Sub-suction and Over-suction These are two inlet conditions that are distinguished by the position of the pump relative to the free surface of the water in the tank. Below is a practical explanation, with references to the relevant points and the main operational implications.
Table of Contents
Regulatory references cited in the response
The applicable standard cited is UNI EN 12845:2020, specifically:
- Section 10.6.1 (inlet conditions);
- Section 10.6.2, with a focus on:
- 10.6.2.2 (sub-section);
- 10.6.2.3 (overhead panel).
Reference is also made to Chapter 8 of UNI/TR 11438:2016, which addresses the same topic.
How to Identify the Bottom and Top Beams (Reference Plane)
To distinguish between the two conditions, we start with a “basic” configuration in which the reference plane is the horizontal plane passing through the center of the pump’s inlet.
From here, the hydraulic conditions BELOW and ABOVE the weir can be directly determined by observing the relationship between:
- pump section (inlet/shaft, depending on the diagram in question);
- free water surface level in the reservoir (including the minimum level).
Sub-case: Why is it considered an “intrinsically safe” condition?
The submersed condition is described as inherently safe because the pump is always located below the free surface of the water. During inlet, this results in a favorable energy condition due to:
- atmospheric pressure;
- severity.
The text also states that NPSHd values are typically always > 10.33 m.w.c. In practice, the inlet system is more “stable” and less prone to problems typically associated with unfavorable conditions.
Overhead doors: factors that make their use inadvisable
The ” soprabattente ” condition is presented as a solution linked to several factors that strongly discourage its use.
The starting point is that, in the inlet, the pump relies solely on the energy derived from atmospheric pressure. In the text , this is reflected in NPSHd values that are typically well below 10.33 m.w.c.
Furthermore, the design calls for the inlet line to be maintained at a pressure lower than atmospheric pressure to allow water to flow upward—a condition that makes certain construction and maintenance aspects more sensitive.
Typical operational risks associated with the upper tank (air, boiling, performance)
In the supply line, a vacuum in the pipe means that any connection (such as a flanged or threaded connection) can become a potential source of air ingress, because:
- The pressure inside the pipe is significantly lower than the external pressure.
If the pressure then reaches the vapor pressure, water may boil at room temperature, resulting in the formation of a two-phase fluid. According to reports, when this fluid enters the pump:
- would prevent us from providing the expected services;
- This would result in damage to the pump.
In summary, the overflow increases the likelihood of conditions that could cause the system to deviate from its design Q/H.
Bottom Valve: When It’s Needed and Why It Becomes “Indispensable”
The text highlights a practical point: the inlet line can remain full only if it is equipped with a check valve.
If this component does not function properly (for example, due to a leak), the pump and the inlet line may be drained down to the free surface level. The resulting consequence is clear: the pump will be unable to deliver the design flow rate (Q/H).
For this reason, it is stated that in the case of the SOPRABATTENTE, the bottom valve is:
- not only necessary, but also essential.
Required limits and conditions (preference for the sub-batten)
The response states that the standard recommends using the “under-sash” configuration, subject to the following limitations:
- At least two-thirds of the tank’s capacity must be above the pump’s axis.
- The minimum level cannot be more than 2 meters below the pump shaft.
- If the minimum level is below the pump shaft, a foot valve must be installed.
Finally, it is noted that, in the case of the SOPRABATTENTE, the maximum distance between the pump axis and the free surface is 3.2 m.
Conclusion
“Under-inlet” and “over-inlet ” are not just definitions: they describe two inlet conditions with concrete consequences for reliability and performance. In the text, the under-pressure condition is presented as the preferred and safer solution, while the over-pressure condition is associated with issues related to negative pressure, air ingress, possible two-phase flow, and the need for a foot valve.
