A pump must produce enough pressure differential to overcome the head loss caused by system resistance in pipes, valves, and fittings. Understanding the factors that affect a pump’s performance is crucial to selecting a pump that will meet your operational needs and budget.
One important factor to consider when calculating the flow rate of your floating pump is shaft speed. Changes in shaft speed are directly proportional to changes in the pump’s output, assuming other factors remain constant. For example, if you decrease the speed of your pump by 10% while keeping all other variables constant, you will experience a similar reduction in the pump’s head discharge.
Another important factor to consider is the available net positive suction head available (NPSHA), which is defined by your pump’s process piping. The NPSHA must be greater than the required flow of your pump (NPSHR) in order to avoid cavitation. You will see this on the pump curves as a red line above the dotted line.
To determine the flow rate of your floating pump, simply connect a head-discharge measurement apparatus to the pump’s discharge piping. The measurement device must be properly sized and pressure-rated in order to measure the full range of pressures and flows that your pump is capable of producing. Close the regulating valve slowly, and record the shut-off head (remember to only close the regulating valve as far as it will go so that your pump can still operate under no-flow conditions). This will allow you to calculate the pump’s flow rate.