Page 31 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

The definition of NPSHr may change in the future. A pump is in a
definite state of cavitation with the 3% total head loss definition. Many
pump users want a more explicit definition of NPSHr, and higher
NPSHa safety margins to avoid inadequate NPSHa and cavitation
altogether.

The pump manufacturers publish the NPSHr values on their pump
curves. We’re saying that the NPSH reading is one of the components
of your pump curves. We’ll see this in Chapter 7 on Pump Curves. If
you want to know the NPSHr of your pump, the easiest method is to
read it on your pump curve. It’s a number that changes normally with a
change in flow. When the NPSHr is mentioned in pump literature, it is
normally the value at the best efficiency point. Then, you’ll be
interested in knowing exactly where your pump is operating on its
curve.
If you don’t have your pump curve, you can determine the NPSH of
your pump with the following formula:

Nl’SHy = ATM + PBS + HV - HvP
Where: ATM = the atmospheric pressure at the elevation of the
installation expressed in feet of head.
Pgs = the suction pressure gauge reading taken at the pump
centerline and converted into feet of head.
Hv = Velocity Head = V2/2g where: V = the velocity of the
fluid moving through the pipes measured in feet per second,
and ‘g’ = the acceleration of gravity (32.16 ft/sec).
Hvp = the vapor pressure of the fluid expressed in feet of
head. The vapor pressure is tied to the fluid temperature.
The easiest thing to do is to get the pump curve from the manufacturer
because it has the NPSHr listed at different flows. Nowadays, you can
get the pump curve on the Internet with an e-mail to the manufacturer,
you can send a fax, or request the curve in the mail or with a local call
to the pump representative or distributor. If you wanted to verify the
NPSHr on your pump, you’ll need a complete set of instrumentation: a
barometer gauge, compound pressure gauges corrected to the
centerline of the pump, a flow meter, a velocity meter, and a
thermometer. Definitely, it’s easier to get the curve from your supplier.

Definition N PSHa (ava i la ble)

This is the energy in the fluid at the suction connection of the pump
over and above the liquid’s vapor pressure. It is a characteristic of the
system and we say that the NPSHa should be greater than the NPSHr
(NPSHa > NPSHr).

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