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.
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.
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Until it was withdrawn fairly recently, the Roundhouse Millie model was one that had a gas fired pot boiler.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..........
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.
- artfull dodger
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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.
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..........
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.
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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