Gas burner for pot boiler

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jefran
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Gas burner for pot boiler

Post by jefran » Fri Jul 15, 2022 6:24 pm

I have been overhauling the 16mm 2-6-4 tank engine I made 30 years ago but which hasn't run for 20 years. She hadn't been touched in all that time and something had seized so I ended up dismantling everything above the frames to get at the trouble and then much of the valve gear to be able to sort it out. All that is done and I had the chassis on blocks running on air in both directions. I retested the (pot) boiler, sorted out the radio control and finally got to the point when a steam test was in order; I went outside, filled the tank with butane and lit the burner: it made its usual gentle roar, but after a minute or two started making surging-like noises and went out. It was a little breezy today, but things weren't any different when I went in the garage. The burner is basically a piece of 1/2" copper tube with a jet (from a small blowlamp) in one end, some air holes just in front of that and then a load of slots cut across at about 1/8" intervals in the portion inside the external firebox/shroud. The end is closed with a brass disc silver soldered in. Gas is taken from the bottom of the tank and passes round the edge of the firebox to a needle valve and thence to the jet. This arrangement worked quite well as I recall, but the copper tube, being kept well annealed by the heat, is very soft and the metal between the slots distorts very easily, so some slots are wider than others. I suspect that I am going to have to make a new burner, but wonder if a different style night be better, fine holes rather than slots, stainless tube rather than copper, gauze over the tube? What is current thinking on how to arrange poker burners for pot boilers?

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Re: Gas burner for pot boiler

Post by TonyW » Fri Jul 15, 2022 6:44 pm

jefran wrote:
Fri Jul 15, 2022 6:24 pm
Gas is taken from the bottom of the tank...
That's unusual. Normal practice is to come off the top of the tank to avoid liquid gas being fed to the burner. It causes surging when that happens.
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Re: Gas burner for pot boiler

Post by jefran » Fri Jul 15, 2022 7:17 pm

That was the advice when I made it, and the pipe round the firebox is to ensure that it all vaporises. It has occurred to me that one thing is different from previous runs; because I didn't have much gas, the tank wasn't completely full of liquid.

The fire has been the most troublesome part of this whole project - I have 2 or 3 meths burners that I couldn't make work.

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Re: Gas burner for pot boiler

Post by jefran » Sat Jul 16, 2022 10:57 am

To work out whether the problem is the burner proper or the supply I made a temporary connection direct to a canister of gas; it was nearly empty, so not much liquid to make its orientation very important, but I kept it pointing generally uphill. There was still some inconsistency in burning, but it only went out once in 20 minutes or so, so that was a definite improvement. The burner remains noisy, easily competing with the loco's exhaust and I noticed some blue flames just in front of the jet, visible through the air holes. Is that a matter of design, or something more simple? The burner was never silent, but I never noticed those flames before, though if the loco was performing well then I wouldn't have looked.

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Re: Gas burner for pot boiler

Post by GTB » Sat Jul 16, 2022 1:59 pm

jefran wrote:
Sat Jul 16, 2022 10:57 am
The burner remains noisy, easily competing with the loco's exhaust and I noticed some blue flames just in front of the jet, visible through the air holes. Is that a matter of design, or something more simple?
There was a very good reason that Brian Wilson and Paul Blake titled the chapter on making gas burners 'The Black Art of Butane Burners' in Brian's book 'Steam Trains in Your Garden'.......... The book is currently out of print, but a useful reference for garden scale model locos, if you can find a copy.


The gas isn't supposed to burn inside the tube of a poker burner. If it is, it means the slots are too large, as the flames should only be on the outside of the tube. The slots (or holes) in a poker burner must be small enough for the gas velocity to be greater than the flame velocity to keep the flame on the outside.

I've not built a gas fired pot boiler, so no direct experience to go on. I prefer to use the single flue/poker burner arrangement which has been the common commercial system for some time.

Until it was withdrawn fairly recently, the Roundhouse Millie model was one that had a gas fired pot boiler. It would be worth talking to Roundhouse to see if they still have spare burners for Millie available. If that burner is no longer available, or you want to try building your own, there is a small illustration on their website with a longitudinal section and a cross section of the Millie burner arrangement. Look under the 'Technical' menu for the 'Technical Info' page and then the page named 'Gas Firing Systems'.

Taking liquid butane from the tank and then vaporising it with boiler heat before it gets to the burner, is an excessively complex system that the G1 mob came up with to stop the tank freezing at high gas flow rates. It's a lot simpler to just put the tank in a water bath filled with lukewarm water.

