What exactly is a seal pump and how does it work?

[ehilge]

I've come across seal pumps in a few power plant diagrams recently and I'm confused on what a seal pump actually does. As it was explained to me, the seal pump pumps water into/around a primary pump to seal the primary pump from leaks. However, something seems to be missing from this description. It makes more sense to me to just seal the primary pump with rubber or some other material. So, what exactly is a seal pump, how is it particularly benificial, and how does it work?
Thanks!

 

[turbo]

Have you researched the use of seal water in pumps? If you do, you will find the motivation for providing seal water at XYZ pressure, thus the seal water pumps.