TDH - Total Dynamic Head - This term can be confusing, and there appears to be no "official" definition. TDH often appears in a context where the writer or speaker wishes to convey the following: "head developed at a given flow rate". Because the term can be confusing, Irrigation Craft therefore, does not use the term, but rather, Irrigation Craft expresses pump performance as in the following example:
300 gpm @ 200 Feet of Head, with no elevation or pipe manifold losses or additions.
Thrust - Including Static, Cyclic, Steady State, Transient, Radial, & Axial
DefinitionPressures bearing on the pump and prime mover (motor or engine). Thrust has two modes, Static (pump at rest), and Dynamic (pump operating). Thrust may originate from within the pump, the prime mover, or from the system into which the pump has been installed.
Two Principle Directions of Thrust
Pressure applied to the impeller shaft and bearings, forcing the shaft in a direction parallel (in line with) the shaft.
Axial Thrust is caused by unbalanced pressures on the two sides of the impeller. Axial thrust loads vary with flow rate, system conditions (both intake and discharge), and at start-up and shut-down of the pump.
Thrust bearing failures obviously indicate that there may be an axial thrust problem. Radial thrust bearing failures strongly indicate axial thrust problems because many radial thrust bearings cannot tolerate much movement in the axial direction. Close examination of a bearing by an expert may determine the cause of failure.
Cavitation (especially suction cavitation), can produce cyclic axial thrust forces that can cause fatigue failure of bearings.
Pressure applied to the impeller shaft, attempting to push the shaft in a direction perpendicular (sideways) to the shaft.
Radial thrust is caused by the following:
- Unbalanced pressures surrounding the circumference of the impeller.
- Mechanical Unbalance - Mass unbalance of the impeller itself, including incorrect location of the center of rotation.
- Hydraulic Unbalance - Mass unbalance of the fluid contained within the impeller.
Radial thrust arising from uneven pressures surrounding the impeller, is a steady state thrust with a constant thrust direction and intensity at a given flow rate and speed.
Radial thrust caused by vane passing syndrome is a cyclic load with the vane passing frequency (rotation speed divided by the number of impeller vanes).
Both types of mass unbalance rotate with the impeller, and therefore produce a cyclical radial thrust and vibration at the frequency of the rotation speed, while intensity increases with increased rotation speed.
Frequent seal failures may indicate radial thrust problems. Although shafts and casings can be made to withstand high radial thrust loads, seals often remain vulnerable to vibration caused by radial thrust loads.
Follow the Link Below to Learn About Thrust In-Depth
Tilting Pad Bearing - See Kingsbury Bearing
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