Figure 4
is an example of an individual pump
performance curve. The required
NPSH is located at the bottom of
this figure, separate from the rest
of the performance curve. Keep in
mind that not all pump curves are the
same and vary by manufacturer. In
Figure 4, the blue curve is for an 8-in.
impeller diameter. The green curve
is for a pump efficiency of 64% and
the red curve is for 30 BHP. In most
pump curves, engineers could not
read the BHP accurately; so instead,
we recommend calculating the BHP
manually using the pump efficiency
according to Equation (12) below.
Power and efficiency
Brake horsepower (BHP) is the actual
horsepower delivered to the
pump shaft. To find the BHP for a
viscous liquid
(BHPvis), use Equation
(9), after calculating the break
horsepower for the equivalent water
values (BHPw, TDHwater and Qwater)
and efficiency (p,w) from the pump
curve using Equation (12).
(12)
To determine the electricity cost
for operating the pump, use Equations
(13), (14) and (15). Equation
(13) converts the BHP of your pump
to the input power or electricity consumption.
Determining the power
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Chemical Engineering January 2018

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