Needle valve

Needle valve
Giffard steam water injector (B-needle valve).
A needle valve.

A needle valve is a type of valve 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.

Contents

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 (the control input) makes for a very small and precise change radially (affecting the resultant flow). Needle valves may be used in vacuum systems, when a very precise control of gas flow is required, at low pressure,[1] 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 (controlling the fuel level within the 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. Small, simple needle valves are often used as bleed valves in hot water heating applications.

Unlike a ball valve, or valves with a rising stem, it is not easy to tell from examining the handle position whether the valve is open or closed.

See also

References

  1. ^ Ward, L.; Bunn, J.P. (1967). Introduction to the Theory and Practice of High Vacuum Technology. Butterworths. pp. 125–126. 

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Look at other dictionaries:

  • needle valve — n. a thin, tapered valve that is usually precisely set to control the flow of fluid through a cone shaped opening: used esp. in carburetors …   English World dictionary

  • needle valve — adatinė sklendė statusas T sritis automatika atitikmenys: angl. needle valve vok. Nadelventil, n rus. игольчатый вентиль, m pranc. valve à pointeau, f …   Automatikos terminų žodynas

  • needle valve — adatinis vožtuvas statusas T sritis automatika atitikmenys: angl. needle valve vok. Nadelventil, n rus. игольчатый клапан, m pranc. soupape à pointeau, f …   Automatikos terminų žodynas

  • needle valve — /ˈnidl vælv/ (say needl valv) noun a valve with a needle like part, a fine adjustment, or a small opening, especially a valve in which the opening is controlled by a needle like or conical point which fits into a conical seat …  

  • needle valve — A fluid control valve that uses a long tapered needle to finely control the amount of fluid passing through the orifice. The more the needle is in, the lesser is the amount of flow, and vice versa. It can be adjusted as required …   Aviation dictionary

  • needle valve — [1] A valve with a needle shaped pin that can be moved to control the flow of a fluid. [2] The valve, in a carburetor, which controls the amount of gasoline flowing from the pump to the float chamber …   Dictionary of automotive terms

  • needle valve — noun A type of valve having a small orifice and a needle like plunger, allowing precise regulation of generally small flow rates …   Wiktionary

  • needle valve — noun : a valve consisting essentially of a slender pointed rod or needle fitting into a conoidal seat and capable of fine adjustment …   Useful english dictionary

  • needle valve — Mach. a valve with a needlelike part, a fine adjustment, or a small opening, esp. a valve in which the opening is controlled by a needlelike or conical point that fits into a conical seat. [1900 05] * * * …   Universalium

  • needle valve — nee′dle valve n. mac a valve with a needlelike part, a fine adjustment, or a small opening • Etymology: 1900–05 …   From formal English to slang

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