Large, medium and small operations have
an extensive inventory of equipment requiring an uninterrupted
power source for operation; an unscheduled shutdown is a disaster. Because
plants and facilities runs around the clock, 365 days a year, downtime is equivalent to
lost revenue.
To prevent such losses, all electrical components in
the plant can be scanned with a thermal infrared imaging camera for suspect hot spots.
Some areas that can be thermally scanned are circuit breakers, transformers, fuses, disconnects switches, bus,
panels, etc.
Once the problem has been detected with an Infrared imager, proper measures can then
be applied for correction. hence the term; Predictive /
preventive maintenance.
Companies use infrared imagers and
thermographers to detect hot spots in electrical equipment in plants. By
scanning substations, distribution lighting panels, and electrical motors. Problems are easily and quickly eliminated before they cause system
failure. The results: avoidance of costly operations downtime.
By taking a thermograph of site electrical panels, thermographers
develop and read a "heat picture"
which reveals components that are overloaded or may become faulty. Unlike normal component
operating conditions, faulty components exhibit readily detectable temperature increases
over the ambient temperature profile.
Thermography verifies that electrical connections are properly made and
maintained.
Thermography also detects
hot spots that might be overlooked by visual inspections. Recently, in a U.S factory,
staff quickly interrupted the power supply to an above ground trailer when infrared test equipment detected a hot spot
registering 123 degrees over the baseline ambient temperature. A
fire could have started, resulting in the probable loss of valuable
records and equipment, had the problem not been uncovered.
Most electrical problems within industrial facilities are manifested or are
accompanied by temperature changes as an effect prior to failure. For this reason IR thermography has become
an integral part of most predictive / preventative maintenance programs. Infrared
cameras can pick up small changes in temperature not visible to the human eye. It is
a non contact, non destructive, and fairly simple method of detecting impending electrical problems.
It is widely known that as temperature
in a conductor rises, so does it's resistance.
Conversely, as resistance increases (in most conductors) temperature rises. The majority of thermal
electrical problems involve improper torque specifications or improper installation at the
junction points. A loosely torqued connector effectively reduces the surface area in
which current can flow and consequently an increase in the contact resistance. Oxidation build up at the connection
point can also cause a rise in resistance. The origin of most conductor, insulation,
and component problems can be traced to a poor connection
using an infra-red camera.
Visual image
Thermal image
Overview of pro / predictive and
preventive maintenance
Preventive maintenance (PM) is also known as time-interval maintenance,
scheduled maintenance, time-based maintenance, and time-based inspection. It is based on
calendar time or equipment running hours. Intervals are based on the worst, or the average
deterioration rate, depending on operator experience and philosophy. If intervals are
based on the anticipated worst case, much of the maintenance program will be redundant. On
the other hand, if intervals are based on the average deterioration rate, some unscheduled
maintenance and shutdowns must be accepted.
Predictive maintenance (PdM), which is based on equipment condition, is also known as
condition-based maintenance, performance-based maintenance, and condition monitoring. It
uses projected data or trends from condition monitoring techniques to determine the
trouble-free service life of equipment, thereby eliminating redundant maintenance and
unscheduled shutdowns. These techniques monitor deterioration, processing conditions, and
specific events that precede the development of equipment faults or failures. Unlike
inspection, condition monitoring provides information about the component or system
without requiring shutdown and dismantling. Inspection sometimes introduces defects that
would not otherwise have existed.
Pro-Active maintenance is based on taking predictive maintenance one step further and is
also known as reliability or productive maintenance. It is a maintenance philosophy where
maximum equipment operating time and availability are achieved through root cause failure
analysis, precision installations, precision repair requirements, and training. For root
cause failure analysis, the question constantly asked is "Why did this equipment
fail?" Once this question is answered, long term solutions can be implemented.
Precision installation, precision repair, and training are maintenance practices designed
to prevent failures before they occur. The first step in implementing a pro-active
maintenance program is to use preventive and predictive maintenance techniques.
