Advice on a boiler burner

[HMEL]

Thanks HMEL. The problem "in my head" - is simply a major discrepancy between the books on model boilers and my previous (all be it decades ago) design work including some pressure vessel work.

• Compressive forces on tubes - not covered by all the texts on boiler design that I have read.
• No consideration in Boiler design texts for stress concentration factors... that are a routine consideration for structural design engineers. (or so I was taught?).

e.g. a book I have been using for 20 years or more (written in the 1960s) has a lot of good advice on design, yet nowhere does he consider compressive strength of tubes with "atmosphere" inside and "boiler pressure" outside. As is well known amongst Engineers, the performance of anything "real" (I.E. not the mathematician's perfect shape) has factors that de-rate the design performance of an object - although they may have been derived mathematically or empirically, they must be considered. E.g. ovality, dents or distortion, built-in "stress-raisers" such as holes for gauges, fire-doors, pipe-work, etc.
A question: How should I consider a fire-door hole affects the strength of the firebox tube in a vertical boiler? - It is a gap in the "hoop" for hoop strength calculations, and as such raises stress in other parts of the tube that is under pressure from the water jacketing the firebox? The "shape distortion" - and subsequent non-uniform stress distribution - will have a significant effect of the collapse pressure of this tube. How can I factor this into the tube material stress determination?
View attachment 124807
Many years ago I collected photocopies of pages from text books as a "quick guide" when I was doing calculations in work. I think these text books still hold truth. Yet in all the texts and Regulations I have seen relating to Boiler design, many things - such as stress raising "holes" in shells - are not considered in the design of steam boilers for modellers. - Yet the rules still apply of "good design". IMHO: Steam is natural, and has no brain to decide when to escape a boiler or not. If the boiler isn't "good enough" - by design or manufacture - or mis-used, then steam will escape, and being naughty, will hurt people. I am allergic to hurt, especially by steam. It makes me scream. So I want to correctly design boilers.

• The first Hydraulic test should first prove the Design is sound. Thereafter confirm the manufacture and materials are correctly applied. (Well, that was normal practice when I was a Designer.) Regulations seem to be "confused" about this?

Additionally, where a design may have a factor of 8 (for safety) - or other value - built-in to the calculations to reduce the stress at NWP from "UTS" to "reasonable" in the calculations (or other methodology), the Regulations only require an Hydraulic test at up to 2 x the NWP. As the Safety valve shall inhibit pressures over 106% of the NWP (or other - depending on "Nationality" of the Regulations), why do we not test at twice the "106% of NWP"?
In a technical article by Kozo Hiraoka, he explains (very well) how the ASME regulations require the consideration of temperature on the strength of Copper, such that a boiler to be worked at 100psi shall be designed using 3000psi as the maximum permissible stress for the silver soldered copper components and boiler construction. But when cold (room temperature The copper has an equivalent maximum permissible stress of 6700psi. Should we not then at ambient temperature be hydraulically testing the Design by taking a boiler to at least 2.23 times the NWP - to at least prove it can work safely at 100psi? (Even NASA can get calculations wrong... as proven by un-planned destructive testing! "I am not alone".). He mentions that the UK test is only 2 x the NWP, and the ASME test is to 1.5 times the NWP...
In my humble opinion, we are not proving the Design with these tests, only that there are no major defects in manufacture. That is OK where a Design is already proven. But where a design is not proven, as with an inherited boiler of unknown design or NWP, surely someone (like me?) should do the sums to determine the SAFE NWP when bringing it back to service after any repairs?
Finally, in their own homes people do a lot of things that are not legislated, or permissible in the public domain. Yet all the guys I know belong to clubs that do have competent Engineers as members for advice, and for testing and certifying boilers. The clubs also hold certified test equipment, procedures for testing, etc.. (Usually above the minimum required by their insurers, for use of members' models in public).
My lack of knowledge is the "standard good practice" for determining by calculation, the strength of various components of a boiler subjected to external pressure. - Internal pressure in tubes is easy to manage. A bit high and the tube pops-out any dents!. But in compression, tubes collapse catastrophically. So I want to do the sums before I destroy boilers. It is just a bit more than "Rocket science" - (They only apply internal pressure to tanks, etc. Which is why they land with some residual gases inside the tanks... and sometimes go "Bang").
Thanks for any advice.
K2

With regard to the safety valve pressure test that number is to insure the safety valve lifts within the pressure it is designed to protect.

