There are
dozens of predictive maintenance technologies, and some have become
standards in many industries. Those "standard" technologies include
vibration analysis, ultrasound, oil analysis, wear-particle analysis,
and thermography. Following are descriptions of the ways maintenance
professionals have traditionally used these predictive technologies as
well as applications you may not have considered.
Vibration analysis is used primarily with rotating equipment to find
problems such as misalignment, out-of-balance conditions, and bearing
defects. Prior to using vibration analysis, maintenance technicians had
to wait until a bearing failed to realize there was a problem. Using
vibration analysis, however, periodic readings are taken and recorded.
Maintenance personnel then compare these readings to a baseline. When
wear reaches a certain level, the bearing is scheduled for replacement,
before it fails. This reduces the amount of reactive maintenance and
ensures the replacement occurs with minimum impact on the production or
facility schedule.
Other
applications include extending vibration analysis to other kinds of
mechanical components. For example, some maintenance departments have
used vibration analysis to help in isolating problems with belts and
sheaves. They also have used the technology on gear drives to help find
defects or rapid wear problems. And they have even used vibration
analysis to monitor non-mechanical problems in fluid power systems, for
example.
While
ultrasound is the most common way to detect cavitation problems in
hydraulic pumps, reading the vibration signature of a pump can detect
the problem, too. Also, restrictions or disturbances in a fluid handling
system creates turbulence and unique vibration signatures that can help
identify the problem.
Ultrasound
functions primarily for leak detection, particularly for steam and air
leaks. These leaks can be expensive and many companies let them go
unnoticed.
The principle
of ultrasound is simple. Most leakage problems produce a range of
sounds. The sounds, when properly detected and measured, provide the
user with the location and severity of the leak. Common applications for
ultrasound include leak detection for pneumatic and other gas systems,
vacuum systems, gaskets and seals, and steam traps. Ultrasound also
detects valve blow-through.
Since many
small leaks are difficult to find simply by listening for the leak, the
ultrasound technique helps technicians discover the many small leaks
that add up to significant losses over time.
Other
applications include the detection of electrical problems. Loose
connections in junction boxes and bus bars can be monitored for the
sounds of arcing. This technique is useful in power distribution centers
and motor control rooms.
Ultrasound
also serves for inspections of electrical switchgear and overhead
transmission lines, where routine inspection is time consuming and
hazardous. These areas are monitored for corona discharge. When the
instruments "hear" the discharge, technicians can quickly find the
problem with little time wasted. Thus, technicians find small problems
before they become critical and cause equipment failure.
Oil and
Wear-Particle Analysis. Some people equate oil analysis and
wear-particle analysis. Actually, they are two very different
technologies.
Oil Analysis determines the condition of a lubricant. Wear-particle
analysis determines the condition of equipment based on the
concentration of wear particles in the lubricant.
If, for
example, a technician suspects that the lubricant in a gear case
contains water perhaps from being left open during an equipment wash
down an oil sample could be drawn and checked for water. The decision
could then be made to change the lubricant or not, based on the results
of the test, rather than than speculation. This ensures that the correct
maintenance is conducted not too much (resulting in high maintenance
costs) and not too little (resulting in unnecessary breakdowns).
Some companies
do not concern themselves with minute quantities of water. Some test for
water simply by heating a flat surface to between 200°C and 250°C and
sprinkling a small quantity of oil on the surface. If it bubbles and
spits, they know that it has too much water in it. This is not as
accurate as performing the analysis, since even as little water content
as 0.02% can create abnormal wear and rapid deterioration of the
equipment. Testing oil samples for water content is not a luxury; it is
a cost-effective practice.
To cite
another example, consider a gear case that is showing signs of abnormal
wear, e.g., noise or overheating. An oil sample could be checked for
wear particles. Considering the types and condition of particles found,
it is possible to isolate a number of possible problems and their
causes, e.g., operating the equipment beyond design speed or capacity or
filter failure. Once the problem has been identified, the appropriate
maintenance action can be scheduled, again with minimum impact on
operations or the facility.
Other
applications will entail analysis of a lubricant itself or the
wear-particles in the lubricant but there are, nevertheless, some unique
applications that employ these tests. For example, wear particles can
show when there is insufficient lubrication. "Insufficient lubrication"
does not necessarily mean the absence of a lubricant in a system. The
lubrication system on an enclosed drive, for example, could have a
clogged spray nozzle, preventing proper lubrication from reaching a hard
to-inspect area. While the visible part of the drive may be getting
proper lubrication, the one area that is lacking lubrication would
produce wear particles that indicate that condition. The samples can
also indicate conditions such as additive failure, lubricant
contamination or excessive loading that exceeds the rating of the
lubricant.
Thermography serves primarily to find electrical components that are
hotter than normal. Such a condition usually indicates wear or
looseness. Thus, thermography allows technicians to perform maintenance
on only the electrical components that need attention without requiring
that all components get the same level of attention.
In utilities,
for example, the correct torque is essential on electrical components to
ensure that no heat is generated from a loose connection. Before
thermography, it was necessary for each connection in a control panel to
be checked manually for correct torque. Using thermography only the
connections that are hot receive attention. This reduces the staff
necessary to perform preventive maintenance on the connections.
Other
applications include the monitoring of outdoor wiring such as overhead
transmission lines, which wear due to environmental conditions.
Thermography also serves to measure transformer temperatures to find
problems indicated when certain areas are hotter than others. In
addition, thermography supports maintenance in industries that have high
temperature processes. The technology helps pinpoint areas where
refractory material is wearing and allows repairs prior to catastrophic
failures.
Another
less-used application for thermography is checking coupling alignment
without major shutdowns of the equipment. As a misaligned coupling
rotates, it generates heat. The greater the temperature difference, the
greater the misalignment. Using thermography, maintenance personnel can
observe the temperature rise across a coupling. Some companies have used
this technique long enough to have developed profiles of the temperature
rise for each type of coupling. Using this profile, they can determine
the amount of misalignment (not what plane it is in). Then, the
technicians can proactively schedule the coupling for realignment.
Making
decisions: In some cases, production or operations personnel protest
when the maintenance department runs a predictive analysis on a critical
piece of equipment and wants to take it down for a proactive repair.
When this happens, why not get a second opinion?
Many
predictive techniques can be used to check for the same problem. For
example, a problem indicated by an oil sample could also be checked by
vibration analysis or thermography. An electrical problem indicated by
ultrasound can also be checked by thermography. By crosschecking using
two
predictive technologies, all departments can be satisfied that they
will make the right decision.
