What is Needle Valve?
A needle valve is a type of valve with a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates.
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 gas like a propane needle valve does.
Construction and operation of Needle Valve
An instrument needle valve uses a tapered pin to gradually open a space for fine control of flow. The flow can be controlled and regulated with the use of a spindle. Needle valves have a relatively small orifice with a long, tapered seat, and a needle-shaped plunger on the end of a screw, which exactly fits the seat.
As the screw is turned and the plunger retracted, flow between the seat and the plunger is possible; however, until the plunger is completely retracted, the fluid flow is significantly impeded. Since it takes many turns of the fine-threaded screw to retract the plunger, precise regulation of the flow rate is easily possible.
The virtue of the needle valve is from the vernier effect of the ratio between the needle’s length and its diameter, or the difference in diameter between needle and seat. A long travel axially (the control input) makes for a very small and precise change radially (affecting the resultant flow).
Needle valves may also be used in vacuum systems, when precise control of gas flow is required, at low pressure, such as when filling gas-filled vacuum tubes, gas lasers, and similar devices.
How Does Needle Valves Work?
Needle valves can operate manually or automatically. Manually operated needle valves use the handwheel to control the distance between the plunger and the valve seat. When the handwheel is turned in one direction, the plunger is lifted to open the valve and allow fluid to pass through. When the handwheel is turned in the other direction, the plunger moves closer to the seat to decrease the flow rate or close the valve.
Automated needle valves are connected to a hydraulic motor or an air actuator that automatically opens and closes the valve. The motor or actuator will adjust the plunger’s position according to timers or external performance data gathered when monitoring the machinery.
Both manually operated and automated needle valves provide precise control of the flow rate. The handwheel is finely threaded, which means it takes multiple turns to adjust the position of the plunger. As a result, a needle valve can help you better regulate the flow rate of fluid in the system.
Different Types of Needle Valves
A manually operated threaded needle valve (described above) is the most common type of needle valve. However, two additional variants are motorized and angle.
- Motorized Needle Valve: These needle valves don’t have a handle for manual operation, but instead 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 output from the inlet by 90 degrees instead of in-line. The same operation concept applies, but the 90-degree angle can allow for better system integration.
Uses of Needle Valve
Needle valves are usually used in flow metering applications, especially when a constant, calibrated, low flow rate must be maintained for some time, such as the idle fuel flow in a carburetor.
Note that the float valve of a carburetor (controlling the fuel level within the carburetor) is not a needle valve, although it is commonly described as one. It uses a bluntly conical needle, but it seats against a square edged seat rather than a matching cone. The intention here is to obtain a well-defined seat between two narrow mating surfaces, giving firm shutoff of the flow from only a light float pressure.
Needle valves are also commonly used to provide shut-off for the pressure gauge or on applicators to shut off the supply of NH3 to the knives in anhydrous ammonia (NH3) applications.
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.
Since the orifice is small and the force advantage of the fine threaded stem is high, needle valves are usually easy to shut off completely, with merely “finger-tight” pressure. The spindle and/or seat of a needle valve, especially ones made from brass, are easily damaged by excessive turning force when shutting off the flow.
Small, simple needle valves are often used as bleed valves in water-heating applications.
Unlike a ball valve or valves with a rising stem, it is not easy to tell from examining the handle position whether the valve is open or closed.
Advantages Of the Needle Valve
- The flow control at a low rate is very accurate
- Needle valves are smaller in size
- They can be used for throttling purpose
- Flow rates can be adjusted accurately
- Needle valves are used as bleed valves for hot water heating
- These valves can shut off easily
Disadvantages Of Needle Valves
- Pressure loss is high in needle valves, because of the high restriction of the fluid flow.
- This valve is used for low flow rates, because of the small passage flow between the seat and needle.
- Damage could happen to the seat and the needle if the fluid has suspended solids.
- We won’t be able to tell if the valve is open or closed by examining the handle position
- Because of the excessive turning force while shutting off the seat of the needle valve could be damaged
