True submersible pumps are surface mounted self-priming pumps that pump water from beneath (usually up to 12M) using no return valve but are usually stored above (in some cases up to 40M). They are stored above the water and may either be exposed directly to the environment, or inside a structure like a pump house, and operated electrically, hydraulically, or electronically. As far as how the pumps work, they serve the same purpose as their surface-mounted counterparts. They pump the water up through a vacuum tube to the external environment. The pump discharge, which may also contain air bubbles, directs the water through a discharge pipe, which is connected to a drain pipe. However, while the pump is below the surface, air cannot be pumped out.
A self-priming pump may have a different design, compared to a submersible pump. For instance, it might have a sealed chamber with a very large seal, rather than a very small seal. Also, the seal might be "rolled". This means that it has a series of chambers, rather than just one large chamber. Usually, this type of pump has a sealed unit that is connected to a sealed end off of a float that forms the external section of the pump. This float has a sealed end cap that has a sealed volume, the volume of which is determined by the total discharge pressure.
What is the difference between surface mounted and submersible pumps? Well, surface mounted pumps, also referred to as gravity pumps, function with the aid of gravity. This means that they draw water directly from a surface. These types of pumps function by drawing water up into the pump's holding chamber. Once in the holding chamber, it is gravity-fed and directed into the discharge tubing.
Submersible sump pumps, on the other hand, the function with the help of fluid pressure. This means that these pumps draw water into the pit and then direct it towards the drain pipe, rather than letting it fill the pit itself. The submersible pump is connected to a series of drains, each connected to an appropriate drainage pipe. If one drain is clogged, the others will take over until it is cleared.
Another difference is found in how the operating modes work. Self-priming pumps can run on the manual mode, which uses motor power for pumping the fluid. The submersible pumps, on the other hand, run on self-priming fluid. While this fluid functions like regular water, it contains an additive that primes the pump for better water circulation. Self-priming submersible pumps are often more expensive than their submersible counterparts.
However, in some cases, the cost of the pump outweighs the benefits. For example, in a food processing factory, the need to quickly move large volumes of liquid is crucial to proper operation. In such cases, the need for fast pumping is a crucial factor, especially when mixing different materials. Lastly, although they use motor power to move fluids, submersible pumps can also be powered manually. If you have a manual pump but no access to electricity, you can opt to use centrifugal pumps.