PAGE 34
Commercial Water
Goulds Water Technology
TECHNICAL DATA – NPSH
NPSH
The minimum operating values that can be reached
at the pump suction end are limited by the onset of
cavitation.
Cavitation is the formation of vapor-
Ŷ
lled cavities within
liquids where the pressure is locally reduced to a criti-
cal value, or where the local pressure is equal to, or
N
ust
below the vapor pressure of the liquid.
The vapor-
Ŷ
lled cavities
ŷ
ow with the current and when
they reach a higher pressure ares the vapor contained
in the cavities condenses. The cavities collide, generat-
ing pressure waves that are transmitted to the walls.
These, being sub
N
ected to stress cycles, gradually
become deformed and yield due to fatigue. This phe-
nomenon, characterized by a metallic noise produced
by the hammering on the pipe walls, is called incipient
cavitation.
The damage caused by cavitation may be magni
Ŷ
ed by
electrochemical corrosion and a local rise in tempera-
ture due to the plastic deformation of the walls. The
materials that offer the highest resistance to heat and
corrosion are alloy steels, especially austenitic steel.
The conditions that trigger cavitation may be assessed
by calculating the total net suction head, referred to in
technical literature with the acronym NPSH (Net Posi-
tive Suction Head).
The NPSH represents the total energy (expressed in
feet) of the liquid measured at suction under condi-
tions of incipient cavitation, excluding the vapor pres-
sure (expressed in feet) that the liquid has at the pump
inlet.
To
Ŷ
nd the static height (hz) at which to install the
machine under safe conditions, the following formula
must be veri
Ŷ
ed:
h
p
+ h
z
ű
(NPSHr + 2 ft) + h
f
+ h
pv
where:
h
p
is the absolute pressure applied to the free liquid
surface in the suction tank, expressed in feet of
liquid
hp is the quotient between the barometric
pressure and the speci
Ŷ
c weight of the liquid.
h
z
is the suction lift between the pump axis and the
free liquid surface in the suction tank, expressed in
feet
hz is negative when the liquid level is lower
than the pump axis.
h
f
is
the
ŷ
ow resistance in the suction line and its ac-
cessories, such as:
Ŷ
ttings, foot valve, gate valve,
elbows, etc.
h
pv
is the vapor pressure of the liquid at the operating
temperature, expressed in feet of the liquid. hpv is
the quotient between the Pv vapor pressure and
the liquid
ő
s speci
Ŷ
c weight.
0.5
is the safety factor.
The maximum possible suction head for installation
depends on the value of the atmospheric pressure
(i.e. the elevation above sea level at which the pump is
installed) and the temperature of the liquid.
To help the user, with reference to water temperature
(40ºF) and to the elevation above sea level, the follow-
ing tables show the drop in hydraulic pressure head in
relation to the elevation above sea level, and the suc-
tion loss in relation to temperature.
Water Temperature (ºF)
68 104 140 176 194 230 248
Suction Loss (ft)
-.7 2.3 6.6 16.4 24.3 50.5 70.5
Elevation Above Sea Level (ft)
1600 3300 4900 6500 8200 9800
Suction Loss (ft)
1.8 3.6 5.4 7.2 9.0 10.8
To reduce it to a minimum, especially in cases of high
suction head (over 13 – 16 feet) or within the operat-
ing limits with high
ŷ
ow rates, we recommend using a
suction line having a larger diameter than that of the
pump’s suction port. It is always a good idea to posi-
tion the pump as close as possible to the liquid to be
pumped.
