Solar Thermal Power Stations Solar Thermal Power Stations Are a Prime Application for Canned Motor Pumps

Editor: Dr. Jörg Kempf

Many industrial processes require heat energy to be moved from one place to another via a heat transfer fluid: typically water and steam at temperatures up to 200 °C, and heat transfer oils for the range 200–450 °C. Pumping fluids reliably at such high temperatures is a challenge that sealless canned motor pumps can meet. The new generation of solar thermal power stations provides a practical example.

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Fig. 1: Single stage canned motor pump, type CNPK 250-630 (Pictures: Hermetic)
Fig. 1: Single stage canned motor pump, type CNPK 250-630 (Pictures: Hermetic)

Pumping applications at temperatures up to 320 °C generally use conventional chemical pumps driven by standard electric motors, with a thermal barrier between pump and motor. This works well as long as compensators are included to isolate the pump nozzles from mechanical forces and torques transmitted through the piping. However, cost pressures often result in the compensators being omitted, and the resulting loads can distort the pump casing.

For temperatures in the range 320–450 °C, sealless pumps are often a better solution. The appropriate standard is API 685, first published in 2002 to provide sealless (mag-drive and canned motor) equivalents to API 610 single-stage chemical pumps. API 685 pump have their mounting feet centered on the axle — a practice that has been standard for boiler feed pumps for decades — so that thermal expansion of the pump casing is evenly distributed upwards and downwards.

Canned motor pumps are sealless centrifugal pumps of monobloc design, driven by an asynchronous canned motor mounted on a common shaft. In conventional designs, the motor stator is protected from corrosion by a shroud or “can” made from non-magnetic material. Part of the pumped flow is used to cool the motor and to lubricate the two identical hydrodynamic bearings. After passing through the gap between the rotor and stator, this cooling flow returns to the pressure side of the impeller via the hollow shaft.

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