ASHRAE Journal - June 2024 - 28

COLUMN ENGINEER'S NOTEBOOK
FIGURE 5 A visual on the NPSH situation for the case study pump.
16
14
12
10
8
6
4
2
Saturation Pressure, psia, vs. Temperature, ° F
Ambient Pressure, 14,700
Allowable Suction Line
Pressure Drop = 8.9 psi or
20.563 ft w.c.
Required Pressure at Pump
Suction, 5.791
NPSHR = 12.0 ft w.c
or 5.195 psia
Saturation Pressure of
Circulated Water, 0.597
20
40
60
80
100
120
140
Temperature, °F
A Graphical Representation of NPSHA
Frequently, " a picture is worth a thousand words, "
and in my case, that turned out to be true when I was
learning about NPSHA. Figure 4 is the starting point for
our picture. The key component is a plot of saturation
temperature vs. pressure (the red line) which represents
the data in the first two columns of the steam table in
Online Figure 2.
To that, I have added lines showing the saturation
temperature and pressure for 85°F (29°C) water at
sea level. I chose 85°F (29°C) because it is a common
FIGURE 6 A visual representation of NPSHR and NPSHA for the case study pump.
Saturation Pressure, psia vs. Temperature, °F
16
14
12
10
8
6
4
2
Ambient pressure, 14,700
Allowable Suction Line
Pressure Drop = 8.9 psi or
20.563 ft w.c.
Required Pressure at Pump
Suction, 5.791
NPSHR = 12.0 ft w.c
or 5.195 psia
Saturation Pressure of
Circulated Water, 0.597
20
40
60
80
100
120
140
Temperature, °F
28
ASHRAE JOURNAL ashrae.org J U N E 2024
NPSHA = 10.8 ft w.c
or 4.663 psia
Saturation Temperature of
Circulated Water, 85
Saturation Pressure vs.
Temperature
Actual Suction Line
Pressure Drop = 9.4
psi or 21.807 ft w.c.
160
180
200
220
240
Saturation Temperature at
Ambient Pressure, 212.00
Saturation Temperature of
Circulated Water, 85
Saturation Pressure vs.
Temperature
design-day condenser
water supply
temperature.
Reading the chart, we
can see that the pressure
drop between the cold
basin and the lowest
pressure point in the
pump impeller would
need to be 14.103 psi
(101.35 kPa -4.1 kPa) to
reach the saturation
pressure for 85°F (29°C)
water and trigger
cavitation.
With the correct tools
and knowledge, you can
configure and size the
piping system so that
at design flow, you would have less than the 14.103 psi
loss based on the physical dimensions of the piping run
and published data for the pressure drop through pipes,
fittings and other components. But you must rely on the
pump manufacturer to tell you what the pressure drop
would be from the suction flange to the low pressure
point in their impeller because that is a component of
the overall allowable pressure drop and would reduce
the allowable loss in your piping run.
One way to think of NPSHR is that it's the pump
manufacturer's way of
saying:
Actual Pressure at Pump
Suction, 5.260
Saturation Temperature at
Ambient Pressure, 212.00
160
180
200
220
240
Due to the design of
my impeller, I will need a
portion of the allowable
pressure drop identified in
Figure 4 to accommodate
the pressure drop that occurs
in the pump from the suction
flange to the lowest pressure
point in the impeller.
The NPSHR curve is
that statement for the
flow range of the pump.
For the pump in our case
study, from Figure 3, we
can see that at the design
flow rate of 1,400 gpm
Pressure, psia
Pressure, psia
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ASHRAE Journal - June 2024

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