The minimum flow rate for the AOR is typically determined by the “start of suction recirculation” for high and very high suction energy pumps, especially those with low NPSH margins. The start of suction recirculation generally ranges from 40 to 100 percent of the BEP flow (see Fig. 2).
Discharge recirculation is similar to suction recirculation but occurs at the impeller/casing discharge at some reduced flow rate below the pump BEP. Although not a common problem, it can do damage to some high (discharge) energy pumps. According to the Hydraulic Institute, pumps with heads greater than 650 ft/stage that require more than 300 hp/stage, have high (discharge) energy. Ideally, the pump manufacturer will specify the minimum flow rate that will avoid operation in the discharge recirculation region for high discharge energy pumps when damage is of concern.
Barring a high suction or discharge energy situation, one or more of the following factors (see Fig. 3) will generally determine the pump AOR:
Temperature rise: All pumps will experience an excessive temperature rise (and possible failure) at some reduced flow rate if operated there for an extended period of time. So lacking any other destructive minimum flow factor, temperature rise will determine the pump minimum flow value. The value of this minimum flow rate is typically below about 10 percent of the BEP.
Bearing life: At flow rates above and below the BEP, the velocities and pressures around the casing volute become non-uniform (see Fig. 4), which can greatly increase the radial loads on the impeller (as discussed in Pump Tips, WW, December 2007). As a result, bearing life normally decreases as the pump flow moves away from the pump BEP (especially with single volute casing pumps). Because of this, pumps should not be operated at flow rates below or above that which will yield the minimum acceptable bearing life for the application.
Shaft fatigue failure: Non-uniform volute pressures away from the BEP can also cause cyclic loads on the shaft, which in some cases can exceed the fatigue limit. Shaft stress can be further increased by any turbulence and cavitation that is created in the casing and impeller, as discussed above. This can even be the factor that determines the pump AOR, depending on the severity of these unsteady and oscillating loads and their impact on the shaft, mechanical seal, and bearings.
Horsepower limit: Pump power draw normally limits the maximum allowable AOR flow rate. However, for high specific speed pumps above about 4,500 to 5,000, the input horsepower actually increases with low flow rates below the BEP. This HP increase can become quite excessive with very high specific speed values. Pump users will often restrict the minimum flow rate for these high specific speed pumps to reduce the pump pressure and/or required input power, and to avoid the requirement for large motors and/or stronger (thicker-walled) piping.
Conclusion
As can be seen from the above, a key factor in optimizing pump efficiency, reliability and life cycle cost is to determine the optimum pump flow range. This requires establishing the pump BEP, POR and AOR (in addition to the true system performance) in order to maximize operation in the POR and avoid any extended operation outside the AOR. The pump manufacturer should always be contacted for these values.
About the Author: Allan R. Budris, P.E., is an independent consulting engineer who specializes in training, failure analysis, troubleshooting, reliability, efficiency audits and litigation support on pumps and pumping systems. With offices in Washington, N.J., he can be contacted via e-mail at [email protected].
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