With respect to the copper silver soldered joint it represents a dissimilar metal bond and credit should not be given to the full strength of a good copper alloy. I think the countries that have seen the most failures tend to look at recommendations differently.

Most of the publish recommendations are based on a long history of experienced failures. To achieve that safety margin we use math to get to those minimum standards.
And believe me I have seen well trained engineers argue over the correct way to analyze some of these problems. This also goes for recommendations as to what pressure should be used for hydro testing and the temperature at which it is to occur. You can actually seal off a leak if the water is hot enough during a test. Inspectors do not like that trick.

After looking at your model and if I were doing the calculation for the door I would treat it as a penetration of two cylinders. The external shell and the inner shell. Because one wall sits close the flame it will operate at a different temperature. Thermal expansion will be interesting but because the dimensions are short are probably not going to cause a lot of problems. The ring forming the door should be fairly thick and will act as a large stay bolt for this little unit. I suppose you could calculate the external pressure limitations but I dont think it would be necessary if its at least the same thickness as the shell walls.

Now personally if I was to build this little unit the door located there would go and I would use a refractory fire box to sit the boiler on and put the door in that refractory wall. Keep the heat transfer the same size but make it a little taller and save myself some issues with calculation. However it is just the way I would do it. doesn't necessarily make it better but sure would be easier to build.

Take Care and good luck with your project.

 

[Steamchick]

Thanks Mr. Heslop,
I am particularly concerned also about how to accommodate the stress raisers where there are "holes" in the cylinder wall.
"Back-in-the-day" when I was a Design Engineer, I would have use a stress concentration factor derived from some empirical table in a text book ... which for a 1 1/2" fire-door hole in a 4" long x 5" diameter tube would give a stress concentration factor of around 2.7. - The premise being that the tube connecting the inner jacket (Firebox tube) to the outer jacket (Outer shell) shall not affect the jacket tube for calculation of strength of the jacket tube. In reality there will some some other stresses in the silvered soldered joint, but not necessarily calculable.
Nowhere have I seen such stress concentration factors applied to boiler work? But it was regular business in the office where I worked. (It was a business that started in the 1920s. when holes in riveted plates were always assessed as weakening the plates! - It was where all service failures were found! - A ship's hull is a pressure vessel subjected to the external pressure from the sea - and when partly crushed by a wave - if plates are too thin - the failure is usually a tear starting at rivet holes. - Or so I was taught?).
I had an inherited (scrap) boiler that had run a steam model "for some time" - but from the number of lead solder repairs what what was known as a "Bad job". I cut it into pieces to re-use the materials. The internal boiler pressure had peeled open riveted flange joints - caulked with plumber's lead solder - and the fire tube - with cross-tubes - had failed many times from the repairs I could see, always at the simple joint between the fire-tube and cross-tube. Ergo the cross-tube solder did not support the fire-tube, but the stress at the silver-soldered joint was had cracked: Perhaps this was from some differential expansion, but more likely from the external pressure on the fire-tube and stress-raiser of the increased stress local to the cross-tube hole (between 2.5 and 3 times the hoop stress?)?
So I am suspicious of "designs" that do not consider stress raisers. I do not mean to criticise your input Mr. Heslop, but think the "x8 safety factor involved " is "the saving factor" here. - I would just de-rate it by between 2.5 and 3 for the stress concentration where cross-tubes are fitted - to a new FOS of 2.5~3.2. - Hence "the job didn't fail".
I do appreciate your good advice! Thanks.

