Tap (valve)

Valves of this type using a cylinder rather than a cone are sometimes encountered, but using a cone allows a tight fit to be made even with moderate manufacturing tolerances. Zabuton are often used for sitting on tatami floors. A hole through the cone allows the fluid to pass if it is lined up with the openings in the socket through which the fluid enters and leaves; turning the cone using the handle rotates the passage away, presenting the fluid with the unbroken surface of the cone through which it cannot pass. In Japanese, a zabuton (za, sitting + futon) is a cushion for sitting on. A cone valve consists of a shallowly-tapering cone in a tight-fitting socket placed across the flow of the fluid. Quite often these futons are marketed as "earth friendly," especially since they do not contain the chemicals with which some conventional bedding is manufactured. They can be identified by their range of only 90º between fully-on and fully-off - usually when the handle is in line with the pipe the valve is on, and when the handle is across the pipe it is closed. Additionally, these alternative futons tend to be hand-made.

Cone valves are usually found in gas taps (and, incidently, the cask beer taps referred to above). While still not traditional futons, they lack the springs and synthetic casings of traditional mattresses and tend to be constructed primarily out of cotton. In the UK this type of tap normally has a wheel-shaped handle rather than a crutch or capstan handle. There is, however, a growing market in Japan for high quality bedding made in the Western style. There is no resistance to flow when the tap is fully open, but this type of tap rarely gives a perfect seal when closed. Most Japanese people would not recognize a Western-style "futon" as a futon. Gate valves use a metal disc the same diameter as the pipe which is screwed into place perpendicularly to the flow, cutting it off. Western-style futons are a cheap alternative to a bed or other furniture, and are often sold in sets that include the mattress and frame.

For high pressure domestic water systems this does not matter, but for low pressure systems where flowrate is important, such as a shower fed by a storage tank, a "stop tap" or, in engineering terms, a "gate valve" is preferred. They are usually placed on a configurable frame for dual use as a bed and a couch, but are not intended to be stored away during the day. This is called a "globe valve" in engineering and, while it gives a leak-proof seal and good fine adjustment of flow, the tortuous S-shaped path the water is forced to follow offers a significant obstruction to the flow. They are usually filled with foam as well as batting, often in several layers, and are almost always much thicker and larger than Japanese futons, resembling a traditional mattress in size. Most taps use a soft washer which is screwed down onto a seat in order to stop the flow. Western futons are different from Japanese futons in several ways. One reason that most beer taps are not designed for adjustable flow is that the beer itself is damaged by the pressure drop in a choked-flow valve: holding a beer tap partially open causes the beer to foam vigorously, ruining the pour. In addition, many Japanese people beat their futons regularly using a special tool, traditionally made from bamboo, resembling a Western carpet beater.

At very low flow settings, the viscosity of the water becomes important and the pressure drop (and hissing noise) vanish; at full flow settings, parasitic drag in the pipes becomes important and the water again becomes quiet. Futons must be aired in sunlight regularly, especially if not put away during the day. Bubbles of cool water vapor form and collapse at the restriction, causing the familiar hissing sound. Futons are designed to be placed on tatami flooring, and are traditionally folded away and stored in a closet during the day to allow the tatami to breathe and to allow for flexibility in the use of the room. At intermediate flow settings the pressure at the valve restriction drops nearly to zero from the venturi effect; in water taps, this causes the water to boil momentarily at room temperature as it passes through the restriction. Such sets can be purchased for under 10,000 yen. The choked flow rate is independent of the viscosity or temperature of the fluid or gas in the pipe, and depends only weakly on the supply pressure, so that flow rate is stable at a given setting. They are often sold in sets which include the futon mattress (shikibuton), a comforter (kakebuton) or blanket (mōfu), a summer blanket resembling a large towel (towelket), and pillow (makura), generally filled with beans, buckwheat chaff or plastic beads.

Turning the knob or working the lever sets the flow rate by adjusting the size of an opening in the valve assembly, giving rise to choked flow through the narrow opening in the valve. Japanese futons are flat, about 5 cm (2 in) thick with a fabric exterior stuffed with cotton or synthetic batting. Most water and gas taps have adjustable flow. They are sold in Japan at specialty stores called futon-ya, and are also available at department stores. Although a gas tap may be a valve that releases any gas, the word is most commonly used to refer to taps that control the flow of natural gas in the home (for gas fires) or in school science laboratories (for Bunsen burners). A futon (布団) is a type of mattress that makes up a Japanese bed. A "beer tap" now may be one of several items:.

This may be because the word was originally coined for the wooden valve in traditional barrels. While in other contexts, depending on location, a "tap" may be a "faucet", "valve" or "spigot", the use of "tap" for beer is almost universal. This convention applies in the UK too, but many installations exist where it has been ignored. In some countries there is a 'standard' arrangement of hot/cold taps: for example in the United States the hot tap is generally on the left.

Mixer taps may have a red-blue stripe or arrows indicating which side will give hot and which cold. The cold tap generally has a blue or green indicator and/or is labeled C or Cold. In English-speaking countries, the hot tap generally has a red indicator and/or is labeled H or Hot. If separate taps are fitted, it may not be immediately clear which tap is hot and which is cold.

Latest designs do this using a built in thermostat. This is a single, more complex, valve whose handle moves up and down to control the amount of water flow and from side to side to control the temperature of the water (achieved by mixing the hot and cold water together). In kitchens, and in the US and many other places, mixer taps are often used instead. Water for baths, sinks and basins can be provided by separate hot and cold taps; this arrangement is common in the UK, particularly in toilets.

. In American English the usage is sometimes more specialised, with the term tap restricted to uses such as beer taps and the word "faucet" used for water outlets; although some Americans use "tap" in the broader sense as well. For example, in Commonwealth English the word is used for any everyday type of valve, particularly the fittings on bathtubs and sinks. A tap is a valve for controlling the release of a liquid or gas.

In an analogy to controlling the flow of a fluid, tap can also refer to drawing electricity from a certain winding in an electrical transformer. To tap a vessel containing liquid metal is to remove the liquid from the vessel, even if no valve as such is utilised in the process.