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A magnetic sensor to measure wear in centrifugal pumps Khoie, Ramin


On average, centrifugal pumps consume between 25% and 60% of the total consumed electrical energy inside process plants. Erosion inside open-impeller centrifugal pumps leads to a reduction in pump efficiency and occasional plant downtime. This work demonstrates a new concept for an online instrument capable of monitoring wear with the objective of improving the maintenance scheduling of centrifugal pumps and the prevention of unexpected failure through a predictive maintenance system. A magnetic wear sensor is designed and fabricated that allows for wear measurement while the pump is in operation. This sensor can be installed on existing centrifugal pumps and does not require any pump modifications. Wear mostly occurs on the tip of the impeller blades reducing the thickness of the impeller which in turn increases the gap width between the impeller and the side plate inside the pump housing, from 0.65 mm (no wear) to 2.50 mm for maximum allowable wear on the pump used for prototyping. By using a magnetic circuit with the pump and its components, wear is estimated by measuring the change in the width of the varying gap between the impeller and the side plate. To assemble the magnetic circuit, a high-permeability clamping mechanism with a relative permeability of 10,000 is designed and fabricated along with a magnetic coil excited using a 1.0 V AC voltage signal at 70 Hz to drive flux through the circuit. As wear occurs, the total reluctance of the magnetic circuit increases causing the inductance of the coil to drop. The coil's inductance is also a function of the impeller's angular position. To estimate wear, data is collected at a sampling frequency of 500 kHz and then assessed in the frequency domain after fast Fourier transform (FFT). The amplitude of the FFT signal at the frequency correlated with the pump's rotational speed is then considered to estimate wear. For a data sampling time of one second the sensor has a signal to noise ratio of 17.8 dB with an average sensitivity of 0.022 mV/mm and a resolution of 0.38 mm.

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