Thread: LPG neutral flame

Originally Posted by phelum

Hi,

I'm using an oxy/LPG torch for brazing. The torch has a standard acet tip (size 20) rather than a special LPG tip.

The problem I have with LPG is that I can't tell whether I have a neutral flame because there is no second cone as there is with acetylene.

Can anybody tell me how to correctly adjust the flame and what I should look for ? Currently I turn the oxygen up until the cone stops shrinking. If I keep increasing the oxygen the hiss gets louder and the cone becomes rough and less well-defined.

Also, where is the hottest part of an LPG flame ? Just after the cone ?

Thanks,

Assuming that you are using comet style gear you need 554 tips not the 551 tips that are used for acetylene.
Secondly you use a slightly oxidising flame when brazing not neutral.
Not 100% certain on lpg flame adjustment but I would suggest that you are fairly close to the mark in the procedure that you are using as what you have described mirrors what would be observed using acetylene, (louder hiss etc). Generally when using acetylene the shortening of the cone signifies the start of the oxidising range of the flame.
It sounds like you are brazing something fairly substantial if you are using a no 20 tip so concentrated heat will be of less importance than overall heating of the work.
Have a play and see what works best.

Originally Posted by Karl Robbers

Assuming that you are using comet style gear you need 554 tips not the 551 tips that are used for acetylene.
Secondly you use a slightly oxidising flame when brazing not neutral.

I am using Comet gear and have belled the end of the tip slightly so it's like an LPG tip. This does help.

Today I was silver brazing some smaller brass work and used a standard size 15 tip. What I could see is that increasing the oxygen shortens the cone to a point and then the cone goes fuzzy and lengthens. Presumably the sweet spot is just before (mix wise) the cone is shortest.

Originally Posted by Karl Robbers

It sounds like you are brazing something fairly substantial if you are using a no 20 tip so concentrated heat will be of less importance than overall heating of the work.

It's a model boiler made from 3mm copper. I have to have it in a hearth with a background LPG burner keeping it warm. Otherwise the local heat required is extreme and there's too much risk of melting bronze fittings or even the copper.

Thanks for your reply. If I could afford a low-velocity LPG mixer and heating tip I would because the gas velocity from the welding tip blows the flux and even the braze away if I'm not careful.

Thanks,

Hi mate, I have a very old CIG Colt oxy-lpg set. In the past I have used a cutting tip for brazing heavy parts. I found it gives a nice soft flame with plenty of heat.

Originally Posted by jack620

Hi mate, I have a very old CIG Colt oxy-lpg set. In the past I have used a cutting tip for brazing heavy parts. I found it gives a nice soft flame with plenty of heat.

Thanks, this is a good tip (no pun intended). I borrowed a Colt handle with the cutting tip and it worked well. Also, I've tried my Comet O/A cutting tip and it doesn't work nearly as well. The Colt O/P tip has more fuel nozzles and they're bigger.

My O/A cutting tip is size 12. I'll get an O/P tip (say size 20) and give it a go.

Cheers,

Originally Posted by phelum

Thanks, this is a good tip (no pun intended). I borrowed a Colt handle with the cutting tip and it worked well. Also, I've tried my Comet O/A cutting tip and it doesn't work nearly as well. The Colt O/P tip has more fuel nozzles and they're bigger.

My O/A cutting tip is size 12. I'll get an O/P tip (say size 20) and give it a go.

Cheers,

One thing concerns me with your posts so far and that is thet you seem to be confused regarding the tips to use with LPG. Oxy Acetylene tips are not designed for LPG use. I cannot comment on the welding tips, but the cutting nozzles are totally different and are in fact a two piece design for Oxy LPG use. Type 551, 91 and 41 for Acetylene and 554, 94 and 44 for LPG. The use of a cutting tip certainly has merit, perhaps even a heating tip may be the best for large workpieces.
They are two totally different fuel gasses, (for info LPG requires roughly 4 times as much oxygen to be mixed compared to Acetylene) and the appropriate gear should be used. An added degree of difficulty if your boiler has much size in it will be the conductivity of the copper removing heat rapidly fron the working area.
Just make sure that you attain a good strong joint as if your boiler should let go then I an sure the results will be unpleasant to say the least.

Originally Posted by Karl Robbers

One thing concerns me with your posts so far and that is thet you seem to be confused regarding the tips to use with LPG. Oxy Acetylene tips are not designed for LPG use.

Hi Karl,
Thanks for your concern here. I might be sillier than I realise. I was going to buy an LPG tip (size 15) but it seemed a bit small and a few people told me the only difference is that the end of the orifice is belled presumably to reduce the gas velocity as it leaves the tip. So I put a 60° countersink on the end of my size 20 tip and polished it. If I use my size 15 O/A tip (unmodified) I can see the difference and can blow the flame out with too much oxygen. Am I missing something here ?

With the cutting tips I borrowed a Colt handle with its LPG cutting tip and this worked well and was the only tip small enough to get at joints inside the boiler. When I tried my Comet cutting tip (type 41 size 12) I couldn't get nearly as much heat out of it. So I've ordered a type 44 size 20 cutting nozzle for my Comet. Does this sound reasonable ?

Would I be better off trying a heating tip ? There is one that will fit my mixer but it's not marked as specifically for LPG. There are LPG heating tips but I'd also need a barrel and injector and it all gets pretty expensive.