Also Thanks HMEL. Very sound advice. The design I am studying is an existing boiler owned by Emers (in Australia, so Australian regs apply). The fire-door interconnection is drawn "thick" but not dimensioned. The fire-box wall and outer shell are both 3mm. The current NWP is 95psi - but I have challenged that due to the "seemingly thin" firebox tube. Hence I am trying to do "better" calculations and "prove myself wrong" in suggesting he should de-rate the NWP of the boiler. Your feedback is very useful.
Sorry for the long replies!
K2
Any more professional advice please?
Regards,
K2

 

[Steamchick]

Back to the discussion of "which burner is appropriate?"
As I make gas burners - because I have needed some and didn't originally know what was a bad burner or a good burner... - I have recently had a chance to compare a meths burner (simple vapour-flame burner) with a ceramic burner.
Based on the "same running of the engine on the same boiler" I deduced a couple of factors that allow me to compare different burner in different boiler designs.
Putting it simply:
On a simple tank with burner beneath:
Meths Vapour burner: - heating =
25.5BTU/hr/sq.in. from hot gas conduction

Ceramic burner:
42.9BTU/hr/sq.in by Radiation
21.6BTU/hr/sq.in by conduction.

The ceramic burner "of the same power of steam in the boiler" as the meths burner, has "lost" some heat by radiant heat so the flue gas is cooler: expressed as the reduction of 25.5 => 21.6BTU/hr/sq.in.
So in any boiler, I can now determine the most efficient type of burner between a non-radiant and radiant burner:
I.E. heat absorbed by the boiler = radiant heat x area receiving the radiant heat + conducted heat x area receiving the conducted heat. A kettle, with large radiant absorption area and relatively small surface area for conduction benefits greatly from a radiant burner. But a multi-flue vertical boiler hardly benefits, because of the large surface area of the conduction of heat into the walls of the flue tubes.
Hope this is useful to the boilermakers?
K2

 

[Steamchick]

HMEL (n' all). Re: Posts #36 and Mr. Heslop's post #38.
I have been studying the ASME codes a bit, regards the Hydrostatic test pressure: Just so you have it in "black and white".
I am using a paper by Kozo Hiaoka - who directly quotes the various ASME regulations:
Section VIII "Pressure vessels" of ASME codes:
Code requirement UG-99:
The hydrostatic test pressure shall be at least 1.3 times the Maximum allowable working pressure (I.E. "NWP") multiplied by the lowest ratio of the stress value Sr on the stress value Sd for the design temperature:
I.E. the formula:
Pr = 1.3P x Sr/Sd
Where:
Pr = hydrostatic test pressure, psi (or kPa),
P = Max. allowable working pressure, psi (kPa),
Sr = Max. allowable stress value at the test temperature, psi. (kPa),
Sd = Max. allowable stress value at the Design temperature, psi. (kPa)
So, in the case of a boiler with 100psi "NWP": (metal temperature is 393deg.F.):
Sr = Max. allowable stress at TEST temperature ((e.g. 68deg.F.) is 6700 psi tensile, or 1914psi compressive (using the factor from Mr. Heslop):
Sd = Max. allowable stress at Design temperature is 3142 psi tensile, or 898psi compressive (using the factor from Mr. Heslop):
Therefore the ratio of stresses for converting the stress at the temperature at NWP to the temperature at "room ambient" is: 6700/3142 = 2.132,
So the Test temperature shall be 1.3 x 2.132 = 2.772 times the NWP => 2.772 x 100 = 277psi.
Or, if compressive stress values are used, the stress ratio is still 1914/898 = 2.77.

In the case of a boiler I have studied, the Fire-box skirt inner tube limits the boiler to 50psi NWP, due to the external pressure upon that tube: - instead of the 95psi NWP that was being used (Calculated by studying all the tubes as if they had internal pressure?). The new test pressure becomes:
Pr = Sr/Sd x 1.3 x P = 6700/3950 x 1.3 x 50psi = 110psi.
And, in accordance with the Code requirement PG-67:
The SAFETY valve shall prevent the boiler pressure exceeding 106% of the NWP: I.E. 53psi.

