Hydrocarbon Processing - January 2021 - 23

Valves, Pumps and
Turbomachinery

A. J. S. Rathore, Indorama Eleme Petrochemicals
& Fertilizers Ltd., Port Harcourt, Rivers, Nigeria

Use the correct lube oil grade
to avoid equipment failures
Lubrication is imperative to the performance of rotating equipment. Each
year, millions of bearings fail due to poor
lubrication, the most common cause
of bearing failure. An estimated 36% of
bearings suffer lubrication failure-studies have shown that actual lubrication
cost is just 1%-3% of the total maintenance budget, while 40% of maintenance
cost is linked to poor lubrication.
Poor lubrication includes:
* Inappropriate lubricant
* Improper quantity
* Inadequate relubrication intervals
* Contamination.
The primary functions of lubricant
are to reduce friction and wear, heat dissipation, protection against corrosion,
and provide a sealing effect by creating
an oil film between mating surfaces.
However, depending on the operating
temperature and the process, a lubricant
is expected to provide other properties
that are as equally important and aid in
extending the life of the equipment.
Many different varieties and formulations of grease and lube oils are available
in the market-every lubricant cannot
perform every job, and each lubricant
must be selected specifically for the
equipment. Key parameters to be considered include load, speed, environmental
conditions and operating temperature,
which determine the viscosity, additives,
non-reactivity to metals being lubricated
and other properties. Choosing the right
lubricant is critical for an application and
should not be replicated, as every application is different.
Case study. This case study is about sin-

gle-stage centrifugal OH1 pump responsible for pumping high-temperature thermic fluid used as a heating medium in the
process. The pump is directly coupled to
a 75-kW motor, which gives the pump a
speed of 2,975 rpm. A pair of 7311 angu-

FIG. 1. Failed bearings that are not properly lubricated can result in the complete seizure of the
pump as well as secondary damages.

lar contact ball bearings are mounted in
back-to-back arrangement, with thrust
bearings and a cylindrical roller bearing
N212 as the radial bearing. The lube oil
used for the bearings was Mobil SHC
626, which is synthetic oil with an ISO
viscosity grade of VG 68 and a viscosity
index of 165. The operating temperature
measured on the bearing housing was
70°C (158°F) and the discharge pressure
generated by the pump was 10 kg/cm2.
The pump experienced frequent
bearing failures: five failures in 3 yr made
it a serious reliability concern. Every failure was similar and resulted in complete
seizure of the pump. This was negatively
affecting spares consumption, maintenance costs and, most importantly, plant
reliability figures. The measured vibration velocity was always below 2.5 mm/
sec, eliminating unbalance, looseness,
misalignment or any such low-order issues as probable causes. Conversely,
high-frequency indicators, such as ac-

celeration and enveloping, were always
in the alarming zone with symptoms of
lubrication starvation, even for a newly
overhauled pump. No oil leakage from
the pump was detected and the oil quality was always properly maintained without any contamination. Frequent oil
flushing was carried out, but no positive
results were realized.
Observations. When examining the
failed bearings-like those in FIG. 1-it
appeared that the bearings were not getting properly lubricated. Every failure
was similar and resulted in the complete
seizure of the pump bearings as well as
secondary damages. Process parameters
were verified to address any operational
deviation, but the pump was found to be
operating within the integrity operating
window. Vibration analysis suggested
lubrication starvation with high values
of acceleration and enveloping even after a complete overhaul of the pump. Oil
Hydrocarbon Processing | JANUARY 2021 23

Hydrocarbon Processing - January 2021

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