Hydrocarbon Processing - December 2021 - 79

Valves, Pumps and Turbomachinery
operate for a long time in these conditions.
However, in SAGD operation, it
is likely that the BFW pump will operate
near the minimum flow or midpoint
flow, due to the lesser steam demand for
injection to the well.
One BFW pump is common for two
OTSGs; therefore, the minimum turndown
of the steam generator must be
considered in the sizing of the BFW
pumps. For example, if the minimum
turndown of one steam generator is
50 m3
pump is 100 m3
run at 150 m3
/hr and the minimum flow of the
/hr, then the pump will
/hr at minimum, which
may be far from the BEP. This is because,
at reduced capacity, pulsations
will increase due to the greater head and
turbulence energy. Force response is
significant for high-energy pumps such
as BFW pumps. Apart from high vibrations,
operation under high turndown
conditions causes an increase on both
radial and axial loads on the bearings.
Pump selection. The ranges for AOR
and POR (see FIG. 3) indicated on the
vendor's preliminary performance curve
and data sheet are preliminary and must
be validated by the purchaser and owner.
The customer should ask the vendor to
provide real test and vibration data for
the same model in a previous job, as well
as for a model of similar size.
The owner and the engineering
company should witness the operating
condition of the pump after installation
and talk to the end users directly about
vibration data after site installation
and commencement of operation. The
vendor manufacturing and test facility
should be inspected, as well, since some
vendors have only an assembly and test
shop and run their design and engineering
from another country. Warranty
terms should be carefully addressed and
consider the risk of unacceptable bearing
housing and shaft displacement vibration
during factory performance or
mechanical running tests.
In addition, measured force response
resonance testing should be carried out to
determine the system's natural frequencies
and dampening characteristics. Impact
testing identifies natural frequencies
and the resulting mode shapes at various
frequencies of the pump. Upon obtaining
natural frequency data, a designer can
modify structural, pedestal, skid design,
etc., to shift the natural frequencies away
TABLE 3. ISO 10816-7 Vibration allowable level at site and shop
for Category 1 and 2 services
Category 1 (critical application),
> 200 kW
POR in situ
POR in situ
POR FAT
POR FAT
3.5 mm/sec RMS
4.4 mm/sec RMS
4.3 mm/sec RMS
5 mm/sec RMS
Category 2 (pump for less critical
or general application), < 200 kW
4.2 mm/sec RMS
5.2 mm/sec RMS
5.2 mm/sec RMS
6.1 mm/sec RMS
TABLE 4. Allowable vibration limit based on API 610, ISO 10816-7 and Hydraulic
Institute 9.6.4
Region
AOR FAT
POR FAT
API 610
3.048 mm/sec RMS
3.9624 mm/sec RMS
from the expected operating speeds.
Acceptance criteria. API 610 divides
the operating flow range of centrifugal
pumps into two regions, as shown in
FIG. 3. At all points of the two regions, the
shaft displacement vibration and bearing
housing vibration must meet API 610
limits. If the pump does not meet the
API 610 vibration limit, then the vendor
may ask to test the pump at the site, as
per ISO 10816-7 or the Hydraulic Institute's
vibration limit. Note, however, that
pumps not in compliance with API 610
are in a major non-conformance situation.
TABLE 4 shows a comparison for vibration
limit based on API 610 (Table 7
in the 11th Ed.), ISO 10816-7 (Annex A)
and Hydraulic Institute 9.6.4 (for critical
application, P > 200 kW).
Pump support pedestal. Pump pedestals
should not be tall and should be
" full box " design. The full box design may
be filled with grout at the site to add mass,
if necessary. The vendor may decide to
increase the pedestal height due to the
skid design, a slope requirement for the
lube oil return line, the centerline of motor
coupling or another valid reason.
The dimensions of the pedestal
should be compared with the dimensions
shown for the proposed model in
the catalog. No industry standard has
been established for pedestal height for
BB3 and BB5 centrifugal pumps; height
is per manufacturer standard. However,
a lower center of gravity for pump assembly
is recommended for better rigidity
and stability. Therefore, the vendor
design should provide a pedestal height
that is sufficient to ensure access to all
ISO 10816-7
4.3 mm/sec RMS
5 mm/sec RMS
Hydraulic Institute 9.6.4
4.3 mm/sec RMS
5.59 mm/sec RMS
nozzles around the casing, including casing
drains. Proper gravity drainage of the
return oil line can be achieved by installation
of the oil console out of a pump skid
in a lower elevation.
Bearing housing pedestal stiffness.
It is recommended to increase the stiffness
or rigidity of the bearing housing.
This can be achieved by a metal-to-metal
fit and increased surface contact for the
bearing housing flange with a drip pocket
flange. Also, additional ribs should be
provided for the drip pocket supporting
the bearing housing.
Bearing housing material. The
weight or material of the bearing housing
should be increased by changing
the material and/or thickness. Bearing
housing made with carbon steel material
(e.g., A261 WCB) should be validated for
proper vibration measurement data during
installation at the site. Materials with
better vibration dampening capacity,
such as cast iron, should be considered.
The vendor should confirm if the option
of cast iron construction for the bearing
housing is feasible. Also, a study for separate
bearing housing design (or, alternatively,
bearing housing that is integral
with the casing) should be conducted.
Isolation and damping. A dynamic
absorber is a device designed to counteract
an initial exciting force by creating the
same resonate frequency through its own
vibratory force. Attaching the dynamic
absorber in series with the resonate bearing
pedestal creates an anti-node for the
bearing pedestal, thereby significantly reducing
the resonate vibration amplitude.
Acoustic studies. BFW pumps are eiHydrocarbon
Processing | DECEMBER 2021 79
Hydrocarbon Processing - December 2021

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