A circulating water pump works just as any centrifugal pump works: by turning an impeller to increase the velocity of a fluid as it passes over the impeller vanes. The high-velocity water is turned into high-pressure water when going through the diffusion part of the casing. The most common casing design is the vertical in-line type.
A circulating water pump is a pump designed to move water through a closed system. A closed system runs in a loop with the pump discharge pipe meeting back at the inlet to the pump, normally without being exposed to any atmospheric pressure, according to PumpScout. In a closed system, where there is never any exposure to atmospheric pressure, an expansion tank must be used. The expansion tank is a closed tank connected to the suction pipe of the pump. The expansion tank uses a diaphragm in the middle, and the area above the diaphragm is filled with air at a specific pressure. When water is passing through a closed loop system it is usually being heated or cooled, which makes the water contract or expand. The air in the area above the diaphragm adjusts and allows for the expansion or contraction of the water. Without the expansion tank, the water would create extreme amounts of pressure and most likely burst the water pipes. These types of pumps are usually found in heating and cooling applications as well as pools, spas and ponds.