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G'day, Steamchick. Many, many thanks for your input, especially the discussion of compressively stressed copper tube. I do have Kozo Hiraoka's book 'The Pennsy Switcher' and he uses larger flues in its coal-fired boiler. He reckons that fewer large flues are to be preferred. That suits me and my gas intentions, except that he calls up flues of .787" OD x .047" wall thickness, which he says should be doubled if in 7-1/4" gauge, (.094"). Both of these seem a bit light to me at 80 psi., and in fact, I'm considering using .125"x1.75"x450 long tubes for my flues, but I have my doubts about the compressive stress at those sizes, especially as we are, I understand, talking about annealed copper tube. Another project I have on the back-burner, is rebuilding a 6" dia. stationary boiler I built years ago, which has a 4-1/2" (approx) inner firebox. All from 3mm copper tube and the inner firebox has bulged inwards: the project is rebuilding and installing radial stays at the firebox. The design was from Model Boilers and Boilermaking, by K N Harris, design no 9, page 163. As you say, better to use somebody else's design.....hmmmm.....
My Sweet Pea-type boiler, with round firebox, will certainly be stayed. That's one of the 'improvements'!
I've been fiddling for what seems to be ages with gas burners/burner configurations and as you say, enough primary air is THE issue and nozzles are 'interesting', shall we say. I'm pretty competent with 0.4mm holes! I have a 'range' of jets in 0.4, 0,.5, 0.6, 0.8mm etc sizes and all sorts of weird and wonderful gadgetry, to supposedly control air. I have quite a few venturis, but my reading on these seems to show that a pipe reducer might do the same job! My venturis are 'sophisticated' by comparison, fabricated from 2mm copper, finally shaped with a very nice(!) steel taper mandrel (2" dia) and a fly press and die. They look a bit like the bell of a small trumpet.
It seems, too, that my 'in flue' burners might be better served with what I've seen called a 'glow cones' and I've tried these, but they're a bugger to make and support in the flue and they are relatively short-lived. I have a burner 'stick', which has burner 'mounts' along it, intended to have about eight gas nozzles evenly spaced along the flue, but I haven't yet tried it out in a flue. I suspect it'll probably self-extinguish and just how to support it in the flue is interesting, too.
Further on the flue size discussion: A correction. Kozo Hiraoka, at the end of the Pennsy Switcher book, goes into more detail about the 7-1/4" gauge version and does not, as I have said, simply 'double' the sizes. While that is true of most other bits and pieces, he does revise the boiler flues, in that he specifies seven flues out of 1"x.078", not three as in the smaller version. He also downrates the pressure of the larger boiler to 70 PSI, with, he reckons, little difference in the boiler's performance. I'd like to rate my boiler(s) at 100 PSI., which again raises questions about the compression ability of even my 1.75"x .125" thick tube, at 450mm long. Using flue tube burners or glow cones I can't see how my flues can be effectively stayed.
And Again! I hadn't 'loaded' the pix etc Steamchick sent re the flue tube burners: my apologies. Good stuff, those and certainly offer a few more insights. 'My' newer boilers are intended for gas, i.e., 'gas specific'. I have considered the loco type with gas burner in the 'conventional' firebox and this is still possible, to some extent, but the flue tube burners and the simplicity of the gas-specific cylinder have most appeal.
Hi Wazrus, I am interested in your investigations for a "sensible" gas fire boiler. I recently (this year) had discussions with a VERY clever guy in Spain. A Professional plant engineer for Chemical plant, he has made a 5" Pacific loco - non-conventional - for gas firing. Joan Llutch is the guy. He is on the (Spanish) web with lots of information and videos of him driving his finished loco.

The Burner he uses is a 4" diameter x 4" high unit he got from Bekeart., who now make various smaller sizes as well as full industrial sizes. › ~ › mediaPremix gas burner solutions - Bekaert

Also I have read the design notes by Susan Parker of her design of boiler. There is a lot more on the internet - as some things do get updated - and her whole design document is a hundred pages or so (if memory serves?). I made a short note - see attached - as a personal reminder of her ideas. Check her web pages for lots more detail, including strengthening work on the fire-tubes.
She adheres to Australian Miniture Boiler regs: › Record › 1728816AMBSC code, part 1, issue 7 copper boilers / Australian ...

Hope this is useful?


