Small sizes of globe valves fitted with a finely tapered plug are known as needle valves. Needle valves are used to control the flow precisely. Needle valves are used in food industries, pharmaceutical industries where precision is important. Needle valves are also used in some components of automatic combustion control systems where very precise flow regulation is necessary.
Design of Needle valves:
The working of the needle valve is similar to the working of the globe valve. When the valve is actuated to open the needle plug will perpendicularly move away from the seat. To close the valve the needle will get seated in the opening perfectly. As the plug is needle-shaped the valve can provide much precise throttling positions. This description also applies to any type of valve incorporating a tapered needle having axial movement relative to the axis of a concentric orifice and thus controlling the effective opening of the orifice.
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 swivelling in the stem provides the necessary rotation for seating without damage. The bar stock body is illustrated in Figure:
Three basic configurations are shown in Figure 1: (A) is a simple screw-down valve; (B) is an oblique version, offering a more direct flow path: (C) is another form where the controlled outlet flow is at right angles to the main flow.In these basic versions, a threaded needle is shown, the thread itself acting as a seal to eliminate leakage past the needle. This is normally quite satisfactory in very small sizes or needle, although a more leak-tight arrangement is to mount the needle in an externally threaded end piece (D).
This end piece can also act as a grip for adjustment of the needle. Sealing, in this case, can be further improved if necessary by incorporating an internal seal. such as a 0-ring. The other common form of the needle valve is the float-controlled, carburettor-type valve.
Applications:
- Automatic combustion control systems
- Used in pharmaceutical and food industries.
- For precise flow regulation is necessary.
- Most constant pressure pump governors have needle valves to minimize the effects of fluctuations in pump discharge pressure.
- Used in situations where the flow must be gradually halted
- Needle valves can also be used for precise throttling service.
