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Difference between revisions of "Needle Valves"
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[[File:Needle valve.jpg|thumb|right|Needle Valves]] | |||
[[File:Needle Valve1.jpg|thumb|right|Needle Valves]] | |||
'''Needle Valves''' are [[Valves]] having a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates. | |||
==Construction and operation== | |||
A needle valve has a relatively small orifice with a long, tapered seat, and a needle-shaped plunger, on the end of a screw, which exactly fits this seat.As the screw is turned and the plunger retracted, flow between the seat and the plunger is possible; however, until the plunger is completely retracted the fluid flow is significantly impeded. Since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is possible. | |||
The virtue of the needle valve is from the vernier effect of the ratio between the needle's length and its diameter, or the difference in diameter between needle and seat. A long travel axially makes for a very small and precise change radially . Needle valves may be used in vacuum systems, when a very precise control of gas flow is required, at low pressure,such as when filling gas-filled vacuum tubes,gas lasers and similar devices. | |||
==Uses== | |||
Needle valves are usually used in flow metering applications, especially when a constant, calibrated, low flow rate must be maintained for some time, such as the idle fuel flow in a carburetor.Note that the float valve of a carburetor is not a needle valve, although it is commonly described as one. It uses a bluntly conical needle, but it seats against a square-edged seat rather than a matching cone. The intention here is to obtain a well-defined seat between two narrow mating surfaces, giving firm shutoff of the flow from only a light float pressure. | |||
Since flow rates are low and many turns of the valve stem are required to completely open or close, needle valves are not used for simple shutoff applications. | |||
Since the orifice is small and the force advantage of the fine-threaded stem is high, needle valves are usually easy to shut off completely, with merely finger tight pressure. The spindle and/or seat of a needle valve, especially one made from brass, are easily damaged by excessive turning force when shutting off the flow. | |||
Small, simple needle valves are often used as bleed valves in hot water heating applications. | |||
Unlike [[Ball Valves]], or valves with a rising stem, it is not easy to tell from examining the handle position whether the valve is open or closed. | |||
==Sources== | |||
[http://en.wikipedia.org/wiki/Needle_valve Wikipedia Needle valve] | |||