Thread: Oxy-LPG cutting

Earlier this year I picked up a brand new "TradeFlame" brand Oxy-LPG cutting and welding kit, with the intention of using it mainly for heating and occasional cutting. I haven't done a huge amount, but enough oxy-acetylene cutting over the years so was comfortable with how to use a cutting torch. I'd prefer acetylene but couldn't justify the additional bottle cost when I've got plenty of LPG around.

Well this damn oxy-lpg cutting caper has had me stuffed. Didn't seem to matter what I did with it I just couldn't get it to cut well.
I know the lpg flame is different, and the hottest part is further out from the tip than with acetylene but that wasn't the issue. Most of the time I couldn't even get the torch adjusted to where it wouldn't pop and flame out when you hit the cutting lever. Pressures were set up as per the chart in the kit for the particular cutting tip, and I'd set them while flowing gas.

Finally I had to walk away and consult Youtube before the torch ended up in the rafters....
After a watching a few unhelpful videos, I came across this one:
https://www.youtube.com/watch?v=f2amO0T3vkQ
Its a bit long winded but if you jump to 5:50 for the procedure to set up the torch prior to cutting. Subtly different to what I would have done with oxy-acetylene - particularly the way he described a 3-stage increase of fuel/oxy to get to the correct final setting.

Maybe its just me having done it incorrectly all these years, but with acetylene cutting I'd always just light the torch, open the fuel valve until its a reasonable flame (somewhere between sooty and flamethrower), open the oxy to get nice neutral heating cones, then check again/adjust with the cutting lever pressed - then get cutting. Same theory doesn't work with LPG from my experience.

Hope it helps someone else.

Steve

Hi Steve, It's been a while since I've done any LPG cutting, but what he has shown is pretty well how I remember it. It's totally different to Oxy Acetylene.
From memory you should also get a neater cut.
Kryn

Hi Steve,

Some years ago I had a need to cut some large pieces of steel. I bought an oxy lpg kit S/H, mainly for the same reasons about cost.

I had never done any acetylene cutting before this and so I was flying blind.

I stumbled upon that same video you listed above. It helped me no end. But I still did not manage to get a neat cut.

I probably never will because I just don't use it enough to get good at.

Simon

Sent from my SM-G970F using Tapatalk

I realized after I’d actually got it working and cut up the 16mm plate “box” that was the focus of all the effort that I still had the largest tip on the torch. I’d put it on earlier to see if it made things any easier.
According to the destructions it should be good for 75mm plate!!
Explains why I used about half a D size oxy for about 2m of cut

I need to have a bit more of a play and write some notes for next time. Like you Simon, I’ll not use it enough to get proficient, but a great option to have when you just need something thick cut that won’t fit in the bandsaw.

Steve

I have not used my lpg tips for a number of years but i dont remember having any problems cutting 25mm plate, it was a bit slower than acetylene but alot cheaper.
I used a comet 3 torch with lpg nozzles and being a little naughty even used the acetylene gauge with a fitting for the lpg bottle.
My BIL has for some time used lpg for profile cutting and often cuts 400mm plate, i think he said he can cut up to 600mm

This is a welcome discussion.

A couple of things I'm not sure of with oxy/LPG cutting. The oxy/LPG cutting nozzle is of course different to an oxy/acetylene cutting nozzle but why is it so, presumably its something to do with how the gases mix?

When increasing the LPG and oxy in stages you can get a situation when each of the mini nozzles has its own well defined mini flame within the whole flame, is that significant, does it tell me anything about the neutrality of the whole flame?

Just my 5 cents worth, early this year I dumped my old, old oxy set and went out and purchased a Hypertherm Powermax 45XP plasma cutter and a 12 CFM air compressor.
What a beautiful handy tool and sure a tad pricey but worth every cent and we have a Mig and a Tig welder so I've given the oxy welding a miss forever.

Originally Posted by Specialized29er

Just my 5 cents worth, early this year I dumped my old, old oxy set and went out and purchased a Hypertherm Powermax 45XP plasma cutter and a 12 CFM air compressor.
What a beautiful handy tool and sure a tad pricey but worth every cent and we have a Mig and a Tig welder so I've given the oxy welding a miss forever.

