Page 36 - Know and Understand Centrifugal Pumps

NPSH, Net Positive Suction Head

NPSHa = Ha + Hsl -Hvp - Hf - Hi
NPSHa = 33.9 + 15.0 -0.839 - 2.0 - 2.0
NPSHa = 44.061 feet

The curve of the pump in this service should show an NPSHr of less
than 44 ft at the duty point. And the purpose of this pump is to drain
this tank, lowering its level. If we don't want inadequate NPSHa and
the possible resulting cavitation to start during the process we should
consider a second Hs2 with the tank empty. The other factors remain
the same. At the end of the process, we have:
NPSHa = Ha + Hs2 - Hvp - Hf - Hi
NPSHa = 33.9 + 6.0 - 0.839 - 2.0 - 2.0
NPSHa = 35.061 feet

To avoid stress from inadequate NPSHa during the draining process,
we should consult the pump curve and be sure that the NPSHr is less
than 35 ft at the duty point.
Now let's consider Figure 2-2. This is a pump in suction lift draining
an opened tank that's 8 feet below the pump centerline. This pump is
installed high on a mountain at 7,000 feet above sea level. The Ha is
26.2 feet. The Hsl is -8.0 feet. The water temperature is 50" F, so the
Hvp is 0.411. The Hf is 1 foot and the Hi is 2.0. According to the
information:
NPSHa = Ha + Hsl - Hvp - Hf - Hi
NPSHa = 26.2 + (-8.0) - 0.411 - 1.0 - 2.0
NPSHa = 14.8 feet
The curve of the pump in this service should show a NPSHr of less
than 14 feet at the duty point. The purpose of this pump is to drain this
tank down to 14 feet below the pump without cavitating. Let's
consider a second static head, Hs~, of -14 feet. The other factors would
remain the same:

Open Tank
Ha = 26.2

Temp. = 50 *F
Flgum 2-2
Hvp = 0.41 1
Ficlure 2-2

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