Why do washbasins and bidets have a double drain hole? What is the overflow and how does it work

The bathroom fixture drainbut in reality also kitchen elements, can have two points that act as a path for moving the water away towards the sewer. One of these two is called in common jargon “overflow” and serves to prevent water from exceeding a safety threshold in order to avoid flooding. It is therefore a safety measure against the possible leakage of excess water from the accumulation space available to it. In fact, it could happen, perhaps accidentally, that you leave the main drain point closed and forget to turn off the water. In these cases, the overflow always ensures the removal of excess water and the conveyance of this water into the main drainage pipe.

What is the sink overflow for and how does it work?

In essence, an overflow drain is a connection path to the main draincharacterized by an opening located in the upper area of ​​the sanitary ware on which it is inserted. The passage of water from the overflow can occur in various ways. For example, in ceramic sanitary fixtures there is usually no actual connection pipe, rather a hollow part inside the sanitary fixture, connected to the main drain and equipped with a perforated end area for the entry of excess water. In other cases, however (for example aluminum kitchen sinks), we find a real pipe that connects to the main drain.

The size of these openings, and possibly the diameter of the pipe, they cannot be arbitrarybut rather they must be such as to guarantee the flow of water without causing a further rise in the water level in the accumulation area. Otherwise, there would be a progressive increase in the accumulated water which could nullify the hoped-for protection!

Where does it download? The unloading and loading points of the system

In general, each element of the plumbing system that you deal with has a so-called loading point and unloading point:

• The point of load it is connected to the local water network, from where the drinking water that serves the home is taken.
• The point of I unloadhowever, is connected to the sewer system, via a series of additional elements present within the plumbing system.

The discharge removes the used water in an organized manner up to the seweravoiding uncontrolled dispersion. A siphon is usually connected to each drain which prevents bad odors from rising from the sewer via a hydraulic plug. The system is therefore completed with these overflow safety points, which always guarantee an outflow of excess water.

What is phoronomy

Wanting to talk about it with a more technical meaning, the overflow it can be identified as a particular hydraulic device that is studied in detail in a branch of hydraulic engineering called phoronomy. Foronomy studies the motion of fluids through holes, in order to identify, in relation to the geometry and surrounding hydraulic conditions, the so-called flow rates necessary for the sizing of the drains. Technically, these openings are called lights and can be distinguished into:

• Lights a swing, when the water surrounding the light always covers all the available area;
• Lights a overflow, when only part of the light is actually in contact with the fluid to be removed.

An overflow that works well is identifiable as an overflow lightor simply I collapse. There are several of these, with different shapes and sizes, and they are also used inside artificial lakes, as a support for the discharges of a dam. There are also examples of light that can work both with swing and weir, such as the very famous surface discharge, called in glass (or also spillway), located in Lake Berryessa, California. In these cases, however, unlike the overflow found in bathroom fixtures, the project involves a series of calculations to optimize the dimensions of the drain to maximize the flow rates removed.

To find out more, here is a video ad hoc on the topic: