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I really wish I had access to cast iron now... How do you think steel cylinders on a bronze piston would do as a compromise?
I reckon should be OK. But watch clearances, as the differentioal expansion between bronze and steel could cause a seizure when it gets to steam temperatures...
But after steaming, wash through with WD40 - The FLUSH THAT OUT with a preservative oil. Car engine oil has a LOT of chemical stuff added, including corrosion inhibitors, anti-acids (WD40 is acidic and promotes corrosion if left too long...), etc. I use a light engine oil... (5W or 7.5W). Whatever I have for the car!
I am really not interested in getting it above 1000 RPM, being as it is not easily balanced, and it will start beating itself to a pulp above those speeds (I put 120PSI to it once... . I will never run this dude in public (I'm willing to stake my own life .... so public certification will probably never be an issue.
Ill also weld me up a 3/16 in thick firebox just in the case it does decide to go. I don't know how to calculate strength of a blast shield, but I have a hard time believing that would come apart given sufficient venting.

As for the boiler design itself, I recently found a couple steel compressor tanks, about 4 inches diameter and 12 inches long, from what you are saying, and adjusting for less RPM and being single action, It sounds like one of these could work. Please correct me if I am wrong.
I don't currently have any plans for powering anything with it, but I am definitely open to ideas for novelty pieces.
I am also very interested in what you called a "super heater" I have thought about running the output of the boiler back through the firebox (steel line of course) to make sure there were no droplets left, is this what you are talking about?
Hi TIG, I guess that pseudonym means you are a TIG welder? Certified or something? - Or just a hobby?
Just to feedback on your post:
I guess you read my comments about running on air... as you said you whacked 120psi into your poor little engine... when it threw a real Wobbler!
As to certification, I guess you have house insurance, Life insurance, Health insurance, etc. In the event of something unexpected going wrong, you may want these to pay out. An uncertified boiler "accident" will prevent any payment, whereas a certified boiler just may help get some money from those insurance merchants?
Welding the firebox... if the boiler goes "bang in a big way, the firebox won't be an issue. It is the steam that does the damage. I'll explain below.
I have asked a few more questions about these air cylinders below.
Super heating the steam - I have said some more below on this subject.
I hope this explanation helps you understand a bit more about Boilers and steam?
Most hobby boilers are made of Copper in small sizes, and silver soldered, and the regulations cover these. Making steel boilers is usually in larger sizes, such as 6" diameter and above on traction engines - where welded boilers are more practical to make than silver soldered copper. But even full sized locomotives had copper boilers, as a hundred years ago it was cheaper to make a copper boiler that would last without corroding, than to make 3 steel boilers for the life of a locomotive. But by the 1950s most full sized boilers were steel. With corrosion inhibitors they can be very practical.
Are you able to make a welded air receiver? A boiler is a lot like that, but with a lot more fittings - such as water level gauge, clack-valve for the feedwater (a non-return valve), etc.
But the key to most steam boilers is the large surface area for heat transfer from "flames" to Steam. Just putting an air receiver in a blow-torch flame does not make a good boiler.
The other thing you need to grasp is that an air receiver failing will split the metal, but then only make a loud bang as the air powers a shock wave ... But if a steam boiler fails... :- if you are lucky there is only a large hiss of steam, that can cook flesh like an oxy-acetylene cutting torch (except that Steam is invisible, unlike most flames!), or if if goes "Bang" the stored energy in the hot steam cooks everything with 3rd degree burns about as effectively as the flash burns from a gas explosion, or dynamite. Oh, and just like the Bang from compressed air, the shock wave will deafen you (burst ear-drums), so you will be the only one who cannot hear your screams. So we are careful to design and make steam boilers to the regulations or approved designs, so they DO NOT FAIL. It is common sense, in my book.
Safety sermon apart, to run at only 1000rpm, needs only 1/3rd of the size of boiler I suggested, so a 3" diameter boiler 7 or 8" long with a few kW of gas flame beneath would power the engine - and possibly drive a small dynamo to light an LED or 2. (Maybe 1000th of the input heat? These engines are not powerful, despite all the" power" we stuff into them!).
So a design converting a steel compressed air cylinder, with suitable connections for Safety valve, gauges, feed and steam take off is feasible, but I have not seen any modellers doing that. Here's a couple of images of popular commercial boilers, for powering engines similar in size to yours. As you can see, there are more fittings than on an air receiver, so the whole thing is just more complex. I have included a view of the array of 3 to 6 tubes filled with water beneath the main copper drum, in the space below where the gas fire lives, and where most of the steam is generated.


