It is the same principle we use to get fuel to the carburetor of your automobile. Now we will look at the cross section of a centrifugal pump and these three formulas will explain why mechanical seals have so much trouble with shaft deflection. This picture describes a volute pump because the impeller is not in the center of the casing. You will note that there is less clearance between the impeller and the cut water than there is between the impeller and the rest of the casing. You will also note that this area is increasing as you move from the cutwater, around the casing, to the discharge nozzle.
Circular pumps have an equal area around the impeller.
They are used to pump larger quantities of liquid, without having to create a head. The volute design is the most popular design because it will produce a head. When we removed the packing from a centrifugal pump we lost a big part of the shaft support system. It therefore becomes very important that we keep the forces equal around the impeller to prevent shaft displacement. If the force increases on one side of the impeller it will deflect the attached shaft and interfere with the performance of the mechanical seal and pump bearings .