Factors Affecting Low Flow Rate Pump Operation

Collectively, the following broadly classed factors have been recognized as the major contributors to low flow pump problems.

Thermal Factors The increase in temperature of the liquid within the pump is directly related to the pump's efficiency. The energy that is available to heat the flowing liquid and the pump casing is basically the difference between the power input to the pump (brake horsepower) and the useful work done by the pump (liquid horsepower). At low flow conditions, centrifugal pumps are very inefficient and a significant amount of input energy is lost and heats the liquid and the pump assembly. Refer to Subsection 2.3.1 and Chapter 12 for more discussion on thermal effects.

Hydraulic Instabilities When the pump is operating significantly below the BEP, flow streamlines (that is, patterns) within the pump change considerably from the rated design streamlines. Fluid eddies are most likely to develop at the inlet and discharge of the impeller resulting in flashing, cavitation, and shock waves that often produce vibration and serious component erosion. This phenomenon is classically known as internal recirculation. It can occur at the pump inlet (suction) and discharge. Refer to Subsection 2.3.1 and 2.3.2 for a more in-depth discussion on this topic.

Mechanical Loads As the flowrate through the pump decreases, steady state loads increase and superimposed dynamic cyclic loads appear radially and axially on the impeller and shaft. The dynamic cyclic component increases significantly when recirculation within the pump occurs. Bearing damage, shaft and impeller breakage, and rubbing wear on casing, impeller and wear rings can occur. See Subsections 2.3.2 and 2.3.3 for discussion of this subject.

Axial-flow and mixed-flow pumps with high specific speed produce comparatively higher head and take comparatively more power at low flow. A bypass system may be necessary not only to reduce component loading and stress but also to prevent motor overload. See Subsection 2.3.1 and Section 8.1 for discussion.

Abrasive Fluids Liquids containing a large amount of abrasive particles, such as sand or ash, must flow continuously through the pump. If flow decreases, the particles can circulate inside the pump passages and quickly erode the impeller casing, wear rings, and shaft.

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