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Hi Wazrus, another bit of my research:
K.N. Harris Design no9 firebox.
By my calculations, for a FOS of 8.8 this is limited to 40psi NWP. (5.25"OD x .125" thk). But then I applied the required SCF of 3.3 for the coal hole penetration into the side of the firebox and this reduced the NWP to: 12psi, in order to get a FOS of 8.5!!
You are using .125"x 1.75"x 450 long tubes for your flues (is the 450 in mm or inches?). I reckon good for NWP = 100 psi with a FOS = 10. Excellent!
You mention flues out of 1"x.078" - I reckon these have a FOS of ~15 at 70psi, and FOS of 10.8 at 100psi. - so OK by me? - without any side penetrations. - But do they have cross-tubes? - in which case the FOS at 70psi becomes an "irregular" 4.7! - so the NWP should be only 39psi!!!
Maybe copying "old" designs is not always good for current regulations.
But if the other people's designs tell you how much flue area for a particular design of boiler then maybe there is some useful information? - e.g. other designs of boiler using gas firing?
Steamchick: your 'handle' intrigues me, as it seems you are male, despite the 'chick'! I'm trying not to be rude or invasive. Ho ho ho. Nevertheless,
did you ever spend time on Bougainville Island? I knew an English bloke there named Chick, who was a train nut. Couldn't possibly be.....However, I note your references to the Spanish bloke and I'll certainly have a look at his stuff. Speaking of others involved with gas-specific boilers, there's another bloke in Finland who has a couple of operating gas-fired locos. If I recall, his website was called sauhnalati, or something very like that and I did make contact with him yonks ago and that's where I saw the 'glow cone' (his words) idea. I did in fact try out the cones (not in a loco, but in a !-1/2" flue) and I could see the idea working, but, as I said, the cones were a bugger to make and support and didn't last very long at all and most of the 'glowing' seemed to be at the end. So I started casting around for 'something else'.
That vertical boiler I built was my first and it turned out quite a reasonable, tight job (ahem!) . I was a bit miffed when I saw the firebox walls buckling inwards. And it was all built to the 'words and music'!! I fired it once or twice on coal, but quickly went to gas, using an adaptation of what the culinary set call a 'mongolian cooker', which is a set of about eight burners arranged around a cast-iron ring. The cast ring wouldn't fit in the firebox, so I rebuilt the 'idea', into a much more compact burner head, which produced a nearly ideal flame: the flames were just blue-tinged, in a nice, tidy ball. In fact, the flame was almost invisible.
As per your Spanish contact, the Finnish bloke can be seen driving his 'gassies', too. Jan-Eric was his name, as i just now recalled.
I will try to include some pix in future, but I'm pretty much computer-illiterate and you'll have to grin and bear it.
Hi Wazrus: The "Chick" is from the family name - going back to 1605... and ignoring all the "Fowl"-named like "KFC" I am the original Ken Chicken (Not to be confused with "Kenticky Chicken"!) I.E. The English family has just over 7000 on the whole family tree (over the 400-odd years). But many countries have their own family trees going that far back as in Japan I met a "Niwatori-San", and in France a "Madame Poullet". Your mate "Chick" was just a N. American twist on the name I guess?
Just like Smith and Weaver, we were named because our ancestors were Chicken farmers.
Possibly the most populous family name, as Googel tells us there are 27 Billion Chickens on this planet.... ( typical googelism, as there are only 8 Billion humans!). ;)
And yes, I am male - for the record.

Back to burners: Ceramic burners - as used for grills for cooking, or in model boilers, burn out - but if kept below 950 degrees C this takes a long time (>1000 hours?). Over that temperature (I had one white hot for 10 minutes!) it accelerates the ageing.

Stainless steel is good for 1050 deg. C. Above that it deteriorates (burns away) rapidly. But that is what we get cheaply as modellers.
Special materials used industrially can run up to 1450 deg.C. - But it is really Expensive - as you can only buy industrial quantities of "orders over $100" and 5 sq.m area. - which is hundreds of $..!
If you can get hold of Nichrome electrical resistance wire you can weave your own fabric that will last last longer than wire containing iron...
But I am trying to develop a simple and cheap wire construction (to easily replaced, and not expensive) so am working on stainless steel wire mesh in a cylinder (I may consider a cone for "flame structure" reasons) which is covered in cheap stainless steel wire wool. Cheap because it can be bought for stuffing car silencers. Apparently, some people using stainless steel report using 2 or 3 "cones" - or whatever shape - of S/S mesh per season, but some say 1/year is enough. That - to me - is a serviceable part , especially if relatively cheap (Less cost than the fuel burnt).
Keep in touch,


