A centrifugal pump is basically a rotating shovel for liquid. Each rotation it expels a donut of liquid. The volume of the donut represents how many gallons per minute the pump delivers. The liquid is thrown off the vane tips. At the center of the shaft there is no relative motion but the liquid there moves out to replace the liquid thrown off the tip. This creates a low pressure area at the shaft center, which is also the liquid inlet (pump suction). External pressure on the liquid supply, which may only be atmospheric pressure, forces more liquid into the pump suction. The amount of velocity of the liquid as it leaves the pump determines how much head (or pressure) the pump will develop. This is determined by the diameter of the vane and how many revolutions per minute it makes (shaft speed). h=V5/2g for you engineers. Pumps are designed around a flow rate which determines how big the case must be to efficiently handle the quantity of water desired. This is indicated by the inlet and outlet pipe sizes but there can be considerable variation. If it gets impractical to make an impeller large enough in diameter to get the head desired, two or more impellers ( stages) can be incorporated into one housing. This is very common in water well pumps where the pump must go down a hole. It is very hard to get a pump with a diameter larger than the hole to go in without the use of a hole stretcher. This device is large, dangerous, and illegal in most states. Ask your well