Pump starts automatically; motor power selection
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How should the motor power required for automatic startup of the pump be considered? For example: the rated flow rate is 40, and the maximum flow rate of the selected impeller is 70. Should the motor power be determined based on the power at a flow rate of 70, or based on the power at 1.2 times the rated flow rate (60)?
It depends on the degree to which the valve is open. If it’s fully open, then 70 is definitely required; if it’s opened to the limited flow rate of 40, then 40 can be used as well
The standard practice among major engineering companies is to select the motor for the self-priming pump based on the end of the curve (maximum flow rate). However, my personal opinion is that if the pump is under some backpressure – that is, either the valve in the outlet line is not fully open or the outlet line is filled with liquid – then there is no need to select a motor rated for the maximum flow rate; it is sufficient to choose a motor with a capacity of 10–25% higher (depending on the power).