Tap (valve)
From Wikipedia, the free encyclopedia
A tap is a valve for controlling the release of a liquid or gas. In the British Isles and normally in the Commonwealth the word is used for any everyday type of valve, particularly the fittings that control water supply to bathtubs and sinks. In the U.S. the usage is sometimes more specialised, with the term "tap" restricted to uses such as beer taps and the word faucet being used for water outlets; however some Americans use "tap" in the broader sense as well.
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[edit] Water taps
The physical characteristic which differentiates a spigot from other valves is the lack of any type of a mechanical thread or fastener on the outlet.
Water for baths, sinks and basins can be provided by separate hot and cold taps; this arrangement is common in the UK, particularly in bathrooms/lavatories. In kitchens, in the U.S., and in many other places, mixer taps are often used instead. In this case, hot and cold water from the two valves is mixed together before reaching the outlet, allowing the water to emerge at any temperature between that of the hot and cold water supplies.
For baths and showers, mixer taps frequently incorporate some sort of pressure balancing feature so that the hot/cold mixture ratio will not be affected by transient changes in the pressure of one or the other of the supplies. This helps avoid scalding or uncomfortable chilling as other water loads occur (such as the flushing of a toilet).
Rather than two separate valves, mixer taps frequently use 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. Especially for baths and showers, the latest designs do this using a built in thermostat. These are known as thermostatic mixing valves, or TMVs, and can be mechanical or electronic.
Mixer taps are more difficult to fit in the UK than in other countries because traditional British plumbing provides hot and cold water at different pressures.
If separate taps are fitted, it may not be immediately clear which tap is hot and which is cold. The hot tap generally has a red indicator while the cold tap generally has a blue or green indicator. In English-speaking countries, the taps are frequently also labeled with an "H" or "C". Mixer taps may have a red-blue stripe or arrows indicating which side will give hot and which cold.
In some countries there is a 'standard' arrangement of hot/cold taps: for example in the United States and Canada, the hot tap is on the left by building code requirements. This convention applies in the UK too, but many installations exist where it has been ignored. Mis-assembly of some single-valve mixer taps will exchange hot and cold even if the fixture has been plumbed correctly. With Moen faucets, this can be remedied as simply as removing the handle and rotating the valve stem 180 degrees.
Most handles on Residential homes are connected to the valve shaft and fastened down with a screw. Although on most commercial and industrial applications they are fitted with a removable key called a "loose key" or "Water key" which has a square peg and a square ended key to turn off and on the water. You can also take off the "Loose key" to prevent vandals from turning on the water. In older building before the "Loose key" was invented for some landlords or caretakers to take off the handle of a residential tap, which had teeth that would meet up with the cogs on the valve shaft. This Teeth and cog system is still used on most modern faucets. Although most of the time a "Loose key" is on industrial and commercial applications sometimes you may see a "Loose key" on homes by the seashore to prevent guests from washing the sand off their feet. Some “Loose keys” are done in decorative patterns for residential use.
[edit] Beer taps
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 may be because the word was originally coined for the wooden valve in traditional barrels. A "beer tap" now may be one of several items:
- Pressure-dispense bar tap
- Almost universally in modern times, bulk beer is supplied in kegs that are served with the aid of external pressure. In a normal bar dispense system, this pressure comes from a cylinder of carbon dioxide (or occasionally nitrogen) which forces the beer out of the keg and up a narrow tube to the bar. At the end of this tube is a valve built into a fixture (usually somewhat decorative) on the bar. This is the beer tap, and opening it with a small lever causes beer, pushed by the gas from the cylinder, to flow into the glass.
- Portable keg tap
- Sometimes, beer kegs designed to be connected to the above system are instead used on their own, perhaps at a party or outdoor event. In this case, a self-contained portable tap is required that allows beer to be served straight from the keg. Because the keg system uses pressure to force the beer up and out of the keg, these taps must have a means of supplying it. The typical "picnic tap" uses a hand pump to push air into the keg; this will cause the beer to spoil faster but is perfectly acceptable when it will be consumed in a short time. Portable taps with small CO2 cylinders are also available.
- Cask beer tap
- Beers brewed and served in the traditional way (typically cask ale) do not use artificial gas. Taps for cask beer are simple on-off valves that are hammered into the end of the cask (see keystone for details). When beer is served directly from the cask ("by gravity"), as at beer festivals and some pubs, it simply flows out of the tap and into the glass. When the cask is stored in the cellar and served from the bar, as in most pubs, the beer line is screwed onto the tap and the beer is sucked through it by a hand-operated low-pressure pump on the bar. The taps used are the same, and in beer-line setups the first pint is often poured from the cask as for "gravity", for tasting, before the line is connected. Cask beer taps can be brass (now discouraged for fear of lead contamination), stainless steel (good, but expensive), plastic (acceptable, and cheaper), and wood (to be avoided if possible).
[edit] Gas taps
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 fuel gas (natural gas or, historically, coal gas, syngas, etc.) in the home (for gas fires or other appliances) or in laboratories (for Bunsen burners).
[edit] Physics of taps
Most water and gas taps have adjustable flow. 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. 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. 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. Bubbles of cool water vapor form and collapse at the restriction, causing the familiar hissing sound. 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.
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.
[edit] Tap mechanisms
Most older taps use a soft rubber washer which is screwed down onto a valve seat in order to stop the flow. This is called a "globe valve" in engineering and, while it gives a leak-proof seal and good fine adjustment of flow, both the rubber washer and the valve seat are subject to wear (and for the seat, corrosion) over time, leading to leakage (see photo). The washer can be replaced and the valve seat resurfaced (at least a few times), but globe valves are never maintenance-free.
Also, the tortuous S-shaped path the water is forced to follow offers a significant obstruction to the flow. 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.
Gate valves use a metal disc the same diameter as the pipe which is screwed into place perpendicularly to the flow, cutting it off. There is no resistance to flow when the tap is fully open, but this type of tap rarely gives a perfect seal when closed. In the UK this type of tap normally has a wheel-shaped handle rather than a crutch or capstan handle.
Cone valves or ball valves are another alternative. These are commonly-found as the service shut-off valves in more-expensive water systems and usually found in gas taps (and, incidentally, the cask beer taps referred to above). They can be identified by their range of motion -- 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. A cone valve consists of a shallowly-tapering cone in a tight-fitting socket placed across the flow of the fluid. A ball valve uses a spherical ball instead. In either case, a hole through the cone or ball 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. 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. The ball in ball valves rotates within plastic seats.
Modern bathroom and kitchen taps often use ceramic or plastic surfaces sliding against other spring-loaded ceramic surfaces or plastic washers. These tend to require far less maintenance than traditional globe valves and when maintenance is required, the entire interior of the valve is usually replaced, often as a single pre-assembled cartridge.
Of the trio of well-respected faucet manufacturers in North American plumbing circles, Moen and American Standard use cartridges (Moen's being O-ring based, American Standard's being ceramic), while Delta uses easily-replaced rubber seats facing the cartridge(s). Each design has its advantages: Moen cartridges tend to be easiest to find, American Standard cartridges have nearly infinite lifespan in sediment-free municipal water, and Delta's rubber seats tend to be most forgiving of sediment in well water.
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