I started off with oxy welding over 40 years ago, but its been at least 10yrs since I've done any now. From memory I even tried to give the old oxy set away a few years back but got no takers!
The main reason for getting this oxy-LPG kit was heating. I've got a MAPP torch for small stuff but it just doesn't put out enough heat for anything a bit bigger.

That Hypertherm looks like a nice bit of kit, but as you say its pricey at around $3K.
I've got a cheap 35A Aldi plasma cutter which does a nice job on thinner stuff, but it's maxed out around 10-12mm thick. Very useful for the $300 price, and great for cutting off fittings and brackets that you can't get a grinder into.

Steve

Originally Posted by Specialized29er

Just my 5 cents worth, early this year I dumped my old, old oxy set and went out and purchased a Hypertherm Powermax 45XP plasma cutter and a 12 CFM air compressor.
What a beautiful handy tool and sure a tad pricey but worth every cent and we have a Mig and a Tig welder so I've given the oxy welding a miss forever.

Yep - I bought a Chinese 120A plasma cutter, that solved cutting plate 25mm and thinner. Supposed to do up to 35mm, haven't tried it on thicker than 25mm yet.

Not in the same league for quality and efficiency as the Hypertherm units but 1/6 of the cost so - shrug.

I also have oxy-LPG and yes is is a bit of an art if you're used to oxy-acetylene. I had to cut 40mm plate with mine, got the job done but the cut looked like it was gnawed through by a bunch of rats OD'ing on meth. Good thing a 9" angle grinder removes metal fast...

PDW

Originally Posted by pdw

got the job done but the cut looked like it was gnawed through by a bunch of rats od'ing on meth.

03ac48ad-1eb6-599f-abcb-40043de23a26.image.jpg
u wot mate?!
Come 'ere and i'll fight ya!

oi maaaate... can I bum a durry?

Real 3 phase would be nice, but no chance of running a 120A cutter at my place in the foreseeable future unless I run a decent generator

I had another play tonight. Fitted the nozzle for 12-20mm thickness and it cut happily, but I reckon your rats must have jumped ship and taken up residence at my place as the quality was pretty rough. No idea where they're getting the meth from.....

Bob Ward - what I found tonight was that with the line pressures set correctly and the oxy valve at the base of the handpiece opened right up, after working up through the "3 stages" of fuelling I got to the point where the individual cones were nice and clean/defined, but when I pressed the cutting lever they got slightly longer and slightly yellow.
Much more like what I'd expect with acetylene than what I had when messing around last night with the bigger nozzle.
I reduced the fuel slightly until the cones cleaned up again and it cut well.

No idea how that fits in with the neutrality of the whole flame, but for cutting it probably doesn't really matter. The heating flame is mostly there to get the initial temperature, then once you hit the cutting lever the pure oxygen does all the work.

I've read that there's no real difference in the cutting capacity/quality between LPG and acetylene once you're actually cutting, and its mainly that the LPG is slower to preheat before you can initiate the cut which makes LPG slower overall.

Steve

Originally Posted by OxxAndBert

The heating flame is mostly there to get the initial temperature, then once you hit the cutting lever the pure oxygen does all the work.

Way back when as a party tick I would turn the acetylene off once I started cutting. Childish perhaps, but it did turn disbelievers into believers on the whole "oxygen is doing all the work" discussion. "No No, you aren't melting it out of the way you are burning it"

Originally Posted by OxxAndBert

No idea how that fits in with the neutrality of the whole flame, but for cutting it probably doesn't really matter. The heating flame is mostly there to get the initial temperature, then once you hit the cutting lever the pure oxygen does all the work.

I've read that there's no real difference in the cutting capacity/quality between LPG and acetylene once you're actually cutting, and its mainly that the LPG is slower to preheat before you can initiate the cut which makes LPG slower overall.

Steve

A neutral flame is in between 'oxidising' and 'reducing'- ie balanced/clean combustion.
For acetylene the equation is 2 C₂H₂ + 5 O₂ = 4 CO₂ + 2 H₂O ie Burn 2 units of acetylene in 5 units of Oxygen, and get 4 units of CO2 and 2 units of water after combustion.
For LPG it's a bit different as it's a mix of propane and butane- not as chemically pure for the purpose of a simple reaction like above.

