Seeking info

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Hello guys. Is there a minimum gauge for wall thickness of small model boilers ? we are about to start building a small vertical boiler to run a small stationary engine. Ive been reading through the probems page and I don't want to build a bomb !
The Boiler we are going to make is 76mm o/d , not sure of the gauge yet till it arrives and the steam chamber is 130mm tall, it will be completely Silver soldered and all the fittings will be top hat and fitted from inside the boiler .The center flue tube is 25.4mm and 1.6mm thick, the water tubes through the flue are 16mm and 1.5 thick, the end caps will be 1.5 thick.

Best regards Sutty

Screenshot 2022-07-17 at 13.26.29.png
Screenshot 2022-07-17 at 15.57.35.png
 

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
1,345
Reaction score
881
Location
Viet Nam

Hi sutty !

Hello guys. Is there a minimum gauge for wall thickness of small model boilers ? we are about to start building a small vertical boiler to run a small stationary engine. Ive been reading through the probems page and I don't want to build a bomb !

That seems to be a difficult question.
I think the Thickness of the boiler depends on the pressure you want it to produce
I think boiler making depends a lot on the engine - how big is the engine? How much pressure does it need to run? .... , with the same size but too much friction it needs 5 bar instead of just 2 bar ...
When I make "boiler" (I make it very simple , and my engine is also very simple)
I make the engine first, then try engine with compressed air, if the engine runs with 1 -> 2 bar pressure then I will make the boiler and try with about 8 bar pressure - is the maximum pressure of my compressor , if it is ok then I set the valve safe about 5 bar
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
919
Reaction score
332

Hi sutty !



That seems to be a difficult question.
I think the Thickness of the boiler depends on the pressure you want it to produce
I think boiler making depends a lot on the engine - how big is the engine? How much pressure does it need to run? .... , with the same size but too much friction it needs 5 bar instead of just 2 bar ...
When I make "boiler" (I make it very simple , and my engine is also very simple)
I make the engine first, then try engine with compressed air, if the engine runs with 1 -> 2 bar pressure then I will make the boiler and try with about 8 bar pressure - is the maximum pressure of my compressor , if it is ok then I set the valve safe about 5 bar

This method is not recommended. Calculate the thickness of the copper pipe/copper plate if roll to the pipe. Pressure testing with water inside with pump and manometer without the safety valve on boiler double pressure of the maximum working pressure. If use air or steam pressure, the boiler can explode and you can be injured. Download the book as I wrote in post #3 for your health security.
 

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Thanks guys, The engine we are going to build is a Stuart S 50, from castings. I've been looking on the net but can't find anything on the pressure it needs to run.
Stuart models sell a made up unit with the engine and a HB4 boiler, the boiler has a 60psi relief valve fitted.
Jens, thank you for that, i'm ordering the books.
Best regards sutty
 

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Jens , I did try that but just got to a site called ToAz, couldn't find anything to download or open.

Regards sutty
 

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Jens, thank you very much. That worked, now I have some reading to do. I will post some pix of the journey.

Best regards sutty
 
Joined
Apr 2, 2020
Messages
2,843
Reaction score
892
Location
Sunderland , UK.
Hi Sutty, Mechanicboy has good advice.
I also can help - and can independently check against ASME limits (Which make it a lot more "precise" - yet a bit more complicated - than the simple text books mentioned) that had previously been used when the "text books" were written.
ASME is abased on the physical attributes of copper, that it is much weaker at the temperature of "Steam pressure" than when cold. Also the strength in compression (e.g. all tubes surrounded by water or steam) is far less than in tension - again at the temperatures of the hot water and steam. So ASME works on "STRESS" calculations and real material properties - with safe limits built-in.
I'll study your boiler when you know the gauge of copper "wall thickness" of each tube) you'll have. I GUESS the weakest part is the central flue tube... Did your design come with any calculations to prove the strength of the boiler?
Regards,
K2
 
Joined
Apr 2, 2020
Messages
2,843
Reaction score
892
Location
Sunderland , UK.
Hi again Sutty:
The central flue tube works out to have: A FACTOR OF SAFETY of
4.6 for 50psi - THIS IS NG. The standard is a FOS of >8. So I suggest you de-rate the boiler to 30psi: then the FOS = 8.3 which is OK for ASME. This would mean a test pressure of 60psi.
if YOU NEED MORE PRESSURE, YOU NEED MORE TUBE WALL THICKNESS.
I'll check shell and "ends" later...