Thanks,

Originally Posted by phelum

Hi Karl,
Thanks for your concern here. I might be sillier than I realise. I was going to buy an LPG tip (size 15) but it seemed a bit small and a few people told me the only difference is that the end of the orifice is belled presumably to reduce the gas velocity as it leaves the tip. So I put a 60° countersink on the end of my size 20 tip and polished it. If I use my size 15 O/A tip (unmodified) I can see the difference and can blow the flame out with too much oxygen. Am I missing something here ?

With the cutting tips I borrowed a Colt handle with its LPG cutting tip and this worked well and was the only tip small enough to get at joints inside the boiler. When I tried my Comet cutting tip (type 41 size 12) I couldn't get nearly as much heat out of it. So I've ordered a type 44 size 20 cutting nozzle for my Comet. Does this sound reasonable ?

Would I be better off trying a heating tip ? There is one that will fit my mixer but it's not marked as specifically for LPG. There are LPG heating tips but I'd also need a barrel and injector and it all gets pretty expensive.

Thanks,

I don't think you are stupid or silly at all, but perhaps you haven't got all the relevant information.
I haven't personally used Oxy/LPG welding tips but have used the cutting tips. A quick look at the Cigweld web site leads me to believe that the mixer is the same for either Acetylene or LPG but the tips are different. To be honest, I've never seen anyone use OXY/LPG for anything other than heating and cutting - that doesn't mean it won't work though.
Are you building a boiler from scratch or just putting fittings in an existing boiler?
The reason I ask is that if making a boiler from scratch I would probably try and get the whole thing hot, if inserting fitttings I am thinking I would lean towards a high heat input and working in a localised area. Are you actually brazing or are you silver soldering, (what filler rod and flux are you using)? How big overall is this boiler and what pressure will it run at? Are you working to a recognised design or just winging it? Please remember that steam at pressure is unforgiving.
As you have found out copper has the ability to transfer heat really efficiently, (think copper base cookware), this is working against you when trying to join this material by conducting the heat away from where you need it to melt your filler wire. You mention your flux being blown away by the flame. This is generally caused by "driving" your tip too hard ie. opening your valves on the handpiece too far, causing an excess flow of gas, usually solved by using a larger size tip.
My gut feeling is that you may be better off in this case with Oxy/Acetylene as it has a greater heat input in a more concentrated fashion which may overcome the heat transfer issues that I reckon you are dealing with. Putting the heat in faster than the material can conduct it away would be the best way to explain this. That of course would be yet another expense so I can understand your reluctance.
A photo of your boiler would be helpful to properly understand what is going on.

Originally Posted by Karl Robbers

A quick look at the Cigweld web site leads me to believe that the mixer is the same for either Acetylene or LPG but the tips are different.

Quite true. I can even get the small heating tip (with barrel) and fit it to my mixer. But if I have to go any bigger then it gets much more expensive.

Originally Posted by Karl Robbers

Are you building a boiler from scratch or just putting fittings in an existing boiler?

I'm working on an existing boiler and I'm silver brazing using SBA45. The boiler is about 500mm long and 120mm diameter and made from 3mm copper. To fix anything I have to heat the whole thing using an LPG burner in a hearth and then use the oxy/LPG torch to work on a particular area. It might be possible to heat the local area only but I doubt I could do it. It's easy to melt phosphorus bronze fittings and even copper. So I rely on background heat to reduce the temperature differential.

Thanks for your tip about driving the tip too hard. I'll try some alternatives when they turn up hopefully tomorrow.

For a boiler like this one to be approved it has to be an approved design and go through multiple inspections during construction and then hydrostatic and steam tests once finished. The boiler passed the hydrostatic test years ago but never got to the steam test. Since I've now done work on it I have to go back a few steps in the testing process. Yes, safety is paramount. I've never seen a toy boiler explode and never want to. Attached is a picture of the boiler undergoing a leak test.

Cheers,

I've now tried silver brazing on 3mm thick copper with three tips and post this review. All this was done with my oxy/LPG torch and a big LPG burner as a background heat source.

First I tried a size 32 O/A welding tip. This was modified by belling the end of the nozzle with a countersink and then polishing the flared hole. The size 32 tip is okay but the gas velocity is still too high. The flame cone is rough and untidy. It was worse before I belled the nozzle.

Next I tried a size 20 Oxy/LPG cutting tip. The heating area is still well defined and the gas velocity is much lower. Gas consumption is lower than when using the size 32 welding tip. The silver braze flows to a smoother skin here presumably due to the lower gas velocity. Another advantage here is that the flame cones are hairy when the oxygen is too low and this makes it easy to adjust the mixture and get a neutral flame.

Finally I tried a heating tip (type 551, size 8 x 12). This is for O/A rather than LPG but it still works well. The LPG heating tips need a different mixer and barrel and are therfore expensive to try. The heating area is larger and it's useful for heating silver-brazed bushes so I can remove them. This tip uses a lot of gas and the heated area is too large for my brazing jobs. The gas mixture can be adjusted to get clean flame cones.

The cutting tip is the best of the three above. For small jobs I borrow a Colt handle with an LPG cutting tip which also works well.

My problem now is that the joints have been brazed so many times that the boiler now looks like a stick welder scratch pad. The silver braze in the joints has changed (probably all the cadmium boiling off) and it's harder to melt and has a longer melting range.

Hope this helps someone,

Thanks for the info. I've been meaning to buy myself a larger oxy/lpg tip, but after your experience I think I'll just stick with the cutting tip. At least until I can afford an oxy acetylene setup.