It seemed as though nothing could stop the squealing.
Not that the Monarch Cement Co.’s huge ball mill wasn’t already loud.
Powered by a 5,000-HP motor, it pulverizes 100 tons of clinker (a
burned mixture of limestone and shale) per hour. But the squealing was
not what Randy Riebel wanted to hear. As electrical supervisor at
Monarch’s plant in Humboldt, Kan., he knew the noise meant the motor’s
bearings were going — again.
In fact, the sound of chronic bearing damage was all too familiar at
the plant, which has the capacity to produce 1,300,000 tons of cement a
year. Since 2001, when the ball mill was new, its motor bearings had
been replaced three times.
“We kept greasing those bearings, but they kept on squealing,” Riebel
recalls. “We knew that if we waited too long, the bearing race walls
would become fluted like they had in the past, and we weren’t looking
forward to another replacement because of all the expense and downtime.
It takes at least 10 days to pull that motor — it’s a major production.
Sometimes we have to hire help, rent a hoist to put it on a truck, and
take it away to be rebuilt. So [in summer of 2009] I decided to try
something else.”
The “something else” was the AEGIS iPRO bearing protection ring, manufactured by Electro Static Technology
(EST). By safely channeling harmful electrical currents away from
bearings to the ground, the company says it extends the lives of
medium-voltage motors and generators, thus improving the reliability of
entire systems. It is available in a range of sizes to accommodate
generator/motor shafts up to 30 inches in diameter.
The ring is suited for medium-voltage motors that drive pumps,
compressors, mixers, conveyors and other machinery used in mining,
wastewater treatment, petrochemical refining and many other
high-current applications. It also protects the bearings of generators
in both utility and on-site power generation systems.
Riebel had been discussing electrical bearing damage with Scott
Wilkins, manager of motor-shop operations for Independent Electric
Machinery Co. (IEMCO), a local motor and equipment repair shop. Wilkins
recommended the iPRO, and Riebel had IEMCO install two of them on the
ball mill motor. While for most large motors EST recommends installing
one in the drive end and insulation on the non-drive end, for some
large motors — especially those that do not have insulation designed
into them or where insulation cannot be easily installed — EST
recommends installing rings at both the drive end and the non-drive end
of the motor.
Riebel and Wilkins chose the iPRO split-ring model, which is designed
to facilitate field retrofits. The mating halves of each iPRO were
installed around the motor shaft without the need to decouple the motor
from the mill. Because IEMCO deal with large motors routinely, its
personnel are well aware of the severe damage shaft currents can cause
to motor bearings.
Mitigating electrical bearing damage
If not diverted, shaft voltages can discharge through bearings, pitting
the balls and race walls. Without long-term bearing protection,
concentrated pitting at regular intervals along a race wall can cause
washboard-like ridges called fluting, a source of noise and vibration.
The eventual result is motor failure.
Ironically, the company says, some products designed to protect
bearings, such as conventional spring-loaded grounding brushes, require
extensive maintenance themselves. Others, such as insulation and
ceramic bearings, can shift damage to connected equipment.
To boost the electron-transfer rate, the iPRO’s entire inner
circumference is lined with multiple rows of specially engineered,
conductive micro fibers. Locked securely in the ring’s patented AEGIS
FiberLock channel, these micro fibers surround the motor shaft,
providing millions of discharge points for harmful shaft currents and
creating the path of least resistance that effectively diverts these
currents away from bearings to ground.
A widespread problem
Contractors and retail home-improvement stores in six states depend on
the Monarch plant. Some of the cement is sold in bulk, some in bags.
And some of it is further processed by the subsidiaries, which
fabricate building products or add stone and sand to produce ready-mix
concrete.
When Monarch was founded in 1908, chunks of blasted limestone (“shot
rock”) up to four feet across were loaded by hand into mule-drawn
carts. Now this limestone is moved by huge front-end loaders, 50-ton
dump trucks and conveyors to be processed by a series of
computer-controlled crushers, kilns and mills until it is as fine as
face powder.
Most of the processing machinery is powered by electric motors, and the
problem of chronic bearing damage is by no means limited to the plant’s
ball mills. Many of the motors are controlled by variable frequency
drives (VFDs), which induce additional high-frequency currents on motor
shafts. A fan or pump motor tends to use less power if the input is
modulated by a VFD, but the benefits of improved efficiency are lost if
the motor keeps breaking down.
Such breakdowns were recurring headaches for Riebel, but because the
two rings installed in 2009 appear to be protecting the bearings of the
ball-mill motor, he has since had IEMCO install the iPRO on nine more
motors that had to be removed from service.
A case in point is a VFD-controlled cooler-vent fan where the 300-HP
motor had to be replaced frequently for almost eight years. Every time,
the kiln had to be shut down for at least a day.
“We’d send the pulled motor out to be rebuilt, but then three to six
months later, we’d have to do the same thing all over again,” Riebel
says. “We didn’t really realize what the problem was. There wasn’t much
information available about electrical bearing damage. We just knew
that bearings would fail and the motor would overheat, but we were not
looking to see why. Again and again, we just sent the motor out, got it
rebuilt and put it back in service. We didn’t know the root cause.”
It is now Monarch policy to have IEMCO add the iPRO ring in the shop
whenever a VFD-controlled Monarch fan motor is overhauled. Another such
installation was on the 2,250-HP motor for an induced-draft (ID) fan
that pulls kiln-heated air through a roller mill to dry the limestone
and shale during the raw grinding process. Other motors include four at
the plant’s kilns, where air is forced in and out: two 2,000-HP ID fan
motors and two 1,000-HP dust-collecting fan motors.
Also, because a cement plant is a very dusty place and many motors are
outdoors, Monarch has begun to specify that some of its new motors must
come equipped with the AEGIS Severe-Duty SGR bearing isolator shaft
grounding ring, another EST product, which has a built-in IP56
non-contact isolation seal to provide extra protection from dust, water
and other contaminants.
Monarch maintenance manager Mark Pily authorized the purchase of the
plant’s first motor with a factory-installed Severe-Duty SGR after
consulting IEMCO’s Wilkins. As of this writing, a 200-HP air-compressor
motor is the only such motor in operation at the plant.
“We want to keep the bearings clean because we push that motor really
hard,” Riebel explains. “We usually lose that motor because of winding
failure. I think most of that is caused by the bearings starting to
fail, which causes the motor to overload.”
An ounce of prevention …
Using a voltage probe and oscilloscope, Riebel periodically takes shaft
voltage readings on all the plant’s motors with grounding rings. He is
pleased with the results because the readings are low, indicating that
the rings have reduced potentially damaging shaft voltages. Riebel also
gives a high grade to the service the plant has received from IEMCO:
“In my opinion, they are by far the best shop we’ve worked with, and
we’ve worked with most every shop in the area.”
Time will tell exactly how much money the rings will save Monarch
overall, but Riebel is convinced the iPRO provides effective, long-term
bearing protection that reduces the costs of downtime and motor
maintenance.
“So far, so good,” he says. “On the 5,000-HP, since the last set of
bearings only lasted a year, chances are we would have noticed problems
by now, but we haven’t had any — no squealing.”
This is an edited article provided by Electro Static Technology. For more information, visit www.est-aegis.com.
It seemed as though nothing could stop the squealing.
Mitigating electrical bearing damage
