Pump head and flow rate
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Facing a pump that has been delivered to the site but not yet connected with piping, how do you specifically understand the relationship between its head and flow rate? Look at the impeller: if water is pumped straight out (without the pipe connected to it), at which H-Q point is it located? Is it high flow rate or high head? Will the motor be in an overloaded state? Will it exceed the rated flow rate? Theory corresponds to practice.
The head and flow rate indicated on the pump nameplate are the values at rated power. At this point, by adjusting the opening degree of the pump’s outlet valve, it is possible to change the relationship between head and flow rate. This relationship depends on the motor’s shaft power as well as the size of the impeller. Of course, the curve charts provided by the pump manufacturer are the results of tests conducted before the pumps leave the factory; you can use these performance curves to verify the actual behavior of the pump. If there are discrepancies, it indicates that the pump does not meet the required standards.
If there is no valve at the pump outlet, it becomes impossible to control the outlet flow rate. This results in a high flow rate that can cause the motor to overload (this is certainly the case if the density of the fluid is low; if the density is similar to that of water, then there is no problem). In such a situation, the flow rate will exceed the rated value. The H-Q point is achieved by controlling the pump outlet flow rate to reach the design point.
If the pump is equipped with a different motor while the impeller remains unchanged, its H-Q performance curve will change, right?
Reply to 3# Sudan: If a pump is equipped with a different motor while the impeller remains unchanged, the H-Q characteristic curve will change, right?
1. Pumping water directly is unfavorable for startup and can cause overload； 2. Obviously, it involves a high flow rate. It is usually controlled by an outlet valve; if the outlet is fully open at the design point, there will only be a high starting current, but this generally does not happen ； 3. It is also necessary to consider what type of pump to use; positive displacement pumps are not suitable.