The pump suction intake must be submerged sufficiently to avoid the formation of vortexes on the liquid surface of the suction tank. These vortexes can take many shapes and forms
The formation of vortexes between the pump suction intake and the suction tank fluid surface causes air to enter the pump suction. This mixture of air and water in the pump reduces the pump capacity. The formation of such vortexes must therefore be avoided. There is a relationship between the intake velocity at the suction intake, and the submergence (S) of the intake.
There are many possible intake design geometries, a few are shown in the next Figure; they all have in common a minimum requirement for submergence to avoid the formation of vortexes.
Here is a picture of a submersion problem, the pump is an axial flow pump whose intake is insufficiently submerged causing a vortex to form and air to be entrained into the pump suction.
The Hydraulic Institute publishes a guide on Pump Intake Design that provides detail recommendations.
The Goulds pump company has an excellent technical section in their catalogue, you can view the table of contents here, it provides recommendations on pump intake design and baffles that can eliminate vortex formation.
I have made a video showing vortex formation in a miniature pump system.
Applets are programs based on the java language that are designed to run on your computer using the Java Run Time environment.
The minimum submersion of a pump inlet is based on tests that were done by the Hydraulic Institute www. pumps.org and this help page gives a brief description of the method devised to calculate it. You can find more information in The Hydraulic Institute's Pump Intake Design ANSI/HI 9.8-1998 standard.
This applet is designed to help you find the minimum submersion required to avoid the formation of a vortex in the pump suction tank and therfore air entrainment in the pump. Tests by the Hydraulic Institute have shown that the relationship between the minimum submergence S , the diameter D and the flow rate q are:
The velocity v is given by:
The velocity is limited to between 2 and 8 ft/s. The recommended velocity rate is 5.5 ft/s. The flow rates are limited to between 25 USgpm and 300000 USgpm.
By clicking on the - Other Configurations - button, you can see various suction configurations where this submergence criteria will also apply.