Needle Valves vs Gate Valves 3

[adjalabi]

Apart from the construction and manner of operation of the closure member, are there any other differences between gate valves and needle valves? Manual or actuated... or any other distinguishing feature asides?

 

[controlnovice]

From Widipedia:
A gate valve is a valve that opens by lifting a round or rectangular gate out of the path of the fluid.

A needle valve is a type of valve usually used in flow metering applications. It is similar in operation to a gate valve but tends to use a smaller, more precisely controllable orifice. When the handle is turned, a plunger slowly descends to block flow.

Manual means that a person has to phisically open/close the valve by turning a handle or wheel. Actuated means the valve is opened/closed by a mechanical, or electrical, source.

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This is normally the space where people post something insightful.

 

[zdas04]

The key difference is called "linearity". For a needle valve, the flow vs. valve opening is nearly linear (e.g., if the valve is 30% open then it will pass about 30% of it's maximum flow).

The linearity of a gate valve is nearly non-existant. A gate valve will pass 100% of maximum flow at about 25% open. At 10% open it will pass about 5% of maximum. A plot of flow vs. percent open (for a given set of pressures and temperatures) is sharply non-linear. If you ever try to throttle with a gate valve you'll see that the quiver in the valve position will give you a wildly varying mass flow rate.

I've never seen a case where a gate valve had a legitimate place in any measurement application.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Ashereng]

adjalabi,

I take it you are looking for generalities. If so, here are some more:

Gate valves are usually used as on/off valves in main lines. As zdas04 mentioned, they do not throttle well. Needle valves are usually used to control a flow - usually a fine flow.

Gate valves are usually used in large pipes, main process lines, etc. Needle valves are usually used in small pipes (2" and less), and usually not in process lines. Needles are also used in front of instruments (instead of a gauge valve).

I have seen gate valves in "manual" and "remote actuated". I usually see needle valves in "manual" only.

 

[JimCasey]

Gate valves are designed to pose the least possible restriction to the flow when they are open. They then have an unobstructed flow path. The only valve with less drop would be a full-port ball valve or possibly a split-body butterfly valve with a thin single-piece vane (Keystone 99, for example) It'd probably possible to find a small gate valve but they are of questionable merit below 1" NPS and highly rare smaller than 1/2"

Needle valves are a special case of globe valves where the ports are drilled. 1/2 NPS would be on the large end for a needle valve, at least for the port size, but there are scads of them with 1/4" ports and smaller, even if they have 1" connection sizes. The flow path is tortuous and discontinuous, both in direction and in cross-sectional area. Needle valves are used where restriction to the flow is either not a problem or is actually wanted. Needle valves are good for regulation of the flow because the valve plug can be manufactured with a long <needle> taper, so there is very high resolution of the flow vs valve-stem position. Blunt valve plugs are also available, and these are NOT useful for precise regulation but usually have a little more capacity.

Needle valves can be manually actuated (Whitey instrument valves, for example) or they can be automated
( Flowserve/Kammer low-flow control valves for instance)

 

[Ashereng]

Pinch valves also provide for mimimal pressure loss (equivalent to pressure loss in same length of pipe).

 

[Floatingproduction]

Dear adjalabi,
needle valve can only be installed in Gas line and the Gate valve is usually controls the fluids

 

[zdas04]

Floatingproduction,
Your post makes no sense and it seems to be making an assertion that you can only use needle valves in gas flow. This is patently wrong. Using the word "control" in reference to gate valves is misleading since they often "redirect" flows "control" generally means to proportion flows. Please clairify and/or expand your statemnts.

David

 

[Floatingproduction]

zdas04
if you have to control gas in a high pressure you cant use gate valve i aint talking about low pressure things

 

[JimCasey]

FLoatingproduction:
You're right. You can't use a gate valve for control. Anywhere.
If you try to modulate with a gate valve you will experience extremely poor control, like David said. Soon, all the jingly pieces in the valve will spontaneously disassemble and find their way to something expensive to repair downstream. Such bad things will in fact happen faster at higher pressures.

If you are trying to control a process by modulation, you need a valve designed for control. One of the primary features of a successful modulating control valve is that the trim pieces are supported by rigid guiding mechanisms. Gate valves are NOT rigidly guided. THe trim pieces are free to jingle around a bit so that they find the seat and nestle into place. This makes them shut off pretty tight with metal-to-metal surfaces, but that's all they can do. Rigidly guided control valves conversely do not shut off as tight because the trims cannot nestle into the seating surface.

Needle valves almost universally have a guide bushing in the bonnet, or the plug is guided in the seat ring with characterization of the Cv vs travel by a tapered flat ground on one side of the plug. GLobe valves can be top-guided, stem-guided, or cage-guided. Rotary control valves have a trunnion bearing on both sides of the flow stream.