Environmental Engineering Reference
In-Depth Information
The Hydraulic Institute (1994) defined NPSH as “the total suction
head in feet of liquid absolute determined at the suction nozzle and
referred to datum less the vapor pressure of the liquid in feet absolute.”
This defines the NPSH available (NPSHA) for the pump. (Note that NPSHA
is the actual water energy at the inlet.) The important point here is that a
pump will run satisfactorily if the NPSHA equals or exceeds the NPSHR.
Most authorities recommend that the NPSHA be at least 2 ft absolute or
10% larger than the NPSHR, whichever number is larger.
Note:
With regard to NPSHR, contrary to popular belief water is not
sucked into a pump. A positive head (normally atmospheric pressure)
must push the water into the impeller (i.e., flood the impeller). NPSHR is
the minimum water energy required at the inlet by the pump for satisfac-
tory operation. The pump manufacturer usually specifies NPSHR.
It is important to point out that if NPSHA is less than NPSHR, the
water will cavitate.
Cavitation
is the vaporization of fluid within the cas-
ing or suction line. If the water pressure is less than the vapor pres-
sure, pockets of vapor will form. As vapor pockets reach the surface of
the impeller, the local high water pressure will collapse them, causing
noise, vibration, and possible structural damage to the pump.
3.2.11.1 Calculating NPSHA
In the following two examples, we demonstrate how to calculate
NPSH for two real-world situations: (1) determining NPSHA for an open-
top water tank or a municipal water storage tank with a roof and cor-
rectly sized vent, and (2) determining the NPSHA for a suction lift from
an open reservoir.
3.2.11.1.1 nPSha: atmospheric Tank
The following calculation may be used for an open-top water tank
or a municipal water storage tank with a roof and correctly sized vent,
as shown in Figure 3.4 and Figure 3.5. The formula for calculating
NPSHA is:
NPSHA =
P
a
+
h
-
P
v
-
h
e
-
h
f
(3.25)
where:
P
a
= atmospheric pressure in absolute or pressure of gases against
the surface of the water.
h
= weight of the liquid column from the surface of the water to the
center of the pump suction nozzle in feet absolute.
P
v
= vapor pressure in absolute of water at a given temperature.
h
e
= entrance losses in feet absolute.
h
f
= friction losses in the suction line in feet absolute.
