Magnetic Couplings, NPSH Margin & Reverse Runaway Speed


Written by:
The Hydraulic Institute
Published:
April 26, 2013

Q. What are the effects of increasing temperature on the magnetic couplings in a rotary sealless pump?
A. Eddy currents in the containment shell are one source of heat in a sealless pump. Heat is also generated by liquid friction as the inner magnet moves through the liquid inside the containment shell.

Magnetic couplings undergo some loss of torque capability with increasing temperature. Additionally, each permanent magnet material has a unique Curie point, which is the temperature at which the material loses all magnetism. Below the Curie point, two ranges are referred to as reversible temperature and irreversible temperature.

Reversible losses naturally occur in the normal rated temperature range of the coupling. The coupling will return to full strength when it cools to ambient temperature. Between the rated temperature of the coupling and the Curie point is a range in which the magnets lose a percentage of their strength permanently as a function of time and temperature. End users should consult the manufacturer for specific information about coupling torque versus temperature prior to sizing the magnetic coupling for a given application. The generally recognized, useful temperature limits vary with the magnet type and grade.

End users should always check with the manufacturer before using magnetic couplings close to these limits because magnet temperatures are higher than the pumped fluid or the environment. When pumping liquids that are particularly sensitive to temperature, or when operating near the vaporization point, the internal temperature rise profile should be obtained from the manufacturer.

For more information on magnetic coupling temperature effects, see ANSI/HI 4.1-4.6 Sealless, Magnetically Driven Rotary Pumps for Nomenclature, Definitions, Application, Operation, and Test.

Q. What information is available to determine the appropriate net positive suction head (NPSH) margin for my rotodynamic pump?
A. The determination of an appropriate NPSH margin considers factors that impact performance and service life of the pump. An inadequate NPSH margin may affect pump head, noise and vibration. Pump service life may be reduced because of material erosion and damage to bearings or seals.

Recommended margin ratios can vary by pump type and application, with higher values applying to pumps with higher operating speeds and/or continuous operation outside the preferred operating range of the pump.

A greater NPSH margin is not detrimental to the pump but may not be desirable. Specifying a higher margin may result in a non-optimal pump selection that will add costs to the pumping equipment (larger/slower pumps or pumps with inducers), reduced efficiency, or a reduced operating range because a higher suction specific speed pump was selected. Requiring a greater suction head to increase the NPSH margin may also increase the cost of the pumping station structure.

The recommended use of NPSH margin involves a known pump design having fixed NPSH3 characteristics that result in a reasonable and safe value of suction specific speed. In such a situation, the NPSH margin is applied to the NPSH3 at the flow rate of interest to obtain the minimal value of net positive suction head available (NPSHA). The use of a higher value of NPSH margin under these circumstances generally results in more conservative conditions for the pump. If the recommended NPSH margin cannot be obtained, then choosing a lower operating speed for the pump for a fixed flow rate will generally result in a conservative selection.

End users are cautioned with regard to obtaining the NPSH margin by specifying pumps with higher suction specific speeds that have lower NPSH3 values. Higher suction specific speed pump designs are more likely to experience objectionable noise and a narrower operating range  compared to lower suction specific speed pump designs. Poor suction conditions may result in flow separation and distorted flow at the impeller inlet, which can adversely affect the NPSHA of the pump.

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