Needle valve how it works
The needle valve is another type of globe valve which is used for accurate throttling in high-pressure and/or high temperature service. Needle valves are designed for small diameter lines requiring fine throttling of gases, steam, low-viscosity oil, water,
or any other light liquid. The valve consists of a sharp, pointed (needle-shaped) stem that controls flow through the seat.
Needle Valve working principle
Needle valves open and close an orifice like valve opening with a tapered needle shaped disc that raises and lowers with the spin of a hand-wheel or with the help of any automatic control mechanism. Figure on the right shows a needle valve cross sectional view, design, and components. The hand-wheel (A) is connected to the plunger, also called as stem (F).
When one turn the handle, the plunger moves up or down based on the threads (C). The locking nut (B) prevents the plunger from fully unscrewing. As the plunger moves down, the tapered pointed end (I) comes into contact with the valve seat to fully seal the valve opening (H). Often, the valve seat is also tapered. The valve’s needle-like plunger fits into the seat. Because of this unique design, a needle valve can precisely and accurately control how much fluid passes through the valve.
Needle Valve Uses/Application
- Needle valves are used in low-pressure hydraulic systems, chemical processing, and other gas and liquid services.
- Needle valves are frequently used as metering valves. Metering valves are used for extremely fine flow control.
- Needle valves are used to control flow and protect delicate gauges from damage caused by sudden pressure surges of liquids and gases.
- Needle valves are also sometimes used in vacuum systems, in order to release a controlled yet steady supply of gas without a great amount of pressure.
- Needle valves are used in flow-metering applications, especially when a constant, calibrated, low flow rate must be maintained for some time.
- 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.
- Small, simple needle valves are often used as bleed valves in water-heating applications.
- One of the most commonly used location for needle valves is to monitor gas flow, like propane, in a system.
Needle valve Construction and Design
Needle Valve Selection Requirements
When selecting a needle valve, four main characteristics as per application requirements
- Material of Construction for example – Carbon steel , SS 316 , SS306
- Fluid Pressure For example – Oil , Water , Chemical etc
- Port or Orifice Size
- Fluid Temperature.