Needle Valve - How They Work
Figure 1: Needle valve
A needle valve is used to accurately control flow rates of clean gasses or liquids. Gradual adjustments of the stem and plunger enable smooth flow control. These valves generally have low flow rates and a relatively significant pressure drop from the inlet to the outlet. Standard port sizes for needle valves range from 1/8” to 2” (2 to 12 mm). The most common application of a needle valve is to control gas flow, such as propane in a tank.
Table of contents
- How needle valves work
- Different types of needle valves
- Needle valve selection
- Needle valve symbol
- Needle valve vs. other valves
- Needle valve advantages and disadvantages
- Needle valve uses
How needle valves work
A needle valve’s plunger has a tapered end that raises and lowers with the spin of a handle to open or close an orifice. Figure 2 shows the cross-sectional view of a needle valve, its design, and various components. The handle (Figure 2 labeled A) is connected to the plunger, called a stem (Figure 2 labeled B). When the handle turns, the plunger moves up or down through the threads (Figure 2 labeled C). The locking nut (Figure 2 labeled D) prevents the plunger from fully unscrewing. As the plunger moves down, the pointed (tapered) end (Figure 2 labeled E) comes into contact with the valve seat to fully seal the orifice (Figure 2 labeled F). Often, the valve seat is also tapered. This allows for accurate flow rate control.
There are various options and sizes available to connect the needle valve to a pipe or hose through the port connection (Figure 2 labeled G) on the input and output. For example, threaded, flanged, or welded. The bonnet is connected to the valve housing (Figure 2 labeled H), which can be made from different materials like brass or stainless steel.
Figure 2: Needle valve cross-section view and components: handle (A), stem (B), threads (C), packing nut (D), tapered end (E), orifice (F), port connection (G), and valve housing (H).
Different types of needle valves
A manually operated threaded needle valve (described in the previous section) is the most common type of needle valve. Two additional variants are:
- Motorized needle valve: These needle valves use an electric or pneumatic actuator to connect to and turn the plunger. These can be remotely controlled, used in a closed loop system, and/or put on a timer to accurately control the flow rate remotely.
- Angle needle valve: These needle valves turn the flow from the inlet by 90 degrees instead of an in-line flow. The same operation concept applies, but the 90-degree angle allows for better system integration. Figure 3 shows an example of an angle needle valve.
Figure 3: Angle needle valve
Needle valve selection
When selecting a needle valve, four main characteristics and/or application requirements must be considered: material, pressure, size, and temperature.
The needle valve housing material (Figure 2 labeled I) is specified according to the application. The most common valve housing materials are brass and stainless steel due to their range of chemical resistance; however, there are also other materials available for special applications.
Brass needle valves are suitable for hydraulic systems, drinking water, high-temperature applications, and gas piping. They are ideal for neutral and non-corrosive media, with the most common implementation for drinking water applications. However, they should not be used for saltwater (seawater), distilled water, acids, or chlorides.
Stainless steel needle valves have excellent overall chemical resistance to almost any media. The material is very durable and can withstand high temperatures and pressures. It is suitable for aggressive and corrosive media like seawater, unless the stainless steel will be continuously exposed to the saltwater because it will corrode.
Plastic needle valves are used to finely adjust pure and corrosive fluids. However, plastic cannot withstand high temperature or pressure and is typically used for applications under 60 degree celsius.
Understanding the application requirements for pressure in a system is important for selecting the correct needle valve. Robust needle valves can handle pressure up to 4,000 to 5,000 psi (275 up to 413 bar) at 100° Fahrenheit (38°Celcius). When higher pressures are required, like for high-pressure hydraulic applications, high-performance valves are available that can handle up to 10000 psi (689 bar) pressure at 100°Fahrenheit (38°Celcius). The vacuum needle design can handle low pressure.
Port & orifice size
Needle valves are available in a wide range of port and orifice sizes. The connection ports can have male or female ends. Common thread standards used are NPT (National American Pipe Thread), BSP (British Standard Pipe), or metric (ISO standards). The most commonly used needle valves range from 2 to 12 mm or 1/8” to 2”. Ensuring the correct port size and orifice size ensures efficient flow and system operation with fewer chances of wear and/or leakage.
Needle valves are suitable for high- or low-temperature applications. Specify the valve housing and packing/sealing material appropriately, especially for extreme temperatures, to ensure it can withstand the requirements. The two most common sealing materials are PTFE (Teflon) for a temperature range of -65° Fahrenheit to 450° Fahrenheit (-54° Celcius to 232°Celcius) and PEEK (Polyether Ether Ketone) for increased temperature resistance up to 600° Fahrenheit (315 ° C).
Needle valve symbol
Figure 4 shows the symbol for a needle valve. The schematic includes the standard 2-way valve symbol with an arrow indicating it is a needle valve.
Figure 4: Needle valve p&id symbol
Needle valve vs other valves
Needle valve vs ball valve
A ball valve has a ball with a hole at its center. The ball is rotated 90 degrees clockwise or anticlockwise to open or close the valve. Needle valves utilize a needle-shaped plunger with a tapered end to block or pass the fluid through the valve. The threaded stem aids fine control of the plunger positioning.
Ball valves are ideal for on-off shutoff applications, and these valves are typically not used for throttling purposes. Needle valves are suitable for those areas where precise fluid control is essential, for example, maintaining an ideal low fluid flow in a carburetor.
Needle valve vs gauge valve
Gauge valves are used to separate a process from instrumentation, for example, transducers or gauges. Gauge valves are mainly used alongside block and bleed valves. Needle valves provide means for isolating an instrument from a process or for fluid throttling applications.
Needle valve advantages and disadvantages
- Accurate flow control, especially for applications involving a low flow rate
- Small size
- High pressure loss because of the high restriction of fluid flow.
- The needle and seat may get damaged if the fluid has suspended solids.
- Examining the needle valve handle does not reveal whether the valve is open or closed.
Needle valve uses
Needle valves have a use across all fluid control applications due to their ability to precisely control flow rates. By selecting the appropriate valve material and size for temperature and pressure, they can be used for almost any clean gas or fluid. One of the most common uses of needle valves is to monitor gas flow, like propane, in a system. A propane needle valve shuts off the flow accurately and/or controls the flow rate, thereby checking the burn rate for several applications.
Needle valves find extensive applications in the following areas:
- Needle valves are used in dams, reservoirs, power plants, and water distribution systems for controlling water pressure and flow rate.
- Needle valves are employed in automated combustion control systems to regulate fuel flow.
- These valves are commonly used to shut off a pressure gauge or control the flow rate in flow metering devices.
- Needle valves control the flow of air to cylinders to control piston speed.
Read our article on needle valve applications for a comprehensive list of areas where needle valves are used.
What is a needle valve?
A needle valve is a throttling valve that is widely used to precisely control the flow rate of fluids.
What is a needle valve used for?
A needle valve regulates fluid flow through an appliance or system. The valve includes a small plunger with a shape similar to a needle with a tapered end.
Does a needle valve change fluid pressure?
Needle valves are used to regulate fluid flow and pressure within a system.