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Wednesday, January 28, 2009

Centrifugal Pump With Magnetic Bearing

Magnetic bearing maintain the rotor of a pump in suspension through the force of attraction of a magnetic circuit. Thus although they bear up to 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 on the shaft, and the shaft itself. The stator consists of electromagnet 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 (26 μm) of the desired position.

Magnetic bearings are found in small, high speed turbomachinery such as high speed, multistage, axial flow turbomelecular vacuum pumps. They were introduced into large turbomachinery in the nearly 1980s, mainly in gas compressors and truboexpanders. Their use and acceptance has grown slowly but steady since then. Pump applications of a signings 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 economics of scale associated with production quantities are required to make these systems affordable.

Two representative magnetic bearing equipped pumps are summarize in the table below. One is a multistage boiler feedwater pump and the other a single stage double suction hydrocarbon process pump. The multistage pump was retrofitted with magnetic bearings, together with another identical pump that still contains the oil lubricated bearing, both installed in an electric generating station. The magnetic bearing pump is not encumbered with the usual complexity of a bearing lubrication system.

Parameter

Multistage Pump

Single Stage Pump

Power, hp (MW)

610 (0.46)

800 (0.6)

Rated speed, rpm

3580

1780

Shaft weight, lb (kN)

800 (3.6)

Thrust end: 930 (4.1)

Drive end: 1415 (6.3)

Thrust bearing design load, lb (kN)

4000 (17.8)

4000 (17.8)

Number of Stages

8

1

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