Table 2: Field Pump Suction Energy Comparison
Click here to enlarge imageIn the case of the problem submersible pump, the “Very High Suction Energy” for the new pump was further aggravated by an elbow at the pump suction, and the fact that the system forced the pump to operate in the low flow suction recirculation region (which can require NPSH Margin Ratios over 10 to eliminate all cavitation under these conditions). This greatly increases the NPSH Margin Ratio values shown in figure 2 (which only apply to flow rates above the start of suction recirculation). These NPSH Margin Ratios should probably be increased by a factor of 2 to 2.5 in the suction recirculation region. So even with actual NPSH Margin Ratios above 3.3, the impeller experienced cavitation damage.
The vertical turbine pump had a different problem. The greater amount of cavitation caused by the lack of vane overlap, and the lower NPSH Margin Ratio (1.2) for this application caused the liberation of an excessive amount of dissolved air to become entrained air. This entrained air entered the bowl bearings, which caused them to fail prematurely.
So what should be the take-away from all of this? I think it is that, in addition to calculating the Suction Energy, Suction Energy Ratio and NPSH Margin Ratio for all new pump applications (and retrofits), special care should be taken to determine if the particular pump has at least 15 degrees of impeller vane overlap, and how it effects the Suction Energy Intensity. Remember that the amount of vane overlap decreases with increasing impeller trim, so the determination must be made for the specific impeller diameter for the application. WW
References: “Pump User’s Handbook – Life Extension”, by Heinz P. Bloch & Allan R. Budris, 2nd Edition, 2006, The Fairmont Press, Inc.
“Effects of Entrained Air, NPSH Margin, and Suction Piping on Cavitation in Centrifugal Pumps”, Allan R. Budris & Philip A. Mayleben, Proceedings of the 15th International Pump Users Symposium.
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, NJ, he can be contacted via e-mail at [email protected].
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