POWER May 2020 - 35

PLANT EQUIPMENT
impact test on the rotor in question.
To prevent fan parts from cracking
over time, fan manufacturers can
stiffen or reinforce parts. Using thicker
components increases stiffness, and
thus frequency. Using stronger materials
can also reduce damage from
fatigue in high-pressure and high-temperature
applications.
Another rotor dynamics evaluation
tool for fans is Finite Element Analysis
(FEA). This computer analysis technique
can help determine what parts of
the rotors may be sensitive to normal
operating frequencies and may experience
excessive stress once the fan
is operational. This type of analysis is
important for rotor development. The
manufacturer considers the results
based on the speed at which the fan
will operate and the maximum speed
to which the fan will be exposed. The
study determines if the fan's geometry
needs adjusting, confirmed with a mechanical
test. Care must be taken during
testing to update the analysis and
to duplicate the test conditions.
There are rotor modes that involve
the fan shaft. These conditions mimic
operation at shaft
critical speed, but
they are a special case of rotor mode
shape. When a well-designed fan on a
good foundation appears to operate at
its critical speed, this is the likely culprit.
Bearings: The Lifeblood of
Industrial Fans
Industrial fans utilize one of two types of
bearings: anti-friction or sleeve.
Anti-friction
bearings commonly include
a set of hardened balls or rollers
that rotate within an outer and inner
ring. Anti-friction bearings are frequently
found on low- to medium-temperature
fans and are the most common bearing
type used on heavy industrial fans.
Sleeve bearings are hydrodynamic
bearings in which a thin wedge of oil
supports the shaft. The bearing surfaces
remain in place, and the shaft moves
on the oil wedge inside the bearing. A
lubricating ring brings oil to the shaft
as it turns inside the bearing. Water is
often circulated through the bearing
shell to remove heat coming from the
oil wedge or the process. Forced oil lubrication
is frequently employed when
water-cooling is not available.
Common bearing problems are often
caused by the following:
■ Inadequate Lubrication. To prevent
bearing problems or failure, ensure
May 2020 | POWER
2. To eliminate vibration issues, fans should
be properly balanced before installation at ISO
G6.3 for minimum standard balances and ISO
G2.5 and G1.0 for tighter tolerances. Courtesy:
New York Blower
the bearings have the correct type and
the proper amount of lubricant. Most
importantly, ensure the lubricant can
withstand the bearing operating temperature.
Lubricants that become too
hot will oxidize and cause problems
to the fan and interior equipment.
Over lubrication of anti-friction bearings
can cause them to run at higher
than optimum temperature. Grease
is commonly used as a fan lubricant
because its thickness makes it last
longer. Oil is popular for fans that operate
at higher speeds. Plant technicians
should monitor lubricant levels
regularly to ensure the fan is not depleting
its reserve.
■ Improper
Installation or Deferred
Maintenance. Improperly installed or
poorly maintained bearings can lead
to premature bearing failure. Don't
defer maintenance to save resources.
Operational delays caused by poor
maintenance requirements can dangerously
degrade
equipment.
The
best approach is to adhere to maintenance
schedules to prevent potential
operational problems.
■ Lubricant Contamination. Contaminated
lubricant diminishes the life of a
bearing. Lubricant becomes contaminated
when particulates enter the oil.
Check lubricants regularly to prevent
contaminant build-up in the oil. Darkened
oil color is a symptom of contamination
or oxidation. White oil is an
indicator that water has contaminated
the bearing.
Vibration Frequency Analysis
Any equipment that makes noise or rotates
experiences vibration. Some vibration
is normal and does not reduce fan
productivity. However, rapidly increasing
or unusually high vibration ampliwww.powermag.com
■
Dust or particulates that move through
the fan can adhere to the rotor and
change its center of mass, causing the
rotor to become unbalanced (Figure
3). Ask the fan's manufacturer to determine
if the rotor can be rebalanced
in the field. Sometimes, cleaning is all
that is required.
■ Dust moving through a fan can remove
material from the rotor. This
changes its center of mass, resulting
in unbalance, which is correctable in
the field.
■ Cracking in a fan rotor can eject material,
resulting in a rapid increase in
vibration. This can lead to severe damage
if not caught promptly.
Reliable industrial fans are a necessity
in many facilities. Selecting the right fan
and components for the application, and
working in tandem with fan experts to
determine the type of fan and internal
components needed position facilities
for long-term success. ■
-Steve Nelson is a technical service
group member with New York Blower.
35
3. Erosion is a concern in some fan applications.
Fans with chromium carbide lines
protect against erosion for clinker cooling processes.
Courtesy: New York Blower
tudes can be a symptom of mechanical
defects that need immediate attention
for the safety of personnel and downtime
mitigation.
The vibration frequency can provide
a clue to its source, and vibration at a
frequency of one times rotating speed
is indicative of unbalance (Figure 2).
Other present frequencies can provide a
trained vibration analyst with information
that can reveal the cause of the problem.
Using a vibration analyzer, analysts can
investigate specific vibration frequencies
with a variable filter or fast Fourier transform
(FFT) processor.
A primary cause of fan vibration is unbalanced
rotors. They can become unbalanced
by the following:
http://www.powermag.com
POWER May 2020

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