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A couple of pictures of other burners I have tried to develop... But I reckon I can only make 1/3rd of the heat of "the best".
  1. First is a burner made from lots of 1mm holes in a tin, covered in wire mesh.
  2. Second is a 4" diameter x 4" high copy of the Bekaert burner for Joan Llutch... but using cheap S/S mesh and covered in cheap S/S wire wool. - This burner has been improved with the addition of a venturi air intake (I can give it "Excess air" so can now tune the mixture and gas pressure)
  3. I studied Jan Eric Nystrom's burner, and tried to improve on it - and failed .... here are 3 stages of my development in the attempt.
Your Mongolian burner is a good approach for intense heat, but your description of a "ball of blue luminous gas" suggests it is choking from back pressure and not getting enough air to form a series of Blow-lamp - type flames from each hole in each nozzle. So you are venting a lot of fuel as Carbon Monoxide up the flue.... from the lack of air and combustion. I.E. too much gas and not enough air. Please don't breath the exhaust - it can kill!


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Looking at the very prolific US sites, many there have made, seemingly successfully, what they call 'pipe' burners, which seem, from pix, to be little more than a tube or pipe with slots cut across the body. My effort at one of those resulted in a most spectacular wall of orange flame. There wasn't enough air(?) but it was not in a flue, but in free air. This of course led to experiments with various means of air 'induction'(!), (the venturis I touched upon) and also a wacky device which included an automotive type of butterfly arrangement. Success rate? Nil.
It seems as though I'll just have to wear the 'glow cones'. I do have a fair bit of stainless steel mesh. And I might also have to arrange the boiler to allow quick and easy replacement of the cones. Looking at your pix, the 'ball of flame' I mentioned was not at all unlike your P8272325, at least with its colours.
I'm certainly hanging in there with the 'burner in flue' approach and i do have some 38x1mm threading tackle, so I'll be messing around with removeable end caps carrying the burner assembly, whatever form that might take. For this, I'm planning to have the flue tubes(s) extend past the 'backhead' and thread the projecting end. Perhaps, too it might be easier to allow cone replacement from the 'other' (smokebox) end, as there'll be a convenient access door. A thought occurs that maybe, if hollow, longitudinal stays were used, extra draught from the engine exhaust, through the stays, could possibly augment the primary air supply. Then , that gets away from the inherent simplicity of the cylinder with two or three large flues. But there'll have to be stays anyway, so what the hell...
We used to be known as 'model and experimental engineers': I'm sort of re-incarnating the 'experimental'... yeah, right.....
Just finished a quickish look at Susan's design and that's almost the bee's knees, isn't it? Two things come to mind, the first is the burner head arrangement and jet size. I doubt if I'd include a feedwater heater. A shortcoming with my Sweet Pea boiler was what seemed to me to be a lack of steam space, so in my rebuild, I have added a dome, in which I'll house the regulator. Otherwise, though, I'll go with Susan's words and music and as you say, let's not re-invent the wheel. Between you and Susan, there isn't much room for any theoretical input from me, so it's over to the explosions department, but not right away. The medicos have intervened yet again.
Hi Wazrus. Good luck with medicos. I've just finished a course of radiotherapy blasts! Now getting through the after effects, so body tired, but brain glad for the stimulation you provided! Thanks. It makes sitting around more entertaining.
Explaining the burners in that pic. The side-stalks of mini-blow-lamp flames are good clean combustion with good concentrated heat. That is what you should get from the Mongolian burner. The balloon of blue CO combustion above the jets of flame is basically excess gas, due to insufficient air ingress at the jet.
In your vertical boiler firebox, the major issue is the cross-sectional area of the flues. Too little CSA means back-pressure, then that reduces the efficiency of the air taken in at the gas jets, so less air means Surplus CO with too little air for fast combustion, and the consequence is a balloon of poor combustion of CO gas that stops much of the heat flow into the boiler.
BUT, that may not be a problem as you managed to implode the firebox. (# 67). POSSIBLY you can recover the firebox with the imploded firebox wall, by deft use of hammer and drift after annealing the copper. To anneal, you just need to raise the copper so it develops black oxide but does not glow red at all. Otherwise you risk undoing all the silver soldering!
Having repaired a few boilers, it is much easier making from new!
I've been called lots of things, but never stimulating! Like you, one needs something other than sitting around waiting for god.
The vertical boiler's flues were very much to the words and music. I don't recall and haven't looked at the number, but they were about 8mm dia and lots of 'em. I do recall brazing them, as there was a bit of trepidation, but all went quite well and there's a good ring of solder around each. Yes I whacked the firebox wall to very small effect and then reasoned(!) that the 'implosion' might have work-hardened (AKA strengthened) the copper. At least to some extent.
Undoing silver solder isn't as easy as it sounds. My 'original' Sweet Pea boiler re-uses quite a bit of the platework and dismantling the thing was a real exercise in finding out just how tenacious silver solder is! The Pea boiler has, I reckon, one of the more awkward bits of flanging - albeit circular - being the throatplate, where the flange 'reverses' almost immediately. I had to make up a full steel flanging dolly. So maybe, just maybe, it was worth the dismantling problems.