My Garratt was built by Brian Wilson with two flues and Roundhouse burners in the boiler, with the gas tank in a water bath on the rear engine unit. Works like a charm while delivering twice the gas flow of single burner systems and only needs one simple control valve.

Personally I dislike the slot type of poker burner (still used by Accucraft) and I use rows of holes like the Roundhouse type, which runs much more quietly. I've made about half a dozen in two sizes to date, a larger one for a 3/4" flue in a 2" boiler and a smaller one for a 5/8" flue in 1.75" or 1.5" boilers. A properly working burner should be cooled by the gas and air flow, so if the burner tube glows red there is something wrong. Mine are all made from brass, which is easier to work with and the oldest ones show some surface oxidation, but no heat damage.

Another option for gas firing a pot boiler is a ceramic burner, which some people swear by. The only one I've built was for use with a vertical boiler, but it was unreliable in service and I finally replaced it with a poker burner. Your mileage may vary..........

Have fun,
Graeme

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Re: Gas burner for pot boiler

Post by jefran » Sat Jul 16, 2022 4:42 pm

Graeme,

Thank you for a most informative and comprehensive reply. I had suspected that the slots were too big, they may have always been so, but some of them are larger now than they were before. Having made everything apart from the handrail knobs, dome cover, feed check valve, fasteners and radio control, I would prefer to make the replacement burner; looking at the Millie burner it is 10mm in diameter with 3 rows of 26 holes that look to be around 1.2mm diameter at about 2.5mm pitch. Can I ask what size of hole you are using in your flue burners? Since I made this loco (30 years ago) I have had some experience with flame arresters and it has just occurred to me that end of line arrester is doing something relevant in making sure that any fire doesn't get inside the pipe, and that the minimum experimental safe gap for the gas concerned might be a good start. The book seems very hard to find, but I shall keep looking.

Thanks again,
Andrew

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Re: Gas burner for pot boiler

Post by artfull dodger » Sun Jul 17, 2022 3:00 am

I am running a Bix ceramic gas burner set up in my O gauge Corgi/Bassett-Lowke Stanier Mogul. Rather than try to fab up something, which I am not that good at. I used their set up designed for the Mamod SL1 locomotive and adapted it to the O gauge loco. Gas valve is fed from the top of the tank in the typical fashion, tank is mounted upright in the tender. Someday I will order a bespoke tank that has the same capacity but fits down in the tender better making it less obtrusive. I keep the gas turned right down till the ceramic burner goes radient, then I turn it up but not so much that the flame wraps around the boiler and makes the hand rails glow! This has helped this engine run much better outdoors where the stock vaporizing alcohol burner struggles on a calm day, let alone any type of breeze. I also did this to a friends Crimson red LMS version and this has prevented the black sooting of the paint work on the boiler from the alcohol fuel.
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Re: Gas burner for pot boiler

Post by GTB » Sun Jul 17, 2022 3:56 pm

jefran wrote:
Sat Jul 16, 2022 4:42 pm
I would prefer to make the replacement burner; looking at the Millie burner it is 10mm in diameter with 3 rows of 26 holes that look to be around 1.2mm diameter at about 2.5mm pitch.
I suspected you might decide to go down the scratch building path.........

My large burners are basically a copy of a Roundhouse internal flue burner with a different mounting system. Like the RH GBI burner they have three rows of 1.4 mm holes, with 16 holes per row at 2.5 mm pitch and the centre row offset longitudinally by half a pitch. I drill them in the mill, as my bench drill wanders around too much when drilling small holes.

Sounds like you have access to a Roundhouse Millie burner, so it would be better measure that up accurately and to start with whatever hole size they use in an exterior burner. It's a lot easier to make a hole bigger if necessary, than make it smaller.

Flue poker burners have an interesting property, in that they only light inside a flue and won't work properly in free air. If they do work outside, they won't work properly in a flue. It wouldn't surprise me if an exterior burner was the other way round and needs different dimensions in some parts to work properly. The firebox arrangement presumably has some effect as well.

Something else to keep in mind is jet size. Butane burners have a narrow operating range and there is a goldilocks zone for the jet size. Roundhouse use a 0.20 mm jet and I stick with using RH jets in my large burners. My small burners all use a 0.15 mm jet.

It took three attempts to make my first working burner and along the way I found that using a jet one size larger meant the boiler could not be stopped fron continuously blowing off. Using a jet one size smaller and the boiler never reached operating pressure.

That lot should keep you occupied for a while. Let us know how things work out.

Regards,
Graeme

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Re: Gas burner for pot boiler

Post by jefran » Sun Jul 17, 2022 5:59 pm

Thanks very much - I will make a start when I can get hold of a suitable piece of tube. I found some good pictures of a Millie burner, rather than a real one, so I will use your sizes as a starting point and keep you posted on the results.

