Features of a Needle Valve

Needle Valve Features and Terminology 

Needle valves are one of the most prominently used flow adjustment valves. In part, this is because of the incredible range of options that needle valves offer. Needle valves allow for precision control of flow, with the ability to make fine adjustments. In comparison, ball valves are typically used to either open or close flow, with much less precision flow control when opened. This article presents needle valve options and demonstrates how they alter valve function or compatibility. 

Seats  

The seat of the valve is the sealing surface where the needle tip closes the valve. The industry commonly designates these with one of two terms.

Hard Seats: Hard seats are made of metal and are often integral to the body material. This firm sealing surface works well for high pressure because the two surfaces won't bend or flex against each other. However, tiny scratches in the metal surface may let lightweight gases like natural gas or helium leak through.  

Hard seats also work better in high-heat situations because metal can handle much higher temperatures than most plastics. 

Soft Seats: Soft seats are typically made of various soft materials, such as polymers and elastomers. Soft materials allow the valve tip to slightly compress the seat, which makes a tighter seal for lighter fluids. 

Soft seats reduce maximum pressure capability as their major tradeoff. Other problems that can arise come from the users themselves.  

Since the seats are soft, it is possible for an operator to over-tighten the bonnet of the valve and crush the seat. This, especially if repeated, can cause permanent deformation of the seat. Eventually, soft seat valves that are overcranked will need replacement. Some valves allow you to replace just the soft seats, while others require complete replacement. 

System designers should be careful to select soft seat material that is chemically compatible with system media. For this reason, spec sheets should list all valve materials. 

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Stem Tip Shape  

Stem tip shape helps to determine the flow characteristics of the valve. The three most common shapes are Vee Stem, Regulating Stem, Blunt Stem Tips.  

• Vee tip - this stem tip is the most common shape. It uses a medium taper tip that controls flow well without limiting the maximum flow the valve.    

• Regulating tip – engineers generally use this choice for instances needing precise flow control. The regulating stem's shape is similar to the vee tip, but encompasses a much narrower angle. This gives an operator fine control, but limits the maximum flow of the valve with its narrow orifice.   

• Blunt tip – is better for coarse flow adjustments. This is a great option for rapid closing of the flow. The blunt face of the tip engages with an almost flat seat. While this will close the flow quickly, it is not ideal for precise flow control.  

Stem Tip Rotation  

Rotating tip design: When operators turn the handle, they rotate both the valve stem and its directly connected tip. As the tip meets the seat, turning the handle continues rotating it to tighten the seal. This high friction rotation can gall the sealing surfaces.

Engineers invented non-rotating stems to combat this potential issue. 

Non-rotating design: These tips introduce a swivel point near the tip that allows the stem tip to stop rotating when it makes contact with the seat. The result is a smooth and straight application of pressure that seals the valve without risk of rotational damage. 

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Roddable?  

This term relates to the internal flow path of the valve. Rodding a valve is the process of forcing a stiff cleaning tool through the internal workings of the valve to clear debris. Rodding requires that the internal flow path be relatively straight. The industry considers valves with a straight flow path of large enough size "roddable".  

Other similar terms are “pickable” and “piggable”.  

• Pickable is a nearly interchangeable term.  

• Piggable typically means larger valves that can be cleaned by pushing a flexible wipe called a 'pig' through the system under pressure to remove all contents and residue. These pigs require the inner structure of the valves and tubing to be fairly smooth and snag free. Otherwise, pieces of the soft pig may lodge within the valve. 

Flow Direction  

Most needle valves feature a flow direction indicator on the side. Manufacturers typically indicate this with a small arrow pointing in the flow direction. But why does flow direction matter?

• Will the valve not close if there is back pressure? 

• Will reverse flow damage the valve? 

The answer is that flow direction DOES matter. However, installing it backwards won't cause immediate damage if flow reverses for a short time. In fact, it may be quite a while before you see any negative side effects.  

The major risk of running a needle valve backwards for extended periods is shortening the life of the seats in the valve. Media flowing the “wrong” way will cause turbulence within the valve. Especially with semi-abrasive media, the media could erode the seat material as it flows around the needle tip. Valve longevity could take a hit from frequent or constant reversed flow.   

Final Thoughts  

Hopefully, these small "tips" and explanations have helped you understand needle valve features and their terminology a little better. If you have any unanswered questions or you think we may have missed something, drop us a recommendation below. We're always looking to expand the knowledge base here at Mako Products! 

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Shop Our needle valve selection now! 

- Superlok Needle Valves

- Mako Threaded Needle Valves

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