Monday, August 5, 2019

MINIMUM SUBMERGENCE REQUIREMENT IN PUMPS

Minimum submergence, as the term suggests, is the minimum depth of pump inlet bell or minimum distance to water surface required from the inlet bell of a pump. This is very important to prevent the formation of vortices during operation.
If the submergence is reduced, initially water swirl is formed. Gradually, as the level comes down, surface vortices are formed. Again as the level decreases, strong vortex with a dye core is formed, which induces turbulence at the suction. Lastly, vortex with a hollow/air core is formed sucking air inside. This might lead to the loss of efficiency of the pump and cavitation due to the presence of air.
Pumps are made to handle liquids only. Any air entrapment will reduce pumping efficiency. If the volume of air is large, the pump stops pumping. To remove air from the pump casing and suction line, generally, we do priming before starting a pump.

The term is of greater importance while designing a pump intake as per Hydraulic Institute standards. In case of designing a river or large water body intake, the minimum water level recorded at the site during the driest seasons (sometimes, forecasting of annual decrease in water level will help) to be considered to calculate minimum submergence.

Minimum submergence S is given as,

S/D = 1 + 2.3 Fd

S = Minimum Submergence
D = Inlet Bell Diameter
Fd = Froude's number = V/sqrt(gD)
V = Velocity of flow at suction (can be determined from flow rate, Q of the pump and inlet bell diameter, Q=AV)
g = Acceleration due to gravity = 9.81 m/s2

Effects on Pump
  1. Vortex formation induces turbulence flow through the pump, impacting the pump bearings.
  2. Air entrapment leads to cavitation, reduction in pumping efficiency and sometimes loss of prime.

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