Hydrocarbon Processing - December 2021 - 76

Valves, Pumps and Turbomachinery
the vibration peaks helps determine the
potential source or cause of the vibration.
The vibration phase is the angle of the
vibration issue, which is used to determine
the dynamic balance and resonance
problems (i.e., rotor critical speed).
To determine the cause of vibration,
the vibration data can be shown in a
wave-form graph, or spectrum. The wave
form is a presentation for vibration data
in the time domain, and the spectrum is
a presentation for the frequency domain.
To clarify and simplify the vibration data
representation, it is normal practice to
use the spectrum, which shows frequency
in the horizontal axis and amplitude
in the vertical axis. The amplitude can
be displacement, velocity or acceleration.
The displacement curve is difficult
to read at higher frequencies. The velocity
curve is the most common curve for
the vibration spectrum. The vibration
amplitude scales can be linear or logarithmic.
By measuring vibration amplitude
in three directions and knowing the
frequency and pump speed, the cause of
pump vibration can be determined by referring
to the frequency diagnostic chart.
BFW pumps factory test. FIG. 2 shows
a typical BFW pump performance test
setup. According to API 610 (Clause
6.9.3), for pumps with hydrodynamic
bearings and proximity probes (e.g.,
BFW pumps), both vibrations at the
bearings housings in three different directions
and shaft displacement probes
must be measured and meet API 610 allowable
vibration limits during a factory
performance test.
API 610 requires a factory performance
test of the pumps as a mandatory
test; however, a mechanical running test
is an optional test and is subject to the
purchaser's decision. A comparison between
a mechanical running test and a
performance test of the pumps is summarized
in TABLE 2.
The proxy probes acceptance criteapply
only to
ria in Table 8 of API 6102
hydrodynamic journal (sleeve) bearings,
which are used for radial loads in BFW
pumps. Hydrodynamic tilt-pad type
thrust bearings do not have guidelines
for acceptance criteria of shaft displacement
in API 610. The values for axial
movement are normally provided in the
instrument alarm setpoints schedule for
the pump. Alarm and shutdown values
for thrust bearings are identified by the
bearing manufacturer.
Low-pressure BFW pumps installed
in series at the suction of high-pressure
TABLE 2. Comparison between factory performance test and mechanical running test for BFW pumps as per API 6102
Criterion
Purpose
Performance test
To confirm that the pump performance and impeller
design are based on the original quoted performance
curve and to validate that the proposed model is a
proven model in terms of characteristics of the impeller
Test duration
Test point
Not a specific value in API; however, it normally
takes about 1 hr
1. Shutoff
2. Minimum continuous stable flow
3. Between 95% and 99% of rated flow
4. Between rated flow and 105% of rated flow
5. Approximately the best efficiency flow
6. End of allowable operating region (FIG. 3)
Driver
Test components
used
Test driver-normally a shop motor
Normally pump, job vibration proximity probes,
job bearings and job mechanical seals are used
in a test stand with a test instruments control
system and shop oil system
Vibration test
* During the performance test, overall vibration
measurements over a range of 5 Hz-1,000 Hz and
a Fast Fourier Transform (FFT) spectrum must be
made at each test point except shutoff
* Vibration measurements shall be made at the
following locations:
1. On the bearing housing(s) or equivalent
location(s) of all pumps, at the axial,
horizontal and vertical positions
2. On the shaft of pumps with hydrodynamic
bearings with proximity probes
Power
measurement
By using a torque meter, which has a length higher
than the job coupling
76 DECEMBER 2021 | HydrocarbonProcessing.com
Similar to
performance test
Power is normally calculated due to limited
space for the installation of the torque meter
between the pump and driver
Mechanical running test
Bare shaft
Combined running test
To confirm that pump components work satisfactorily and that
all components inside the skid are interacting and working properly
together in a combined running test
Minimum 4 hr, normally after stabilization of oil and seal temperatures
Only between 95% and 99% of rated flow (FIG. 3)
Can be test driver or
job driver
Similar to
performance test
Job driver
Pump, job driver, job skid, job mechanical
seal, job coupling, job instrumentation and
control system, job vibration proximity probes,
job oil system and seal coolers; however,
depends on component availability, and other
project constraints may be different
Normally only with
proximity probes
Normally only with proximity probes

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Hydrocarbon Processing - December 2021

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