Page 39 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

6. Reduce the losses (Hf) of the connections and fittings in the suction
piping. For wheel actuation valves, maybe globe valves could be
converted into gate valves. For quarter turn valves, butterfly valves
could be replaced with ball valves. A totally open butterfly valve still
has the post and wings in the flow path. Maybe convert short radius
elbows into long radius elbows. If you had two or three consecutive
elbows, maybe you could use a flexible ‘S’ connection. This reduces
Hf.
7. Eliminate some elbows. If the suction piping has multiple elbows,
you can bet that some of those elbows are canceling themselves, and
are not needed. This reduces Hf.
8. Lower the temperature of the fluid in the suction. This reduces the
Hvp.
If you cannot increase the NPSHa of the system, maybe you could
reduce the NPSHr of the pump, by:

Change to a pump with a larger suction diameter. For example,
convert a 1 x 2 x 8 pump, into a 2 x 3 x 8 pump. The larger pump
would have a reduced NPSHr. You need to keep the same impeller
diameter (8 inch) to maintain the discharge head and pressures, but
you would be converting the 2 inch suction nozzle into a 3 inch
suction nozzle. This would reduce the fluid velocity entering into
the pump, and therefore the Hf and Hi.
Install a small booster pump into the suction piping. The booster
pump would have a reduced NPSHr for the system feeding it, and
the discharge head of the booster pump would increase the Ha to
the primary pump.
3. Increase the diameter of the eye of enclosed impellers. This reduces
Hi.
4. Ream out and polish the suction throat and pathway to the
impeller. This is normally the roughest casting inside the pump.
Center the suction nozzle on a lathe and open the diameter of the
pathway toward the impeller. This lowers the existing NPSHr of
your pump, reducing the Hi.
5. Use an impeller inducer. An impeller inducer looks like a corkscrew
device that fits onto the center hub of the primary impeller and
extends down the suction throat of the pump. It is actually a small
axial flow impeller that accelerates the fluid toward the primary
impeller from further down the suction throat of the pump. Some
inducers bolt onto the impeller and others are cast into the main
impeller. The inducer has a low NPSHr for the system feeding it,
and it increases the Ha to the primary impeller.

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