As I said, I'll try to find some of the pikkies I've taken of the burners and other bits and pieces.
As for the medicos, I've had a couple of moles diagnosed as 'not very nice': not dangerous -yet - but worth watching, so I spent most of today at their behest and tomorrow I'm to be fitted with a sleep doover, by another clutch of medicos. Can't imagine sleeping - or trying to sleep - with the bloody thing!
I got fitted for a CPAP machine a couple of years ago. I diidn't think I needed it until I saw my oxygen levels during a sleep study. So I reluctantly started using the CPAP. Now I find that I sleep much better when I use it, even though I still hate the mask. So for me, it's a net benefit.
Hi Wazrus,
Does the vertical boiler still exist, or has it been "re-cycled"?
My view on that design is that it was suitable for NWP = 12psi, before the failure you experienced. (I.E. it could fail at anything over 103psi.). Assuming you "Failed" it at 100psi, that relates to a factor of Safety of 8.3 - so in fact I would expect it to have failed, if not at the NWP = 100psi when the safety valve blows, but either at the 104psi Max. steam test pressure, or at the hydraulic test if you went much above 100psi. (The ASME test pressure for a NWP = 100psi boiler shall be 277psi !). But that is on Annealed copper, so if it failed at a higher pressure than 100psi, then it was likely that it was work hardened to some (unknown) degree.
It is often mis-understood that if a boiler can achieve the Hydraulic test, then it meets the Regulations. In fact, the hydraulic test for ASME only simulates 1.3 times the NWP, but the regulations require the design to withstand at least 8 times the NWP. (Based on stress calculations).
I reckon that there are quite a number of boilers that cannot withstand 8 times the NWP. In reality, this means there are some boilers being operated at a reduced factor of Safety. I simply ask anyone to think, and decide if they want to sit 3 feet away from a loco that contains more stored energy than a hand-grenade, and risk (through a catastrophic failure) scalding hot steam venting that will permanently damage humans and cause extreme pain.
As the Sweet Pea design is "well proven" by the number of examples that exist and have not failed, it appears to be "a good design". But, I would suggest you take the experience from your vertical boiler and get design calculations made of all the material thicknesses, and if required, increase any thicknesses to something at or above the calculated minimum. Much easier to do now, before you make the boiler!
I haven't read the Australian regs. (Don't appear to be on-line). but guess they are equivalent to the ASME regs for USA. Maybe you need to get a copy from AMBSC code, part 1, issue 7 copper boilers / Australian Miniature Boiler Safety Committee | National Library of Australia ?
I did read an interesting article that may help you?
G'day, Steamchick
The vertical boiler still exists, yes, but it's very much on my list of 'things to do' (endless list) which includes radial staying. As I said and we've discussed, bashing out the bulges ought to be, in some measure, possible, but....I'm prepared to risk the brazing coming undone. After that came the Sweet Pea boiler and it was thoroughly stayed, which made the construction just that much more difficult. Besides the longitudinal stays, I added radials as per AMBSC specs. There were no
issues with firebox bulges. While it may be a 'well proven' design, the Pea boiler has no fans in Australia, at least none from the AMBSC and there is an alternative design, vaunted as 'essential'. After the vertical boiler, I began using only 4mm copper in all my creations, if nothing else, erring on the side of safety.
It wasn't the hydraulic test which caused the bulges, or at least that's my contention: the AMBSC stipulates an hydraulic test to 2x NWP and this is what I did. I do have copies of the AMBSC codes, copper and steel.
And for cds4byu, today is CPAP day, which for me, like getting older, has nothing to recommend it.
Hi Wazrus. The 2 difficulties with "old fashioned" 2 x NWP for the hydraulic test is that:
  1. It does NOT consider the effect of temperature on the strength of Copper. If temperature deterioration of the strength of Copper is considered, you need to factor the pressure by 6700/3142 - I.E. you need 2.13 times the NWP just to get the same % of limiting stress in the boiler as it will see at NWP. So that hydraulic test would only simulate about 80 psi steam pressure. All you are doing with that test is satisfying the inspector that the boiler won't easily fall apart, and it doesn't leak. BUT when you do the steam test at 104% of the NWP (104psi) you are getting to the stress level where you should still have a factor of 8 for Safety on the stress in the boiler. That boiler obviously did not as it buckled.
  2. The service hydraulic test at 2 x NWP is superceded in ASME to consider permissable stress at steam temperatures. But this is still only a Service test. It is not intended to prove a design factor of safety of 8. To do that, you would need a test at 8 x NWP = 800psi. Susan Parker did that with her design. But for those of us "Innocent" of design parameters we follow the designs of old - which we assume will meet the regulations and service conditions that we will meet. Sadly, not all "old designs" are good enough for today's regulations, and Real Life - as you experienced.
I think that as there are improvements to the Sweet Pea Boiler you will do well to adopt them.
But get the calculations done by your club engineer - or someone capable - and be sure your design is OK before you cut metal.

