Page 30 - Know and Understand Centrifugal Pumps

NPSH, Net Positive Suction Head

Think about it this way. When we see a magician pulling a rabbit out of a hat, in all
probability there's a rabbit hidden in a secret compartment inside the top hat, or the
rabbit is hidden in the magician's coat sleeve. The rabbit does not appear
spontaneously. Isn't it interesting that magicians all wear long sleeved topcoats? They
always reach into a 'top hat' for the rabbit. When I see a magician pull a rhinoceros
from a frisbee, then maybe 1'11 believe in magic. There is illusion, but there is no
magic. Likewise with a pump, the energy must be in the fluid for the impeller to
convert it.
Equally, if your body requires more oxygen than the available oxygen in the
atmosphere, then you would be asphyxiated. There must be more oxygen available in
the air than the oxygen you consume.

To express the quantity of energy available in the liquid entering into
the pump, the unit of measure for NPSH is feet of head or elevation in
the pump suction. The pump has its NPSHr, or Net Positive Suction
Head Required. The system, meaning all pipe, tanks and connections
on the suction side of the pump has the NPSHa, or the Net Positive
Suction Head Available. There should always be more NPSHa in the
system than the NPSHr of the pump. Let's look at them, beginning
with what the pump requires:

Definition of NPSHr (required)

It is the energy in the liquid required to overcome the friction losses
from the suction nozzle to the eye of the impeller without causing
vaporization. It is a characteristic of the pump and is indicated on the
pump's curve. It varies by design, size, and the operating conditions. It
is determined by a lift test, producing a negative pressure in inches of
mercury and converted into feet of required NPSH.

L

An easy way to understand NPSHr is to call it the minimum suction pressure I
necessary to keep the pumped fluid in a liquid state.

According to the Standards of the Hydraulic Institute, a suction lift test
is performed on the pump and the pressure in the suction vessel is
lowered to the point where the pump suffers a 3% loss in total head.
This point is called the NPSHr of the pump. Some pump
manufacturers perform a similar test by closing a suction valve on a test
pump and other manufacturers lower the suction elevation.