ASHRAE Journal - June 2024 - 22

ENGINEER'S NOTEBOOK
COLUMN
David Sellers
Part 1
Net Positive
Suction Head
BY DAVID SELLERS, P.E., MEMBER ASHRAE
Net positive suction head (NPSH) is a complex concept, and the piping on the project
shown in Figure 1 suggested that the concept was not fully understood and embraced
by the project team. It took me a while to feel comfortable with NPSH. This column is
the fi rst of two parts, the goal of which is to share the NPSH lessons learned over the
years in the hopes others might benefi t from the insights.
FIGURE 1 Positive pressure at the pump suction will go a long way toward
preventing NPSH problems. The fact that the pump suction line for this system
rises above the cold basin level means that the pressure in the suction line is
subatmospheric from the start. Pressure drops in the piping will compound the
problem. Visit http://tinyurl.com/ASHRAE-NPSH01 to tour.
The blue condenser water piping confi guration
leaving the counterfl ow cooling towers in Figure 1 was
just one component of the problem we observed on that
recent site visit. The linked tour (http://tinyurl.com/
ASHRAE-NPSH01) will allow you to observe several other
contributors, but none as obvious as shown in Figure 1.
Pump Suction
Piping Rises Above
the Cold Basin
Condenser
Pumps
Tower Cold
Basin
Equalizer
Line
Preventing Cavitation
NPSH is all about preventing cavitation-a
phenomenon that happens with pumps (and control
valves and propellers on ships) due to the reduction in
static pressure that occurs as a fl uid accelerates to pass
through the smaller cross-sectional area represented
by the eye (and immediate vicinity) of the impeller. It
can ruin the pump impeller, control valve plug or ship
propeller. Most people describe it as sounding like rocks
bouncing around inside the pump. You can hear an
example of it in the tour associated with Figure 1.
Bernoulli's equation (a simplifi ed form of the steady
fl ow energy equation) suggests if the fl uid needs to
accelerate (increase kinetic energy), the energy to
David Sellers, P.E., is senior engineer at Facility Dynamics Engineering.
22
ASHRAE JOURNAL ashrae.org J U N E 2 0 2 4
http://tinyurl.com/ASHRAE-NPSH01 http://tinyurl.com/ASHRAE-NPSH01 http://tinyurl.com/ASHRAE-NPSH01 http://www.ashrae.org

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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