World class maintenancePredictive maintenance techniques
ArcelorMittal Gent chooses to apply situation-dependent or predictive maintenance. The operating situation of the installation therefore determines the time of the maintenance activity, and not the defects (as with curative maintenance), nor the duration since the last maintenance (as with preventive maintenance).
Repairs after an untimely failure due to a defect, or too early replacement before parts have reached the end of their working life, are then prevented. Maintenance should only take place at the ideal time: only when this is required by the situation of the machine. Perfect control, based on knowledge, and the following up of the operating situation of the installation, prevents extra costs due to failures or premature replacements.
For predictive maintenance, we must be able to measure and to detect the operating situation. There are three possible ways, depending on the application:
- vibration measurements for rotating machines
- thermography for static electrical boxes
- oil analysis for electrical distribution transformers
Electrical motors drive pumps, ventilators, conveyor belts and suchlike. The rotation of the motor and the driven appliance cause vibrations from which we can derive much information about the state of the installation. We register vibrations using measuring pins pressed against the machine in different places. We use both axial and radial measuring points.
The visual representation of these measurements results in interpreting and diagnosis:
- the amplitude of the vibrations in the frequency spectrum
- the course of divergence on a time basis
The interpretation of vibration measurements identifies the following problems, for example:
- vibrations at the frequency of the rotational speed and multiples of it, particularly radial with twice the rotational speed:
-> alignment problem
- peaks in the frequency spectrum at not whole multiples of the rotational speed:
-> bearing damage
- vibrations at multiples of 100 Hz
-> electrical problems
The comparison of consecutive vibration measurements gives insight into the severity and evolution of the situation, and helps determine the follow-up period and the necessity to take action.
Bearing damage, ventilator reversing rougher
The powerful processors and the high technology, ultrafast measuring sensors at our disposal, considerably accelerate traditional arithmetic analysis methods such as FFT. In this way, we are practically immediately given indicators for interpretation, advice and action and we avoid untimely breakdowns.
Distribution boards and control cabinets ensure power for and control of the electrically driven and controlled installations. Problems occur due to local heating up as a result of poor conductivity. Greater electrical resistance increases the temperature of the conductor. In the longer term this can lead to:
- the breakage of the conductor with a power interruption and stoppage as a result
- fire due to excessive heat generation with all the resulting damage
- accelerated degradation of electrical control units due to a too high operating temperature with expensive repairs as a result
Heat emits infrared radiation. An infrared camera generates a temperature image of the installation. A logical colour presentation of the measured temperature helps us to easily locate areas of heat. The defect is identified by comparing the heat image with the visual recording of a digital camera.
Heating of cable shoe fusing, pickling shop
At ArcelorMittal Gent some 30 oil-cooled transformers distribute electrical power from the 150 kV-inputs to the 6 kV-outputs to the machine rooms.
Perfect monitoring is required in view of the costs and the importance of safety and production. We therefore examine oil samples.
The following analyses give an image of the internal situation and form the basis for an estimate of the remaining life span:
- dissolved gases in oil are an indicator of the paper insulation
- water content, acidity and disruptive voltage determine the degradation of the oil
- furan derivatives give a precise picture of the course of the level of degradation of the insulation
Correct interpreting gives a basis for deciding on the treatment or replacement of the oil, and gives information about the time for replacement of the transformer.
The interpreting of the more than 120,000 annual measurements has significant advantages:
- no breakdowns, so no production loss and no problems with delivery periods
- increased production times by excluding unwarranted maintenance
- the avoidance of expensive repair costs and the lengthening of the life span by timely damage detection
These predictive techniques with the application of high technology measurements result in proactive correction. The benefits due to assured operation are obvious.