The only thing I don't like is the lack of insulation over the top and ends of the boiler, so 1/2 the gas heat escapes in warming the room, instead of making steam!
These boilers usually have a single or double run of steam pipe through the fire-box, to dry the steam before it gets to the engine, Not exactly superheated, but just enough to help considerably.
Here's a vertical boiler (Much smaller fire, so less power per lb of copper!):
This is a boiler kit supplied by "PM Research" - here's a picture showing what is inside the boiler as flue tubes where the steam is generated from the hot gas from the fire:

Perhaps you can give some more details of the air cylinder you think may suit? What is the primary use (air, at what pressure? what cylinder dimensions? Wall thickness, etc?). Have you examined it closely and carefully for any rusting on the inside and outside? If rusted, please do not use as a boiler unless you have had it shot-blasted, ultra-sonically tested, and crack checked. Rust (corrosion) attacks weak spots, creates stress raisers and generally erodes the safety margin almost to nil, before most people even consider it to be something to look at. (Safety margin at "Nil" is when it leaks air!). Most "cheap" air cylinders are without an internal zinc coating for corrosion resistance, and all compressed air is WET, with distilled water (condensate) that is more corrosive than water we drink. So I would expect the air cylinder you have to be rusted = No good.

Hope this gives you a better idea of what is involved in making a decent model boiler?
Here's some of mine... (rotate to view as required).
3" Bench horizontal boiler: NWP = 40psi.
Dads boiler 1.JPG
Yarrow: 1 1/2" drum, NWP = 25psi
P5080599.JPGSmall vertical boiler 2" diameter NWP = 67psi.P5080601.JPG

And here is a large stainless steel boiler for a 5" gauge loco - NWP 100psi - for the truly ambitious modeler!

P.S. Maybe your TIG skills can TIG-weld a 1/8"~ 3/16"mm thick copper boiler? - There are professionally made commercial boilers that are TIG welded copper, so with the skills and certification these are OK as well. Or can you silver solder thick copper with your TIG set?).
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I really wish I had access to cast iron now... How do you think steel cylinders on a bronze piston would do as a compromise?
I use cast iron weights for projects. It is easy to machine compared to A36 steel.

Hi TIG, I guess that pseudonym means you are a TIG welder? Certified or something? - Or just a hobby?
P.S. Maybe your TIG skills can TIG-weld a 1/8"~ 3/16"mm thick copper boiler? - There are professionally made commercial boilers that are TIG welded copper, so with the skills and certification these are OK as well. Or can you silver solder thick copper with your TIG set?).

