Minimum acetylene tank size for frame building?

[edoz]

I started with the smallest size tanks the welding supply store carried (they litterally both fit in a 26l backpack) , and I was able to fillet braze just fine. I got a bunch of practice joints and my first couple of frames out them, then I traded up when they were empty.

[jclay]

Bumping this thread because many are unaware of the safety implications related to use of acetylene cylinders that are too small for the fuel demands made of them. I definitely operated outside of recommended safe practices for years.

Posts 15 and 17 accurately summarize the older and current recommendations. In post 10 I noted that a Meco #5 was OK for use with my AC3 (75 cu ft stamped) cylinder. If adjusted properly (soot just disappears from flame prior to addition of oxygen) it would consume acetylene at an excessive rate and risk aspirating acetone into the regulator, hose and torch. That could damage the diaphragm and hose, and in extreme cases cause them to rupture. Now I know why the flame from the #5 and 6 Meco tips occasionally started getting dirty, different and rough, as compared to using the 3 or 4, when driven hard; acetone was bleeding through and igniting along with the acetylene. The flame was distinctly different and uncomfortable so I dialed it back or used a smaller tip, but I didn't know why or what I was risking.

The Smith (now Miller) O/A instruction book and catalog have consumption rates for their tips which, when used in conjunction with Mark Stonich's tip size chart, can provide guidance for other tip brands:

Welding Tip Size Comparison Chart

[jclay]

image.jpg

[EricKeller]

Huh, I guess I'll have to retire my aw207 until I get propane. I have a 209, but I bought that before I realized it would over-draw the tank. I never thought that the variations I was seeing were a result of burning acetone, good post.

[FTMN]

How does the pressure impact consumption?

Tip size obviously determines the consumption rate, but I would think the pressure would too. According to the Smith chart my AW203 puts out 3.2 CFH, and my AW205 puts out 6 CFH, but both those rates are determined by Smith when using a pressure of 10psi. I use significantly lower pressures; typically in the 3-4 psi range when I am fillet brazing.

I use a 40CF acetylene tank, so I shouldn't exceed 4 CFH. The AW203 tip is within the 1/10 rate rule, even at 10psi. I would also think I'm OK with the AW205 too, since I'm using lower pressures.

Thoughts?

[czernysz]

Good question.
I've been wondering who to ask. Also, the range of flame adjustment with any individual tip size and pressure setting must impact withdrawal rates. Smiths withdrawal rates for their welding tips are rated at 10 psi both regulators, with gas consumption representing the average volumes of gases consumed when acetylene is added until sooty smoke just disappears from the acetylene and prior to opening the oxygen valve and adjusting to a neutral flame . The Aw207 tip consumes 12 Scfh on both gasses.
If I'm running 5 psi regulator pressure, is the withdrawal rate halved?
Curiously,one of my welding book sets the safe acetylene withdrawal rate at 1/5 of cylinder volume (The Science and Practice of Welding, volume 2, tenth edition). Most welding manufacturers are still showing 1/7 on their websites.
I wonder what happened?

[czernysz]

Does anyone have a list of withdrawal rates for the Smiths aw series tips that Miller no longer makes?
Aw206, aw208 etc. I've misplaced mine.
Much appreciated.
Thank you.

[Mark Kelly]

How does the pressure impact consumption?

Tip size obviously determines the consumption rate, but I would think the pressure would too.

Thoughts?

If flow followed the Hagen Poiseuille Law, you'd expect it to be proportional to (P_in² -P_out²)/P_out (with all pressures as absolute, not gauge) and to the fourth power of tip diameter.

With the data given this doesn't appear to be the case, flow rate at 70 kPa is given approximately by 0.17 * diameter^2.3 (flow rate in m³ /min, diameter in mm).

[Mark Kelly]

Thinking about the above, one possibility is that the control section of the tip increases in length as the tip diameter gets larger. This would explain most of the difference observed, the rest could simply be that the flow resistance of the rest of the torch assembly becomes proportionally more important as the tip diamter increases, which seems logical.

Assuming the pressure relationship applies, the flow rate at 35 kPa would be expected to be 0.82 / 1.89 or about 40% of the flow at 70 kPa

[jclay]

Does anyone have a list of withdrawal rates for the Smiths aw series tips that Miller no longer makes?
Aw206, aw208 etc. I've misplaced mine.
Much appreciated.
Thank you.

Post 23, above.

[czernysz]

Post 23, above.

Fantastic. It's like magic.
Thank you.

[jclay]

Good question.
I've been wondering who to ask. Also, the range of flame adjustment with any individual tip size and pressure setting must impact withdrawal rates. Smiths withdrawal rates for their welding tips are rated at 10 psi both regulators, with gas consumption representing the average volumes of gases consumed when acetylene is added until sooty smoke just disappears from the acetylene and prior to opening the oxygen valve and adjusting to a neutral flame . The Aw207 tip consumes 12 Scfh on both gasses.
If I'm running 5 psi regulator pressure, is the withdrawal rate halved?
Curiously,one of my welding book sets the safe acetylene withdrawal rate at 1/5 of cylinder volume (The Science and Practice of Welding, volume 2, tenth edition). Most welding manufacturers are still showing 1/7 on their websites.
I wonder what happened?

