The Polyethylene plant of Carmel Olefins employs a stirred reactor.
The stirrer-motor assembly has four bearings.
The bearings of the electrical motor and the stirrer upper bearing are operated without liquid lubrication. The lubrication is provided by drops of lubricant contained in the flow of ethylene entering the reactor.
The operation life cycle of the stirred bearing was limited to 2,500 hours. The short life cycle of the stirred reactor bearings was the result of the specific structure of the equipment and the harsh operation conditions of polyethylene production.
After 2500 hours of operation the standard procedure was to stop production, take out the stirrer and replace it with a new one.
Frequently visual checking of the stirrer after dismantling showed that the bearings were in normal condition and it was possible to continue operating the stirrer. On other occasions severe degradations of the bearing conditions was revealed.
The Process Control instrumentation in place at Carmel Olefin was not sufficient to make a determination as to when it was necessity to replace the stirrer. The plant’s practice was to listen to the reactor noise to detect changes in bearing performance.
THE EXTENSION OF STIRRER LIFE CYCLE
The operating period of the stirrer for this case study was over eight life cycles. After two of them the visual inspection discovered some wear. Two cycles were terminated early due to pending bearing failures.
The installed system is based on induction motor phase angle analysis. It measures the electrical motor current and voltage, and further detects the motor phase. The hardware measuring the current and voltage is installed in the Plant MCC.
Phase angle signals are proportional to the current values of electrical motor torque.
The torque could be considered as a sum of two components: a mean value which is proportional to the load and another one which is proportional to friction losses in bearings and load fluctuations. The stirrer load fluctuations are relatively small. During spectral analysis the constant component disappears and frequency spectrum of variable component, is obtained.
Analyses of these spectra reveals small fluctuations of torque caused by improper performance of bearings.
Signal processing follows these small deformations. It is done by a special technology that eliminates the linear dependencies between various condition indicators and introduces weighting factors for relatively small but diagnostically important indicators.
Phase angle fractals are another signature applied for stirrer diagnostics. Phase Angle Fractal represents a polar diagram of a motor torque as a function of a rotor angle position. It is an accumulated diagram obtained in the course of a number of shaft revolutions. Fractals are associated with fluctuations of peak 1
The monitoring results show that using PSE’s monitoring system allowed Carmel Olefin to extend stirred life cycle by 25% and perform three yearly production interrupts instead of four, practiced earlier, and successfully prevented catastrophic failure on two occasions.
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