Xylem GLSSV2 SSV Series Vertical Multi-Stage Pumps D1 - Manual (Page 64)

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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-filled cavities within liquids
where the pressure is locally reduced to a critical value, or where
the local pressure is equal to, or just below the vapor pressure of
the liquid.
The vapor-filled cavities flow with the current and when they
reach a higher pressure ares the vapor contained in the cavities
condenses. The cavities collide, generating pressure waves that
are transmitted to the walls. These, being subjected to stress
cycles, gradually become deformed and yield due to fatigue. This
phenomenon, characterized by a metallic noise produced by the
hammering on the pipe walls, is called incipient cavitation.
The damage caused by cavitation may be magnified by electro-
chemical corrosion and a local rise in temperature due to the plas-
tic deformation of the walls. The materials that offer the highest
resistance to heat and corrosion are alloy steels, especially austen-
itic 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 Positive Suction Head).
The NPSH represents the total energy (expressed in feet) of the
liquid measured at suction under conditions of incipient cavita-
tion, excluding the vapor pressure (expressed in feet) that the
liquid has at the pump inlet.
To find the static height (hz) at which to install the machine under
safe conditions, the following formula must be verified:
h
p
+ h
z
≥ (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 specific
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 flow resistance in the suction line and its accessories,
such as: fittings, 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‘s
specific 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 following tables show the
drop in hydraulic pressure head in relation to the elevation above
sea level, and the suction 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 operating limits with high
flow 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 position the pump as close as possible to the liquid to be
pumped.