Graham Jones Larry Hawkins Paul Cooper

Magnetic bearings maintain the rotor of a pump in suspension through the forces of attraction of a magnetic circuit. Thus, although they bear up the weight and hydraulic loads of the impellers and the shaft, they are not really bearings in the traditional sense of the rotating and stationary surfaces bearing on one another. The supporting magnet circuit for each bearing includes stationary magnets in a stator surrounding the shaft, a laminated rotor that fits on the shaft, and the shaft itself. The stator consists of electromagnets in the traditional heteropolar design, and if a homopolar design is employed, permanent magnets can be added. Sensors monitor the position of the shaft and signal a controller to adjust the magnetic loads to keep the shaft to within about 0.001 in (25 mm) of the desired position.

Magnetic bearings are found in small, high-speed turbomachinery such as high-speed, multistage, axial-flow turbomolecular vacuum pumps1. They were introduced into large turbomachinery in the early 1980s, mainly in gas compressors and turboexpanders. Their use and acceptance has grown slowly but steadily since then2. Pump applications of a significant size have appeared and have confirmed the general position that magnetic bearings can provide a technically sound bearing with maintenance and operating advantages, including zero wear. However, due to the technical complexity of magnetic bearing systems, the economies of scale associated with production quantities are required to make these systems affordable.

Two representative magnetic-bearing-equipped pumps are summarized in Table 1. One is a multistage boiler feedwater pump3-6 and the other a single-stage double-suction hydrocarbon process pump7. The multistage pump was retrofitted with magnetic bearings (as shown in Figure 1) and is shown in Figure 2, together with another identical pump that still contains the oil-lubricated bearings—both installed in an electric generating station. The magnetic-bearing pump is not encumbered with the usual complexity of a bearing lubrication system.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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