Establishing Minimum Pump Flow Requirements

The bypass system designer must know the minimum pump flow specified by the pump manufacturer in order to properly design a bypass system. The four previously discussed topics should be evaluated in detail by the pump manufacturer to establish the minimum flowrate specification. Minimum flowrate specifications are generally established through a combination of analytical and experimental techniques coupled with field performance data.

Thermal Considerations The maximum allowable temperature rise of the pump is primarily based on two points: the permissible pump casing and shaft thermal growth and the flash point temperature of the pumpage. Pump manufacturers use analytical, laboratory and field data to validate their thermal analysis to ensure that pumps do not seize within the allowable temperature operating ranges. Refer to Subsection 2.3.1 for temperature rise calculations.

Applications involving extremely high or low temperature fluids may require more in-depth analysis to determine if individual component thermal growth is the limiting factor in determining minimum flowrate. Additionally, certain chemicals, which polymerize or solidify at particular temperatures, may establish the minimum flowrate specification. Chapter 12 and Subsection 2.3.1 provide more detail on this subject.

Hydraulic Considerations The minimum bypass flow requirement for most pumps is based on minimum continuous stable flowrate, a hydraulic criterion, rather than a temperature rise. Pump internal recirculation will occur at both the impeller inlet and impeller outlet as flowrates are reduced. Internal recirculation will occur at flowrates well above those that cause temperature concerns. Refer to Subsection 2.3.1 for a detailed evaluation of this topic.

Mechanical Considerations It is necessary to know how head, radial thrust, axial thrust and power vary with capacity before deciding on minimum allowable flow. Bearing capacity, motor rating, and stresses in drive and driven components are important influences.

Abrasive Wear Considerations Relatively high bypass flowrates may be required to protect the pump against abrasives in the liquid. Heavy wear can occur at flows below 85% of the best efficiency point. The designer must establish the minimum pump flow specification using the pump manufacturer's recommendation and his experience with comparable pumps and liquid/solid mixtures.

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