Why do pumps really exists?

It does make sense centrifugal compressors exist, because gases can compress.

We all know that liquids don't compress like gas. Then why do centrifugal or reciprocating pumps exists?

See even a the Wikipedia article on Centrifugal Pumps.

• They exist because people make them. People make them because they generate flow. Aug 9 '15 at 6:59
• @EddinNomar: are you asking why do pumps exist or how is it possible for them to exist - as in the physics behind how can pumps operate the way they do? Because the reason why they exist is to move liquids either laterally and/or upwards to a higher gravitational potential.
– Fred
Aug 9 '15 at 8:27

hm...

I presume the term does not compress is what kicked of this question.

Right?

The task of a pump is to store energy in the fluid (potential energy in the form of pressure). This stored energy can be used for different purposes (e.g. @Fred comment). As you already mentioned there is a quite interesting feature of fluids. Some are compressible some aren't. This means whereas some will change their volume under pressure some won't.*

As you might have guessed this influences the design of pumps. Depending on the fluid and its characteristics (one of which is compressibility, you might have a look for the Mach-Number, $Ma$).

*) As pointed out by @Reza-Afzalan this statement is a simplification or rather exaggeration since all fluids are compressible but the volume change of fluids is usually negligible small.

All real materials are compressible. If you look at thermodynamics tables, even small change in pressure causes density change in fluid. But as an engineer, when working on pumps you can omit small and effectless changes in density of fluids. So pumps, compressors, and fans all do the same thing: they compress material and create flow.

• you are right, my answer might have been to simple with respect to volume change. I added a section to my answer, thanks! Aug 11 '15 at 5:35
• But a truly incompressible liquid can happily be pumped - as rul30 says "adding energy" is the end aim. How this is manifested varies. Aug 11 '15 at 13:30
• As you stated, the compression is often irrelevant, but pumps all significantly pressurize and/or create flow.
– Rick
May 26 '17 at 13:02