A Rotary Air Compressor liquid ring vacuum pump comes with an impeller with blades mounted on a center hub located in a cylindrical body but off-set from the center.
In Body 1 to the proper, you can see the blades near the top of the pump are nearer to the outside wall than at the sides and bottom level of the pump. You can observe this more obviously in Figure 2.
The impeller sits between two end plates (port plates) that have shaped holes cut into them called ports.
The pump requires a liquid (also called the sealant) to create vacuum the following. Prior to starting the pump, it should be partially filled with the liquid sealant. The liquid 
can be water (rendering it a water band pump), oil or a solvent, dependant on the application.