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Wednesday, May 20, 2009

Shaft Elongation in Vertical Pumps

The elongation of a vertical pump shaft is caused by three separate phenomena:
  1. The tensile stress caused by the weight of the rotor.
  2. Tensile stress caused by axial thrust
  3. The thermal expansion of the thrust
In most cases the tensile stress created by the axial thrust is several times greater than that created by the weight. In typical example of a 16,000 gpm pump designed for a 175-ft head and 5o ft long, the elongation caused by the weight of a 1600 lb (726 kg) impeller will be of the other of 0.0033 in. The elongation caused by the axial thrust will be approximately 0.0315 in.

The elongation caused by thermal expansion has to be considered from two angles. Firs, if the shaft and the stationary parts are built of materials that have essentially the same coefficient of expansion, both will expand equally and no significantly the same range of temperature in which they were assembled, no significant relative expansion will take place, even if dissimilar coefficients expansion are involved. Whatever the case, the pump manufacturer take these factors into consideration by providing the necessary vertical and play between the stationary and rotating pump components.

Load of foundations of Vertical Pumps

It the motor support is integral with the pump discharge column, and if the hydraulic thrust is carried by the motor thrust bearing, this thrust is not additive to the deadweight of the pump and of its motor plus the weight of the water contained in the pump, insofar as the load on the foundations is concerned. This is because the pump and motor mounted in this fashion form a self contained entity and all internal forces and stresses are balanced within the entity. If the pump and motor is supported separately, however, and joined by rigid coupling that transmits the pump hydraulic thrust to the motor thrust bearing, the foundation will carry the following heads when the pump is running.