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Product Catalogue

Needle valve [2009-07-21]

A needle valve is a type of valve having a small orifice and a threaded, needle-like plunger. It allows precise regulation of flow, although it is generally used for, and is capable of, only relatively small flow rates. Needle valves are similar in design and operation to the globe valve. Instead of a disk, a needle valve has a long tapered point at the end of the valve stem that is lowered through the seat to restrict or block flow. A cross-sectional view of a needle valve is illustrated in figure 6-8. The long taper of the valve element permits a much smaller seating surface area than that of the globe valve; therefore, the needle valve is more suitable as a throttle valve.
A needle valve has a relatively small orifice with a long, tapered, conical seat. A needle-shaped plunger, on the end of a screw, 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. Therefore, and since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is possible. Needle valves are often designed with a metal needle (generally brass, bronze, or stainless or other alloys of steel) and an elastomeric seat (generally PVC, CPVC, PTFE, or a wide range of brand name plastics and thermoplastics). While this is the most common form, valves are available that have metal - metal, plastic – plastic, or plastic – metal needles and seats. These variations are usually designed with specific applications in mind, especially situations where corrosion, high or low temperatures or extensive wear are possible. In such cases, it is best to consult with the manufacturer to find which type of valve is best for the application at hand.
Needle valves are used in almost every industry in an incredibly wide range of applications - anywhere control or metering of steam, air, gas, oil, water or other non-viscous liquids is required. They can be found in every industry from aerospace to zoological sciences, every service from gas and liquid dispensation to instrumentation control and cooling to power generation.
These small valves are widely used to accurately regulate the flow of liquids and gases at low flow rates. The fine threading of the stem and the large seat area allow for precise resistance to flow. Needle valves are used to control flow into delicate gauges, which might be damaged by sudden surges of fluid under pressure. Needle valves are also used in situations where the flow must be gradually brought to a halt and at other points where precise adjustments of flow are necessary or where a small flow rate is desired. They can be used as both on/off valves and for throttling service.
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. 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. Also needle valves should be avoided in applications where the media is viscous, or in the dispensation of slurries. The small flow orifice

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