Hydrocarbon Processing - December 2021 - 78
Valves, Pumps and Turbomachinery
prioritized, and so that adequate resources
are allocated to minimize the impact of
adverse consequences.
Pump vibration can have many different
causes. The main causes of vibration
for centrifugal pumps have been reviewed
by experts in literature.4 FIG. 4 shows the
main causes of vibration for BFW pumps
in a " fishbone diagram. " 4
As FIG. 4 shows,
vibration observed during factory performance
or the mechanical running test
can be classified into two main categories:
mechanical and hydraulic.
The hydraulic forces (i.e., vane pass induced)
are significant and normally take
place in the BFW pumps. The effect of
hydraulic forces may be magnified due to
a resonance response within the structure.
When the pump is moved from the performance
test stand to a job skid for the mechanical
running test, the amplitude and
composition of the vibration may change,
even though no change was made to the
pump itself (e.g., impeller trimming).
The conclusion is that the interaction
with the mounting structure should be
accepted as the root cause, rather than
the forces themselves, if the level of bearing
housing vibration is still outside of
API limits. The hydraulic forces will
not be removed by proper installation
of equipment and piping. The vendor
may refer to the weight of grouting that
will be added at the site; however, the
increased force of hydraulic pulsations
will be absorbed via the bearing housing
and pedestals, and not the weight of
the skid. If these elements are not rigid,
then the effect of the rigid base will be
Vibration measurements at site test
compared to factory test. The vendor
may argue that vibration at the site after
proper installation will decrease compared
to shop measurements. According
to ISO 10816-7, 1st Ed., for Category 1
and 2 services, the allowable limit of vibration
at the preferred operating region
(see " POR " in FIG. 3) for the site test is
lower than for the shop test, as shown in
TABLE 3. This may be because ISO makes
consideration for temporary foundation
and piping during the shop test. However,
this is on the basis of presumption
that a permanent installation at the site is
perfect and free from any design or construction
deficiency.
The author believes that this may not
be a valid assumption, based on several
installations. Under site conditions, vibration
levels of BFW pumps are often
expected to be higher compared with test
bench conditions at the shop due to a few
key reasons:
1. Operating temperature for the
service at the site is higher than
the test water at the factory.
Acoustic response will change at
operating conditions due to the
lower density of liquid compared
marginal, at best. The hydraulic forces
will be dampened by modifications to
the impeller and diffuser. Increased clearance
between the impeller tip and the
volute tongue may help reduce hydraulic
pulsations due to the vane pass; however,
this may have a negative impact on pump
head and efficiency.
with the density
during the test.
2. Increased hydraulic pulsations at
the vane pass and other multiple
frequencies due to increased
recirculation during turndown
may excite structural and acoustic
frequencies of the suction
and discharge piping. Due to
space limitations and other site
constraints, it may not be possible
to ensure an adequate separation
margin with such excitation
frequencies and their harmonics.
Operating any pump at flowrates
outside of the best efficiency
will increase hydraulic induced
forces due to recirculation. Every
effort should be made to avoid
resonance of the system during
the design stage.
3. Nozzle loads under operating
conditions at the site are
expected to be higher.
Practical mitigations for pump vibration
during shop performance
test. Mitigations of vibration risks during
factory test are divided into several
main categories, as detailed in the following
sections.
Pump and minimum flow sizing.
Vane pass hydraulic forces are present
in all centrifugal pumps to some degree,
and the effects worsen when the pump
operates at flowrates outside of the best
efficiency. For this reason, the minimum
continuous stable flow (MCSF) for
pumps must be sized correctly. MCSF
is defined as the point where the pump
vibration meets the allowable vibration
limit.1
If the pump has a vibration at minFIG.
4. Main causes of vibration for a BFW pumps in a fishbone diagram.4
78 DECEMBER 2021 | HydrocarbonProcessing.com
imum flow, then the vendor may propose
increasing the minimum flow as an acceptable
remedy, which will decrease the
AOR (FIG. 3), and the pump will not be
the same product as that originally sold
by the vendor.
On the other hand, during the sizing
of the BFW pumps, special attention
must be given during the early production
stage to the size, duty and turndown
of the minimum flow recycle valve and
cooler, as well as the OTSG quantity,
turndown and minimum steam demand.
The vendor may say that the pump has
an unacceptable vibration at the minimum
flow or close to the midpoint,
without assuming that the pump will
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Hydrocarbon Processing - December 2021
Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2021
Contents
Hydrocarbon Processing - December 2021 - Cover1
Hydrocarbon Processing - December 2021 - Cover2
Hydrocarbon Processing - December 2021 - Contents
Hydrocarbon Processing - December 2021 - 4
Hydrocarbon Processing - December 2021 - 5
Hydrocarbon Processing - December 2021 - 6
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Hydrocarbon Processing - December 2021 - Cover3
Hydrocarbon Processing - December 2021 - Cover4
Hydrocarbon Processing - December 2021 - GP-1
Hydrocarbon Processing - December 2021 - GP-2
Hydrocarbon Processing - December 2021 - GP-3
Hydrocarbon Processing - December 2021 - GP-4
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Hydrocarbon Processing - December 2021 - GP-40
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