Lessons learned the hard way concerning
centrifugal pump repairs.
Bob Boelens is the author of this
article. He can be reached by E-mail at mailto:<email@example.com>
All centrifugal pumps -
- When repairing the cutwater area make sure that you retain the
proper minimum clearance between the impeller and the cutwater.
This minimum is 4% of the impeller diameter on impellers up to 14"
(355 mm), and 6% of the impeller diameter on impellers above 14"
- If bearings have failed catastrophically or are "locked up" be
sure to take a precision measurement of the bores that the
bearings fit into and a precision measurement of the shaft
journals that the bearings are mounted on.
End suction pumps -
- When fabricating large bore seal chambers make the new chamber
as long as possible to place the seal as close to the bearing as
possible. This will reduce the amount of shaft deflection that the
seal is subjected to.
- Take a close look at how the thrust bearing (usually the
bearing closest to the coupling end of the pump) is retained. Make
sure that the bearing is held securely in place when you
reassemble the pump.
- When eliminating packing sleeves and replacing them with solid
shafts be sure to use a shafting material that is as corrosion
resistant as the original sleeve.
- One of the bearings on the pump has to be allowed to "float"
in order to accommodate thermal growth of the rotating assembly.
Make sure that the fit on the "floating" bearing will allow the
bearing to move easily. If the bearing cannot easily move it will
be subjected to excessive thrust loads and fail prematurely.
Split case double suction pumps -
- Some of these pumps use "sleeve nuts" to lock the sleeves in
place on the shaft. When mounting mechanical seals on these sleeve
nuts be sure to check the sleeve nut outside diameters for
concentricity after assembling the rotating element.
- The impellers on these pumps are usually held centered in the
pump case via the position of the sleeves. If the impeller is not
properly centered in the pump casing there will be a hydraulic
imbalance that could result in excessive thrust loads on the pump
- When converting these pumps from packing to mechanical seals
make sure that the sleeves are sealed either against the shaft or
the impeller. Failure to do this could result in a leak path under
the sleeve to the atmosphere.
- Most of these pumps use a double volute design. If you are
repairing the cutwater areas on these pumps make sure that the
cutwaters are exactly 180 degrees apart. Failure to do this will
result in shaft deflection due to uneven hydraulic loads on the
- These pumps often use diffuser fins in the pump case in the
suction path of the pump. These fins are designed to reduce
turbulence as the fluid enters the impeller. If these fins have
been worn they must be repaired to their original shape in order
for the pump to operate smoothly.
- The stuffing box areas on all split case pumps deserve special
attention. When cutting or installing casing gaskets make sure
that the gasket is at least flush with the stuffing box face. Many
OEM gaskets are too short in this area.
- Pay close attention to the condition of the gasket surfaces on
the pump casing adjacent to the stuffing box. This area is subject
to more corrosion than the rest of the pump casing due to
frictional heat caused by packing and the presence of oxygen
around the packing outside diameter.
- Impeller wear rings should have their final outside diameter
machined after the wear rings have been installed on the impeller
if possible. This eliminates any "out of round" conditions due to
set screw distortion, welding or rough handling.
- The fit between the casing wear ring outside diameter and the
pump casing should have no clearance. Any clearance in this area
will cause internal recirculation and rapid casing erosion.
- When purchasing casing rings, check them for concentricity
before installing them. These parts are often damaged in shipping
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