I hope this is helpful. I thank both HMEL and Mr. Heslop for their input as it is very valuable.
Cheers! - And happy and safe steaming!
K2
P.S. The usual note: "Please correct me if I am wrong".

 

[Zeb]

I guess what I do like about the ceramics is that they seem to have heat more evenly distributed. Any benefits to the long-term boiler health even if it comes up to temperature slower?

 

[Steamchick]

Hi Zeb, Thanks for your comments, but I'm not sure I agree with the comments you make?
In simple terms, the boiler is heated by the heat from the gases burned in the fire-box, which is always the hottest region. In a comparison of heating methods, when using gas the same jet will make the same "gas heat" - whatever type of burner is used. But there is a difference in the balance of heat that the boiler takes-up from the hot gases and radiant heat.
If totally heated by "flames", then there will be a gradient of heating from the hottest (near the flames) to the coolest (by the exhaust chimney), so the copper and water will be responsible for keeping the near uniformity of the temperature rise in the boiler metal.
If a coal fire "of the same power" is used, then there is a large radiant part in the firebox, and a lot of burning gas in the flues, when the fire is drawn by a blower.... so actually, the "forced" fire will probably give the highest heat into the fire-box, as well as good heating in the flues, yet it is still balanced from a "warm-up expansion stress" perspective by the conduction of the copper and the water inside the boiler. The ceramic is a compromise somewhere between these 2. The "flat face of radiant heat" will pump more heat into the surfaces that directly "see" the radiant heat (mostly the fire-box), and there is a lower proportion of the heat supplied into the flues, yet the warm-up stresses will be very similar to the other heating forms due the the conductivity of the copper and the circulation of heat by the boiler water. I would expect two identical boilers, one fired by a flame burner, the other by a ceramic, to be heated faster by the ceramic because of the improved efficiency of heat flow into the boiler fabric when using a radiant heating element. So I find your "slower" comment peculiar? (Perhaps you have compared burners with different jets?).
But frankly, I do not think the boiler sees much "differential stress" when warming by any method. A dry boiler would be a much different matter, though, and the firebox would heat faster from any radiant heater than by "flames and hot gas" alone.
But you imply that the ceramic is more controllable - with gas, you can turn down the heat to warm the boiler "slower" - and on a large boiler this may help extend the life of the structure. But I personally think ("IMHO") that the failure of a good boiler after a lifetime of use is more likely to be from corrosion (bi-metal corrosion at joints?) and fatigue of joints from repeated heating and cooling cycles. In making and repairing boilers, I have experienced problems of the stresses developed by differential heating (heating the outside but not flue tubes) causing differential contraction as the boiler cools, causing cracking of joints where I have been a bit mean with the silver solder, on "poorly designed" joints. e.g. a simple flue tube through a drilled hole. When repaired with more flux and silver solder to form a small bead, the extra strength has been adequate to cope with these stresses. (Fully insulating the boiler after soldering so the cooling is much slower and heat dissipates around the boiler,also helps considerably to reduce the differential cooling stresses and cracking of joints.) Good joint design is paramount!
So I would conclude by suggesting uniform overall heating is much better than local heating when manufacturing a boiler, but hardly significant (IMHO) during normal warm-up and cooling of the boiler.
Does that help you?
K2

 

[timo_gross]

Altho' CO itself is odorless, the other carbon related unburnt fumes should always be a clue to the poison presence.

Trust me on this! Forget all the hopes of a "experience, feel or divine detection". I have been in 220 ppm CO and.... The only thing you notice is the gasmonitor getting all upset telling you to rush to the emergency exit and wait on the roof. You will not feel a thing you do not even notice that your brain is getting slow and you are dizzy. No clues. Mentioned head aches you will have the next day.
When running a melting furnace or playing with any sort of fire smelling some fume is no indicator for anything. The potential risk starts with every fire inside.

You simply wake up with a bad head and an oxygen mask on your face, and a worried-looking emergency crew!

