Read the PDF : Ethanol Plant Stops Motor Bearing Damage _ Chemical Processing CASE STUDY
Proper shaft grounding thwarts premature failures
By Kent Glasser, Red Trail Energy
Dec 11, 2017

Something was destroying the motor bearings of the centrifuges and thermal oxidizer fan at Red Trail Energy’s
ethanol plant in Richardson, N.D. Maintenance personnel realized the bearings on these large motors were
failing long before they should. The high cost of replacing the bearings soon caught the attention of the plant
manager, who called in a specialist — Scott Fisher of Sustainable Grounding Systems — to determine what was
causing the damage.

Fisher had worked at the Red Trail plant before when a North Dakota electrical contractor, Ystaas Electric
Services, brought him in to evaluate the plant’s grounding systems. Ystaas enlisted Fisher’s services when it
discovered stray currents on the cabinets of variable frequency drives (VFDs) that control the speeds of various
motors in the production process.

An expert on the causes and prevention of equipment damage due to stray currents and inadequate grounding,
Fisher is well known for his work with major companies involved in oil exploration, drilling and refining;
petrochemical processing; grain elevators; and even the Los Angeles train system. He learned his craft while
working in Europe for a company that was involved with grounding systems for various applications, including
high-speed rail and military installations.

“Without proper grounding, stray electrical currents can do tremendous damage. In fact, these types of currents
are becoming a huge problem and dealing with them is a highly specialized field. Because they are not
maintenance-oriented, most electrical contractors don’t know how to solve this type of problem. They can get
system resistance down to the 25-ohm level required by National Electric Code. I was trained to get systems
down to 5 ohms or less,” explains Fisher.

The Ethanol Plant

Red Trail Energy’s Richardson plant produces 65 million gallons of
ethanol annually from 25 million bushels of corn through a dry-milling
process. The process generates coproducts including 135,000–
140,000 tons of dried distillers grain and 80,000 tons of modified wet
cake each year.

The dry-milling process starts by grinding dried corn into flour, which
is stirred with water to produce mash. The addition of enzymes to the
mash converts starch to dextrose. The mash, after processing in a
high-temperature cooker to reduce bacteria levels, goes to fermenters
where yeast is added to transform sugar into ethanol and carbon
dioxide.

During fermentation, which takes about 40–50 hours, agitation keeps
the mash cool and facilitates the activity of the yeast. After
fermentation, the resulting “beer” goes to distillation columns to
separate the ethanol from the remaining stillage. The ethanol then
gets concentrated to 190 proof through conventional distillation and
dehydrated to approximately 200 proof in a molecular sieve system.

Next, the stillage passes through a centrifuge that separates coarse grain from solubles, which then are
concentrated to about 30% solids through evaporation, resulting in condensed distillers solubles or “syrup.” The
coarse grain and syrup then are dried together to produce dried distillers grains with solubles, a high-quality
nutritious livestock feed.

Tackling the Problem

When Fisher arrived at the plant in May 2017, he took shaft voltage readings on the 30-hp and 150-hp centrifuge
motors and the 400-hp fan motor for the plant’s thermal oxidizer. These motors exhibited premature bearing
damage — long before reaching the L10 life of their bearings, i.e., the life that 90% of the bearings should
achieve. He knew right away the culprits were stray currents from VFD-induced voltages that were building up on
motor shafts and discharging through motor bearings — and just how to deal with them.
VFD-induced shaft currents discharge to ground along the path of least resistance. Without long-term shaft
grounding, that path typically is through the motor bearings. So, Fisher recommended installing Aegis shaft
grounding rings on all motors controlled by VFDs to channel these harmful discharges away from bearings and
safely to ground.

He chose split Aegis SGR Rings for the 30-hp and 150-hp centrifuge motors. These rings come in mating halves
that allow fast, easy installation on an in-service motor without having to decouple the motor from attached
equipment. After removing any dirt, corrosion and paint that might interfere with adhesion and conductivity, Fisher
installed the rings using conductive epoxy.

Because of the 400-hp motor’s large size, Fisher opted for an Aegis Pro Series Ring. Designed for large low- and
medium-voltage motors, Pro Series Rings have six rows of conductive microfiber brushes that provide high
current capacity. Fisher installed the larger split Pro Ring using universal Pro Series mounting brackets.
Once the installations were completed in July 2017, he took follow-up readings on the running motors to check
shaft voltage levels. Readings that were as high as 19 volts before had dropped to less than 1.3 volts (too low to
cause premature bearing damage) after the installation of the rings.

In addition, Fisher recommended the added protection of electrically “bonding” all motors to an electrolytic deepwell ground rod system that would guarantee stable resistance through the changing seasons.

“Red Trail has not had any problems since we installed the Aegis rings — no more high-pitched squealing from
damaged bearings,” notes Fisher. “And costly downtime has been minimized.”

Fast Payback

Red Trail Energy has initiated an ongoing program to improve control of its production process by running all
motors on VFDs. This not only will assist operators in fine-tuning the process but also will allow “soft” starting of
motors after a process shutdown. Red Trail has found through experience that the stresses from restarting the
system can lead to critical events with unforeseen consequences. Using VFDs to soft start motors can minimize
these stresses. In addition, protecting VFD-controlled motors with Aegis rings forestalls unplanned shutdowns
due to motor bearing failures.

A shutdown due to the failure of a key motor could result in the loss of one or more batches, costing hundreds of
thousands of dollars in lost revenue. The considerable savings from avoiding such shutdowns have convinced
Red Trail’s management to put Aegis rings on critical process motors. Installing the rings makes sense from both
an operational and financial standpoint, notes Fisher: “They pay for themselves in short order.”

Written by : KENT GLASSER – maintenance manager for Red Trail Energy, Richardson, N.D. Email him at
kentg@redtrailenergy.com.