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What is the difference between best efficiency point and operating point for pumps?

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In a pumping system, a system curve can be derived based on the static head required to lift up the fluid and variable head due to possible head losses. The pump curves which relate the performance of the pumping to head against discharge can be obtained from pump suppliers. When the system curve is superimposed on the pump curve, the intersection point is defined as the operating point (or duty point). The operating point may not be necessarily the same as the best efficiency point. The best efficiency point is a function of the pump itself and it is the point of lowest internal friction inside the pump during pumping. These losses are induced by adverse pressure, shock losses and friction.

Losses due to adverse pressure gradient occur in pumps as the pressure of flow increases from the inlet to the outlet of pumps and the flow travels from a region of low pressure to high pressure. As such, it causes the formation of shear layers and flow separation. Flow oscillation may also occur which accounts for the noise and vibration of pumps. The effect of adverse pressure gradient is more significant in low flow condition.

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For shock losses, they are induced when the inflow into pumps is not radial and contains swirl. In an ideal situation, the flow within the pump should be parallel to the impellers such that the flow angle is very close to the impeller angle. The deviation of the above situation from design causes energy losses and vibration.

head vs flow

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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