What is self-priming pump and what is its working principle？

For normal centrifugal pump, if the suction surface is below the impeller, it needs to fill water when starting the pump, which is very inconvenient. In order to store water in the pump, bottom valve is installed at the inlet of suction pipe, and when the pump works, bottom valve will lead to great hydraulic loss. But for the so-called self-sucking pump, it does not need to fill water when starting the pump (it still needs to fill water for the first starting after installation), and after short-time running, the pump can suck water and can be put into normal use.