Gas welding and the 1/7 rule

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I'm a newbe to gas welding and I read everywhere that you should not consume more than 1/7 of the acetylene cylinder capacity per hour, or the gas will become unstable. My question is, how can I ensure that I'm consuming within the 1/7 per hour limit? none of what I read so far explain this very well. I have the portable Lincoln Electric gas outfit from Home Depot.
Reply:Alll welding or heating tips require a certain CFH gas flow.  If a cylinder has 140 CF in it, you can withdraw 20 CFH before it will become unstable. This is why a Acetylene Gauge has 15#'s on the outlet side, so as not to withdraw to fast.  So check the tip size and how much it will draw, before use.  When using a Type 11 Size 15 Rosebud, you cannot use a small Acetylene, because it requires more volume than the cylinder will deliver and the Tip will backfire.  There is as much to OF cutting and welding as there is to Electric welding or cutting.  Like there are 14.7 CUFT to 1`# of calcium Carbide, when making acetylene gas and pumping it into a cylinder.  Look at the TW (tare weight)weight on the Cylinder and then weight it.  Multiply the difference b 14.7 and that is how many CUFT you have in the cylinder.  and On On On On  SMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:It's not that it becomes unstable. What happens is the tank will freeze up, causing flashbacks. Also you will draw acetone into your regulator and destroy the seals.Miller EconotigCutmaster 38Yes ma'am, that IS a screwdriver in my pocket!
Reply:this is something that i have never worried about nor have i ever met anyone who does.  in theory its bad, but unless you're using a big assed rosebud and have a really small bottle, its really not something to worry about.
Reply:Roger that rule!!  1 to 10 can work, but when welding, you will get pops from the acetone.  Please be carefull.Various GrindersVictor Journeyman torch200cf Acet. 250cf oxygenLincoln 175 plus/alpha2 gunLincoln v205t tigLincoln 350mpEsab 650 plasmaWhen you can get up in the morning, Its a good day.Live each day like its your last.
Reply:Didn't your welding set come with a little booklet that tells the cfh flow for each tip size at the recommending pressures. If I remember right the only tips that that you can use with an MC tank and still comply with the 1/7 rule are size 0 and 00. You can get by using a small cutting torch tip for a little bit such as cutting a bolt or heating a frozen nut.
Reply:Originally Posted by tresiDidn't your welding set come with a little booklet that tells the cfh flow for each tip size at the recommending pressures. If I remember right the only tips that that you can use with an MC tank and still comply with the 1/7 rule are size 0 and 00. You can get by using a small cutting torch tip for a little bit such as cutting a bolt or heating a frozen nut.
Reply:Well, actually the rules of thumb for with drawal rates and the rules of thumb concerning exceeding x-lbs of pressure on the working side are separate issues. The 1/7 and/or 1/10 guidelines are to prevent drafting acetone out of the tank. The working pressure guidelines are because of the fact that once removed from the acetone, acetylene tends to become unstable above certain pressures. That's why it was in acetone to start with.
Reply:The 1/7 rule has beenh in effect for at least 50 years, I know of, and it is dangerous, thats why the mfg. of equipment are careful when recommending the cfh for the tips and the 15# outlet is in effect at all times.  John  SMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:Sandy is correct: there is NO relation between the 1/7 capacity rule and the 15PSI rule other than the incidental one that the acetone is there to stabilize the acetylene. None.Long explanation warning:The 1/7 rule has to do will pulling acetone from the tank-- the acetylene is dissolved in the acetone very much like CO2 is dissolved in carbonated beverages. At slow rates, the gas works its way to the surface while still in solution and comes out at the surface, the same way that a bottle that isn't capped tightly goes flat. Once a (temperature dependent) concentration of gas forms above the liquid, the gas comes out of solution and redissolves at the same rate (chemists call this an equilibrium.) As you allow gas out of the head space in the bottle, the pressure goes down, and gas redissolves more slowly than it comes out of solution, tending to restore the gas concentration in the head space.As long as the withdrawl rate is slow, there is essentially no 'bubbling' at the acetone surface. If the withdrawl rate is too high, you get the same effect as a diet soda (diet has no sugar, and doesn't foam up easily. Acetone doesn't foam easily, either), where as the bubbles surface and break, they spatter liquid. This liquid can be carried over through the