Our expert responds #11

QUESTION
What is meant by Vertical Turbine Pumps (VTP) submergence?

ANSWER
In point 4.4.4.4 g) of UNI EN 12845, it is required that the value of the minimum immersion height (submergence) is indicated for submerged type pumps.
In the case of VTPs, the submergence (S) is the minimum depth of immersion of the pump to avoid the formation of vortices on the open water of the tank; it varies according to the size of the pump and the rotation speed. As an indication, it corresponds to two and a half times the diameter (D) of the hydraulic part but must, in any case, be commensurate with the NPSH conditions required by the pump.
In the next figure, we highlight the phenomenon, whose manifestation we want to avoid.

First of all, we want to avoid dragging air into the suction pipe (or in the case of VTPs, directly inside the pump) due to the vortexing movements that are generated on the surface of the storage tank. These vortices are caused by an insufficient overlap of the head of the initial end of the pipe, and/or by an excessive speed in the pipeline. Having taken care to respect the speed values imposed by the standard, only the insufficiency of the hydraulic head remains as a possible cause of the intake of air.

In order to correctly design, from a technical point of view and in compliance with point 9.3.5 of UNI EN 12845, the water intake in the VTPs, it is therefore necessary to keep a minimum hydraulic head equal to the submergence S above the suction mouth. Below this level, the volume stored in the tank cannot be used for the purpose of calculating the useful volume.