TYPES OF VALVES
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. 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.