POWER February 2021 - 26

FUNDAMENTALS
Understanding Net Positive
Suction Head Requirements
for Pumps
A centrifugal pump that performs well under steady-state conditions may be found
to cavitate when transitioning between operating modes. Resolving a transient
cavitation problem can seem daunting because many thermodynamic and
dimensional factors are involved. This article will help the reader understand the
causes of pump transient cavitation, present a simple screening technique for
identifying poor designs, and describe what corrective measures to take.
Michael F. Czyszczewski, PE
P
ump cavitation occurs when a decrease
in suction pressure causes
the liquid to begin boiling. Vapor
bubbles form and violently implode
when they enter higher pressure regions
of the pump. This creates noise, pitting,
erosion, flashing, and vibration. If the
bubbles collect and form into a vapor
cavity, the pump can experience a water
hammer when the cavity collapses. The
accumulated damage from these shortterm
transient events can eventually
cause a catastrophic pump failure.
What Is Pump NPSH?
Anyone who has sized a pump should
be familiar with the term net positive
suction head (NPSH). Simply put, pump
NPSH is the excess head (or pressure)
exerted on the pump's
suction
that
keeps the liquid from boiling. The available
NPSH (NPSHa) for a pump is given
in units of feet-absolute and is defined
as follows:
NPSHa = (HP
where HP
- HVPA
) + (HST
- HFS
)
is the absolute pressure on the
surface of the liquid source upstream of
the pump converted to head units; HVPA
is the pump suction absolute vapor pressure
(the pressure at which a liquid at a
particular temperature begins to vaporize)
converted to head units; HST
is the
static head on the pump suction (the
vertical distance between the minimum
liquid level of the source and the pump
impeller's centerline); and HFS
is the head
loss due to friction in the suction piping
at the maximum flowrate. Note that
head and pressure terms are often both
used when discussing NPSH. Use the
26
sometimes use a 1% or 0% criterion
and occasionally, the pump's NPSHr
curve includes a manufacturer's margin.
If the basis for your NPSHr curve is not
clear, the manufacturer should be asked
to clarify what it represents.
If the pump is required to provide
its full design flow capacity without
any cavitation loss, NPSHa must exceed
NPSHr by an appropriate margin:
NPSHa ≥ margin x NPSHr. The margin
required to compensate for a 3% output
loss can be calculated as follows:
margin = [(slope x QFW
100)] + 1.0
x 3) / (NPSHr x
www.powermag.com
margin of 1.8 might be used for the design
point, and 1.5 at runout.
Note that the margin recommendations
do not account for unique
operating conditions or equipment arrangements.
If there are uncertainties in
your design that could increase the potential
for transient cavitation, a greater
margin would be warranted. Use the
largest of the margins applicable for
your pump's service when checking for
adequate NPSHa.
Pumps Susceptible to NPSH
Transients
A NPSH transient occurs when the
POWER | February 2021
liquid specific volume (V) in units of ft3/lb,
to convert head in feet to pressure in psi:
psi = feet / (144 x V)
NPSH Margin
The manufacturer determines the required
NPSH (NPSHr) for a pump in units
of feet-absolute, by steady-state testing
the pump with water. NPSHr increases
with flowrate, and typically represents
the point at which a 3% loss in pump
output occurs due to cavitation (called
NPSH3). Unfortunately, manufacturers
where the slope of the pump's NPSHr
curve at the maximum pump flow (QFW
)
is ∆H/∆Q. This margin is typically a factor
of about 1.2.
The American National Standards
Institute (ANSI)/Hydraulic Institute (HI)
standard ANSI/HI 9.6.1-2017 " Rotodynamic
Pumps Guideline for NPSH
Margin " gives recommendations on
how much margin to use based on the
pump's service. Additionally, some engineering
firms apply system specific
margins based on their experience. For
example, when sizing a FW pump, a
Theoretically, any liquid pump can
experience NPSH transients. However,
systems that pump low-pressure,
high-temperature liquids are most
likely to experience problems.
http://www.powermag.com

POWER February 2021

Table of Contents for the Digital Edition of POWER February 2021

Contents
POWER February 2021 - Cover1
POWER February 2021 - Cover2
POWER February 2021 - Contents
POWER February 2021 - 2
POWER February 2021 - 3
POWER February 2021 - 4
POWER February 2021 - 5
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POWER February 2021 - Cover3
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