Yes, My pseudonym refers to my welding hobby. I'm not certified, but I have welded a pair of racing carts for SAE mini Baja, and passed the sample tests that they require for tech inspection. I have never worked with copper, so I think for now it would be better for me to stick to steel and be diligent with draining the tank and rust preventatives. I guess I could build my own from scratch, as I could overbuild it significantly, and it takes the possibility of rust out of the equation; an idea I like quite a lot! (I have a family friend with a missing face because of a car radiator) However, maybe a commercially built tank would be safer because someone smarter than I has already looked at the blueprints? The tank that I am thinking about is off of a compressor and had a blow off valve set to 125PSI, and and a single output port. Ill probably look up the make and model for an exact rating. If I am running at 25PSI, that should give me a 5X margin of safety.
Also I was thinking I would extend the firebox over the top and ends of the boiler. I should have said that in my last post, made myself look a bit of a fool didn't I? The "firebox" (if it would still be called that, I don't know) should therefore contain any failure of the pressure vessel shouldn't it? I would have to direct the air intake holes and exhaust away from where I will be sitting, as I would expect both to become steam outs in the case of failure. Ive also looked up the regulations that you spoke of and will be reading them, no sense to go into this ignorant in any way.
Most of how you describe your situation is what I had guessed, just better to know for sure. I have many books and texts on Copper Boilers, but nothing on steel. As the Air Pressure vessel codes will have a factor of safety on the cylinder to be able to have the blow-off at 125psi, based on the strength of the weakest part of the cylinder's design and steel used, then your additional FOS of 5 seems to be reasonable, for steel boiler regs/standards - I GUESS! - As I don't know these standards.
But good for you to read the regs. to understand this properly.
I do feel that as "Ignorance is Bliss" in the old adage, in this modern world we should apply "Ignorance is 'dead' bliss", but Knowledge can delete the 'dead' word... - KNOWING it is safe is true Bliss!
The Regs are just the Federal Gov'ts perspective from the best Engineers they have (Usually seconded from industry and universities) so we don't need to decide what is safe, just do as they say and we should be OK (except for the odd things that even brains at NASA - "Rocket Scientists" that is - get wrong!).
You mention making the firebox as a complete cover over the boiler tank... A very good idea, especially with a shell of insulation over that (The insulation that goes between any hot surface and cover plates - called "cleading" - need only be air, wool, cotton, fibreglass, corrugated cardboard, wood, High tech silicon wool insulation, etc. etc...). If there is a passage between the boiler and the surround where the exhaust gases flow, then this provides more scope for the exhaust gases to cool and give their heat to the boiler and make steam... Most of my boilers have about 1/2" of either gas space, insulation or both around them which greatly improves the efficiency of the boiler. - It makes them look huge, when the copper inside is half the volume! I can give lots of information on insulation alone, but first just think about how and where to add bosses for all the boiler fittings you need to add.
  • A safety valve - set to be on full blow at NWP + 6% pressure, at FULL FIRE, when the steam outlet is closed. - BUT it must not be blowing at NWP. This valve is usually sited at the highest point of the boiler, in a space that is directly where steam is generated (NOT away some distance down a pipe run!).
  • A water gauge: So you can monitor the water level: Too high and you'll pump water into the engine (called "Priming"), too low, and the fire can overheat certain parts of the boiler (Such as an exposed silver soldered joint in most copper boilers) and cause a failure.
  • A "Clack valve" - This is a non-return valve where the water is admitted from the pump system. It prevents steam from getting back down to the pump and heating it up, possibly affecting the durability of seals, etc. The admitted water should not directly impinge on any surface exposed to flames or hot gases, but should always mix with water that is hot. If it sprays into steam, then a sudden drop of steam pressure can occur at each stroke of the pump, when the cold water spray instantly cools the steam in the ullage of the boiler. This is inconvenient in use.
  • A steam collector. As the boiling water generates a lot of spray and mist (water droplets of all sizes) the steam collector need to be arranged to minimise the carry over of the wet droplets in the steam. It is simply "power lost" that does not drive an engine, but can cause major problems in an engine. It means water is lost through the engine without being able to do the work that the steam does, and needs replacing by the pump, which takes more heat, etc...
  • A Regulator (a valve that is adjustable for the steam fed to the engine). This is usually directly located adjacent to the outlet of the steam collector. For some model boats I do not have a regulator, as the flame-size and use of steam are balanced for the speed of the boat. But I still have a steam collector.
  • A drain point and "blow-down valve". This uses the reserve of steam at the end of a session of use, to violently blow the remaining (HOT!) water and steam and any residues from the water, out of the boiler after the fire has been extinguished. If using anything other than de-mineralised water, in a non-corrosive (copper) boiler, then there will be salts that become saturated in the water and drop out as calcification and dust. Even some rusting of steel can occur, and will be washed out by blowing-down. It uses the heat of the hot metal to keep boiling the water as it is blown-down, and the violent production of steam causes the sediments to be disturbed and washed-out. The drain pipe from the blow-down valve MUST be directed away from people, into a collector (I use a large plastic bottle with vent hole).
  • A connection for a pressure gauge. You need a good one so you can trust the readings, and it should have a condensate loop so steam condenses and prevents steam getting directly into the gauge (Hot gauges can be damaged and not read accurately!). It should NOT be on any connection such as the Safety Valve, steam take-off, or water-feed point, and is usually at the highest point of the boiler.
Planning the boiler arrangement is very important, as you don't get a second chance when the boiler doesn't work as required!
There is a catch here... Called "Welding control".
Bosses welded to the shell must be of an adequate design to reduce any stress concentration in the boiler shell, you must NOT tap directly into the boiler shell for these connections, and often reinforcement around the boss is necessary. ASME regulations state in calculations (of hoop stress, etc.) that a stress concentration factor of 3.3 shall be applied to all penetrations of shells. As well, all welded zones must be suitably heat treated after welding to relieve the heat affected zone of temperature generated hardening or annealing or other stress inducing discontinuities. NO undercutting of welding is permitted. Starting and finishing of welds on pressure vessels must be on disposable tokens, not on the pressure vessel itself. And it is good to make test pieces using the identical materials, thicknesses, weld preparations and processes as for the boiler, and do destructive testing of them... Keep for reference, signed and dated for the date the welding was performed. The boiler shall have a unique identification number, marked on a NON-stressed part that is integral with the boiler. An independent and qualified welding examiner should approve the methods, materials, test pieces and boiler, before pressure testing. The welding rules for steel pressure vessels are very comprehensive about size, welding method, shape, etc. - All just for your safety. I am sure you'll study the welding regs. before striking an arc...
Hope this helps?
- If there is anyone able to add to (or correct?) my advice, then I welcome it. I do not have a lifetime of making pressure vessels and boilers in steel, just a couple of years when I needed to know a few things. = I am NOT an expert an this field, just passing-on some of what I have had to learn...
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welcome to the site I have a son in tenn also lived and worked there . I’m a former toile maker and retired mech engineer done top fuel dragsters and funny cars Precision TIG Welding was a good side line until med issues left me with double vision I’ve taught it for a while still have my machine trying to tech son and grand son but there is something missing . I think it comes from playing competitive sorts where each day or game you try to play harder better , smarter And never quit. Sometimes just forcing a little better just because you can .
Hello, I'm a young man looking for people who know more than I do. I've always been fascinated in the building of things, whether it be boats, planes, trains, or automobiles, and the more insane they are the better. Whether they go fast, accelerate hard, do something that none else do, or just keep going for a hundred years, that's where I'm at.
I've restored a 1963 Ford Fairlane, built a plywood john boat from scratch, built a gas/electric hybrid moped, and currently working on a 1975 Corvette.

At the heart of every one of these... The engine.

All my life I have wanted to build an engine from scratch, and thanks to finally getting access to a machine shop, and my classes ebbing for a second, I finally succeeded. Granted, its not much of an engine, being just a single cylinder wobbler, but I designed every piece from the ground up, and machined every piece to a level of precision that rather impressed the shop foreman who has been there for 30+ years. All with really no training.

Now it is time to start pushing forward, and move into engines that last longer, make more power, run something useful, and maybe eventually get into the gas or diesel! Perhaps I'll start with a boiler for this little wobbler... Yall will probably see that rather soon.

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