Trap (plumbing)

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Water seal in drain pipe under a sink. Water enters at right, fills the trap, and continues left. Inverted siphoning occurs below the line "A".
Examples of traps

In plumbing, a trap is a device shaped with a bending pipe path to retain fluid to prevent sewer gases from entering buildings while allowing waste to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases from escaping 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 U-bend could not jam, so, unlike the S-bend, it did not need an overflow. The most common of these traps 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 (oriented horizontally). It is also referred to as a sink trap because it is installed under most sinks.

Because of its shape, the trap retains some water after the fixture's use. 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. Toilets almost always have an internal trap.

Because it is a localized low-point in the plumbing, sink traps also tend to capture dense objects (such as jewelry) inadvertently dropped into the sink. Traps also tend to collect hair, sand, and other debris and limit the size of objects that enter the plumbing system, thereby catching oversized objects. For all of these reasons, most traps may be disassembled for cleaning or provide a cleanout feature.

Where a volume of water may be rapidly discharged through the trap, a standpipe may be required to minimize impact to other nearby traps.

History[edit]

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.

Once invented, despite being simple and reasonably reliable, widespread use was slow coming. In Britain, the plumbing needed to support the full use of traps was introduced only when the Thames River, which was being used as an open sewer, forced legislators to pass laws in the 1860s to install closed sewers so as to avoid the objectionable smell reaching the nearby Houses of Parliament. As of 2017, 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 countries that employ them.[1]

Venting and auxiliary devices[edit]

Trap with copper drain pipe at underside of firestop packing in 2 hour fire-resistance rated concrete floor slab
Typical P-trap

Maintaining the water seal is critical to trap operation; traps might dry out, and poor venting can suction or blow water out of the traps. This is usually avoided by venting the drain pipes downstream of the trap; by being vented to the atmosphere outside the building, the drain lines never operate at a pressure much higher or lower than atmospheric pressure. Plumbing codes usually provide strict limitations on how far a trap may be located from the nearest vent stack.

When a vent cannot be provided, codes may allow the use of an air admittance valve instead. These devices avoid negative pressure in the drain pipe by venting room air into the drain pipe (behind the trap). A "Chicago Loop" is another alternative.

When a trap is installed on a fixture that is not routinely used—such as a floor drain—the eventual evaporation of the water in the trap must be considered. In these cases, a trap primer may be installed; these are devices that automatically recharge traps with water to maintain their water seals.

Accepted traps[edit]

In some regions of the United States, "S" traps are no longer accepted by the plumbing codes as these traps tend to siphon dry even when well vented. It may be possible to determine whether a household uses an S- or U-bend by the presence of an overflow pipe outlet.[clarification needed] What is required instead is a P-trap with proper venting. Certain drum-styled traps are also discouraged or banned.[2]

See also[edit]

Notes[edit]

  1. ^ "50 Things That Made the Modern Economy: S-Bend". BBC. Retrieved 2018-11-02.
  2. ^ Saltzman, Reuben (April 18, 2013). "How Bad Are Drum Traps?". The Home Inspector (blog). Star Tribune. Minneapolis. Archived from the original on July 15, 2015. Retrieved December 28, 2016. Includes several photographs of various types of drum traps.

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