Although a properly selected PdM program offers many benefits, the primary reason for
using such a program is to improve earnings by reducing maintenance costs, increasing
production uptime, and improving product quality. Maintenance costs are reduced by
minimizing equipment damage, extending maintenance intervals, eliminating unscheduled
repairs, reducing spare parts inventory, identifying design deficiencies to allow
permanent repairs, and verifying repair quality before equipment is returned to service.
Production capabilities increase by reducing downtime, coordinating equipment outages, and
minimizing the scope of repairs. Condition monitoring is used to predict hazardous
conditions early enough for operators to take remedial action to prevent failures and to
minimize the deterioration rate.
PdM has become an indispensable part of many plants' maintenance planning and shutdown
strategies. Although it can increase safety, improve efficiency, and reduce operating
costs, PdM has received limited use in some installations because operators have been
reluctant to change established practices. This attitude has been rationalized by the
safety and economic penalties that would result from a failure. As more information
becomes available, however, and as experience with condition monitoring is gained, and
safety and economic advantages are demonstrated, the use of condition-based maintenance as
an alternative to time-based maintenance will increase.
Companies that apply PdM report impressive savings. These savings,
however, vary widely. Manufacturing plants reported an average of $6 saved per dollar
spent on PdM. However, companies with well-documented PdM programs reported an average of
$10 saved per dollar spent. DuPont's Sontara plant documented savings during 1989 of $22
per dollar spent on PdM.
About 55 percent of the savings result from production increases
attributed to increased uptime and product quality. The remaining savings are equally
divided between labor reductions and reduced spare parts inventory. The effect of PdM
programs on maintenance cost is illustrated by the experience of several shipping
companies that reduced maintenance manhours 37 percent when PdM replaced PM for fans,
pumps, and alternators.
A study by the U.S. Manufacturing Div. of Mobil Oil showed that when
failure was predicted by monitoring equipment and the unit was allowed to run to failure,
the cost per incident was $445,000, but when the unit was shut down and repaired, the cost
per incident was only $79,000. Mobil documented a 62 percent reduction in rotating
equipment failures and savings of $2 million during the first year PdM programs were
implemented. The study also concluded that 60 percent of refinery equipment failures can
be prevented.
DuPont spends about $1.8 billion annually on maintenance worldwide. In
1987, the corporate manufacturing committee undertook a benchmark study to compare DuPont
with 16 of the best maintained plants outside the company. A corporate maintenance
leadership team, emphasizing PdM, was formed. As a result, the company saved $300
million/year in maintenance costs, with the goal being $500 million annually. In addition,
DuPont mechanics now maintain 37 percent more equipment than they did at the time of the
1987 study. Over the past 5 years, plant uptime has increased from less than 50 percent to
86 percent. Each 1 percent increase in uptime is worth $500,000 in pretax profit. The
success of the PdM program is attributed to a strong management commitment to PdM
principles and to employee training and certification programs. An estimated 10 percent of
the maintenance budget is earmarked for training, with most of the staff members attending
three or more training seminars a year.
Why should I use Predictive Maintenance?
CASE HISTORIES
Filter pumps
For several years, the filter pumps in a plant had a poor maintenance
record. Pump bearing failures and motor rewinds had accounted for more than $90,000 in
maintenance costs. In addition, downtime attributed to the pumps in one year cost the
plant $396,000 in after-tax earnings. After monitoring vibration and process data, PdM
technicians decided to control flow using a variable-speed drive rather than throttling
flow through a control valve. This action eliminated the problems, saving the $600,000
budgeted for revisions to the entire pumping system and reducing energy cost by about
$30,000/year.
Injection pump
After the failure of a deep well injection pump, a PdM program was
implemented. The monitoring program identified the time for replacing the wear rings
before another failure, resulting in savings of $30,000 for each failure prevented.
