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Subject:   Reasons A Pump H-Q Curve may be Different than the Design Curve

From Allan R. Budris and Water World


Actual system H-Q curve not known:

The actual current system H-Q curve may be different than the original system design. Once a plant is commissioned and the plant is put in service, the system head begins to change. In the short term, levels change in the tanks and wells, valves open and close, and filter screens become clogged. As maintenance occurs, pipe schedules are changed, equipment is changed and new equipment is added into the system. In the long term, equipment loses efficiency, scale forms on the internal pipe walls and the plant undergoes expansion and contraction. Even when new, the original calculated system curve may differ from the actual system performance due to the assumptions used in the calculation, such as 10 year old pipe. Any pump change should, therefore, start with the development (confirmation) of the true current pumping system “Head-Capacity” curve, as detailed in the writer’s January 2009 Column on: “Creating an Accurate Pumping System Head-Capacity Curve...“ A field test of the pump total developed head at one or more measured flow rates can help determine the actual (current) pump and system H-Q curves. By developing the true system head-capacity curve, an accurate determination of the current and new pump operating conditions can be established.

Additional references on aging pumps

From Pump System Hydraulic Design 10.2.4 Determination of Pump Operating Points—Single Pump

The system curve is superimposed over the pump curve; (Fig. 10.6). The pump operating points occur at the intersections of the system curves with the pump curves. It should be observed that the operating point will change with time. As the piping ages and becomes rougher, the system curve will become steeper, and the intersecting point with the pump curve will move to the left. Also, as the impeller wears, the pump curve moves downward. Thus, over a period of time, the output capacity of a pump can decrease significantly. See Fig. 10.7. for a visual depiction of these combined effects

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