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Bill736
12-15-2010, 10:43 PM
It seems that the great acetylene debate is now a closed thread, just before I had a semi-relevant question. I haven't used my acetylene tank for months. I keep the cylinder tank valve closed, and the regulator screw all the way out ( zero pressure). I also keep the torch valve at the end of the hose closed. If, however, the tank valve had a very small leak, and the regulator also had a very small leak, then the hose could fill with acetylene , and perhaps become unstable. In fact, such a leak would eventually result in tank pressure in the hose. Should I therefore keep the torch valve cracked open, to vent any gas that might build up in the hose ? ( And yes, the tank is upright). In fact, cracking the oxygen torch valve might also be a good idea, yes ?

TGTool
12-15-2010, 11:35 PM
I believe I saw that recommendation in some documentation just recently. I may have been re-reading the Smiths torch manual. I'm not sure I do that but you make a good point about the possibilities and maybe the rationale for the recommendation.

Evan
12-15-2010, 11:44 PM
Not to worry. An acetylene regulator has a built in pressure relief valve on the hose side that will open at around 50 to 60 psi. That is low enough that the acetylene won't reach the critical pressure for the size of the ID of the hose. Cheap regulators may have a one time rupture disk, good ones have a resettable valve.

SVS
12-16-2010, 12:26 AM
This idea of maximum safe cell size is blowing my mind. Any simple way to explain it?

macona
12-16-2010, 12:29 AM
From what I gathered it has to do with the heat of decomposition. With a small area the heat is absorbed by nearby walls. But in an open space the heat will just continue to build and bad stuff happens.

Evan
12-16-2010, 12:57 AM
It isn't heat related. I first learned about this when looking up information on acetylene powered hypersonic shock tunnels a long time ago. I will have look and see what I can find.

SVS
12-16-2010, 01:02 AM
That makes sense Macona.

Lazlo's "Mine Safety for Dummies" poster mentioned "Store cylinders above freezing" ? Other than threading a regulator into a valve slugged with ice I don't see a reason. (Used torch plenty of times in sub freezing temps.)

On edit:I see there may be more to the story.

Evan
12-16-2010, 01:05 AM
Sorry about this but it is the best I can find on short notice.



Results of five experimental investigations on the initiation, propagation and transmission of detonation have shown that the wave behavior depends on the relative rates of gasdynamic expansion and chemical energy release occurring within the cellular detonation front. The former rate is controlled by the boundary conditions defined by the physical system, while the latter rate depends on the chemical and physical properties of the combustible mixture. The fractional increase zeta in the area of th post-shock stream tube, evaluated over a chemical kinetic distance equal to the cell length, has been identified as a parameter which satisfactorily characterizes the competition between these two rate processes. For zeta less than about 20%, the chemical processes survive the gasdynamic expansion and self-sustained propagation is possible. However, under these supercritical conditions, the wave propagates with a velocity deficit which appears to be a universal and theoretically predictable function of zeta. For zeta greater than 20%, the shock/reaction zone coupling breaks down, resulting in failure of the wave. The critical conditions for the propagation of detonation waves subjected to a wide range of expansion inducing mechanisms, including viscous boundary layers, compressible boundary gases and yielding walls, are all found to be consistent with the 20% criterion. However, the criterion becomes inapplicable as the cell size approaches the characteristic transverse dimension of the geometry.

http://adsabs.harvard.edu/abs/1985PhDT........36M

What it means is that below a certain free space cell size the shock wave outruns the reaction zone so the detonation fails.

Weston Bye
12-16-2010, 05:27 AM
Sorry about this but it is the best I can find on short notice.


Results of five experimental investigations on the initiation, propagation and transmission of detonation have shown that the wave behavior depends on the relative rates of gasdynamic expansion and chemical energy release occurring within the cellular detonation front. The former rate is controlled by the boundary conditions defined by the physical system, while the latter rate depends on the chemical and physical properties of the combustible mixture. The fractional increase zeta in the area of th post-shock stream tube, evaluated over a chemical kinetic distance equal to the cell length, has been identified as a parameter which satisfactorily characterizes the competition between these two rate processes. For zeta less than about 20%, the chemical processes survive the gasdynamic expansion and self-sustained propagation is possible. However, under these supercritical conditions, the wave propagates with a velocity deficit which appears to be a universal and theoretically predictable function of zeta. For zeta greater than 20%, the shock/reaction zone coupling breaks down, resulting in failure of the wave. The critical conditions for the propagation of detonation waves subjected to a wide range of expansion inducing mechanisms, including viscous boundary layers, compressible boundary gases and yielding walls, are all found to be consistent with the 20% criterion. However, the criterion becomes inapplicable as the cell size approaches the characteristic transverse dimension of the geometry.