Refrigeration unit
Excessive vibration was monitored on the pumps and motors of a
refrigeration unit. Analysis of vibration data suggested that the problem was caused by
recirculation within the impeller, as well as misalignment. A small-diameter impeller was
installed and the pump/motor shaft was realigned; excessive vibration was eliminated. In
addition to eliminating potential failure of the system, the changes resulted in a
decrease of amperage from 43 to 33 Amps, providing annual savings for the two pumps of
more than $16,000.
Turbine generator bearings
Oil sampling tests from the bearings of one 7.5 MW turbine generator
showed an increase in tin and lead content. The probe gap (which measures the distance
between a noncontact displacement probe and the shaft) indicated that the shaft in the No.
1 bearing had dropped about 0.03 in. from its original position, suggesting that the
bearing surface was worn or damaged. The bearing was disassembled and severe damage was
found. If the problem had not been detected, the bearing would have failed and repairs
would have cost millions of dollars. In this case, vibration and wear particle analyses
did not flag the problem, but oil and probe gap analyses did.
Generator insulation
Vibration monitoring of a generator on an offshore platform showed an
increasing radial vibration at 2 times synchronous speed, suggesting an electrical
problem. The recommendation was to check all possible electrical systems and, if the
vibration persisted, to replace the generator. Exhaustive electrical tests revealed no
fault. Convinced of the accuracy of the vibration signature, the equipment specialist
requested that the rotor winding be stripped revealing a complete failure of the
insulation. Without the vibration data, the generator would have continued in service,
presenting a danger of burning out and premature replacement.
Compressor gearbox
A steadily rising vibration level on a compressor gearbox indicated a
problem with the pinion gear bearings. When the gearbox was stripped, bearing damage was
observed, the pinion gears were heavily lacquered, and the thrust bearings were scored. If
repairs had not been performed on time, damage could have resulted in higher repair costs,
lost production, and potential safety hazards.
Gas turbine rotor imbalance
Maintenance technicians agreed that the sound of a gas turbine was
different from normal. Real-time vibration monitoring indicated a severe rotor imbalance.
Resources were mobilized and the unit was replaced during the peak of the compressor
season with no production loss. In the workshop, damage to the unit indicated that failure
was imminent. Without this timely diagnosis, a major production loss would have resulted.
Turbine gearbox
Analysis of lubricant oil from a turbine gearbox revealed a steady
increase in iron content from 3 to 152 ppm. Vibration monitoring indicated an increase in
vibration at the gear mesh frequency. The gearbox was replaced, eliminating a potential
hazard if failure had occurred.
Compressor bearing
After 4 weeks of fluctuating vibration levels on a compressor, the
machine was shut down by vibration trips on a Sunday afternoon. Because of production
commitments, operators inhibited the vibration trips. On Monday, a machine monitoring
specialist using a portable vibration monitor discovered a complete loss of bearing
restraint. The compressor was stripped, showing that the free end journal bearing was
completely damaged and the fifth-stage impeller had serious fatigue cracks. Had the
impeller failed, secondary damage would have caused a safety hazard and a long shutdown.
An on-line vibration monitoring system could have prevented this near miss.
Power turbine rotor imbalance
A power turbine was operated for a time with a near trip level of
vibration on the rotor. A consultant diagnosed the problem as misalignment between turbine
and gear. Repair would have required shutting down the unit for an extended period, but
company priorities dictated that the problem not be corrected until a lull in production
occurred. However, a machine condition monitoring specialist performed a detailed
vibration analysis and diagnosed a simple imbalance. The imbalance was corrected in 6
hours, reducing the vibration level to normal. Without the vibration analysis, the unit
could have operated with the imbalance for a long time and experienced severe damage
unnecessarily.
Pump misalignment
A horizontal split case pump had vibration levels of .67 inches per
second overall. Predominant frequency was 2 xrpm indicating misalignment. The motor and
pump bearings were hot to the touch, the motor was audibly noisy, and one of the pump
mechanical seals was leaking. The alignment was checked and found excessive with 15 mils
offset, large angularity and one motor foot had a soft foot of 30 mils. The alignment and
soft foot were corrected. Vibration levels were reduced to .03 inches per second, the
motor and pump bearings no longer ran hot to the touch, motor audible noise was
significantly reduced, and the mechanical seal stopped leaking. The maintenance personnel
canceled the work order to replace the mechanical seal.
