Capturing underground aquifer water and bringing it to the public through a tiny opening in the ground sounds like a job for a super hero! All hail the vertical turbine pump!
These pumps are specifically designed to handle vertical lift pumping applications as mentioned above. But, every hero has its weakness... the vertical turbine pump can lose its powers if its minimum submergence requirements are not met. This post walks you through what minimum submergence is and what can go wrong when it isn’t met, so that you can care for your system’s hero and successfully supply groundwater.
WHAT IS MINIMUM SUBMERGENCE?
The minimum submergence of a vertical turbine pump is the amount of liquid above the pump suction bell that is required to avoid vortex creation. These cyclonic, spinning movements in the water bring about two potential problems: Air entrapment and pre-rotation. If the vortex goes deep enough, a streamline of air may enter the pump and cause cavitation, leading to pump damage and poor performance. With pre-rotation, water enters the pump with rotational movement, either in the correct direction or opposing the pump’s rotation, causing a disturbance in the pump’s dynamic operation and changes in the system parameters. The probability of these issues arising decreases as the level of liquid above the pump inlet increases.
HOW MUCH SUBMERGENCE IS ENOUGH?
Your pump manufacturer will tell you how much submergence is required on the pump's performance curve or the outline drawing. The value they give you is based upon the bell geometry and flow rate. There's also a standard about the subject that provides guidance as well. The standard, ANSI/HI 9.8 Pump Intake Design, presents an empirical formula for the submergence that is based upon the bell diameter in inches (D) and flow rate in gpm (Q).
Submergence (in), S = D + 0.574 x Q / D1.5
Minimum Submergence from ANSI/HI 9.8 Pump Intake Design
While the minimum submergence’s specific purpose is to determine the fluid level that prohibits vortexing, the liquid level is also important in other design factors that need to be considered. To start, the liquid level is used to determine the net positive suction head (NPSH) that is available for the pump to use and for system success, the more, the better. On the contrary, if there's too much liquid above the pump, a siphoning effect can occur within the well where pressure and liquid weight will move the fluid on their own, potentially “assisting” the pump with fluid flow and reducing the system head. Finally, as the working fluid may also be responsible for equipment cooling, a different minimum level may be required for pump protection and should be considered alongside of the vortex-preventing minimum submergence.
Keeping all of these factors in mind will help keep your pump fit and strong, so that it can maintain its hero status in your system.