Hydro’s customer, a nuclear power plant, experienced a severe pump
failure of one of the three installed circulating water makeup pumps.
These are 3-stage vertical turbine pumps running either in stand-alone
or parallel operation as required.
The subject pump failed just 40 days from its commissioning. The failure
manifested itself through high vibration and caused severe scoring of the
pump shaft and wear ring landings, ultimately leading to fatigue failure of the
weld on the discharge head flange.
HydroAire Inc., A Hydro Company l
Chicago, IL
800.223.7867 l
Hydro’s Expertise
on Root Cause Analysis and Problem Solving
Critical Pump Fails
Cracked Flange Cracked Flange Detail
Root Cause Analysis
The OEM changed the impellers from
original Bronze to Stainless Steel in the
failed pump. Thorough investigations of
the pump curves of Stainless Steel &
Bronze impellers showed significant
differences – qualitatively &
quantitatively. As compared to the
Bronze impellers, the Stainless Steel
impeller exhibits less stable head curve
near the duty flow. Also, the Stainless
Steel impellers need higher Brake
Horsepower for the required maximum
duty flow. HydroAire inferred that the
unstable operation at the duty flow
could have caused the pump failure.
The customer accepted & approved
to study the differences between the
geometry of bronze and stainless Steel
impellers.
Reverse Engineering and
Geometry Analysis
HydroAire’s comprehensive Reverse
Engineering process and the results
discovered significant geometry
differences between the Stainless Steel
and Bronze impellers. The differences
in the Stainless Steel impellers are
expected to adversely affect the pump
performance, NPSHR & the minimum
required submergence. The plant also
confirmed that the subject pump was
operated at the time of failure with
minimum submergence that was meant
for the Bronze impeller design.
Therefore, this submergence was not
sufficient enough for the pump with
Stainless Steel impellers which could
have compounded the problems of
unstable operation with high vibrations.
Pump Repair and Upgrade
Instead of repairing the cracks on the
discharge head flange by welding,
Hydro suggested a spool piece
design, manufactured and mounted
on to the discharge head (refer to the
picture shown below). The stiffening
ribs added to this spool provide
additional strength.
Performance Test and
Analysis
Based on HydroAire’s analysis, the plant
decided to replace the Stainless Steel
impellers back to the original Bronze
impeller design and to verify the pump
performance in HydroAire’s test facility.
Test captured extensive performance
data including the vibration on the bowl
flange in submerged condition. The new
curve was analyzed in conjunction with
the plant-provided system requirements
and found the pump with Bronze
impellers to be good for both the
standalone and parallel operations with
the sister pumps already in operation.
The plant found HydroAire’s concluding
report to be sufficient to close its internal
root cause analysis.
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