That said I must add. You will only wake up:

• if someone finds you in time,
• if that someone knows why you passed out (who thinks CO, when the elderly neighbour fell asleep in his garden shed? While playing with a steam engine at 23:00 pm? They will close the door behind you, avoid noise and be happy for you )
• if the someone calls emergency crew ( probalbly related to local inheritance laws )
• if emergency crew arrives in time (related to local infrastructure)
• if emergency crew is guessing CO intoxication

At 2 Euros, Pounds or whatever currency, per IF I would rather get my head aches from canned beer or a cheap bottle wine, than CO.

There are also combined devices that check O2 levels at same time.

I do not want to scare anyone from the hobby and I do not consider myself as a militant "safety enthusiast". I get lectured about safety all the time. But I feel the urge to say just read steamchicks paragraph on CO, and just follow his advice. No panik necessary, but awareness and caution. It is safety at bargain price of 2$ per IF and 1Ct a but.

I have heared stories about head aches from older work colleagues, younger ones had the luxury of beeing forced to wear emergency filter masks and mobile CO monitors. I totally missed out on any 1st hand reports of serious accidents involving CO or low O2 levels.
Too late to buy CO monitor for today,

For less than £10 on the intershop I thought it worth shouting about. - Just so you all don't go and die on me and I'll have no-one to moan to...
K2

I will report how much I paid for it. I promise! Will go home now and have a beer, following steam chicks advise on Saturday.

Greeting Timo

 

[Richard Hed]

Trust me on this! Forget all the hopes of a "experience, feel or divine detection". I have been in 220 ppm CO and.... The only thing you notice is the gasmonitor getting all upset telling you to rush to the emergency exit and wait on the roof. You will not feel a thing you do not even notice that your brain is getting slow and you are dizzy. No clues. Mentioned head aches you will have the next day.
When running a melting furnace or playing with any sort of fire smelling some fume is no indicator for anything. The potential risk starts with every fire inside.


That said I must add. You will only wake up:

• if someone finds you in time,
• if that someone knows why you passed out (who thinks CO, when the elderly neighbour fell asleep in his garden shed? While playing with a steam engine at 23:00 pm? They will close the door behind you, avoid noise and be happy for you )
• if the someone calls emergency crew ( probalbly related to local inheritance laws )
• if emergency crew arrives in time (related to local infrastructure)
• if emergency crew is guessing CO intoxication

At 2 Euros, Pounds or whatever currency, per IF I would rather get my head aches from canned beer or a cheap bottle wine, than CO.

There are also combined devices that check O2 levels at same time.

I do not want to scare anyone from the hobby and I do not consider myself as a militant "safety enthusiast". I get lectured about safety all the time. But I feel the urge to say just read steamchicks paragraph on CO, and just follow his advice. No panik necessary, but awareness and caution. It is safety at bargain price of 2$ per IF and 1Ct a but.

I have heared stories about head aches from older work colleagues, younger ones had the luxury of beeing forced to wear emergency filter masks and mobile CO monitors. I totally missed out on any 1st hand reports of serious accidents involving CO or low O2 levels.
Too late to buy CO monitor for today,



I will report how much I paid for it. I promise! Will go home now and have a beer, following steam chicks advise on Saturday.

Greeting Timo

You are right of course, and number one: why would anyone be doing this indoors?

 

[Zeb]

Great response. Sorry for the lack of inflection in the text. I was inquiring out of ignorance. The fatigue cycling makes sense.

I found this interesting. In that copper does a little worse with fatigue than aluminum (steel has a fatigue limit, so can be cycled indefinitely within reason). I wonder if the heat applied to a boiler might actually stress relieve it a bit if the change over time is slow. Copper is so nice in that it plastically deforms before failure (Trevaskis paper).

