BY LUIS INFANTE & RODOLFO ALVARADO.
A high
energy pump at a water injection station in El Furrial, Venezuela exhibited
extremely high vibration levels prior to an overhaul. It then suffered a
catastrophic failure during startup following overhaul. The hydrodynamic
bundle, rotor, and drive end (DE) bearing suffered damage.
High
energy pump for boiler feed water. Courtesy of Flowserve.
This centrifugal pump is a 3,000 HP, double-case volute, boiler feed water pump type. It has nine stages, outputs 750 gpm of water with suction pressure 1800 psi and discharge pressure 5250 psi. Rated speed was increased from 6000 to 6600 RPM to enhance the hydraulic performance. However, the pump’s actual discharge pressure was about 4,500 psi, well below the target value of 5,000 psi. The coupling was reportedly poorly fitted.
The
increased RPM created rotordynamic concerns of getting closer to a critical
speed, thus the operator wanted to know about the synchronous regime. The
operator also wanted remedial measures and temporary mitigation steps to keep
the pump running for 4-6 months until remedies were finally enforced.
The course
of action for this investigation was clear since the beginning: conduct an
internal clearances analysis and a forced response rotordynamic study.
A thorough
study of internal clearances was conducted. Table 1 shows the results from this
study featuring a comparison between internal (hydrodynamic bundle) clearances
from different data sources, namely: design data, shop measurements, API-610
minimum clearances, and typical clearances for a similar pump with vibratory
problems. As a result, the recommended clearance for the center seal, balance
piston, eye, and impeller hub seals are shown in the bottom part of Table 1.
Due to the
high vibration levels (20 mil DE, 5 mil NDE) reported before the overhaul, a
simulation of the pump’s forced response rotordynamic behavior was attempted using
a trial unbalance weight located in the coupling. A mass-elastic mathematical
model incorporating the effect of the internal seals at 1X and 2X design
clearance (as suggested by API 610) was used for this purpose.
One can
expect a flexible rotor behavior (first critical speed below operating speed)
for this high-speed, long slender shaft, but the stiffening effect provided by
the internal seals (Lomakin’s effect) locates the first critical speed in the
vicinity of 9,000 RPM, well above 6,600 RPM operating speed. Retrofitting the
bearing and coupling with the technology presented below raises this first
critical speed to approximately 10,000 RPM, thus discarding the likelihood for
resonance despite the steep vibration plot obtained during a ramp up vibration
survey.
Table 1:
Analysis of internal clearances.
REMEDIES AND MITIGATION
Our
attention was focused on balancing and the effect on the dynamic stability of
the rotor-bearing-support substructure. A bearing and coupling retrofit was
recommended with a balancing plane on the coupling and an added mass on the
bearing housing. It was discovered that the shaft was excessively long on the
DE. Shortening the shaft together with a reduced moment coupling proved to be
beneficial in keeping rotor synchronous response away from resonances. Such
coupling design has a center of gravity and flex discs moved closer to the
bearing.
Tilting
pad bearing is a good option for stability and support of this slender
high-speed shaft. A pad load distribution analysis showed increased clearances unloading
the top pads and the six-pad arrangement offered the best support. Furthermore,
a lateral stability analysis revealed six-pad bearing to have the best
stability parameters (whirl mode, log dec, amplification factor, undesirable
speeds).
Effective
mitigation measures that ran for a few months with acceptable vibration levels
turned out to be:
1.
Add 150lb to the bearing housing.
2.
Increase internal clearances and
bearing clearances (max 6 mil).
3.
Finning the bearing housing ribs for
enhanced heat dissipation.
4.
Using the coupling as a balancing
plane; adjusting and balancing the coupling.
5.
Correct bearing housing distortion.
CONCLUSIONS
The most
likely cause of failure was attributable to tight clearances found in the
hydrodynamic bundle’s internal seals. OEM design and even API clearances were
considered to be too tight. Reducing internal clearances below API 610
recommendations exposes these pumps to catastrophic failures during start up.
For the impeller eye, we recommend a 50% increase (to 27 mil) over API values
for stages 1–4 and a 25% increase (to 23 mil) for stages 5–9. Clearances for
other rotor locations are shown in Table 1.
Trimming
the shaft in the DE side and installing a reduced moment coupling helps in
separating the operating speed from the first critical envelope, thus reducing
the amplitude of vibration.
Source: turbomachinerymag
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