Hydraulic coverage and a water level below the minimum in the tank can trigger vortices, air entrainment, and a rapid decline in pump performance. Below is a practical example that helps illustrate “what happens” when the conditions required to prevent turbulence and air ingress are not met.
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Context: Storage tanks and the risk of air entrainment
In the context of storage tanks, the issue of hydraulic coverage is linked to a practical consideration: preventing the decrease in the free surface level from generating turbulence and local velocities that are so high as to draw air from the surface into the inlet pipe.
The goal is to maintain conditions that do not promote the formation of surface vortices and, consequently, the entry of air into the pipe that feeds the pump.
Hydraulic Overlap (UNI EN 12845:2020): What to Avoid
This refers to Chapter 9.3, “Storage Tanks,” of UNI EN 12845:2020, and specifically to Section 9.3.5 (referring to FIGURE 4 and TABLE 12). The approach is clear: to define conditions that help prevent air entrainment from the free surface, caused by vortices resulting from turbulence and high local velocities.
In other words: when there isn’t enough coverage, the vortices can “draw in” atmospheric air and carry it through the inlet pipe to the pump.
Case Study: Emptying a Tank Using an Overflow Inlet System
The example shown involves a storage tank being drained, with the pump operating in ABOVE-WATER inlet mode. The sequence illustrates how conditions change as the free-surface level varies relative to the suction pipe.
- Initial level: The loose hair can be identified (in the example) by a mark on the tube.
- Up to a certain level (FIGURE 1): no surface vortices are observed.
- As we go further down (FIGURE 2), vortices begin to appear around the tube.
- Even further down (FIGURE 3): the vortices are fully developed, with a massive inflow of air.
This example clearly illustrates the connection between a drop in water level (and thus reduced hydraulic coverage) and free-surface instability near the Inlet.
From Vortices to Air Entrainment in the Inlet Duct
When, as shown in FIGURE 2, vortices begin to form around the pipe, the increase in local velocity causes air to be drawn into the pipe. Once inside, that air reaches the pump’s inlet.
When the system reaches the condition shown in FIGURE 3, with fully developed vortices, the inlet can become massive. At this point, the inlet no longer carries only water under “clean” conditions, but rather a mixture containing a significant amount of air.
Cavitation and two-phase flow: distinct phenomena, but they can add up
In the case described, since the pipe is ABOVE WATER, cavitation may occur in addition to air entrainment. The process is as follows: the pressure inside the pipe drops below the vapor pressure of water, causing part of the liquid to evaporate at room temperature.
The following can therefore coexist within the pipeline:
- liquid water + gaseous water (formed due to excessive negative pressure: pressure < vapor pressure);
- air coming from the atmosphere, drawn in due to excessive inlet velocity caused by insufficient coverage.
This is an important point: they are two separate phenomena, even though they may occur together and both contribute to functional problems.
Performance Degradation: What Happens to the Q/H Curve
Air entrainment and cavitation can cause (even individually) a decline in pump performance. FIGURE 4 illustrates a possible trend in this example.
The result is critical for the system: it becomes impossible to achieve the Q/H point specified in the design calculations. In operational terms, this means that the pump may not be able to deliver the expected combination of flow rate and Head under design conditions.
Rapid deterioration: impeller, supports, and bearings under stress
If the conditions described persist, the result will be the rapid destruction of the machine. Specifically, the damage may affect:
- impeller and supports, due to the implosion of cavitation bubbles;
- bearings and their mounting locations, due to unbalanced axial forces that cause impeller displacement and abnormal stresses.
In summary: the lack of hydraulic coverage is not just a “hydraulic” issue, but can quickly turn into a mechanical reliability problem.
Prevention: What to Refer to in UNI EN 12845:2020
To prevent the problems described from arising, the text calls for compliance with UNI EN 12845:2020, specifically:
- Section 9.3: Storage Tanks, with a focus on Section 9.3.5, Figure 4, and Table 12, which provide guidelines for preventing the formation of surface vortices;
- Section 10.6 Inlet Conditions, subsection 10.6.2.1, which stipulates that NPSHd must be > r NPSHr + 1 at the maximum flow rate specified for the pump.
These references directly link the monitoring of hydraulic coverage (and vortices) to the verification of inlet conditions.
Conclusion
When the hydraulic head requirement is not met, a drop in the free surface level can lead to vortices, air entrainment, and (in the case of an over-head pump) even cavitation: the result can be a rapid decline in performance and damage to the pump, including deterioration of the impeller and bearings.