Cheers,
K2
 
Joined
Apr 2, 2020
Messages
2,843
Reaction score
892
Location
Sunderland , UK.
Hi Sutty,
Looking at the end plates: 0.625" thick : The LARGEST circle this can withstand SAFELY is 1.25inches, according to the ASME data I have. - So you are just OK for 50psi, and comfortably OK at 30psi.
Looking at the design "as a boiler"...
I have some suggestions:
1- Considering the "Hot Gas" passing up the flue, it had very little pressure driving it, yet there should be "quite a lot" of gases from the burner to get through the 7/8" bore of the flue: Yet your design blocks-off 80% of the passage with the large cross-tubes. I strongly recommend these cross tubes are made as small as possible (say ~1/4" diameter?), as they add a lot to circulation of water within the boiler, yet are more of a problem in this design by blocking the flue, which limits the size of burner you can have.
2- Why have 3 cross tubes, well spaced? - Better to have them closer (1" centres?) and towards the bottom where flue gases are hotter and you can gain more heat for steam.
3- If you can angle the cross tubes, say at 30 ~60 degrees from the horizontal, you will greatly improve the circulation of water in the boiler as the steam bubbles generated in the cross tubes will rise to one end to float to the surface, and draw in fresh (cooler) water at the other end.
I estimate around 25sq.in. of surface for heating. This means it is capable of converting around 0.25 cu.in. of water per minute into steam. This means it can use around 300 BTU/hr. converting hot gas to steam. (not a lot!). Equivalent to the power of an 85W bulb.
You can increase the steaming capacity by (~20%?) adding a ring of (say) 8 x 1/4" OD tubes around the main flue tube: to allow more flue gases and more heating surface between the hot exhaust and the water/steam.
4- Heat source: I advocate a radiant burner. This increases the heat transfer between the burner (glowing red hot) and the underside of the boiler, so the 2 sq in. of underside has maybe 5 times the heat transfer from burning gas to water than with a simple flame. Thus increasing the steaming capacity by maybe a further 20%? Note: When flames "touch" the boiler metal, the temperature drops too low so the gases do not burn on the metal surface, which cannot get hotter than the steam temperature. This un-burned gas then insulates the surface from the actual hot burning gases, so preventing heat transfer ... and you can't make more steam than if the flames naturally end before they touch the metal.
5- Bushes: You MUST have bushes to reinforce all the penetrations, top-plate as well as boiler shell. These should be Phosphor-bronze in copper boilers, as anything else is spoiling the boiler for a little cost of material. BRASS will corrode and "crumble like a cookie" after 20 years or so, I have seen it! Copper is not strong enough to hold a good thread for safety. Stainless steel will not easily and reliably silver solder. (Special fluxes etc. required).
6- You will need a water pump to replenish the boiler water as it is steaming. Most use a simple hand pump for this.
7- You will need a bottom bush adding to "Blow-down" the boiler after use and empty it out. The blow-down valve permits rapid de-pressurisation so any deposits (from scale in the water) are flushed out in this process, otherwise the scale will build-up as a sludge in the bottom of the boiler and the bottom will overheat and burn out one day!

I hope this helps you understand how the boiler should be designed properly, as it is "not just a simple kettle", but as much a part of your steam engine collection as your favourite model. Perhaps the MOST important part for safety? And most modellers address the cosmetic issues to make the boiler look like a thing to be proud of, as they do with their engines. e.g. polished wood cladding (for insulation of the outside shell), polished brass fittings (gauges, etc.)...
Cheers!
K2
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
919
Reaction score
332
Important of all one can think of when making a boiler for the steam engine: How high is the pressure and how much should the boiler deliver per minute to operate a steam engine of a certain size.

Larger heating surface can be achieved with several water pipes or a single pipe that is spun into a spiral with a water pump in one and then the other end goes to the steam engine which is often called a flash boiler. But the water can be used up quickly, so the steam engine should have its own water pump to supply the boiler at all times.

Modelboat with flash steam boiler and hi reving steam engine.
 

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Cheers Steamchick, the boiler casing is 76mm o/d with a 1.3 wall, the flue is 25.4mm dia with a1.65mm wall and the cross tubes are 15.9mm dia with a wall thickness of 1.7mmm. I was going to make the ends from 1.5mm.
I'm still working my way through the book Mechanicboy sent me, interesting reading.
No calculations for any of the parts or materials, didn't realise it was so critical but will use the material we have as a practice piece.
we started this afternoon making the burner parts as the grandson was sent home from school.

Best regards sutty
 

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
K2, just read your last post, I can see a complete re-design coming, what kind of wall thickness for the smaller cross tubes ? can they be machined from solid copper bar ?
what is a radiant burner ?
where are the 8 x 1/4 tubes situated ? in the flue or in the water ?

regards sutty
 
Last edited:

sutty

Well-Known Member
Joined
Jul 17, 2022
Messages
71
Reaction score
38
Location
England
Cheers K2, there's no rush we're away for a couple of weeks so cant do much except read and research.
we've had another go at sorting out the boiler so if you have a look at it and see what you think all 16 gauge except the cross tubes, there are 8 of those, in the firebox they're 10mm x 1mm wall the outer case is 3" and the firebox bit is 2".
I like the look of the burners but I'd like him to practice his silver soldering on the meths burner set up, we've cut out the copper for the tank which is 70 x 80 x 25 with a threaded boss soldered in for the fill point and some 6mm brass tube for the feed to the 3 burners he's half way with the burners and showing a real interest.
 

