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How is it possible to create suction without the use of a vacuum?

For example: As I understand, positive displacement pumps transport fluid by creating a vacuum at their inlet to draw in a distinct volume of fluid down the pressure gradient, before ejecting it under pressure, towards their outlet.

Centrifugal pumps also come under the heading 'vacuum pumps', but do they actually create a vacuum? It seems the definition of a vacuum is often disputed in this context.

Is it possible to design a suction pump that does not need to create a vacuum?

Thanks.

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  • $\begingroup$ youtu.be/iA6enVQvNgw $\endgroup$
    – The Space Guy
    Commented Sep 23, 2021 at 8:44
  • $\begingroup$ This video might help:) $\endgroup$
    – The Space Guy
    Commented Sep 23, 2021 at 8:45
  • $\begingroup$ Pumps work by creating a pressure differential. E.g. if you have a submersed pump with its impeller low down in a fluid well, the pressure at the suction of the pump can be several atmospheres. and there is no vacuum involved. The suction end of a pump (positive displacement or not) only has to create a pressure lower than head of liquid at the inlet to create a gradient. If the inlet pressure is atmospheric then the suction only has to be slightly less than atmospheric. If the inlet pressure is only a few millibar, then the suction has to be something approaching a vacuum. $\endgroup$
    – KDP
    Commented May 2 at 22:18
  • $\begingroup$ Its fairly easy to see why its almost impossible to create a true vacuum in a large enclosed vessel when the suction of the evacuating pump has to be at a lower pressure than the vessel. That last atom in the vessel randomly bounces around until by chance it bounces into the suction of the evacuating pump. (well that and the fact almost all vessels off gas particles from the material that constitutes the walls of the vessel when the pressure approaches a true vacuum). $\endgroup$
    – KDP
    Commented May 2 at 22:22

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Suction means you have a difference of pressure on the two ends of a pipe, so you don't need vacuum but just lower pressure, for example if you suck at a straw to get liquid to your mouth. But of cause you can transport liquids upwards without "suction". So maybe you have to say more exactly what you want.

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