Control of gas flow with Needle valves

Fittings such as elbows, tees, valves and reducers represent a significant component of the pressure loss in most pipe systems. This article discusses the differences between several popular methods for determining the pressure loss through fittings. The methods discussed for fittings are: the equivalent length method, the K method (velocity head method or resistance coefficient method), the two-K method and the three-K method.

Fittings in pipelines such as valves, elbows and changes in pipe size all cause pressure losses to fluid flowing through them. This article describes a comprehensive new range of Valves and fittings. Generally speaking the methods increase in accuracy in the order. A comprehensive new range of Valves and fittings, adapters and fabricated assemblies has been introduced by HP Valves.

Needle valves

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.

Instrument Needle Valve uses a tapered pin to gradually open a space for fine control of flow. The flow can be controlled and regulated with the use of spindle . 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 radically (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, such as when filling gas-filled vacuum tubes, gas lasers and similar devices.

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.