Trap (plumbing)

In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases and chemical fumes from escaping through drains.

It is the addition of a 90 degree fitting on the outlet side of a U-bend, thereby creating a P-like shape (oriented horizontally).

This water creates an air seal that prevents sewer gas from passing from the drain pipes back into the building.

Essentially all plumbing fixtures including sinks, bathtubs, and showers must be equipped with either an internal or external trap.

In Britain, the requirement to use traps was introduced only after the Great Stink in London, in the summer of 1858, when the objectionable smell of the River Thames, which was effectively an open sewer, affected the nearby Houses of Parliament.

As of 2017[update], only about two-thirds of the world population have access to traps,[citation needed] in spite of the evidence that good sewage systems significantly improve economic productivity in places that employ them.

In the United States, plumbing codes usually provide strict limitations on how far a trap may be located from the nearest vent stack.

In some regions of the US, "S" traps are no longer accepted by the building codes as unvented S-traps tend to siphon dry.