Hydrocarbon Processing - December 2021 - 77

Valves, Pumps and Turbomachinery
pumps for net positive suction head
(NPSH) improvement may use anti-friction
bearings (i.e., rolling element bearings).
The bearing vibration cannot be
measured with conventional proximity
probes because the rotor is rigidly fixed
to the bearing, providing no relative vibratory
movement between the shaft and
the bearing. For these types of pumps,
only bearing housing vibrations need to
be measured and compared with the API
610 allowable value for bearing housing
during shop or field tests.
API 610 divides the operating flow
range of the centrifugal pumps into two
regions, as shown in FIG. 3. Centrifugal
pump vibration varies with flow. The vibration
is minimal at the best efficiency
flowrate (BEP) and increases as flow is
increased or decreased.
At all points of the two regions, the
shaft displacement vibration and bearing
housings vibration must meet API 610
limits. Sometimes, the vendor interprets
Clause 8.4.1 of API 610 for pumps with
proximity probes if the bearing housing
vibration is out of the API 610 limit;
however, if shaft probes are within the
limit, then shaft probes data are considered
as the basis for accepting or rejecting
the pump. This is definitely a misinterpretation
of API 610, since both the
vibration data from the bearing housing
and the proximity probes must meet the
API 610 limit. This means that the bearing
housing vibrations are measured in
units of velocity, and shaft displacement
is measured in units of displacement, as
shown in Table 8 of API 610.
Normally, displacement
is
a good
measure at lower frequencies (the
recommended frequency range is
0 cpm-600 cpm). The failure mode is
generally the " stress " caused by the displacement.
Velocity measures how often
the displacement is applied in a given
time period. It is related to the fatigue
mode of failure. Velocity amplitude is a
good unit of measure in the range of 600
cpm-60,000 cpm. Even at a small displacement
amplitude, the repeated motion
can cause fatigue failure.
As an example, consider a pump with
a vibration due to the vane pass frequency.
The vane pass frequency vibration of
a centrifugal pump is a hydrodynamically
induced vibration at a frequency
determined by the number of impeller
vanes times the pump cpm. This means
that the vibration due to the vane pass
frequency has a higher frequency (i.e.,
greater than 600 cpm). It is less noticeable
with shaft displacement probes
readings; therefore, most of the vane pass
vibration issues of centrifugal pumps are
found with bearing housing vibration
readings. This means that, during the
performance test of a BFW pump, acceptable
vibration is measured at 1X, 5X
and 7X, according to shaft displacement
probes data. Shaft probes may not capture
high-frequency vibration over 600
cpm; however, when bearing housing vibration
is measured, the pump has vibration
at 5X and 7X, which is the vane pass
frequency. This is why API 610 requires
pumps with hydrodynamic bearings to
meet the allowable vibration limits for
both the bearing housings vibration and
the shaft displacement data.
Causes for pump vibration during
factory test. The risk of vibration at
flows below and above the BEP is high
for BFW pumps with a speed of 3,000
rpm and above during the factory performance
test. Qualitative (and quantitative,
if required) vibration risk management
information from the shop performance
test should be provided for new BFW
pumps during the bidding stage so that
all risks associated with the proposed
vendor's model can be recognized and
FIG. 2. Typical BFW pump performance test setup.
FIG. 3. Regions of operating flow range of the centrifugal pumps based on API 610.2
Hydrocarbon Processing | DECEMBER 2021 77
Hydrocarbon Processing - December 2021

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