Page 252 - Know and Understand Centrifugal Pumps

Avoiding Wear in Centrifugal Pumps

the original factory setting. This wear represents an 8% decrease in
efficiency. Now the pump is 69% efficient. Let’s do the math with all
other factors constant. This reduction in the efficiency represents an
annual electricity cost of $61,845.60. The additional electricity is six
thousand three hundred ninety four dollars and eighty cents. Four
thousandths wear (0.004 inch) has cost us almost $6,500.00 per year
for just one pump. Just to mention, a new wear ring may cost up to
$60.00 plus the labor to change it (this will never add up to
$6,500.00).
Effective and well planned maintenance can reduce the operating cost
of your pumps and other equipment as this example demonstrates.
With differential pressure gauges on the pump, an amp meter and flow
meter you can determine if strict tolerance parts are worn. This
indicates the need to take the pump into the shop for corrective
procedures. If you don’t do it, you are wasting your annual operating
budget. As we mentioned in Chapter 6, the Wear Rings should be
called Efficiency Rings. Now you know why.

Fluid velocity accelerates wear

Small impellers with high motor speeds may produce the necessary
pump pressure. This type of combination produces high fluid velocities
that will wear pump parts much faster than desirable. This is in the
Affinity Laws. In addition the impeller suffers rapid wear due to high
tip velocities. When a pump is disassembled and excessive wear is found,
95% of the time high velocity fluid is to blame.

Tu rbu I en ce

Uneven wear in parts is often due to turbulence. Bad piping designs or
poorly sized valves can cause turbulence and uneven wear in pumps.
Whenever possible, use straight pipe sections before and after the
pump. Uneven flow creates turbulent flow and excessive wear occurs.

It is not recommended to place an elbow at the suction of any pump
(Figure 16-2, next page). This will cause a turbulent flow into the
pump. If elbows are needed on both sides of the pump, you should use
long radius elbows with flow straighteners. You should have 10 pipes
diameters before the first elbow on the suction piping (Example: If the
pump has a 4 inch suction nozzle, you should respect 40 inch of
straight pipe before the first suction elbow.) Short radius elbows cause
vibrations and pressure imbalances that to lead to wear and
maintenance on the pump.

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