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Gỗ Thông Nhập Khẩu Làm Ván Sàn có tốt không

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Hello,

I have a question about torches:  I was taught at school (using an oxy/propane mix) that the rule for safety is Propane, then light, then add oxygen.  For turning off the torch, the reverse - oxygen then propane - is true. 

Recently, I started a welding course at Central Tech, and the teacher was adamant the Rule used by all gas welders is "A before O, or up you go" (A=Acetylene, O=Oxygen).  He further said the same rule applies when using Propane, and that the reason is that the oxygen 'blows out' any chance of the lit propane travelling back down the pipe. 

Can anyone please explain to me why jewellers use the reverse process from welders?  I've searched the internet, and all I can find are disparate answers on the procedure, but nowhere can i find the reason why jewellers would use the reverse procedure for the same gas/oxygen mix when turning off the torch?  IS there a reason?   The closest I've come to is "otherwise you get a nasty 'pop' sound".  

Thanks in advance! 

Liss.  

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In welding operations you would be using the much larger K cylinders and the concern there is that unburned fuel could be released and ignite unexpectedly so therefore the fuel is shut off first and then the oxygen.  Also remember your work environment, an industrial workplace is no comparison to a small jewelry studio.  In jewelry operations, one would be using the much smaller MC and B cylinders and they would unlikely release any significant volume of unburned fuel.  In the Smith Acetylene/Air torches that "nasty pop" is actually a desired sound as it indicates that the unburned fuel has now been burned off.  I was taught to ignite the fuel first, then add the oxygen and I use the reverse for shutting down the torch, with both my oxy/acetylene and oxy/propane torches.  There are times however that my torch has blown out either from getting too close to the work surface or in melting operations from a bit of slag extinguishing the torch and nothing disastrous has happened.  Just the pop.  I always purge the lines first before reigniting the torch after that's happened just to be on the safe side to make sure that the lines are clear.  Don't laugh, but there is a well known instructor that as a student was terrified of turning off the acetylene air torch because of the popping noise it made and would always get one of her other fellow students to ignite and shut off the torch for her.  Clearly she got over the noise.  Anyways, I think this is a case of it's "it's always been done this way" and maybe no one has ever bothered to change or question it.  I would contact the good folks at Smith Torches, they have been very helpful whenever I've had a question regarding their torches or procedures.

Hi Sonja,

Thank you for your clear and concise reply.  I have done some further research, and I will send an email to Smith - their instruction video is what led my teacher and I to have this debate.  I was taught the same as you, and believe me, having to "reverse" what I do instinctively has been my challenge with this class - especially since the large tanks are less than 5 feet away from me!  I had thought about the environment - my current research has also led me to look at the beginnings of torch use in jewellery.  Namely, the origins of the blowtorch - when the jeweller would literally "blow" air from a hose into the gas.  they effectively "turned off" the oxygen first.  

As to your comment regarding a fellow student,  I have known many students who were afraid of their own torches, but who also got over it.   My welding instructor actually uses that as a benchmark to see who will drop the class - If they can't light and shut off the torch without wigging out, they need to rethink their class choice.  

Hi Lissa, well...now you have me really piqued on this issue because inconsistency is a contributing root cause to accidents.  Industrial welding and jewelry soldering is pretty much the same, just the scale is different.  Bigger tanks, bigger torches, bigger objects.  One would think that procedures should be standardized between the two however, even the torch manufacturers can't agree.  For industrial welding there is a difference with what Smith says vs what Victor says, with Victor being aligned with the jewelry brazing procedure of gas on first, gas off last.  Here is a letter I found online, you may have already come across it in your research:

VICTOR A THERMADYNE Company

September 29, 2003

Re: Shutdown Procedure for Oxy/fuel Cutting Torches

To Whom It May Concern:

Victor Equipment Company recommends a shutdown procedure for oxy/fuel torches that is
different to that recommended by most other manufacturers of gas welding and cutting
apparatus. There are times when this difference in procedure may cause confusion and
concern. From this, the following is an explanation behind the logic of our procedure.
Victor recommends turning off the oxygen first, then the fuel when shutting down an oxy/fuel
torch. The primary reason for this procedural sequence is SAFETY. This can best be
explained with the following three reasons:

