Why doesn’t a positive displacement pump need to be primed before starting?

Generally, positive displacement pumps also require priming, but since they have a self-priming function, operators skip this step for convenience.

Right, self-priming capability: Theoretically, since there is no seal between the pump body and the impeller of a centrifugal pump, it lacks self-priming capability. While the latter is sealed, and transportation takes place through a sealed fit ； My understanding{:1_90:}

How does the impeller of a centrifugal compressor draw in gas?

Reply to 4# Feitian Xiaoyu: It seems a bit like a multi-stage electric fan

Although positive-displacement pumps have self-priming capability, it is still best to fill the pump before using it for the first time, as there is no liquid inside the pump when it is first put into use, which can lead to dry grinding

The working principle of a centrifugal compressor is that as the impeller rotates at high speed, the gas moves along with it; under the effect of centrifugal force, the gas is thrown into the diffuser at the back, creating a vacuum zone at the impeller. Fresh gas from the outside then enters the impeller. The impeller keeps rotating, continuously drawing in and discharging gas, thereby maintaining a continuous flow of gas. Driven by the impeller blades, the gas rotates at high speed along with the impeller; under the effect of centrifugal force, it flows toward the impeller’s outlet. There, it undergoes expansion by the impeller, resulting in an increase in both its pressure energy and kinetic energy. After entering the diffuser, the kinetic energy is further converted into pressure energy. The gas then passes through bends and recirculators to reach the next stage of the impeller, where it is compressed further, thereby bringing the gas pressure to the level required by the process. Put simply, it’s a bit like the blades of a household fan, except that the impeller of a home fan is open and its speed is much lower

What was said on the 3rd floor is correct; positive displacement pumps eliminate the need for a tank pump because they all have self-priming capability!

Why can’t a centrifugal pump draw liquid when it rotates with air in its chamber, while a centrifugal compressor can draw gaseous media when it rotates in air?

Reply to 9# Feitian Xiaoyu: Hehe :) Buddy, gases are different from liquids; gases can diffuse on their own. Think about it: the gas at the inlet of the centrifugal compressor naturally gathers in the low-pressure area at the inlet (due to the external atmospheric pressure or external force). But liquids won’t work, right? Furthermore, a liquid pump has a certain required installation height; without filling the pump, how can the liquid reach the low-pressure area at the center of the impeller? Lol Of course, you’ve also heard of installing the pump at a lower level – by placing it underground, below the liquid surface – so that the liquid can flow on its own to the central area (similar to how gas does).

The density of air is lower than that of liquids, so the centrifugal force is also much smaller. The liquid cannot be expelled, so no low-pressure area is formed, and it’s therefore not possible to draw in more liquid. Naturally, the pump no longer has self-priming capability. The gas binding phenomenon, huh