Giffard steam water injector (B-

needle valve).

A needle valve.

Needle valveFrom Wikipedia, the free encyclopedia

A needle valve is a type of valve having a small port and athreaded, needle-shaped plunger. It allows precise regulation offlow, although it is generally only capable of relatively low flowrates.

Contents

1 Construction and operation2 Uses3 See also4 References5 External links

Construction and operation

An Instrument Needle Valve uses a tapered pin to gradually open aspace for fine control of flow. The flow can be controlled andregulated with the use of a spindle. A needle valve has a relativelysmall orifice with a long, tapered seat, and a needle-shapedplunger on the end of a screw, which exactly fits the 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 takesmany turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate ispossible.

The virtue of the needle valve is from the vernier effect of the ratio between the needle's length and itsdiameter, 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 also be used in vacuum systems, when a 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, lowflow 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 aneedle 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 lightfloat pressure.

Since flow rates are low and many turns of the valve stem are required to completely open or close, needlevalves 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 areusually easy to shut off completely, with merely "finger tight" pressure. The spindle and/or seat of aneedle valve, especially one made from brass, are easily damaged by excessive turning force whenshutting off the flow.

Small, simple needle valves are often used as bleed valves in water heating applications.

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

See also

Ball valveButterfly valveControl valveFlow limiterGate valveGlobe valvePlug valveMass flow controllerPlastic pressure pipe systems

References

2. Understand different types of Needle Valves http://www.instrumentation-fittings.com/needle-valves.html

External links

Needle valve (http://www.piping-engineering.com/needle-valve-regulation-valves.html)

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Categories: Valves Plumbing valves

1. Ward, L.; Bunn, J.P. (1967). Introduction to the Theory and Practice of High Vacuum Technology. Butterworths.pp. 125126.

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