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Re: Gas burner for pot boiler

Post by GTB » Mon Jul 18, 2022 2:42 am

jefran wrote:
Sun Jul 17, 2022 5:59 pm
Thanks very much - I will make a start when I can get hold of a suitable piece of tube.
Roundhouse appear to machine their burners out of the solid and I do the same.

You can't easily get metric brass rod here in OZ, let alone heavy wall tube, so I use a piece of 3/8" round brass and drill it out to 7.5mm. I've got a vague memory that using thin wall tubing such as K&S makes the burner noisier than it needs to be. A thick wall tube is likely more heat resistant as well.

As mentioned, I use the mill to drill the rows of holes. Drilling holes in round tube is a pain at the best of times and I use a 2mm spotting drill to mark the centre pops for the 1.4mm drill.

There's some sort of secret herbs and spices involved in Roundhouse burners. Mine are never quite as silent as RH ones..........

Graeme

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Re: Gas burner for pot boiler

Post by jefran » Mon Jul 18, 2022 3:26 pm

Graeme,
Thank you; that saves me going and trying to get some tube! I will go for bar and try making a simple drilling jig for the tiny holes.

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Re: Gas burner for pot boiler

Post by CharlesMac » Sat Jul 23, 2022 1:16 pm

I have made burners using the method of drilling holes in thick wall tubing. The problem I find is keeping the flame from blowing out or off of the burner when in operation. My solution is to place a roll of #80n mesh screen inside of the burner tube. This will hold the flame to the burner tube under all conditions of operation. I roll the screen around a drill bit just small enough that the roll can be inserted into the tubing. I have made a gas burner for a Roundhouse " Old Colonial " locomotive to replace the original alcohol burner. The burner now gives me 30 plus minutes of operation of this locomotive with no trouble.
I have modified all of my Accucraft locomotives with the mesh in the burner tube. This has made them much quieter and easier to light.
The mesh roll should be long enough to reach the end of the tube and just touch the edge of the air inlet holes outside of the boiler. If the roll needs to be removed for any reason the roll can be pulled out with a pair of needle nose pliers.
My Accucraft "Earl " has had this modification for 13 years with no trouble in operation.

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Re: Gas burner for pot boiler

Post by jefran » Thu Jul 28, 2022 4:21 pm

Charles,
As the mesh shouldn't be in the flame, I suppose that it wouldn't have to be from anything special like Inconel - what are you using?

I finally have some progress to report; as the old burner was made from 1/2" copper tube, and the holes and locations in the external firebox were made to suit, I decided to at least start off with 1/2". I found some 1/2" 16g tube (say 1.6mm wall) on E-bay and made a jig to help drilling the 114 1.2mm holes (3 rows of 38); it took a while to work out the technique for drilling, and the random effect at one end is so bad I would prefer not to show a picture of it. In the end, the best results came from marking out, a dotting punch and holding the drill with about 3/16" poking out of the chuck. I annealed the tube after punching and holding it in a drilling vice managed to drill the last 40 1.1mm holes without breaking a drill compared with about 5 or 6 holes per drill before. I noticed that the air holes just in front of the jet on the Millie burner were a lot smaller than those I had in my old burner so I did the first test with no air holes at all; I got some big yellow flames with the burner in open air. I tried larger holes until I got to 2.8mm and no yellow, so I have stuck at that. I tried it in the external firebox and got a glimpse of blue flames so I have reassembled the whole thing, leaving the feed arrangement as it was for now. It didn't seem to make a difference now whether the canister was upright, lying flat or upside down. Further testing will have to wait until some new O rings arrive as much more gas dripped out of the filling set-up than went into the tank.

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Re: Gas burner for pot boiler

Post by CharlesMac » Fri Jul 29, 2022 2:53 am

I used #80 mesh brass screen as it was available and not very expensive. As the flame burns above the burner tube the screen doesn't get hot enough to melt or deform. My Accucrft " Earl " has had this modification in place for 14 years now with no change in operation or burn through.

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Re: Gas burner for pot boiler

Post by jefran » Sun Jul 31, 2022 4:21 pm

I got the new O rings yesterday and was able to give the new arrangements a better trial. I filled the tank until liquid came out of the vent at the top, opened the needle valve and lit the burner; it is so quiet that the ambient noise outside made it hard to tell whether it was burning, and in the first few minutes it went out a few times, but it then ran nicely for about 20 minutes, with the loco running on blocks. I had to put some more water in a couple of times but the initial tests seem promising. The valve gear needs some adjustment, but I hope to have the loco pulling trains and then we will really see how it's doing.

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