Needle valves perform a number of important functions, including isolating pressure instruments (gauges, switches, transmitters) for their safe and convenient installation (without having to shut down a process), the slow and partial opening of the valve to ensure no damaging pressure spikes on the instrument and the control and regulation of flow.
Needle valves are one of the most common manually operated valves used in many facilities. The term ‘needle’ refers to the valve construction in that the base of the stem is tapered and narrowed like a needle, and seals to the body of the valve by pushing into a seat. Needle valves are small in size and control and regulate pressure and fluid flow with exceptional accuracy.
Major components of the valve type include the body (1), needle/needle tip (9), gland packing (11), bonnet (2), and handle (6).
The operating principle of the instrument needle valve is simple. The handle (6) is connected to the needle stem (9). When the handle is turned, the needle moves up or down based on the internal threads. The locking nut (3) prevents it from fully unscrewing. As the needle moves down, the tapered pointed tip (9) comes into contact with the valve seat to fully seal the orifice. The valve seat is also tapered.
Instrument needle valves offer a high level of precision when trying to regulate pressure and set flow rates. Unlike ball valves which are basically open or closed, instrument needle valve stems are internally threaded which means to properly open or close a valve, it may take a number of multiple turns.
Common instrument needle valve construction uses both an angled or a straight-through flow pattern, through the body of the valve. Other typical valve types using needle valve construction include gauge root valves, block, and bleed valves, and manifolds.
The typical connection of an instrument needle valve is threaded, however, with manifolds, flanged connections are also possible. The connection type will depend on the application.
In slurries and viscous media, the needle valve is avoided as the small orifice can easily be blocked by thick material or solids.
The needle valve body is normally made up of Brass, Carbon Steel, Stainless Steel, or any other specialty alloy materials. Valve seats are usually manufactured from a Metal or a Thermoplastic Material. The selection of the Valve seat material will depend on the media. A general rule of thumb is metal seated valves are used in liquid services; soft seated valves are used in gas services.
Gland packing or Packing material, which seals the needle stem, also requires different materials of construction. Common materials include FKM, PTFE and Graphite (to name a few).
As there is a direct relationship between pressure and heat being applied to the valve, all these material considerations will need to be taken into account to properly account for the application.
There are many advantages for installing an Instrument Needle Valve:
- They allow for the safe and convenient installation of pressure gauges, switches and transmitters
- Throttling, even with a low volume of fluid, is possible
- Flow rates can be adjusted precisely
- They are compact in size and space is generally not an issue during installation
- Simple installation and operation
website