Flowrate is impacted by orifice size, secondary (delivery) pressure AND torch valve adjustment but the direct visual indication that Smith corelates to flowrate is the point at which the soot disappears from an acetylene only flame. Presumably they picked that as a cue easily discernible to the operator and measured the flowrate for each orifice size at that operational point; secondary pressure and orifice size would then become irrelevant to flowrate. Operation at lower secondary pressures shouldn't alter the flowrate required for a incipient soot free, acetylene only flame.

Why? That's a fine question! I may see if OSHA has any hard data on it.

For me the take home message was simply the reminder of the existence of the need to keep the withdrawal rate/tip size in mind wrt cylinder size in use; I had forgotten about it. Figuring out that the reason my largest tips had distinctly different and uncomfortable characteristics was probably due to acetone entrainment and ignition was sobering.

[Mark Kelly]

Operation at lower secondary pressures shouldn't alter the flowrate required for a incipient soot free, acetylene only flame.

I very much doubt that that is true. A high source pressure and heavy valve restriction will result in turbulent flow at lower flow rates than a low source pressure and light valve restriction.

[Mark Kelly]

Thinking about the above <snip>

Assuming the pressure relationship applies, the flow rate at 35 kPa would be expected to be 0.82 / 1.89 or about 40% of the flow at 70 kPa

I should have thought about it for longer.

As John's post above reminds me, the flow rate also depends on the valve setting which I had not included. The result shown is therefore useless.

[jclay]

I very much doubt that that is true. A high source pressure and heavy valve restriction will result in turbulent flow at lower flow rates than a low source pressure and light valve restriction.

The distance between the torch valves and tip orifice, relative to the tube diameter, is great and includes a mixer, the goal of which is turbulent mixing. We could calculate the Reynolds Number to see what flow regime would develop in the tube regardless of the mixer and everything else but it's not worth the trouble; anyway you slice it the flow will be turbulent within the mixer and for some distance downstream of it. We've already bought turbulent flow in that portion of the system, it seems to me. The goal of the smooth contraction adjacent the exit orifice is to straighten the flow/reduce turbulence/maybe even achieve laminar flow through the contraction; I think the different flow regimes are established via the engineered hardware within the torch, mixer and tip.

I understand your point re pressure/flowrate curves as they relate to turbulent vs laminar flow and in industrial settings the valve position vs inlet pressure can make for downstream differences (like a pinched valve with high inlet pressure positioned way to close to a device that requires fully developed turbulent flow), but I think that those are very high order terms in our torches and from a practical perspective can be ignored.

I have been wrong before but that's how I see this situation.

[czernysz]

Flowrate is impacted by orifice size, secondary (delivery) pressure AND torch valve adjustment but the direct visual indication that Smith corelates to flowrate is the point at which the soot disappears from an acetylene only flame. Presumably they picked that as a cue easily discernible to the operator and measured the flowrate for each orifice size at that operational point; secondary pressure and orifice size would then become irrelevant to flowrate. Operation at lower secondary pressures shouldn't alter the flowrate required for a incipient soot free, acetylene only flame.

Why? That's a fine question! I may see if OSHA has any hard data on it.

For me the take home message was simply the reminder of the existence of the need to keep the withdrawal rate/tip size in mind wrt cylinder size in use; I had forgotten about it. Figuring out that the reason my largest tips had distinctly different and uncomfortable characteristics was probably due to acetone entrainment and ignition was sobering.

Thanks. Seems logical. Same flame , same flow-rate.

[jclay]

The distance between the torch valves and tip orifice, relative to the tube diameter, is great and includes a mixer, the goal of which is turbulent mixing. We could calculate the Reynolds Number to see what flow regime would develop in the tube regardless of the mixer and everything else but it's not worth the trouble; anyway you slice it the flow will be turbulent within the mixer and for some distance downstream of it. We've already bought turbulent flow in that portion of the system, it seems to me. The goal of the smooth contraction adjacent the exit orifice is to straighten the flow/reduce turbulence/maybe even achieve laminar flow through the contraction; I think the different flow regimes are established via the engineered hardware within the torch, mixer and tip.

I understand your point re pressure/flowrate curves as they relate to turbulent vs laminar flow and in industrial settings the valve position vs inlet pressure can make for downstream differences (like a pinched valve with high inlet pressure positioned way to close to a device that requires fully developed turbulent flow), but I think that those are very high order terms in our torches and from a practical perspective can be ignored.

I have been wrong before but that's how I see this situation.

My evaluation, expressed a little more clearly, is this: The system is designed to provide mixing (the mixer, hence some degree of turbulence) and a smooth contraction into the tip orifice (straightening the flow/eliminating turbulence). Those elements (particularly the contraction) govern the characteristics of the gas jet leaving the tip orifice and any flowstream peculiarities at the valves would be lost before exiting the tip orifice. The flow characteristics of the jet exiting the tip shouldn't be affected by variations in inlet pressure and valve settings, assuming an adequate flowrate to achieve the soot free, acetylene only flame.

I typically use approximately 5 and 5 psi under flowing conditions with the AW1A. The Meco Midget requires higher pressures, closer to 10 psi, to drive the larger tips hard.