In making and repairing boilers, I have experienced problems of the stresses developed by differential heating (heating the outside but not flue tubes) causing differential contraction as the boiler cools, causing cracking of joints where I have been a bit mean with the silver solder, on "poorly designed" joints. e.g. a simple flue tube through a drilled hole. When repaired with more flux and silver solder to form a small bead, the extra strength has been adequate to cope with these stresses. (Fully insulating the boiler after soldering so the cooling is much slower and heat dissipates around the boiler,also helps considerably to reduce the differential cooling stresses and cracking of joints.) Good joint design is paramount

Good stuff. I picked up a PM research boiler kit and am looking at squeezing, rather than hammering, the rivets to keep the stresses lower (I have a squeezer with a 3" yoke). Regardless, the joints appear to be overengineered for us hobbyists. I have been wondering about the firing method I might try after the fire bricks they provided run out. Great thread.

 

[Steamchick]

Zeb,
I can do the sums and recommend the "optimum" burner for you if you want to mail some boiler dimensions, steam demand (or engine size and use) info to me?
Copper "work hardens" with age, low temperature cycling and small stress loading. But becomes crystaline and brittle with extreme ageing - where the stress is concentrated. OK on Cathedral roof-tops, almost forever, but remember the National Regulations require the calculations to demonstrate the boiler is below 1/8th of the UTS (in tension) and the compressive strength is only 1/4 of the UTS! (or thereabouts). Factor in stress concentrations (which very few bother to do!), and the wall thicknesses can become serious huge... So do the sums (please? - Or i can do them for you?) before making any bits for the boiler. - Unless of course it is a "bought design" and you can be supplied with the calculations for certification by the designer. Many designs in books deliberately don't specify a NWP - and are often not made for today's regulations anyway...
It is a shame when a boiler has been made for 80psi, yet only meets the regulations (after calculation) as a 25psi NWP boiler!
"Engineering = Proper design FIRST, then proper manufacture".
(rant over),
Cheers!
Ken

 

[Zeb]

An optimized burner would be nice! Maybe you could make a recommendation. Here's a video I made reviewing the PM research boiler I'm working on. I'm looking at doing video of the build, in hopes people of your experience can chime in.



My biggest concern with making the wall thicknesses too thick, would be that the copper would no longer fail plastically in an extreme situation (dad is at work and the kids want to try to run engines). And yeah, warbirds are still being flown unnecessarily rough with age-hardened/fatigued aluminum. I had 40+ year old surplus (aluminum) rivets, and they were hard as rock to shoot.

Does the flame type/cleanliness contribute to crystallization if it's dirty? Maybe these questions are too far in the weeds, but they interest me anyways.

 

[Steamchick]

Zeb,
Seeing you have a kit, I think the design will comply with ASME regulations. - But I suggest you should get a certificate of design, or copy of calculations, from the kit supplier to confirm the NWP complies with ASME to show your Boiler tester when the time comes. (Like planning for the baptism 7 months before baby is born! - Plan on success!). This is a simple design but very effective. A regular gas burner (3kW camping stove) is about the best burner, as the wire or ceramic radiant burners will shine straight through all the flue CSA. They may be a few % more efficient, but I would not bother when camp-stoves are well proven by the makers. - And cheap! radiant burners are much better of horizontal boilers where they can "shine" on large surface areas, before the exhaust gases pass through the flue tubes. (e.g. the Stuart boiler that is common in the UK).
Enjoy the silver soldering. You will need to make a fire-brick hearth and it takes a lot of heat (I use 3 Blow-lamps, Around 7~8kW total heat!). And I suggest a leather welder's apron, and leather welder's or blacksmith's gloves and safety glasses are a must.
Thick copper is just better than thin, as stresses are lower, for the same loading.
Enjoy!
K2

 

[Steamchick]

Hi Zeb,
I like the water pump casting, who supplied it? PM Research? - I'll see if it is cheaper than UK stuff with shipping and customs... (Usually varies with the value of the $).
Cheers.
K2

 

[Zeb]

Yes the casting is PM research. You might have to call to get one piecemeal, as they only sell them in the kit. I like working with their castings. They sometimes have flaws but they add a lot of character.

