Effects of wear on pump performance

The four basic quantities in pump performance are Head, Power,
Efficiency and Flow.

 

Effects of internal wear on pump performance

 1.   Wear on vane outer ends
Wear, which reduces the impeller diameter, is most common with
abrasive or corrosive liquids. Head and Power curves are lower for all
flows: the same effect as if a smaller diameter impeller was installed in
the casing. Performance may give flow insufficient to meet production
needs, and in any case the pump will use more electrical energy for a
given flow. A similar effect occurs with open-faced impellers, where the
clearance between the impeller front edges and the casing increases.
This will be particularly pronounced if a second pump of two in parallel
has to be run because either pump can no longer supply the required
flow on its own. If one of the pumps is very worn, running both pumps
together may mean that the worn pump contributes no flow, wasting
energy and possibly damaging the pump. Reverse flow may occur
through the worn pump, causing it to run in reverse. A pump running
backwards is not always obvious at first: most of the noise produced is
from the motor and its cooling fan.


 2   .Internal wear
Wear at the impeller/sealing ring interface (i.e. wear or sealing ring)
allows liquid to recirculate from impeller outlet to suction. This leakage
flow is approximately proportional to the clearance and is approximately
constant over the pump flow range. Internal leakage also occurs
from erosion in the horizontal joint of split-casing multi-stage pumps


3.  Cavitation
Cavitation can cause erosion and even distortion of impellers, and
localised erosion in casings. In split casing pumps, erosion of the halfjoint
can allow bypassing of stages.

4.  Blockage
Impellers can get blocked with deposit, thus restricting flow.

5.  Casing corrosion and/or erosion
Casing corrosion and/or erosion occurs in many pump applications,
and is particularly evident with chemical pumps. It can be monitored
with casing thickness measurements.

6.  Impact damage
Impact damage from foreign objects carried in the liquid.


7.  Leaking external seal
Shorter life than expected can be caused by incorrect installation such
that misalignment or excessive end play occurs between the seal faces.

8.  Maintenance errors
Use of incorrect components such as a thicker casing gasket than
specified, or use of a gasket when none is required, can reduce pump