What Is a Needle Valve and How Does It Work?

A needle valve is a specialized valve typically used in instrument installation applications in the process industries. Needle valves allow for the slow and partial opening of the valve which is desirable in that it does not “shock” the instrument in which it is installed. Needle valves can also be used to regulate flow and pressure given their construction. Let’s explore this valve design in more detail.

What Is a Needle Valve? 

 

There is little chance you can look at a run of process piping and not see at least one needle valve. Needle valves are one of the most common manually operated valves used in any process facility. The term Needle refers to the valve construction in that the base of the stem is pointed somewhat like a needle and seals to the body of the valve by pushing into a seat. Major components of this valve type include the body (7), seat (8), stem seal and backup ring (5,6), stem (4), packing or O-ring (3), bonnet (2), and handle (1).

 

 

 

Unlike most valves requiring only a quarter turn to fully open or close, needle valve stems are threaded such that it may take as many as 20 full turns to completely open or close the valve. This offers high precision when trying to set flow rates or control pressure and allows for a gentle gradual opening or closing of the valve preventing sudden changes in the attached devices. 

 

Common needle valve construction uses a straight-through flow path through the body of the valve, but angle pattern bodies where process fluid takes a 90-degree turn are also available. Other typical valve types using needle valve construction include gauge root valves, manifolds, and block and bleed valves. Process connections in a standard needle valve are typically threaded but may be offered in tube ended compression as well. 2, 3, and 5 valve manifolds used in instrument isolation may have flanged or threaded connections depending on the application.

 

What to Look for in a Needle Valve 

 

Let’s start at the body (7) of the valve and work our way up. Body and bonnet materials are usually the same. Having the right metallurgy for your application is important. Acids and Caustics react differently to different metallurgies. Make sure you pick the right material of construction for your application. Common materials include Steel, Stainless Steel, Hastalloy C276, and Titanium.

 

Inside of your body will be the seat (8) where the stem seals to the body. There are two main types of seats: Soft seated and Metal Seated. A general rule of thumb is metal seated valves are used in liquid services, soft seated valves are used in gas services, but there are exceptions. In soft seated applications having the right seat, the material is vital for the life of the valve. Much like choosing the correct metallurgy for the body, make sure you take into consideration every process constituent when selecting a seat material.

 

Stem seal (5,6) materials are dictated by 1) operating temperature, and 2) process composition. Materials must be compatible with all process components and must be tolerant of these components throughout the valves operating temperature range. There is a myriad of choices for these components, so never settle for what’s generally available. Stem seals will be in constant contact with the process, so do your homework and chose these materials wisely. If there are any questions, we can help.

 

Your stem (4) will be of hardened material and have a particular tip design based on the valve’s application. Common stem tip types include Blunt Vee, Pointed Vee, Regulating (high precision), non-rotating metal, non-rotating PTFE. Seat material type based on process media will dictate which stem types are right for your valve. 

 

Packing material or an O-Ring (3) will seal the stem to the Bonnet (2). O-Ring seals are typically found on non-adjustable bonnet assemblies. Packing material is usually found on adjustable bonnet valves. There are preferences and differences in each design. Work closely with your valve provider to configure the bonnet assembly to work best in your application. 

 

Finally, handle (1) types can be T (shown) or round knob. When needle valves are configured to be used as metering valves to control flow or pressure (as an example when used on a rotameter), knobs may be labeled in such a fashion as to indicate where the valve stem is in relation to full open or fully close. This provides a visual indication so users can open or close the valve in specified increments.

 

Sourcing a Needle Valve That’s up to the Task 

 

We’ve provided a brief explanation here as to what constitutes a needle valve, but there is not nearly enough space to discuss all the variations in configuration offered from the various needle valve manufacturers available. With so many choices and variations, be sure you work with a company familiar with every option available. Gather your process and application information and our applications specialists will be happy to help you configure a valve for your specific application. 

 

Contact a Sagacity Allstream Fabrication Engineering specialist today at (844) 514-9170 or connect with us online to get started on your project. From start to finish, we can help you find the correct valve for your application needs. 

 

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