Attachments

  • Screenshot 2022-07-22 at 14.35.11.png
    Screenshot 2022-07-22 at 14.35.11.png
    114.7 KB · Views: 0
Joined
Apr 2, 2020
Messages
2,843
Reaction score
892
Location
Sunderland , UK.
Hi Sutty,
A bit of time while I await re-charging of my battery saw... so I can continue cutting a 12" trunk. "Timber!" - well, maybe tomorrow?
Back to:
1 - 8 x 1/4 tubes situated - around the flue tubes, through the water and steam. Does this quick (15 seconds) sketch help?
P7222434.JPG


2 - Ceramic burners (radiant burners): From top to bottom:
8 1/2" x 5 1/2"; 4in diameter; 2in square; 4in x 5/8in.
But also there are radiant burners per: Bekeart Brochure - even as small as will fit larger model boilers and locos. These are made from wire mesh or woven wires (knitted fabrics, etc.) in FeCrAlloy, or Nichrome. And I have udes regular burners with wire mesh of old electric element wire to make radiants that glow orange in the regular blue gas flames following guys who do that on small gauge locos.
P8092309.JPG P4172369.JPG P1022321.JPG

3 - To the boiler, based on your materials.
* The main flue tube (most critical item):- the flue is 25.4mm dia with a1.65mm wall - Compressive stress limit is 825psi: AT 50 psi the calculated hoop stress (considering stress concentration factor for the cross tubes) = 1406psi = NO GOOD: but of NWP is reduced to 30psi: Compressive hoop stress becomes 863psi against a spec of 921psi = OK.
THEREFORE I recommend this boiler has a NWP NOT EXCEEDING 30 psi. - Which means the safety valve shall prevent any overpressure at 32psi. (Hydraulic test at 60psi for certification).
* the cross tubes are 15.9mm dia with a wall thickness of 1.7mmm. Compressive hoop stress permitted = 921psi, and hoop stress is 172psi (as there are no penetrations in these tubes, the SCF = 1) = OK for NWP = 30psi:
* The shell: the boiler casing is 76mm o/d with a 1.3 wall -Hoop stress 2982psi for NWP = 30psi, but permitted stress = 4389psi, so OK:
* I was going to make the ends from 1.5mm. - SO you can have a circle of up to 1.65in. (41.9mm) without stays or other support. (CUT a circle this diameter, and fit it anywhere in the end plate without overlapping a tube, or stay, and the design fails). Your plan is for a 1" annulus between the outer shell and main flue, so you cannot fit a 1.65" circle anywhere there: So that is OK for 30 psi NWP.
- But remember, the flue will fail the calculations against the stress permitted by the regulations if you want to certify the boiler for more than 30psi. So even though other parts won't fail, you would encroach on the factor of safety determined by the Federal Government - and fail the certification.

I hope this helps you understand what you can do with these materials in making a boiler.

I also posted a point in #12 about how the 15.9mm cross tubes block most of the gas passage for up the flue. Draw a circle 22mm diameter, now draw the cross tubes (16mm wide) and shade-in with a pencil. The gap at the sides is hardly enough for a candle flame to pass through, never mind the exhaust gases from the burner you'll need to power an engine. You idea of drilling solid 1/4" copper seems OK, except copper is so soft it will just collapse while you are trying to do that. I advise you just buy some copper tube. (I use regular 1/4" or 6mm central heating tube on boilers for 30psi like this one).

I hope this is useful?
If I have made any errors, I hope some reader can advise as I would not want anyone doing the wrong thing with steam boilers. - I am never offended by people correcting me. (there is always a first time?).
Cheers!
Enjoy being a boilermaker!
K2
 

Attachments

  • Bekeart Brochure_Premix gas burner_A4_EN_LR pages.pdf
    578.8 KB · Views: 0
  • Stainless steel mesh radiant for Locomotive firebox1.doc
    1.4 MB · Views: 0
Joined
Apr 2, 2020
Messages
2,843
Reaction score
892
Location
Sunderland , UK.
Hi Again Sutty, I have now seen your post #18: This redesign is a great improvement - as a steam maker:
SO, checking the FIREBOX INNER tube, 2in OD x 16 gauge (For copper: 0.0508in) = 1.29mm.:
AT 30psi NWP: Hoop stress of the FIREBOX tube = 2052psi. PERMITTED STRESS = 921psi - because this is in compression, and has side penetrations for the cross-tubes. THIS IS NG.
Checking further: for a NWP = 14.5psi I have a hoop stress of1016psi against a permitted stress of 1038psi. so 14.5 psi is OK (15psi is JUST over the limit, so NG to Regulations). And you would need a Safety valve set to 15.3 psi max. This may be OK - depending on what engines you are powering. Meths powered boilers are often for little engines that run at just a few psi, so you must decide.
OK?
Ken
 

Latest posts

Top