The Challenge
A plant manager, from a leading
coal-fired plant located in the Midwestern United States, approached us with a
question about a gear coupling failure in its coal pulverizer drives.
Power
Generation Industry background
More than 50 million consumers
and business owners from Ohio to New York and Canada would agree that a
consistent power supply is not only vital to the overall economy, but also for
the comfort and stability of daily life. The major power outage that struck the
U.S. and Canada in mid-August of 2003 cost New Yorkers alone, an estimated $500
million. Although experts point to a dated power grid as the main cause of the
recent outage, every detail of the power generation system, including the smallest component, plays a critical role in maintaining the electric power plant.
Equipment
and Components
Depending on the size, a
single plant can have more than 25 coal pulverizers. Coal pulverizing allows
for less emissions and more efficient use of the fuel. The coal burns to create
the steam that turns the generators and provides electric power. Large electric
motors use a worm gear arrangement to drive the coal pulverizers and a blower
that sends the pulverized coal to the furnaces. The shafts of the motor must be
mechanically connected to the worm gear and blower. The worm gear is a
relatively small, yet vital component. If it fails, the pulverizers and blowers
can no longer operate.
Dissecting the problem
Gear couplings allow for axial
movement of the shafts, which increases the wear on the gears, sleeve bearings
and other components. This wear on the components would cause them to fail
prematurely.
The loss of production
due to downtime was becoming an issue. Bearing replacement costs were in the
thousands of dollars per bearing.
Preventative maintenance
costs associated with gear couplings were also becoming a burden due to time
spent on inspection and lubrication to keep them operative. The power plant’s
rigorous preventative maintenance program required regular disassembly of the gear
couplings for inspection, cleaning and re-lubrication of the gear teeth. In
addition, assessment of the condition of the couplings was a judgement call
unless extensive and time-consuming measurements were performed. Coupling
replacements also require heavy machinery to move the motors.
Old technology, old solution
The existing gear coupling design was typical of the technology
available when the plant was built in the early 1970’s. Where gear couplings
did not limit axial movement of the motor shaft, sleeve bearings were used.
This old technology configuration increased wear and shortened life spans of
both the motor sleeve bearings and the gear couplings.
In other words, increased
sliding velocity of the gear teeth in the gear coupling creates a lot of heat
and wear, just picture yourself rubbing two sticks together. The motor moves a
bit, the gear on the coupling moves a bit, and this motion could be repeated up
to a million times over the course of the day, contributing to premature
failure.
Contemplating a similar
solution
Install gear couplings
with limited end float; however, its maintenance nightmare will continue
because the gear couplings are difficult to assess for wear and the inspection
and replacement of the gear coupling still required removing the motors. The
gear couplings would still have to be disassembled, inspected, cleaned, and
re-greased every time; definitely not an effective solution.
New
Technology, new solution
Install a Disc Coupling.
Unlike gear couplings, the Lovejoy SX Disc Coupling design is inherently
balanced. There is less vibration than the gear coupling as it wears. The disc
coupling performs a limited end float function without special design
considerations for the sleeve bearings. Maintenance-free operation is achieve
because the Lovejoy coupling has no moving parts and requires no lubrication.
This translates into a long coupling life.
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| Actual Pulverizer Solution |
Preventative maintenance
and inspections of the disc coupling’s flexible elements was now possible using
a simple strobe light and while the coal pulverizer is running. Hence, there is
no need for an expensive shutdown and disassembly. Replacement of the disc
packs became a breeze without moving any machinery or disturbing the original
alignment.
Bonus
features
Lovejoy was able to
provide a heavier duty coupling than the gear type in the same space as before.
Nothing had to be moved or modified for installation. Additionally the
component cost was comparable to those of a gear coupling. The Lovejoy’s design
allows for more capacity and misalignment capability than any other disc coupling
available today.
In addition, disc
coupling signals the maintenance staff when it needs replacement – you can
actually see it with your own eyes without any disassembly. In contrast, in
gear coupling is more of judgment call, unless you take it completely off the
machine and measure it extensively. With
the Lovejoy disc coupling, the outer leaf of the disc will crack before the
entire disc pack fails, signaling to the maintenance staff it needs replacing.
Happy
conclusion
The coal pulverizer is critical equipment, if it is not
blowing coal, it is not generating power; needless to say, would you rather
have your pulverizer blow coal or blow off your money?. The disc coupling
solution reduced the maintenance nightmare, and the plant has been experiencing
success for more than a year with the original installation. Plant management
approved the proposal to convert all gear couplings on 24 coal pulverizers to
Lovejoy disc couplings, based upon the proven benefits and demonstrated cost
savings.
