What is a needle valve and what is the purpose of needle valves
The needle valve got its name because of its narrow tapered plug and port arrangement. The needle valve is small in size and the control of the fluid flow can be done accurately. Needle valves can be used if the pressure heads or flow rates must be safely reduced and controlled. The needle valves are capable to control the pressure and flow. Needle valves are linear motion valves, these valves are used in instrument systems for throttling of small volumes. Needle valves have metal to metal seats, but because of the small size, they can be used for positive shut off. These valves are capable to control the volume, the delicate gauges flow can be controlled by needle valve which could be damaged by sudden surges of fluid under pressure, needle valves are used where accurate flow control is needed and they are also used where the small rate of flow is required.
How needle valves are constructed? are needle valves directional
Needle valve consists of a valve body that has an orifice and a tapered stem and it can be screwed in and out to vary the orifice opening. The flow in both directions can be restricted by the help of needle valves. The preferred direction of flow is indicated by an arrow in the valve body and when the flow is in this direction then the pressure loss across the valve will be less. A needle valve with an internal check valve allows for restricted flow in one direction and free flow in the other direction. Needle valves use the stem to change the size of the orifice and this would give us the ability to compensate for changes in differential pressure in order to maintain a constant flow rate through the valve and a constant actuator speed. It would also allow establishing the desired flow rate without removing the fixed orifice.
How does a needle valve work? Is needle valve a globe valve
The spindle ends in a tampered needle, which would move into the valve seat so that it can restrict or prevent the flow of fluid through it. The in and out motion of the needle to and from the valve seat is controlled by the spindle which is operated by hand wheels. The fluid will turn ninety degrees that go through the valve and it will pass through an orifice, the orifice size can be changed by placing the cone according to the seat. The needle-shaped plunger is on the end of the screw so when the screw is turned the plunger will be retracted, then the flow between the plunger and the seat is possible. The fluid flow will be impeded until the plunger is retracted and the flow rate regulation is possible. Some stems which are properly threaded have slow linear movement so many turns are needed for a full flow section and this makes the needle valve suitable for regulating the flow
A needle valve can be considered as a small globe valve, it is a form of a globe valve and they are used for low flow. The only difference between the globe and needle valve is instead of a disk at the end of the valve stem it has a long tapered point. The long taper of the valve allows a smaller seating surface area than the globe valve, so the needle valve is more suitable as a throttle valve.
What are the 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
What are the 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
What are the applications of needle valves
- They can be used on gas bleeder lines
- These valves can be used for sampling lines and for other abrasive services
- Fluid power systems
- These valves are used for flow metering applications
- Needle valves are used in automatic combustion control systems in which accurate flow regulation is needed
- Needle valves are used in constant pressure pump governors in order to reduce the effect of fluctuations in pump discharge pressure
To know more about control valves check the following link