For the CPAP users you will probably need a humidifier if your unit doesn’t already have one built in. Mine didn’t and I had a raging dry throat in the morning. The humidifier was just a pan with a heater under it. Maybe you can use your vertical boiler. Plumb it in to the CPAP line with a heater under it. You could be truly steam powered.
I have attached a copy of the ASME info on doing a hydrostatic test on boilers. Note that it states to do the test at 1.5 or more above the NWP. Note also that this is what the code refers to for full size boilers. I am not sure but in the State of Washington, the NWP max that they allow is 100 psi. I believe that may also be noted in the ASME code for Model Boilers. I did not verify this however. I personally see no reason to go over 150 psi for a hydrostatic test. If there is a leak in a soldered joint, it will be obvious even before the 150 psi is reached. No reason to over stress as this can do more harm than good.


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CPAP, BiPAP, EPAP, APAP. It's quite enough for some cynicism. Tonight, I catch up on a bit of the lost sleep. And their prices! Eye-watering stuff. Shopgeezer: cheeky bugger. And my (hired) blower has a humidifier.
The 'tried and true' maxims I have used and clearly, they've come up short. Extra stays in the Pea boiler didn't do any harm, as well as making it more interesting to dismantle. The gas revised type will certainly use stays in the firebox area. I'm not sure whether or not this post went earlier, but I am attempting to 'attach files' of pikkies of some of the aspects of my endeavours.
pic 002 shows one of the bell-shaped venturi sections being formed in the fly press
pic005 shows the final steel formers, male and female, for the venturis
pic007 a sort of completed venturi: needs a lot more work, but the intake is there...maybe...?
pic 0326 is a collection of jets, sizes varying between 0.4 and 0.8mm
pic0329 is one of the 'burner bars', the large circular sections have threaded seats for a pair of jets. I had envisaged one of the bars would be around 400 long, with eight? or so jets along its length. I have no idea whether the
jet bar' thing will work at all. It's my hope that it may be an alternative to the 'glow cones'.
pic 689 is the Pea boiler, with some new platework and a new dome. The old outer firebox shows some of the 'old' stay positions. patches of silver solder can be seen where they were.
pic 690 shows the pea boiler and my larger creation
pic 691 the two boilers side -by-side again, with a 12" square for scale
pic 696 is a variety of plates, for dome, tubeplate etc, with some strengthening rings, as required around the dome penetrations. The drilled steel rings are bolt pattern templates for drilling of the dome flanges.
The jets gave some heartaches and not only because of the tiny holes, but by the non-availability at least in Australia, of very small hex brass bar. However, on a trip to Hong Kong, I found both 3 and 4mm A/F in shops in Reclamation Street, Kowloon. I would recommend that locality if you're shopping for the oddest of odds and ends. But maybe not in 2021!


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