Sunday, September 2, 2012

Needle Valve - A Metering Valve

What is a Needle Valve

A needle valve is linear motion valve used to make relatively fine adjustments in the amount of fluid flow. 

The distinguishing characteristic of a needle valve is the long, tapered, needle like point on the end of the valve stem. This "needle" acts as a disk. The longer part of the needle is smaller than the orifice in the valve seat and passes through the orifice before the needle seats. This arrangement permits a very gradual increase or decrease in the size of the opening in a needle valve.

Needle valves are often used as component parts of other valves, more complicated valves. For example,needle valves are used in some types of reducing valves.
Needle valve
Needle valve
 Needle Valve Applications

Most constant pressure pump governors have needle valves to minimize the effects of fluctuations in pump discharge pressure. Needle valves are also used in some components of automatic combustion control systems where very precise flow regulation is necessary.

Types of Needle Valve Body Designs

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. 

Needle valve
One advantage of needle valves is fine flow adjustments they offer. 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 in metering valves (needle valves). Therefore, the number of hand wheel turns can be directly correlated to the amount of flow.Needle valve specification should match the application where valve is going o be installed.

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  valve maintenance.
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