valve, into the regulator and lines, etc. Acetone burns fine (not as hot as acetylene), but it will do a number of things that are a problem: It may damage gaskets and seals in the regulator, it will through the flame out of neutral (more oxygen is needed for acetone than acetylene), it may damage hoses, but, most important, there is less acetone for the acetylene to be dissolved in. This can increase the head space in the bottle (kept small so as to reduce the chance of a detonation), as well as screw it up for the refiller (they need to replenish the acetone when it is below a safe level) or for the next user (reduced value, as less acetone will hold less acetylene). Note that a little acetone vapor is carried in the gas anyway, but that is a normal loss and is small. It is a fairly constant and predictable loss.As to the 15PSI rule: Think of an explosive like nitroglycerin. It doesn't need oxygen to go off. It explodes throuh a chemical decomposition, not a burning like gasoline.Acetylene can do the same thing. It does it all of the time. A molecule lets go here, a molecule lets go there. If only one molecule decomposes, not much energy, and if there arn't too many others in the area ready to let go, nothing else happens. As pressure increases, the molecules are forced into closer proximity and hit more often. This increases the chance that when one molecule lets go, another will be nearby that can be triggered to let go by the impact of the pieces (sounds a lot like the description of a nuclear chain reaction, no? Different mechanism--this is chemical, not nuclear--, less energy involved, same concept for propagation). As temperature goes up, the impacts will be harder (and the molecules will, on average, be closer to letting go spontaneously).The 15PSI rule comes about because at the highest temperatures normally used, the decomposition won't run away at 15PSI (which, at sea level is about 30PSI relative to vacuum) The risk goes up quite quickly with pressure, and fairly quickly with temperature. IIRC, at 80 deg F, roughly 45PSI absolute (30PSI on the gauge at sea level) is the point where detonation becomes almost certain. (I can't find the chart in a quick search... I'll look later if I have time)Why doesn't the gas detonate in the valve or the tank head space? Because the ports are kept small enough that as the spontaneous decomposition goes on, the energy is removed by the surrounding metal rapidly enough to prevent running away. If the ports are too large, the likelihood of the decomposition products finding another molecule to trigger before giving the energy away to the metal becomes high enough that runaway is likely (similar to the 'critical mass' concept for nuclear systems, a lot depends on configuration. The critical mass is smaller for a sphere than a cylinder, for example)
Reply:Dug out a few references, and can't edit the previous post, so a couple of fixes and clarifications on my previous post re: the 15PSI rule:At 'normal' temperatures, there is almost no risk of spontaneous detonation, but it can be triggered in several ways, heat being the principle one, shock being another. The level of shock required is pretty high, and is the mechanism that propagates the detonation once initiated. The propagation rate is low enough that the shock wave will damp out and detonation won't propagate at lower pressures. As pressure goes up, propagation becomes more likely. The small passages in acetylene equipment and in arrestors remove energy both by cooling and by damping action.Not sure where I crossed my wires and to get spontaneous decomposition as the trigger. Been a while since I dealt with the chemistry in a direct manner. As to the 80degF at 30PSI being particularly significant, I was thinking of conditions from a particular process.
Reply:Hey, enlpck? I noticed your location and was wondering if you mean "armpit" as you would say "city of Lincoln" you might say "armpit of New Jersey". Just joshing with you, I worked a job in Delanco a while back.SA200,Ranger8,Trailblazer251NT,MM250,Dayton225AC,T  D-XL75,SpoolMate3545SGA100C,HF-15-1  RFCS-14 When I stick it, it stays stuck!
Reply:thank you gentlemen for this very informative post. i guess ive been taking my bottles for granted and havent had any problems so far but i learned some things on this post. again, thank you.
Reply:Originally Posted by mooseyeHey, enlpck? I noticed your location and was wondering if you mean "armpit" as you would say "city of Lincoln" you might say "armpit of New Jersey". Just joshing with you, I worked a job in Delanco a while back.
Reply:Very informative thread, I must say. I had always wondered why every time anyone tried using a rosebud for heating in our shop...there was that incessant backfiring ?? Now I know the reason, we only have small acetylene cylinders on hand. Has anyone ever connected two in tandem to obtain an increased volume ?
