VTP Submergence is the minimum immersion depth required for a Vertical Turbine Pump (VTP) to operate correctly, preventing undesirable phenomena in the inlet. In this answer, I’ll explain what it means, why it’s required, and how it affects the effective volume of the tank.
Table of Contents
What Is VTP Convergence?
The submergence (denoted as S) of Vertical Turbine Pumps (VTP) is the minimum depth to which the pump must be submerged to prevent the formation of vortices at the free surface of the tank.
As part of the requirements, it is specified that the minimum submersion depth must be indicated for submersible pumps. In other words, this is not “optional” information, but a parameter that must be explicitly stated.
Why VTP Convergence Is Important: Vortices and Inlet Air
The primary objective of VTP backflow prevention is to prevent air from being drawn into the inlet or, in the case of VTPs, directly into the pump.
The problem stems from the vortexes that can form on the surface of the storage tank. When vortices form on the free surface, they can create conditions that allow air to enter the Inlet—a phenomenon that designers explicitly seek to prevent.
The causes of vortices: what generates them, according to the text
Vortices on the free edge (which we want to avoid) are attributed to two main causes:
- insufficient water depth at the starting end of the pipe (i.e., not enough water “cover” over the inlet);
- speeding.
The text also highlights a very clear practical implication: if the speed limits specified by the rules are observed, then the primary cause of air ingress is an insufficient hydraulic head.
This logic directly links the VTP convergence to a “minimum level” requirement above the inlet.
What Causes VTP Sommergenza?
The VTP convergence is not a fixed value: it varies depending on:
- pump size;
- rotational speed.
This point is essential because it avoids a common mistake in technical documentation: treating submergence as a “standard” depth applicable to every situation. In the text, however, it is presented as a value that must be correlated with the machine’s characteristics and operating conditions.
Practical Note: VTP Overlap and Diameter D
As a general guideline, VTP Convergence (S) is linked to a simple rule:
- S is approximately two and a half times the diameter (D) of the hydraulic component.
This guideline is useful as a general reference, but the text does not treat it as an “automatic” value that must always be applied: in fact, it is immediately clarified that the submergence must still be commensurate with the conditions required of the pump (see the connection to NPSH in the following paragraph).
Link to the required NPSH (technical consistency of the parameter)
In addition to the guideline regarding diameter, the text emphasizes a requirement for design consistency:
- The VTP head must be commensurate with the NPSH requirements of the pump.
This statement should be interpreted in a very practical sense: even if an indicative value (such as the D ratio) is used, the final choice of submergence depth must be compatible with the pump’s requirements for operation under the expected conditions.
In practice, the VTP Convergence serves as a bridge between:
- the physical phenomenon we want to avoid (vortices and air),
- and the conditions required for the pump (required NPSH).
Impact on the usable volume of the tank (UNI EN 12845, 9.3.5)
From a design perspective, the text identifies a key criterion: to properly design the water inlet for VTPs, a minimum hydraulic head equal to the submergence depth S must be maintained above the inlet.
This has a direct and significant impact on technical documentation as well:
- Below the level corresponding to submergence S, the volume stored in the tank cannot be used for the purpose of calculating the usable volume.
Therefore, the VTP clearance is not merely a “mechanical” or structural dimension: it affects how the actually usable volume is calculated.
How to Use VTP Overlap in Design (Without Forcing It)
To remain consistent with the above, VTP convergence should be treated as a requirement by:
- specify for submersible pumps (minimum submersion depth);
- set to prevent vortices on the free edge and air entrainment;
- evaluate based on the pump size and rotational speed;
- verify that it is compatible with the required NPSH;
- Apply as a minimum layer over the inlet;
- Include this in the calculation of the usable volume, excluding anything below that level.
If you’d like, I can help you turn these points into an internal verification checklist (specifications/report), but I’ll need you to confirm the missing information listed at the bottom.
Conclusion
The VTP Submergence is the minimum submergence depth required to prevent vortices at the free surface and air ingress into the inlet; it depends on the pump’s characteristics and must, in any case, be consistent withthe required NPSH. Furthermore, the entire volume below the submergence level cannot be counted as usable tank volume.
