A needle valve is a type of valve with a small port and a screw thread, needle-shaped plunger. It allows regulation of Fluid dynamics, although it is generally only capable of relatively low flow rates.
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 easily possible.
The virtue of the needle valve is from the vernier scale 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 also be used in , when a precise control of gas flow is required, at low pressure, such as when filling gas-filled vacuum tubes, and similar devices.
Needle valves are also commonly used to provide shut off for the pressure gauge or on applicators to shut off the supply of NH3 to the knives in anhydrous ammonia (NH3) applications.
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 in 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.
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