Replenishing the suction port of the pipeline pump is also a method of regulating the flow rate, which saves some power compared to the outlet throttling. In general, it is undesirable to have air in the liquid being transported, and too much air always presents a risk of losing the pump to its filling head, so this method is rarely used in practice except in a few cases.
By adjusting the flow rate in this way, the required power can be minimized and overheating during flow regulation can be eliminated. Steam turbines and internal combustion engines are easily adaptable to speed regulation at a small additional cost. Various mechanical, magnetic, and hydraulic shifting devices, as well as DC and AC variable speed motors, can be used to adjust the speed. In general, variable speed motors are too expensive and can only be adjusted using adjustable vanes when economical studies of special conditions prove to be worthwhile.
After studying the adjustable guide vanes installed in front of the impeller, it was found that this method is effective for pump adjustment when the specific number of revolutions = 5700 (2.086). The blades produce a positive pre-rotation that reduces heading, flow and efficiency. And for a relatively small adjustment effect only by the blade. In Europe, large-scale energy storage pumps for power generation have successfully applied adjustable outlet diffusion blades. The propeller pump with variable pitch blades has also been successfully studied. At a constant head, and with a relatively small loss of efficiency, a large range of flow variations can be obtained. However, these methods are too complicated and expensive, and thus are greatly limited in practical applications.