Testing and quality

TESTING AND QUALITY
Revealing and interpreting hidden features using dedicated signal analysis
Filter and fit
Based on the signals of only a few sensors, Sulzer Innotec was successful in identifying
the most important natural frequencies and vibration mode shapes of a pump impeller.
The results are, essentially, in good agreement with the FEM calculations performed by
Sulzer Pumps. The measurement results indicate that the mechanical resonances are quite
well damped, so that they will not jeopardize the integrity of the impeller in any way.
1 Positions of strain
gauges and
accelerometers on the
impeller (axial view)
(Source: PhD. thesis
by Stefan Berten)
14
| Sulzer Technical Review 1/2011
In 2009, Sulzer Innotec received an
order from the Sulzer Pumps division
to analyze and assess vibration
signals measured in a pump test stand.
The test pump that was examined is the
reproduction of the last stage of a highpressure
centrifugal pump operated by
a variable-speed drive. This impeller
stage has a rated power of 1.4MW.
The impeller has seven blades and a
diameter of 0.35m. In centrifugal pumps,
rotor-stator interactions and flow separation
phenomena are known to excite
vibrations of stationary or rotating components,
which might result in fatigue
failure. Based on performance monitoring
of a particular pump stage in part-load
SG6
SG7
SG5
SG4
SG8
Acc3
Acc = Accelerometer
SG = Strain gauge
SG1
SG3
Acc1
Acc2
SG2
conditions, Sulzer Pumps assigned an
extensive experimental investigation of
the mechanical and hydraulic oscillations
to the EPFL (École Polytechnique Fédérale
de Lausanne).
Many sensors for pressure, strain,
acceleration, and displacement were
installed in order to record the corresponding
oscillations. All measurements
were performed by the Laboratoire de
Machines Hydrauliques (LMH) of the
EPFL and analyzed, for the most part,
in the context of a PhD. thesis.
Customized analysis of the impeller
mode shapes
The task of Sulzer Innotec consisted of
identifying the operational vibration
mode shapes of the rotating impeller
based on the measurement signals of the
strain gauges and accelerometers
installed on the impeller and assessing
the results and comparing them with the
results from FEM calculations.
For certain operating conditions, a frequency
excited by the stationary components
might coincide with a weakly
damped natural frequency of the impeller
and might result in substantial resonance
amplitudes. Conversely, a lack of strong
resonant vibrations despite excitation at
the resonance frequency indicates that
the corresponding resonance is highly
damped.
The task of Sulzer Innotec was to
analyze the vibration signals of two
speed ramps—one run-up and one coastdown.
Figure 1 indicates the positions
of the eight strain gauges and three
accelerometers installed on the impeller.
Conditioning of strain gauge signals
The recorded strain gauge signals were
considerably affected by noise; substantial
noise sources were sensor noise, bit noise
of the analog digital converter ADC, and
cross talk of external signals. The comparison
of all strain gauge signals showed
that these signals are mostly in phase and
have similar amplitudes. The conclusion
was that the signals were seriously
affected by strong cross talk of an external
signal. Therefore, as a first step of signal
conditioning, the average value of all
strain gauge signals was subtracted from
each individual signal; thereby, the noise
level could be substantially reduced. The
resulting spectrograms now revealed the
typical series of order lines, which result
from the excitation at integer multiples
of the pump rotation frequency. In particular,
the orders 12, 24, and 36 are clearly
prominent, as expected for a pump
equipped with 12 diffuser guide vanes.
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