SPECIAL TEST


Shaft deflection of single volute pump


A large single stage overhang process pump has had multiple mechanical seal
failures due to excessive shaft deflection presumably due to high hydraulic radial
thrust attributable to its single volute lip design.

To reduce the high hydraulic radial load and minimize the shaft deflection it was
decided to modify the casing to simulate a double volute lip design by welding a
piece of metal splitter opposite the single volute lip at the periphery of the volute
cutwater.

Initial calculations were made to estimate the axial thrust values using empirical
thrust factors for both single volute and  double volute configurations based on
using the same model impeller. The equipment was then tested at normal
operating conditions at 1185 RPM. The objectives were:

  • To verify that the modification will result in the expected reduction in
    hydraulic radial thrust.

  • To compare the calculated (or theoretical) radial thrusts with the actual (or
    tested) radial thrusts to validate the widely used empirical thrust factors
    that were used in the thrust calculations.

The pump was run at different flow rates ranging from shut-off (zero flow), to its
best efficiency point (BEP), and to its normal end-of-curve flow. Both the radial
thrusts and shaft deflections were measured at these different flow rates. The
equipment was similarly tested before, and after, the casing modification.
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Hydraulic axial thrust
Hydraulic radial thrust
Hydraulic radial thrust on single volute pump
Balance disk (disc)
Based on the test results, the following observations were made:

  • The volute modification resulted in big reduction in shaft deflection.

  • There was no significant change in its Q/H/E performance resulting from
    the volute modification.

  • As expected, the shaft deflections (both before and after the modification)
    showed a decreasing trend as the flow rates were being increased, and
    the deflections were at minimum values at the BEP.

Additionally, a comparison was made between the calculated shaft deflection and
the measured shaft deflection prior to the volute modification. The comparison
showed that up to about 50% of BEP the empirical thrust factor used in calculating
the hydraulic radial loads of single volute pumps was conservative and resulted in
higher calculated thrust value and shaft deflection. At above 50% of BEP, the
empirical thrust factor was slightly at the low side, but close enough to be
considered in line with the test results.

The casing modification to simulate a double volute lip design, resulted in very
significant improvement to the equipment's mean-time-between-failures (MTBF).

For more details, please contact CPO.

R: shaft deflection 0111
C: trouble-shooting
F: shaft_deflection_test