Chemical Engineering January 2018 - 40

Capacity - 2,850 rpm (50Hz)
Capacity - 3,500 rpm (60Hz)
FIGURE 3. These pump composite curves show the options for Examples 1 and 2
(8)
To prevent cavitation in a pumping
system, NPSHa should be at least
3 ft above the required NPSH value
(denoted by NPSHr) read from the
pump curve for the given TDH and
pumping rate.
(9)
4 x 3 - 8G A70 3,600 rpm
300
250
200
150
100
50
25
Based on Equation (8), there are
several ways to increase the NPSHa
to make a pumping system feasible.
They include the following:
1. Raise the liquid level in the suction
tank (increasing the S term)
2. Lowering the pump location (increasing
the S term)
3. Reducing the frictional loss on the
suction side (by reducing suction
side velocity or pipe length)
4. Pressurizing the suction tank (increase
Ps)
5. Lower vapor pressure by reducing
pumping temperature (reduce Pvp)
Curve: G-3609
Viscosity and pump sizing
Viscosity correction is often overlooked
in pump sizing by new engineers.
As stated previously, all
pump curves are drawn for water
with a viscosity of 1 cP. Therefore,
we need to pay attention to viscosity
corrections to pump performance.
The Durion Co. Inc., now a part of
Flowserve Corp. (Irving, Tex.; www.
flowserve.com), has released a simple
graphical approach. Head and
capacity are not noticeably changed
by viscosity below 4.3 cP at pumping
temperature. Pump efficiency is
reduced when handling liquids with
viscosity over 4.3 cP at pumping
temperature. Using a fluid with a
higher or lower viscosity compared
to water changes the dynamics of
the centrifugal pump. Power consumption
increases rapidly with a
viscosity increase because of reduced
efficiency. In order to select
a pump from standard performance
curves, it is necessary to apply correction
factors to determine the
equivalent pumping rate and total
dynamic head for water before reading
the pump curves.
The graphical approach utilizes
straight lines to determine simple
correction factors for the horsepower,
capacity and total dynamic
head. First, convert the viscosity
units to centistokes (CS) by dividing
the centipoise (cP) value by the
specific gravity. Referring to Figure 2,
start by drawing a straight line from
the calculated total dynamic head
(A) to the flowrate (B). Then, draw a
straight line from the intersection on
line C through the known viscosity in
centistokes (D) until reaching line E.
From line E, one can read the correction
factor for break horsepower
100
200
300
400
500
600
700
800
900
(Chp). From the intersection on line E,
draw a line through point F to line G,
where the correction factors for flowrate
(CQ) and total dynamic head (CH)
can be read. We have automated this
process in the Excel spreadsheet.
After obtaining the correction factors,
Equations (9),
100
200
300
400
500
U.S. gal/min
FIGURE 4. This individual pump performance illustrates Example 2, p. 40
38
600
700
800
900
(10) and (11)
can be used to correct brake horsepower
(BHP) capacity and total dynamic
head (TDH). Specifically, input
the values for the viscous liquid, use
the correction factors read from the
chart, and calculate the equivalent
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2018
Total head - 2,850 rpm (50Hz)
NPSHr, ft
Head, ft
Total head - 3,500 rpm (60Hz)
http://www.flowserve.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering January 2018

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