Reply:Yes, one time I manifolded 26 large acetylene for a flame hardening project with the largest head that union carbide made.  JohnSMAW,GMAW,FCAW,GTAW,SAW,PAC/PAW/OFCand Shielding Gases.  There all here. :
Reply:Originally Posted by enlpckSandy is correct: there is NO relation between the 1/7 capacity rule and the 15PSI rule other than the incidental one that the acetone is there to stabilize the acetylene. None.Long explanation warning:The 1/7 rule has to do will pulling acetone from the tank-- the acetylene is dissolved in the acetone very much like CO2 is dissolved in carbonated beverages. At slow rates, the gas works its way to the surface while still in solution and comes out at the surface, the same way that a bottle that isn't capped tightly goes flat. Once a (temperature dependent) concentration of gas forms above the liquid, the gas comes out of solution and redissolves at the same rate (chemists call this an equilibrium.) As you allow gas out of the head space in the bottle, the pressure goes down, and gas redissolves more slowly than it comes out of solution, tending to restore the gas concentration in the head space.As long as the withdrawl rate is slow, there is essentially no 'bubbling' at the acetone surface. If the withdrawl rate is too high, you get the same effect as a diet soda (diet has no sugar, and doesn't foam up easily. Acetone doesn't foam easily, either), where as the bubbles surface and break, they spatter liquid. This liquid can be carried over through the valve, into the regulator and lines, etc. Acetone burns fine (not as hot as acetylene), but it will do a number of things that are a problem: It may damage gaskets and seals in the regulator, it will through the flame out of neutral (more oxygen is needed for acetone than acetylene), it may damage hoses, but, most important, there is less acetone for the acetylene to be dissolved in. This can increase the head space in the bottle (kept small so as to reduce the chance of a detonation), as well as screw it up for the refiller (they need to replenish the acetone when it is below a safe level) or for the next user (reduced value, as less acetone will hold less acetylene). Note that a little acetone vapor is carried in the gas anyway, but that is a normal loss and is small. It is a fairly constant and predictable loss.As to the 15PSI rule: Think of an explosive like nitroglycerin. It doesn't need oxygen to go off. It explodes throuh a chemical decomposition, not a burning like gasoline.Acetylene can do the same thing. It does it all of the time. A molecule lets go here, a molecule lets go there. If only one molecule decomposes, not much energy, and if there arn't too many others in the area ready to let go, nothing else happens. As pressure increases, the molecules are forced into closer proximity and hit more often. This increases the chance that when one molecule lets go, another will be nearby that can be triggered to let go by the impact of the pieces (sounds a lot like the description of a nuclear chain reaction, no? Different mechanism--this is chemical, not nuclear--, less energy involved, same concept for propagation). As temperature goes up, the impacts will be harder (and the molecules will, on average, be closer to letting go spontaneously).The 15PSI rule comes about because at the highest temperatures normally used, the decomposition won't run away at 15PSI (which, at sea level is about 30PSI relative to vacuum) The risk goes up quite quickly with pressure, and fairly quickly with temperature. IIRC, at 80 deg F, roughly 45PSI absolute (30PSI on the gauge at sea level) is the point where detonation becomes almost certain. (I can't find the chart in a quick search... I'll look later if I have time)Why doesn't the gas detonate in the valve or the tank head space? Because the ports are kept small enough that as the spontaneous decomposition goes on, the energy is removed by the surrounding metal rapidly enough to prevent running away. If the ports are too large, the likelihood of the decomposition products finding another molecule to trigger before giving the energy away to the metal becomes high enough that runaway is likely (similar to the 'critical mass' concept for nuclear systems, a lot depends on configuration. The critical mass is smaller for a sphere than a cylinder, for example)
Reply:Originally Posted by weldgaultIf you knew what you were talking about, you would be dangerious.   John
Reply:Originally Posted by TinbasherVery informative thread, I must say. I had always wondered why every time anyone tried using a rosebud for heating in our shop...there was that incessant backfiring ?? Now I know the reason, we only have small acetylene cylinders on hand. Has anyone ever connected two in tandem to obtain an increased volume ?
Reply:Originally Posted by Joe HCare to elaborate?He has simplified the explanation some, but is pretty much on the money.
Reply:Originally Posted by weldgaultBe happy to, but not on an open forum.  John
Reply:FIRE AND EXPLOSION HAZARDS:*Pure acetylene can ignite by decomposition above 15 psig; therefore, the UEL is 100% if the ignition source isof sufficient intensity.GASEOUS ACETYLENE IS SPONTANEOUSLY COMBUSTIBLE IN AIR AT PRESSURE ABOVE 15 PSI(207 kPa.). It requires a very low ignition energy so that fires which have been extinguished without stoppingthe flow of gas can easily reignite with possible explosive force. Acetylene has a density very similar to that ofair so when leaking it does not readily dissipate. Gas may travel to a source of ignition and flash back.Fires involving acetylene occur occassionally at fusible metal pressure relief plugs at the tops and bottoms ofcylinders, commonly due to hot metal or slag being dropped on the fusible plugs. When the fusible plug releasesa large volume of acetylene will rush out, creating a "roaring" sound. The flame may extend a foot or two awayfrom the cylinder until the pressure is reduced. In some cases, the other end of the cylinder may develop acoating of frost.http://zenstoves.net/MSDS/Acetylene.pdf
Reply:""Acetylene cylinders are special. Acetylene becomes unstable and can explode under a slight shock when its pressure is above 203 kPa (1 kilo Pascal = 1000 Newton per m2) or about 29.4 psi. Therefore safety regulations prohibit its use at a pressure of 105 kPa (15 psi) or higher.""http://www.welding-advisers.com/Oxya...ding-tips.html