ASHRAE Journal - June 2024 - 24
COLUMN ENGINEER'S NOTEBOOK
FIGURE 3 The case study pump curve.
Double Suction, Vertical Split Case Pump Curve
85.00
150
125
100
75
50
25
10 1/2
9 3/4
9 1/2
8 1/2
7
60%
6 in. Suction, 5 in. Discharge, 10-1/2 in. Maximum Impeller Size
82.5%
70%
75%
80%
84.0%
Design System Curve
Design Operating Point
75%
70%
65%
Design NPSHR
400
800
Flow, gpm 1,400
1,200
1,600
which can also damage the impeller (Online Figure 1). It's a
vicious circle. (To see Online Figure 1 and all online figures,
visit https://tinyurl.com/JournalExtras.)
NPSHR vs. NPSHA
One confusing aspect is that NPSH is discussed in two
contexts. One is NPSHR (R = required), which is from the
perspective of the pump and related to the physics of the
pump. The other is NPSHA (A = available) and is related
to the physics of the system.
NPSHR
NPSHR is typically documented by a line on the pump
curve (shown in Figure 3), which is the curve for the
pump associated with the site in Figure 1.
The green line is the NPSHR curve for the pump, and
it can be interpreted to say that at the design flow rate
(1,400 gpm [88 L/s]) the NPSHR is 12 ft w.c. (36 kPa).
Notice NPSHR is not a linear relationship. This is
because pressure drop varies as the square of the flow
most of the time. A pump that is doing just fine one
day might experience cavitation the next day due to
a seemingly inconsequential change in the system
operating characteristic.
For example, the pressure drop due to flow in a
common header for two pumps piped in parallel may
cause the operating point of each pump to be such that
cavitation is not an issue when they both are running.
But, if one pump is cycled off and the other runs out its
curve as a result, the operating point may transition to
a condition where cavitation can occur. (VFDs can be
24
ASHRAE JOURNAL ashrae.org J U N E 2024
2,000
NPSHR
80
40
2,400
applied to mitigate this, which will be
covered in Part 2 of this column.)
For the pump we are considering,
a flow increase of 200 gpm or
13 L/s (about 14%) would increase
the NPSHR from 12 ft w.c. (36 kPa)
to approximately 20 ft w.c. (60 kPa)
(about 66%).
12.00
NPSHA
Understanding NPSHA is where
things start to get a bit more
challenging, at least for me. Bottom
line, NPSHA needs to be equal to or
greater than the NPSHR.
To focus our discussion, I am going
to discuss NPSHA in the context of
a cooling tower, an open system. Thus, atmospheric
pressure at the cold basin becomes the reference
pressure for NPSHA assessment. For a closed system, the
system's fill pressure becomes the reference pressure.
You can extrapolate what follows in that context.
Because NPSHA is a function of the piping system
configuration and the location (because atmospheric
pressure is the reference pressure for an open system,
the reference pressure for a system in Denver will be less
than the reference pressure for a system in Atlantic City)
one way to think of this is to consider and contrast things
that would cause the pressure at the low pressure point
in the pump impeller to increase, relative to things that
would cause the pressure at the low pressure point in
the pump impeller to decrease.
All of this is considered in absolute pressure terms and
in the context of the saturation temperature of the fluid
at the low pressure point in the impeller.
NPSHA Key Components
Reference Pressure
For a cooling tower, the reference pressure is
atmospheric pressure. If I am at a sea level location, for
instance New York City, then the reference pressure is
higher than if I am at a location like Denver.
Bottom line, the higher you go, the lower NPSHA is,
all other things being equal. This means that a nonproblematic
piping configuration on your New York City
project may run into problems when you replicate it for
your Denver project.
Head, ft. w.c.
NPSHR
https://tinyurl.com/JournalExtras
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ASHRAE Journal - June 2024
Table of Contents for the Digital Edition of ASHRAE Journal - June 2024
Contents
ASHRAE Journal - June 2024 - Intro
ASHRAE Journal - June 2024 - CT1
ASHRAE Journal - June 2024 - CT2
ASHRAE Journal - June 2024 - Cover1
ASHRAE Journal - June 2024 - Cover2
ASHRAE Journal - June 2024 - 1
ASHRAE Journal - June 2024 - Contents
ASHRAE Journal - June 2024 - 3
ASHRAE Journal - June 2024 - 4
ASHRAE Journal - June 2024 - 5
ASHRAE Journal - June 2024 - 6
ASHRAE Journal - June 2024 - 7
ASHRAE Journal - June 2024 - 8
ASHRAE Journal - June 2024 - 9
ASHRAE Journal - June 2024 - 10
ASHRAE Journal - June 2024 - 11
ASHRAE Journal - June 2024 - 12
ASHRAE Journal - June 2024 - 13
ASHRAE Journal - June 2024 - 14
ASHRAE Journal - June 2024 - 15
ASHRAE Journal - June 2024 - 16
ASHRAE Journal - June 2024 - 17
ASHRAE Journal - June 2024 - 18
ASHRAE Journal - June 2024 - 19
ASHRAE Journal - June 2024 - 20
ASHRAE Journal - June 2024 - 21
ASHRAE Journal - June 2024 - 22
ASHRAE Journal - June 2024 - 23
ASHRAE Journal - June 2024 - 24
ASHRAE Journal - June 2024 - 25
ASHRAE Journal - June 2024 - 26
ASHRAE Journal - June 2024 - 27
ASHRAE Journal - June 2024 - 28
ASHRAE Journal - June 2024 - 29
ASHRAE Journal - June 2024 - 30
ASHRAE Journal - June 2024 - 31
ASHRAE Journal - June 2024 - 32
ASHRAE Journal - June 2024 - 33
ASHRAE Journal - June 2024 - 34
ASHRAE Journal - June 2024 - 35
ASHRAE Journal - June 2024 - 36
ASHRAE Journal - June 2024 - 37
ASHRAE Journal - June 2024 - 38
ASHRAE Journal - June 2024 - 39
ASHRAE Journal - June 2024 - 40
ASHRAE Journal - June 2024 - 41
ASHRAE Journal - June 2024 - 42
ASHRAE Journal - June 2024 - 43
ASHRAE Journal - June 2024 - 44
ASHRAE Journal - June 2024 - 45
ASHRAE Journal - June 2024 - 46
ASHRAE Journal - June 2024 - 47
ASHRAE Journal - June 2024 - 48
ASHRAE Journal - June 2024 - 49
ASHRAE Journal - June 2024 - 50
ASHRAE Journal - June 2024 - 51
ASHRAE Journal - June 2024 - 52
ASHRAE Journal - June 2024 - 53
ASHRAE Journal - June 2024 - 54
ASHRAE Journal - June 2024 - 55
ASHRAE Journal - June 2024 - 56
ASHRAE Journal - June 2024 - 57
ASHRAE Journal - June 2024 - 58
ASHRAE Journal - June 2024 - 59
ASHRAE Journal - June 2024 - 60
ASHRAE Journal - June 2024 - 61
ASHRAE Journal - June 2024 - 62
ASHRAE Journal - June 2024 - 63
ASHRAE Journal - June 2024 - 64
ASHRAE Journal - June 2024 - 65
ASHRAE Journal - June 2024 - 66
ASHRAE Journal - June 2024 - 67
ASHRAE Journal - June 2024 - 68
ASHRAE Journal - June 2024 - 69
ASHRAE Journal - June 2024 - 70
ASHRAE Journal - June 2024 - 71
ASHRAE Journal - June 2024 - 72
ASHRAE Journal - June 2024 - Cover3
ASHRAE Journal - June 2024 - Cover4
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