Trap (plumbing)

In plumbing, a trap is a U-shaped portion of pipe designed to trap liquid or gas to prevent unwanted flow; most notably sewer gases from entering buildings while allowing waste materials to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases and chemical fumes from escaping through drains. In heating systems, the same feature is used to prevent thermo-siphoning which would allow heat to escape to locations where it is not wanted.

Similarly, some pressure gauges are connected to systems using U bends to maintain a local gas while the system uses liquid.
For decorative effect, they can be disguised as complete loops of pipe, creating more than one U for added efficacy.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0