Why Are U-bends Used in Plumbing?
U-bend pipes are installed under sinks and toilets to provide a water seal against the outside sewer pipes. The U-bend retains a permanent pool of clean water, protects the fixture and prevents any gases, vermin or bacteria from escaping out of the sewers. Gravity holds water to act as a barrier, preventing the escape of sewer gases, while allowing waste to pass through. If a plumbing fixture is not used over a month or so, the water in the trap will evaporate, allowing noxious odors to escape.
The U-bend was invented in the middle of the 18th Century but, for economic reasons, was not widely used until more than 100 years later. Since its introduction the U-bend has been an important factor in preventing infectious illnesses and diseases in large cities. Once invented, in spite of being simple and reasonably reliable, widespread use in sewage was slow coming.
In Britain the plumbing needed to support the full use of traps was introduced only when the Thames, which was being used as an open sewer, forced the legislators to pass laws in the 1860s to install closed sewers so as to avoid the unbearable smell reaching the nearby Houses of Parliament. As of 2017, only around two thirds of the world population has access to traps, in spite of the evidence that good sewage systems significantly improve economic productivity in countries that employ them.
In refinery applications, traps are used to prevent hydrocarbons and other dangerous gases from escaping outside through drains. In domestic applications, traps are typically U, S, Q or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as an S-bend. It was invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880.
The new U-bend could not jam, so, unlike the S-bend, it did not need an overflow. The most common of these traps in houses is referred to as a P-trap. It is the addition of a 90 degree fitting on the outlet side of a U-bend, thereby creating a P-like shape. It is also referred to as a sink trap because it is installed under most house sinks. Because of its shape, the trap retains a small amount of water after the fixture’s use.
This water in the trap creates a seal that prevents sewer gas from passing from the drain pipes back into the occupied space of the building. Essentially all plumbing fixtures including sinks, bathtubs, and toilets must be equipped with either an internal or external trap. Because it is a localized low-point in the plumbing, sink traps also tend to capture heavy objects (such as jewelry) that are inadvertently dropped into the sink.
Traps also tend to collect hair, sand, and other debris and limit the ultimate size of objects that will pass on into the rest of the plumbing, thereby catching oversized objects. For all of these reasons, most traps can either be disassembled for cleaning or they provide some sort of cleanout feature. When a large volume of water may be discharged through the trap, a standpipe may be required to prevent impact to other nearby traps.