Paper machine
A routine vibration survey on a tissue paper machine showed a high
level of vibration on the north fan pump drive motor. The fan pump is a 30,000
gallon-per-minute, horizontal split-case, double-suction pump driven by two 1,000HP DC
motors. The exact rotational speed is dependent on the grade of the paper and the speed of
the paper machine. The pump usually operates around 900 RPM. The DC power supplies for the
drive motors are silicon control rectifiers (SCRS) that convert the three-phase AC input
into DC output.
The pump was installed in the spring of 1988 and had never experienced
a vibration problem. However, survey results showed that the overall vibration level on
the north motor had increased by 1000% to 0.99 inch/second peak velocity in the horizontal
plane. A second set of vibration readings was taken and analyzed primarily because none of
the drive motors on the five fan pumps had ever vibrated at that level.
The rotational speed of the pump was 900 RPM at the time of the survey.
The predominant frequency was 60 Hz, four times the speed of the motor. The peak vibration
did not relate to any bearing frequency. The close amplitudes on both ends of the motor
ruled out defective bearings.
When a DC power supply using SCRs is out of adjustment, a peak at 360
Hz usually appears; occasionally some harmonics of 360 Hz can be seen. The power supply
uses six SCRs to convert the 60 Hz AC current into the DC output. An out-of-adjustment
unit reduces the efficiency of the SCRS, causing them to fire in an incorrect timing
sequence. The result is a 360 Hz (6 x 60 Hz) vibration that can be detected using spectral
analysis. An electrically-induced 60 Hz spike is seldom seen on a DC drive motor. However,
the electrical maintenance group was asked to examine the power supplies as a possible
source of the vibration.
The cause of the vibration was located during the inspection of the DC
power supplies. The paper machine had been down for repairs a week earlier. All of the
machine drives had been field stripped, and all of ther wiring terminals had been cleaned,
serviced, and reconnected. During the reconnection, two of the gate lead wires to the SCRs
were crossed, with the result that the two SCRs fired out of the proper sequence. The
effect differed from that of a bad SCR not firing and was compounded by the two SCRs
firing out of sequence.
The vibration generated by the crossed gate lead wires would have led
to failure of the motor if the problem had not been detected during the vibration survey.
After the leads were properly connected, the vibration in the drive motor disappeared.
Thermal infrared imaging cameras are an essential tool for
effective predictive and preventive programs.
 THERMOGRAPHY
APPLICATIONS:
- Electrical inspections in buildings, plants, facilities, refineries.
- Thermal heatloss inspections for buidings, plants, facilities, refineries.
- Moisture contamination evaluations in buildings, condo's, plants facilities
- Concrete integrity inspections
- Concrete Water Heated floor inspections for leaks and temperature distribution
- Flat roof leak detection for buildings, plants, facilities
- Power generation generator inspections.
- Power Plant boiler flue gas leak detection
- Substation Electrical inspections, tranformers and capacitor evaluation
- Over head urban and rural distribution electrical inspections
- Electrical motor inspections, mecahnical bearing inspections
- Heat ventilation air conditioning equipment evaluation
- Cold Storage cooling losses.
- Refinery process line insulation loss or leak detection
- Refinery process evaluation
- Heat exchanger Quality and efficiency evaluation
- Furnace refractory (insulation) inspections
- Furnace Internal flame evaluation and tube inspections
- Flame propogation explosion analysis.
Additional Reference Materials
Sample predictive
maintenance electrical report
Sample R&D circuit board IR report
Emissivity: An Infrared White Paper
(includes emissivity table)
How Infrared Works: A Primer
Glossary: Infrared Terms
Sierra Pacific Infrared Inspection Site
Click on the logo for the thermal camera index
( More information on
thermal infrared imagers)
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