Advantages of using a needle vaulve in an idle circuit.

[archiphile]

My first post here on IFI in quite sometime...years even. I was showing my partner the manifold assembly I have put together for my new/first gasser and her first question was “Why use a needle vaulve instead of another ball vaulve” in reference to my idle circuit. I told her because I had seen everyone else doing this but that I could not give her a real answer to her question. I have googled and search here and found nothing in the topic so ladies and gents I am all ears. 

[HojPoj]

Needle valves allow very fine control, ball valves not so much.  In other applications you'd have a ball valves inline with a needle valve so that the needle can dialed in to the desired setting,  and the ball valves used for on/off.

[Mikey98118]

Actually, ball valves can be used in the place of needle valves by adding an adjustable stop to limit the valve's motion. Do they have the very fine control of a needle valve? No; but then, how fine a control do you need on an idler circuit? It boils down to a matter of choice; not a matter of need.

[Frozenforge]

This is what happens when you have too much stuff in your fittings and valves drawer

[Frosty]

A thing of beauty. All it lacks is a little polishing and color coding. 

Frosty The Lucky. 

[HojPoj]

10 hours ago, Mikey98118 said:

Actually, ball valves can be used in the place of needle valves by adding an adjustable stop to limit the valve's motion. Do they have the very fine control of a needle valve? No; but then, how fine a control do you need on an idler circuit? It boils down to a matter of choice; not a matter of need.

True, unless the ball valve you select is oversized for the application, at which point the point of first flow creates enough orifice to exceed the desired gas flow (or rather, the difference between a little and a lot of flow is only a couple degrees of rotation).  If using a ball valve for control of the idler circuit, I'd advise erring on the small side to ensure the ability to actually have a flow rate that's less than what's going to the full-flow branch of the circuit.

[archiphile]

So maybe this will clear itself up when I fire it up and start messing around with settings but it is still unclear what the advantages are. I mean it would have saved a sizable chunk of cash to have just put the mechanical stop in. I have spent about 25 man hours searching online and in various supply houses and big box stores to source enogh parts to build the darn thing. My time is worth money too this forge just got real expensive. I am certain I won’t regret all the time money and effort I have spent into this build. Mikey your book is a blessing.

[timgunn1962]

If you look through a ball valve as you open it, you will see it initially closed then, as it starts to open, an eye-shaped opening which gets progressively bigger until the valve is fully open. With a full-bore ball valve, it goes from fully-closed to fully open in 90 degrees. Fully open, there is no restriction whatsoever and the bore of the through-hole is the same size as the pipe it is installed in. Ball valves are intended to have two operating positions: fully-open and fully-closed. The fact that there are intermediate positions is almost incidental.

With a needle valve, there is a profiled needle in a hole. The design usually makes it difficult to see what is going on inside, but as the needle is unscrewed, a very narrow ring-shaped gap first appears, getting wider as the valve is opened further. Typically several turns are necessary to take the valve from its minimum opening (or closed, if it will fully close) to its maximum opening. In most cases, fine threads are chosen specifically to maximize the sensitivity of this adjustment and 10 or more full turns are needed for full travel. Needle valves are intended to control flow and the fact that some can close fully is almost incidental. 

Compare the ball valve, with its quarter turn from fully closed to fully open, with the needle valve, with its 10 turns from fully closed to fully open, and it's fairly easy to see how the needle valve wins when the goal is sensitivity. 

When things go pear-shaped and you need to shut off the gas in a hurry, the needle valve is a lousy choice and the ball valve wins hands down. 

In many cases, it is best to have both: the ball valve to control whether or not gas flows and the needle valve to control how much gas flows when it does.

 

[Mikey98118]

I totally agree.

However the question has not been which valve is best suited for use in an idler circuit, but whether or not a ball valve could be used effectively in an idler circuit at all; I have done so. I have also used needle valves for idler circuits, and found them to be far more precise. On the other hand the ball valve is cheaper and allows a simpler circuit to be constructed, so it comes down down to a choice between nicer performance or easier/cheaper construction. Normally I favor performance of practicallity, but the ball valve option also allows more compact construction because the circuit is so basic. After several years I remain undecided as to which path is "better."

[archiphile]

47 minutes ago, Mikey98118 said:

I totally agree.

However the question has not been which valve is best suited for use in an idler circuit, but whether or not a ball valve could be used effectively in an idler circuit at all; I have done so. I have also used needle valves for idler circuits, and found them to be far more precise. On the other hand the ball valve is cheaper and allows a simpler circuit to be constructed, so it comes down down to a choice between nicer performance or easier/cheaper construction. Normally I favor performance of practicallity, but the ball valve option also allows more compact construction because the circuit is so basic. After several years I remain undecided as to which path is "better."

What is the need for precision? Why is the precision applied to the idle circuit and not the main line?

[ThomasPowers]

Archiphile; how good are you at pouring 1 drop out of a full 55 gallon barrel? Would using an eyedropper be easier? Now if you need to pour 30 gallons out of the barrel; would the eyedropper be a good method?

The primary burner for a forge has a wide range of gas input it will work with, 20 psi or even more may be the difference between the low and high settings.  An idler circuit is like a pilot light for the forge burner and you would prefer to use as little gas as possible and yet not go out while in use so a precision fitting to meter the flow is suggested.  (And at least in all my forges there is a regulator that controls the flow to the main burner and it works quite well for the main burner.)

[Mikey98118]

16 hours ago, HojPoj said:

True, unless the ball valve you select is oversized for the application, at which point the point of first flow creates enough orifice to exceed the desired gas flow (or rather, the difference between a little and a lot of flow is only a couple degrees of rotation).  If using a ball valve for control of the idler circuit, I'd advise erring on the small side to ensure the ability to actually have a flow rate that's less than what's going to the full-flow branch of the circuit.

All excellent points; the smaller the ball valve the better for this use. I used the smallest I could find because I like to keep controls as small as possible, since I like mini-forges. So, I apparently just lucked out. It is better to plan carefully. Thanks for bringing up this matter.

[Frozenforge]

One of the things to remember when talking about all these controls is to have a 2 stage regulator. Otherwise as your bottle cools off from draw and it starts getting low the actual supply pressure drops and a single stage regulator will not maintain a steady pressure. The 2 stage will maintain a constant pressure till the supply pressure approaches the desired regulated pressure.