I've done welding on antique aircraft over the years, so I'm all set. Sometimes I'm a little dull on the safety side though.

*edit
My state also did away with boiler codes over ten years ago. The onus is on the law-abiding citizen to do the job properly at their own risk.

 

[Steamchick]

Hi Zeb,
I managed to find the pump casting kit on the PM website at $41. I'll stick to my own design from raw stock, as the shipping and customs cost would double that cost.
Thanks anyway. Good luck with your boiler for silver soldering.
Another tip:
I use a "Fernox" proprietary product for de-scaling pickle. Use the Yellow granules to make up into a solution (desert spoon to a pint or 2 of tap water) - turns green to blue when neutralised by de-scaling copper. The used solution is OK for electro-plating copper then... Pickling is Necessary between heating operations. Soak overnight, rinse, then you can set-up for the next operation. Is penetrates all the gaps, and dissolves all the black oxide. Much better (IMHO) than "vinegar", citric acid, cheap non-sugar coca-cola and other stuff used as a cheap solution, although they are often used. Good for all the bathroom metal and front-door brass-work. as well - it gleams! Turns brown brass to shiny in seconds. The £10 tub I bought more than 10 years ago is now 1/3rd full. Good value for money to me.
You'll need a tub of flux as well as the pinch provided in the kit. So buy more silver solder and flux before you start. Embarrassing to run out 3/4 of the way through a session.
I use a welding rod to apply flux. Heat the end that would normally go into the electrode holder, holding it at the far end (don't worry about the flux on the rod - it is just insulation here to help keep your gloves from getting hot). - Not red hot but "hot" in your flame and dip into the flux. The powder will partially melt onto the rod when it can be put exactly where you want it, heated with the flame. When the copper is hot enough, the melted flux will run on the surface and wet it - you'll see the de-oxidisation as it wets the copper. Then when the copper is a dull to medium red heat the silver solder will melt and run into the gaps in the joints. Make sure you keep on applying flux, it is chemically cleaning all the while, and only the proper temperature and flux-clean surface will wet properly with the silver solder. When good, you'll see it flow! But overheating will burn the flux and can make porous silver solder, as well as TOXIC fumes (can permanently damage lungs if breathed in). USE VENTILLATION (fresh air blowing from you to the job so fumes go away) - But as a welder you'll understand all that. Try on a bit of copper scrap first so you are familiar with what is happening. As a certified welder you understand what many beginners do not. Practice makes perfect! And pull some trial joints apart to examine the joint and see how good you are, before you do the real boiler material.
This kit looks like a simple set of flue tubes through end plates, so is the simple case for calculation, so I expect the PM people to provide you with some certification of their design for when you go to a boiler tester for testing and the certificate.
Problem boilers seem to pre-date ASME (etc.), or are "Home-made" designs (like mine) where some of the current requirements and calculations were not known at the time of doing the design. Some boilers exist without calculations, so it is really down to the tester's experience and compliance with Regulations to resolve what to do about those. (E.g. the boiler "designed", or just made, by Fred up the road as a 1 1/2 times scaling of the boiler Harry made... "It works and didn't fail the hydraulic test" he says... WRONG! It doesn't have the safety factors required by law.).
Enjoy!
Ken

 

[Zeb]

Thanks for the advice. I did buy extra to practice on, because I know I'll need it!
I think if I were to do my own design to use in a group setting (railroad club), I'd also test the worst of two copies to ultimate failure. I think my most favorite attribute of copper, is that it warns you with a balooning effect.
During aircraft certification, the FAA requires test articles for both ultimate and fatigue testing. The structures gurus have told me calculations come with a lot of noise and are simplified. Finite element analysis uses simplified meshes, and joint fixities are difficult to predict. Empirical tests often reveal the gremlins encountered after several thousand cycles.
Super nerdy stuff!