Acetylene has a way better heat density than LPG, and also acetylene has a way better combustion balance- Oxy:Fuel approx 1.1:1. Whereas Oxy LPG is approx 4:1. What this means is 'bottle for bottle' acetylene performs better.
The oxy jet does the cutting. On an oxy tip, the smaller holes around the circumfrence of the tip are actually called preheating jets. For thicker materials, the central oxy jet increases to allow more gas flow and oxidising effect on the steel to effect the cut.
Getting back to heat density ('enthalpy of combustion' for the nerds amongst us) the superior property of acetylene allows a faster rate of work- for cutting, heating or welding. Spot on steve.
Acetylene has a better draw-off rate too- LPG phase change liquid>gas can be sluggish at high draw off rates or even low draw-off at lower temps.

But yeah, for pure cutting on a budget, oxy LPG does the job for most. Saw some scrappies doing a train dismantle and LPG was the gas of choice. With a man-pak of oxy- the hooks on the side of the pack conveniently hangs 4x 45KG LPG bottles off the side... Manifolded together the LPG draw off is improved and it's a formidable cutting rig for a team of guys.

Originally Posted by Commander_Keen

and also acetylene has a way better combustion balance- Oxy:Fuel approx 1.1:1. Whereas Oxy LPG is approx 4:1. What this means is 'bottle for bottle' acetylene performs better.

1 bottle of Acetylene requires 1.1 bottles of Ox.
1 bottle of LPG requires 4 bottles of Ox.

How is that better? What am I missing? am I just reading it wrong?

Originally Posted by Commander_Keen

Acetylene has a better draw-off rate too- LPG phase change liquid>gas can be sluggish at high draw off rates or even low draw-off at lower temps.

I seem to recall being told there was a limit to the nozzle size you could run on each cylinder size because above that draw off rate there was a risk of acetone getting into the reg. Plausible?

Originally Posted by Stustoys

1 bottle of Acetylene requires 1.1 bottles of Ox.
1 bottle of LPG requires 4 bottles of Ox.
How is that better? What am I missing? am I just reading it wrong?

That's about the correct 'air/fuel' ratio- keep in mind differences to some extent if you are running a neutral, carburising or oxidising flame, and also cylinder sizes do vary. If we were to be scientific about it we'd me measuring the gas ratios in 'Moles'- but tell that the to guy at the gas shop and he'd just look at you funny.
The benefit some may perceive in not renting/buying/filling an acetylene tank may be offset by their higher oxy use.

Originally Posted by Stustoys

I seem to recall being told there was a limit to the nozzle size you could run on each cylinder size because above that draw off rate there was a risk of acetone getting into the reg. Plausible?

very much so, but your mileage may vary.
The science is in the phase change from liquid to gas. Propane is a liquid in the tank, so is acetylene (mixed in acetone to stabilise it, and in a porous mass in the tank). When a liquid converts to a gas the temp drops (think of it as the opposite of compressing air, which gets hot). This is why you may get a frosty cylinder when you put on all 6 burners on the BBQ some times. The temp drop gives less energy for the liquid to want to change state, hence the sluggishness of your bbq for example in cold weather. Acetylene boiling point is -84 °C, whereas LPG is -42°C ie acetylene is more easily converted to a gas than LPG at most temps we see. This becomes a feedback loop when phase change drops temp, and temp drop reduces phase change.

Then there is the draw off rate- which is also a function of the liquid to gas interface on the cylinder cross surface area- the more area the more available space for a phase change.
This is why a acetylene cylinder diameters change as the bottle gets bigger. Also why LPG is short and squat for a BBQ bottle- to give it a fighting chance in cool weather. You can increase draw-off rates by manifolding cylinders- join them together in parallell so that the gas flow is split among all cylinders in the manifold.

Acetone is used to stabilise acetylene- without it it can decompose at higher pressures leading to all invoolved having a 'very bad day'. Acetone tends to want to stay in it's liquid form in a pressurised cylinder, and you'd not want it going thru the reg -makes for terrible combustion, as well as not being good for the diaphragm in the reg.

TL;DR: Yes, cylinder size affects draw-off rates, and hence your ability to properly run larger nozzles in some instances.