An important difference between a submersible pump and a surface pump is the depth with which this device can lift water. Not only large industrial, but even some household well models can cope with a depth of 200 meters. This is because their direct function is simply to push the water away from oneself, creating pressure.
Moreover, the magnitude of this outlet pressure directly depends on engine power and the number of impellers. It is easy to imagine a design in which the impellers act as series-connected pumps.
However, as with any technique, the operation of a submersible pump has several limitations. First of all, this is the pressure that the device experiences on itself when immersed to a depth. The limit is always indicated by the manufacturer in the technical passport. Failure to comply with these standards can lead to damage to the unit, as individual elements, seals, cable connections may simply not withstand external pressure when the pump is submerged below the permissible level.
The second limitation, especially for sufficiently powerful submersible pumps, can be the phenomenon of cavitation. The discharge of water occurs already on the first impeller and is amplified from the wheel to the wheel so that the pressure can reach 10.2 atmospheres at the outlet. To cope with this problem, it is necessary to install a pump taking into account the so-called water backwater of 1-3 m. This will allow to avoid “boiling” of water on the impellers.
Compliance with NPSH is also important for the proper operation of a well pump. This is nothing more than a characteristic of the permissible minimum depth for pump immersion. How this norm works in practice, it will become clear only after differentiating the definitions of the water level in the well. It can be dynamic and static. Static - the level that water occupies in the well at rest, dynamic - this is the limit for water below which it does not fall. As a rule, well pumps should be loaded 1-2 m below the dynamic mark. Firstly, this will ensure the value of NPSH, and secondly, even with maximum pumping, the pump will definitely remain under water. You can find the NPSH value for your pump in specialized catalogs.
It is also not recommended to lower the pump below the specified norms, that is, to the very bottom of the well, and here's why: water intake only from constantly replenished lower layers can lead to stagnation and flowering of the upper layer of water in the well. The risk of stale water entering the pump is very high, and even 5 ml will be enough to spoil a ton of good water. By loading the pump in compliance with all technical recommendations, you will ensure a regular exchange of water masses, and getting only fresh water from the well.