Page 32 - Know and Understand Centrifugal Pumps

NPSH, Net Positive Suction Head

As a general guide the NPSHa should be a minimum 10% above the
NPSHr or 3 feet above the NPSHr, whichever is greater. Other books
and experts indicate that the NPSHa should be 50% greater than the
NPSHr, to avoid incipient cavitation. Again, be prepared for stricter
definitions to NPSHr and higher safety margins on NPSHa.

The NPSHa is in the system. The formula is:
NPSHa = Ha + Hs - Hvp - Hf - Hi
Where: Ha = Atmospheric head (14.7 psi x 2.31) = 33.9 ft. at sea
level. See Properties of Water I in this chapter that considers
atmospheric pressure at different elevations above sea level.
Hs = Static head in feet (positive or negative) of the fluid level
in the suction vessel to the pump centerline.
Hvp = the Vapor head of the fluid expressed in feet. It is a
hnction of the temperature of the liquid. See Properties of
Water I1 in this chapter.
Hf = Friction head or friction losses expressed in feet in the
suction piping and connections.
Hi = Inlet head, or the losses expressed in feet that occur in
the suction throat of the pump up to and including the eye of
the impeller. These losses would not be registered on a suction
pressure gauge. They could be insignificant, or as high as 2
feet. Some pump manufacturers factor them into their new
pumps, and others don’t. Also, changes occur in maintenance
that may alter the Hi. If you don’t know the Hi, call it a safety
factor of 2 feet.
By observing the system, you can calculate the NPSHa within a one or
two point margin. The main idea is to be sure the NPSHa is greater
than the NPSHr of the pump. Remember that the NPSHa only deals
with the suction side of the pump. Let’s go back to that formula:
NPSHa = Ha + Hs - Hvp - Hf - Hi

1. To determine the Ha, atmospheric head, you only need observe the
vessel being drained by the pump. Is it an opened, or vented
atmospheric vessel? Or is it a closed and sealed vessel? If the vessel is
open, then we begin with the atmospheric pressure expressed in
feet, which is 33.9 feet at sea level. The altitude is important. The
atmospheric pressure adds energy to the fluid as it enters the pump.
For closed un-pressurized vessels the Ha is equal to the Hvp and
they cancel themselves. For a closed pressurized vessel remember
that every 10 psia of pressure on a vessel above the vapor head of
the fluid will add 23.1 feet of Ha. To the Ha, we add the Hs.
2. The Hs, static head, is the static height in feet observed from the
level in the vessel to be drained to the centerline of the pump. If the

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