.

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 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

body  design  for  a  needle  valve  

One  type  of  body  design  for  a  needle  valve  is  the  bar  stock  body.    Bar  stock  bodies  are common, and, in globe types, a ball swiveling in the stem provides the necessary rotation for seating without damage.   The bar stock body is illustrated in Figure 21. Figure 21   Bar-Stock Instrument Valve Needle valves are frequently used as metering valves.   Metering valves are used for extremely fine flow control.   The thin disk or orifice allows for linear flow characteristics.   Therefore, the number of handwheel turns can be directly correlated to the amount of flow.  A typical metering valve has a stem with 40 threads per inch. Needle  valves  generally  use  one of  two  styles  of  stem packing:  an  O-ring  with  TFE  backing rings or a TFE packing cylinder.   Needle valves are often equipped with replaceable seats for ease of maintenance.