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Until it was withdrawn fairly recently, the Roundhouse Millie model was one that had a gas fired pot boiler.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.
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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.
Glad to hear that it worked first time. Burner design is very much a 'suck it and see' exercise, as there is no published design data around for garden scale butane burners. What little info that exists is empirical and about building burners for internally fired boilers.jefran wrote: ↑Sun Jul 31, 2022 4:21 pmit 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.
There are two main reasons I use the Roundhouse type of burner. One - they run quietly, especially when turned down after working pressure is reached, so much so that I have trouble hearing them outside. Two - they are more efficient than the older slotted design.
If you don't have machine tools and skills to make burners, you can use mesh to quieten down the slotted type, but it also restricts the gas flow, which reduces heat output.
To be honest, I'd seriously consider ditching the liquid butane feed arrangement for the much simpler gas feed normally used in commercial models. Some of your lighting and running issues may be due to variation in the liquid feed rate.
Drilling all the holes in poker burners is a problem as you found. My bench drill has enough slop in the quill that it is useless for precision work. I use my small mill for most of my drilling work, as it allows for co-ordinate drilling. Next best choice of machine, in the absence of a mill, would be to use a vertical slide in a lathe.
I now use a spotting drill to put each row of hole centres in the metal, as it's hard to centre punch a long straight row of hole centres by hand. Before a friend put me on to using a spotting drill, I used to use a centre drill (slocombe drill) to mark the centres.
Drilling holes in a tube isn't easy, but by holding the tube in a milling vice, carefully setting up and using spotting drills, I've drilled the holes in seven scratchbuilt burners so far and I'm still using the same 1.4 mm drill.
Unfortunately garden scale live steamers can do that. It depends on how big the compressor is.
I use an old airbrush compressor for testing chassis on air. It is rated at 1cfm, so puts out about 28 litres/min of air. A standard Roundhouse 2" x 6" boiler with one of their burners can only generate about 12 litres/min of steam with the burner flat strap and roaring it's head off. At the normal running setting, it will be generating about 6 litres/min of steam and that's all R/H cylinders need to pull a train, if they are working properly.
Get the burner running on the bench to your satisfaction first. I aim for a steaming rate of around 5 ml/min of water being boiled at working pressure and gas usage of about 1 ml/min. Measuring the water usage is easy. If you have a digital balance (the kitchen variety usually measure to the nearest gram) you can weigh the loco before and after filling the boiler, then at the end of the run to find the water usage. Gas can be a bit trickier to weigh in the loco. I usually weigh the gas can before and after filling the tank and use all the gas during the run. Some gas is lost by venting during the fill, but the weights will be near enough in the greater scheme of things. The density of water is 1 g/ml and for liquid butane is 0.6 g/ml, so the volumes are easily calculated from the weight.
If the boiler/burner performance is up to scratch, then look at the cylinders and valve gear. If the loco has run satisfactorily in the past, then the likely cause after 20 years of storage is sticky valves, or deteriorated piston rings, allowing internal steam leaks.
I use Roundhouse burners and cylinders as the design yardstick for my scratchbuilt locos. The burner will use 1 ml of butane to boil 5 ml of water per min and that will steam a pair of 9/16" dia x 5/8" stroke cylinders. That can be scaled down for smaller cylinders and within reason, scaled up for slightly larger ones.
I might be completely off target here, but as you have mentioned that it runs OK on blocks( presuming wheels hanging free) it couldn't be that with weight on the wheels, bearing condition or loco attitude on the rails, is not causing a drag or bind somewhere? Just a thought.
As the loco ran on blocks on steam last week, I am hoping that the problem is the intermittent state of the fire, but I will have a better idea tomorrow.
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