1. One of the most common causes of backfire and sustained backfire (burning inside
the torch) is overheating of the tip or nozzle. The primary cause of overheating is
insufficient flow (velocity) of gases which allows the >5000° F flame to burn on the
surface of the tip or even slightly inside the preheat orifice(s). Proper gas flow with
sufficient velocity causes the flame to burn slightly away from the tip end.
One of the simplest ways of determining adequate fuel flow when using acetylene is
to have a “soot free” flame when the fuel is on and no oxygen is flowing. Thus, if the
operator turns off the oxygen first, and the flame produces soot, he is reminded that
insufficient flow was used. From this, the operator should either increase the flow or
change to a smaller size tip. (This depends on the application.)

2. All valves have the potential of leaking at one time or another. When the fuel is
turned off first, the flame is immediately extinguished, even though there might be a
small fuel gas leak. If the oxygen valve is closed first, the flame will continue to burn
(hopefully without soot!). Then, when the fuel valve is closed, and if a leak is present,
a small flame will continue to burn. This will immediately warn the operator of a
problem.
Neither shutdown procedure will show an oxygen leak, but at least in the latter
method, the operator can detect the more hazardous of the two.

3. As mentioned, other manufacturers recommend closing the fuel first, and then the
oxygen EXCEPT when there is a sustained backfire (burning inside the torch). In
this case, they recommend immediately closing the oxygen valve first.
Humans are creatures of habit. To recommend reversing the shutdown procedure in
an emergency situation would be like telling an automobile driver to use his right
foot on the brake for normal stops, but in the event of an accident, use the left foot. A
few seconds can mark the difference between a safe shutdown and a damaged
torch, and most importantly, possible injury to the operator.
There have been claims that using our method causes soot build-up inside the torch.
Victor has performed many tests and found that the soot build-up that others claim to occur is caused by improper gas flow and pressure settings, not the sequence of closing the valves. This holds true for all equipment that we have tested.
It should also be noted that all industry standards organizations, such as ANSI, AWS,CGA, and NFPA, recommend the operator follow the manufacturer’s instructions for proper operation of the equipment. To this extent, there’s not a “golden rule” in the industry. Since there are basically two different ways of shutting down an oxy/fuel torch, we feel our method provides the safest way.

Sincerely,

David Pryor
Manager, Product Technology and Advancement

From where I am sitting, this reply from Victor Thermodyne makes absolutely the most sense, especially in getting people to reverse what they have been taught to do in an emergency situation.  Firstly how often do people get to "practice" responding to a welding flashback emergency.  As I mentioned, I've had a couple of situations where the torch was extinguished accidently and the first thing I'm reaching for is the oxygen knob on my torch.  Even the placement of the oxygen knob on the Smith Little Torch is not random, it's on the right hand side of the torch.  The majority of people are right handed so hold the torch in their left hand, leaving the dominant right hand to react to the environment.  If the O2 was on the left hand side of the torch , it would be awkward as hell to try and shut it off in an emergency situation.

Smith apparently recommends the gas on first, gas off first procedure for it's industrial welding procedure, HOWEVER....go to the Riogrande Youtube channel and there will be the Smith representative showing the opposite when it comes to the jewelry torches:

Propane: https://www.youtube.com/watch?v=Ih-5fMLMEKc

Acetylene: https://www.youtube.com/watch?v=THgExtwB4fk

It'll be interesting to see what Smith comes back with because they truly are at odds with what they are saying for industrial welding and what they are saying for jewelry brazing/soldering and there must be a reason for it.  However I think Victor is on the right path and maybe it's time for industry to change; the old ways are not always the best ways.  Good luck with your welding course and your new task in reforming the industry, you could be on to something very positive here!

Hmmm, I've casually wondered this too.  I was a welder and have migrated into jewellery.  Besides the startling "pop" sound, I was told as a welder that by turning off your acetylene first then the oxygen, that the flow of oxygen helps keep the tip clean. Since acetylene burns such a dirty sooty flame, if you turn it off last you're leaving all that soot to settle in the tip etc.  Out of habit, from having learned the welder's way first, it is still the way I turn off my jewellery torch, even though propane is a much cleaner flame.

Very interesting to read the letter in Sonja R's response. 

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