What it means is that below a certain free space cell size the shock wave outruns the reaction zone so the detonation fails.

Wow. Reading that quote is enough to induce Turboencabulator glaze over.

gary350
12-16-2010, 08:31 AM
If an accident can happed it will. There are many things to consider.

Once I was using the oxy/act torch it was the end of the day almost time to go home when I ran out of oxygen. I turned everything off and we all went home.

Next morning the temperature had dropped to about 35 degrees the tanks were cold. Remember the oxygen tank had run out so pressure was at ZERO when I turned everything off but now that it is cold things contract the oxygen tank has negative pressure inside. The hoses have NO safety check valves. As I recall both of the knobs on the torch handle were both closed. I turned on the acetylene tank first then I turned on the oxygen valve. There was pressure in the acetylene hose and negative pressure in the oxygen hose. I always turn on the acetylene knob first then light the torch then I turn on the oxygen and make adjustments. Well this day I had forgotten the oxygen was empty when I ignited the torch it sounded like someone hit the oxygen cylinder with a sledge hammer. BOOM. It scared the crap out of me and everyone else too. Next thing that happened smoke starting coming off the outside of the oxygen tank. The orange paint on the cylinder started turning brown and by then the tank was smoking like crazy. I didn't know what was going on but I did realize the tank has to be hot to burn the paint and make smoke. The paint turned very dark brown. I put my hand near the oxygen tank and it felt like it must be about 800 degrees F.

This is what I think happened. The cold oxygen tank with negative pressure sucked in some acetylene gas when I ignited the torch the flame went down the hose and inside the tank where the explosive mixture ignited and exploded. The explosion is what made the tank sound like someone hit it with a sledge hammer. BOOM. The explosion produced enough heat to burn the paint on the tank.

I went straight to the welding supply store and bought safety check valves and to this day I will not use a torch without them.

Evan
12-16-2010, 09:03 AM
I am curious how you managed to draw the tank to zero pressure. Long before that happens you wouldn't have enough pressure to weld or especially cut. Also, the regulator is one way especially with zero or negative pressure on the tank side. When you adjust the pressure it doesn't open a valve, it changes the amount of gas pressure it takes to bleed past the diaphragm. Check valves are intended to prevent pressure waves from damaging the regulator gauges.

If you ignite the acetylene before adjusting the oxygen and the tank went boom how did the acetylene get there through a closed oxygen valve?

gary350
12-16-2010, 09:37 AM
I am curious how you managed to draw the tank to zero pressure. Long before that happens you wouldn't have enough pressure to weld or especially cut. Also, the regulator is one way especially with zero or negative pressure on the tank side. When you adjust the pressure it doesn't open a valve, it changes the amount of gas pressure it takes to bleed past the diaphragm. Check valves are intended to prevent pressure waves from damaging the regulator gauges.

If you ignite the acetylene before adjusting the oxygen and the tank went boom how did the acetylene get there through a closed oxygen valve?

Well pressure was not exactly zero but very close. I was working on something. Time was getting very short. I was trying to finish before time to go home. I knew it would take 15 minutes to change the tank and there was only about 10 minutes left before we go home. I kept turning up the regulator pressure trying to keep the torch going finally when there was no more pressure I started turning the acetylene pressure down. Flame on torch finally got to small I had to stop. I am not sure if I left the regulators at they were or if I changed one or both back where they were.

I don't know how gas went backward through the oxygen regulator unless it was a defective regulator. That was 25 years ago electronic cameras had not been invented yet too bad I didn't get a video of the oxygen tank smoking and the paint turning brown that would be my proof it happened. But then there would still be a skeptic that would claim I threw the tank in a bond fire or something. All I know for sure is it happened I am not 100% sure how it happened. You call Brad Carter, Tommy Scott, Denis Park and talk to them they were there and they saw it too.