Tap (valve) - Tap Mechanisms

Tap Mechanisms

The first screw-down tap mechanism was patented and manufactured by the Rotherham brass founders Guest and Chrimes in 1845. Most older taps use a soft rubber or neoprene 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, also corrosion) over time, so that eventually no tight seal is formed in the closed position, resulting in a leaking tap. 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 flow rate 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 90°—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. In UK English this is usually known as a taper-plug cock. 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.

Hands free infrared proximity sensors are replacing the standard valve. Thermostatically controlled electronic dual-purpose mixing or diverting valves are used within industrial applications to automatically provide liquids as required.

Foot controlled valves are installed within laboratory and healthcare/hospitals.

Modern taps often have aerators at the tip to help save water and reduce splashes. Without an aerator, water usually flows out of the tap in one big stream. An aerator spreads the water flow into many small droplets.

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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Famous quotes containing the word tap:

    A book is like a man—clever and dull, brave and cowardly, beautiful and ugly. For every flowering thought there will be a page like a wet and mangy mongrel, and for every looping flight a tap on the wing and a reminder that wax cannot hold the feathers firm too near the sun.
    John Steinbeck (1902–1968)