Power Engineering Magazine, November, 2002 — WHEN POWER PLANT equipment fails a plant manager's
biggest concern is getting the equipment back into service as quickly
as possible. However, equally important is discovering why the equipment
failed, and if the type of failure can be prevented in the future.
Effective lubrication is extremely critical to all
gearboxes and proper lubrication will help prevent gear and bearing
failures. Many gear and bearing failures result from insufficient
or interrupted lubrication. Maintaining proper lubrication includes
using the proper lubricant, keeping oil clean and free of foreign
materials, and maintaining a sufficient supply of lubricant. Selecting
the correct lubricant will depend upon the gear type, load, speed,
operating temperatures, input power and reduction ratio
Lubrication problems can cause several gear problems.
Scoring and galling are generally caused by oil film breakdown resulting
in metal-to-metal contact. The resulting high temperature causes
tooth surface damage. Continuing to operate the gearing without
adequate lubrication will eventually degrade the gear's tooth profiles
to the point where replacement is the only remedy. Abrasive wear
is also often the result of foreign materials present in the lubricant.
Maintenance professionals have several tools
at their disposal for anticipating and diagnosing gearbox
lubrication problems. One of these tools is oil analysis.
By analyzing particulate content and concentration in the
oil, engineers are able to monitor the condition of an operating
gearbox and the lubricant's condition. Likewise, lubrication
problems can be detected by examining wear patterns on gears.
Gear tooth pitting is characterized by a large
number of very small pits, distributed evenly over the working
surface of a gear. The appearance of such pitting is usually
an indication of gear overload. However, it might also be
indicative of lubrication problems caused by corrosive or
improper lubricant additives.
All power plant equipment generates specific
vibration profiles and their frequency depends upon a machine's
geometry and operating speed. By analyzing shaft vibration,
engineers are able to determine whether the cause of the machine
fault is imbalance, misalignment, wear, bearing defects and/or
gear defects. Once equipment starts vibrating, damage to other
ancillary equipment including piping can occur. Misalignment
and wear are other problems with gearing systems. When misalignment
occurs cracks can develop at the end of a gear tooth causing
helical and bevel gearing to fail.
Wear caused by vibration can cover a broader
range of gear damage, from scoring and galling to abrasive
wear and plastic yielding. Plastic yielding can occur on gears
subjected to heavy, continuous load, as well as gears subject
to intermittent heavy loads or overload.
Reducing Maintenance Costs
A lubrication problem nearly caused a catastrophic
failure in a gear reducer at a coal-fired power generating
station in the mid-Atlantic. Over time, lack of lubrication
caused asphalt buildup on two of the plant's ball mills, clogging
the gear reducer's lubrication system. This resulted in excessive
wear and considerable asphalt buildup on the roots of the
gear, pinion teeth and pinion drip pan. These failures were
so significant that they also damaged the mill pinions and
All gearing should be inspected for wear.
Removing the pinion and damaged gear and repairing
and reinstalling it is a very time consuming, labor intensive
and expensive process. As a result the plant looked at alternative
solutions to reduce the repair cost and downtime. In consultation
with Philadelphia Gear, it was determined that the gear could
be reconditioning without removing it from the unit.
After refurbishing, including the recommendation
of a graphite-based grease for lubrication, the mills have
run problem free. The temperature profiles across the gear
ranged from 144 F to 155 F and the vibration levels were reduced
by 50 percent. Prior to refurbishing the temperatures ranged
from 155 F to 182 F. Because of the success of refurbishing
the first unit, the plant is looking at repeating the same
procedure on a second unit in 2003.
Another project involved a major city in the
Southeastern United States that was using gas turbine generators
to supplement their power grid during times of peak usage.
These units, installed nearly three decades ago, had received
very little maintenance and only periodic oil refills. Because
of inadequate maintenance, the rotating elements, bearings
and pinions all suffered from metal fatigue and severe pitting.
Ultimately the bearings in one of the units failed and the
subsequent vibration destroyed the gears.
After the gearbox was removed it was sent to
a Philadelphia Gear Regional Service Facility for inspection
and cost/benefit analysis. The inspection revealed that repairing
the gearing and bearings within the unit would cost $100,000
and take up to 20 weeks to repair. The alternative was a new
gas turbine that would cost twice as much as the repair and
take 30 weeks to deliver and install. While the problem was
one of improper lubrication, vibration caused the most damage.
Establishing a PM Program
While identifying the cause of equipment failures
can sometimes be as simple as looking closely at the damage,
discovering the root cause of such a problem is considerably
more difficult. In some instances plants do not have the sophisticated
equipment needed to identify shaft vibration anomalies or
analyze oil samples for foreign materials. However, one solution
is outsourcing the preventive maintenance (PM) functions.
Outsourcing PM allows plants to focus on their own core competencies,
the operation of the plant.
When selecting a preventive maintenance provider,
there are several key services that should be included in
any agreement. Any agreement should cover breakdowns, scheduled
maintenance, parts reconditioning, service upgrades, reverse
engineering, alignment and balancing, and on-site or off-site
diagnostic services. Providers should be thoroughly skilled
in performing gearbox failure root cause analysis.
While identifying the cause of equipment failure
is only the first step in establishing an overall preventive
maintenance program, it is an important step. The information
gathered will ultimately serve as the foundation for planning
future preventive maintenance. Such information will also
help avoid making the same mistakes again.