Figure 1: Needle valve
A needle valve is used to accurately control flow rates of clean gasses or fluids. The adjustments are gradual and smooth for controlling the flow rate, however, they can also be used as a reliable shut-off valve. However, they are generally only used for low flow rates and have a relatively large pressure drop from the inlet to the outlet. Common port sizes for needle valves range from 1/8” up to 2”. The most common application is to control the flow of a gas, like a propane needle valve does.
Needle valves open and close an orifice with a tapered end that raises and lowers with the spin of a handle. Figure 2 shows a needle valve cross sectional view, design, and components. The handle (A) is connected to the plunger, also called a stem (F). When you turn the handle, the plunger moves up or down based on the threads (C). The locking nut (B) prevents it from fully unscrewing. As the plunger moves down, the tapered pointed end (I) comes into contact with the valve seat to fully seal the orifice (H). Often, the valve seat is also tapered. There are various options and sizes available to connect it to a pipe or hose through the port connection (G) on the input and output. The bonnet (D) is connected to the valve housing (E), which can be made out of different materials like brass or stainless steel.
Figure 2: Needle valve cross section view and components
Based on the threads, you can very precisely locate the plunger’s tapered end away from the valve seat to accurately control the flow rate. Therefore, by varying the plunger location you will control the flow rate between maximum and zero.
Figure 3: Angle needle valve
A manually operated threaded needle valve (described above) is the most common type of needle valve. However, two additional variants are: motorized and angle.
When selecting a needle valve, four main characteristics and/or application requirements need to be taken into account: material, pressure, size, and temperature.
The needle valve's housing material (Figure 2 callout E) 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 suitable for neutral and non-corrosive media, with the most common implementation being for drinking water applications. However, they should not be used for salt water (seawater), distilled water, acids, or chlorides.
Stainless steel needle valves have very good overall chemical resistance to almost any media. The material is very durable and can withstand high temperature and pressures. It is suitable for aggressive and corrosive media like seawater.
Understanding the application requirements for pressure in your system is important for selecting the correct needle valve. Robust needle valves can handle pressure of up to 4,000 to 5,000 psi (275 up to 413 bar) at 100°F (38°C). 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°F (38°C). Special designs are also available when you need a vacuum needle valve to handle the low pressure.
Needle valves are available in a wide range port and orifice sizes. The connection ports can also be 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 will ensure efficient flow and system operation with less chances of wear and/or leakage.
Needle valves are suitable for high or low temperature applications. You need to 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°F to 450°F (-54° C to 232°C) and PEEK (Polyether Ether Ketone) for increased temperature resistance up to 600°F (315 °C).
You will see needle valves 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 commonly used location for needle valves is to monitor gas flow, like propane, in a system. A propane needle valve can accurately shut it off and/or control the flow rate and effectively the burn rate for applications.
If looking at a schematic, Figure 4 shows a needle valve symbol. It has the standard 2-way valve symbol with an arrow indicating it is a needle valve.
Figure 4: Needle valve symbol