 

[Steamchick]

Yes, even NASA sometimes find their calculations, or manufacture, are imperfect.... and have tested some jobs to destruction that eere not planned to be destroyed.... But since the Egyptians, Greeks, and everyone to "modern day geeks" have experience - and we have Regulations now - to add to the safety factors, so we are (mostly) right.
The proof test shows the real job has been made initially without major flaws, the design test proves the design, the destructive test proves the failure point for the part that was destroyed.... Sometimes this tests the materials rather than the design... So add every bit of experience you can to the job, and you should be OK.
K2

 

[Steamchick]

Hi John,
Sorry to bother you with an old topic.
I have just been in a discussion and this bit of information popped-up.
Is it relevant to you and your boiler certification?
https://ejwinter.com.au/products/ambsc-boiler-codesKen

 

[Emers]

I used those regs to build my boiler almost 4 years ago. It has performed flawlessly since it was certified by our club inspector
It is due for recertification later this year.
Cheers

 

[Steamchick]

Thanks Emers.
Glad to know you could get it certified... From memory (imperferfect?) that was the potential sticking point, that the boiler was a larger size of an existing design, but without increasing the material thicknesses. - Hence I proposed that it needed de-rating (safety valve resetting, etc.) for certification.
What is the current NWP you have certified? - I'm curious, to amend my understanding of this boiler.
I'm glad you are running successfully with it.
I have recently had discussions with one of the Club inspectors, and he is working to new (2018) UK testing rules and guidance notes. Because of "no testing" for 3 years of Covid and other reasons, the new inspector (and rules) are now more rigorous than previously. Almost all the boilers are out-of-certification (2 year re-certification limit) so need full examination and certification as new boilers.
One boiler (belongs to the club) that I initially rated (~2005) at 45 psi, but after recently re-assessing the calculations for the firebox tube (similar to yours) we agreed it should be re-rated for 27psi NWP. - Therefore the test and running conditions (e.g. single hand water feed pump instead of 2 independent filling methods) could be used for a "Small" boiler rated below 3 bar litres. But the engine with this boiler runs satisfactorily at 20psi boiler pressure, so no problems for us.
The inspector did say that if I had not shown the firebox calculations then the testing rules would have permitted him to certify the boiler, but once he had seen the calculations he could not agree to the old rating, as this compromised the required factor of safety. So he certified this at the new rating, as a "new boiler" design. He also commented that too much was left to the verbal agreement between the boiler inspector and the boiler user, as the rules for testing gave a very loose and imprecise guide to "acceptable" calculations.
Unfortunately, the same calculations have also de-rated a boiler from 45psi to 11psi, which means it will only be used on the smallest of engines that run at 10psi - or less. But this boiler was one that was inherited from someone's estate. It was leaking, from the joint of the fire-hole to outer shell and fire-hole to inner shell, and the firebox inner top plate to flue tubes... - The silver soldering had cracked - possible through over-pressure, fatigue, or because of the brittle grade of silver solder? So I repaired it and had used it for a few years (certified at 45psi) before understanding the calculations that have re-rated it to 11psi... But this may explain why the silver solder had cracked!
Close examination of 2 other boilers has condemned them because of distorted plates (copper too thin) that I have confirmed with calculations. - In one case the calculations said the plates were likely to be over-stressed at ~60 psi (limiting stress for annealed copper) and it had been hydraulically tested at over 90psi - but not thoroughly examined afterwards, just checked "it didn't leak". Hence, previously approved for the NWP of 45psi as it "did not leak", but the new examination (new inspector) required the casing to be removed to examine the boiler, as it was his first experience of this boiler.
The inspector did remark that the UK rules do not require calculations of firebox inner walls... possible because any failure of a soldered joint (as has been experienced on Loco boilers occasionally) inside the firebox only causes a sudden big cloud of water vapour around the driver, not an explosion.... (if that is "safe?").
It disturbs me somewhat that I have experienced so many boilers where the recommended safety margin (Factor of safety of "8" by design) is not achieved by many boilers. - Just by "luck"?
But I'm glad our new inspector is more thorough than previous ones... although there are now 3 boilers needing "upgrading/strengthening" to retain their practicality.
Ken