Vertical submersible multistage electric pumps: This is the recommended configuration when you want to reduce the risk of air in the inlet and malfunctions. Let’s look at why, step by step, using a practical approach.*
Table of Contents
The two possible configurations: under-sash and over-sash
From a hydraulic standpoint, there are two possible configurations for operating a pump. The main difference is whether or not the hydraulic potential energy of the water in the inlet tank can be utilized.
In practice, the Zo ratio can be:
- Positive: SOTTOBATTENTE condition
- negative: OVERLAPPING condition
This distinction affects how the pump appears before startup and how the inlet line behaves.
Potential Energy and Zo Quota: What Really Changes
The hydraulic potential energy of the water in the suction tank increases as Zo increases. It’s a simple concept: the more “head” is available, the more the pump can benefit from the inlet conditions.
When Zo is positive (below the waterline), the system’s design helps keep the machine ready to start. When Zo is negative (above the waterline), the pump must already “work” to draw water into its inlet.
This difference is at the root of the typical problems associated with the upper beam.
Under the door: pump primed and ready to go
In the case of the SOTTOBATTENTE system, the inlet line and the machine are always full of water. Consequently:
- The pump is primed.
- The pump is ready to start at any moment
This configuration reduces the likelihood of air pockets forming in the inlet and helps prevent unstable conditions during startup.
Overflow: Why the Bottom Valve Is Needed
In the case of an OVERFLOW system, the machine remains full only if there is a foot valve (a check valve located at the lower end of the Inlet pipe).
This valve allows you to fill the system before the pump starts:
- the inlet line
- the pump itself
Without this condition, priming is not guaranteed, and startup may become problematic.
Vacuum in the inlet and air intake from the atmosphere
In an OVERHEAD configuration, the pump must overcome the force of gravity to draw water toward its inlet flange. To do so, it creates a vacuum in the inlet line, with an internal pressure lower than atmospheric pressure (P < Po).
This situation poses a specific risk: air may be drawn in from the atmosphere through the existing joints in the pipeline and between the valves.
Once air enters, it does not “disappear”; it tends to accumulate and create undesirable operating conditions.
Air Buildup in the System: Precautions for the Inlet Line
Air tends to accumulate at the highest points in the system. For this reason, it is important to ensure that inlet lines always run downward toward the pump, so that there are no high points upstream of the pump.
The goal is simple: to prevent areas from forming where air can become stagnant, because these pockets can lead to malfunctions.
In summary, in overhead installations, the geometry of the inlet piping becomes a critical factor that must be carefully managed.
Vertical, sub-flange, multistage electric pumps: the “high point” inside
In vertical multistage surface pumps, the design is such that a high point is created inside them. Specifically, air can accumulate right at the seal chamber (TM).
This is a key point for understanding the preference for the under-sash design: even if the inlet duct is properly maintained, the machine may have an internal area prone to air buildup.
When air ends up where it shouldn’t, the effects can be immediate and harmful.
What Causes Air to Enter: Mechanical Seals, Performance, and Air Bleeding
The presence of air prevents the mechanical seal from being lubricated, causing it to fail quickly.
Furthermore, the amount of air can become so great that:
- reduce the pump’s performance
- lead to a loss of interest
De-biting can result not only in a complete loss of Q/H output, but also in the mechanical destruction of the rotating parts.
It is therefore a risk not only to performance but also to the integrity of the machine.
Conclusion: When Is It Advisable to Use These Pumps?
For the reasons outlined above, it is advisable to use vertical multistage pumps (and, consequently, the pressurization units that use them) only when a hydraulically UNDERPRESSURE system is planned.
In other words: if the system is set up to keep the inlet line and pump filled with water, this reduces the likelihood of air accumulation and the resulting consequences.
If you’re considering the right configuration for your system, checking the plumbing layout (under-sill or over-sill) is a crucial first step.
