Advice on a boiler burner

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davidyat

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When I tried to build a Rider Ericcson hot air engine (and failed), I used this plan to make 2 sizes of a Sandy C Burner.
Grasshopper
 

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Emers

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I have built a 6 inch vertical boiler and I am trying to get a good burner for it. The firebox is 4.75 inch inside diameter and I am thinking a ceramic burner would be ideal. Can anyone point me to a good design or plans for my boiler.
 

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Hi Emers.
I hope that by "a good burner" you mean a gas burner, burning cleanly and efficiently. Actually, the camping stove burner is probably that, but if you want more power a ceramic may be appropriate. But the various types have limitations:
  1. Open flame ( a camping burner type): Good, if adequate flues and heat-exchange surfaces. Improved by the addition of suitable wire radiant elements above the flames. (just above the limit of the light blue combustion zone).
  2. Ceramic: In a very small firebox, but with a lot of flue area, these are not always the most efficient. But with a large area of boiler exposed in the firebox can be very efficient, especially when there is a lot of exhaust area, but not flue surface area.
  3. Wire mesh radiant: Run at higher temperature than ceramics before they burn-out. Stainless steel wire wool must be considered a consumable in this application. The Industrial elements use special allow wires that I have yet to source...
  4. Coal. By far and away the most powerful burners are coal. Providing the boiler is designed for it and the draughting is well managed.
  5. Porous ceramic media: These burners are only just becoming technically feasible and onto the industrial market. 4 times more powerful than wire mesh or ceramic radiants, they are approaching the power density of coal fires. (Coal and ash being "ceramic, porous and so spread-out they actually mimic the porous ceramic combustion process, whereby the gas is burned inside the lumps of material, as well as in the spaces around ... - Or maybe the new porous ceramic gas burners mimic coal fires?).
  6. Blown coal dust burners. Used in very large power stations, but not known at sizes for model boilers... Probably the most efficient, powerful and controllable burners - if you have a large Babcock boiler and the thousands of tons of coal available, and all the after-treatment for the flue gases....
I have provided a lot of design guidance to Doug for his 3inch boiler: see 3” boiler project. and earlier pages.
There is also stuff attached earlier on the thread that Doug found useful, although I wrote it a while back and it hasn't been corrected.... so if you find anything in error please tell me so I can fix it.
I think a 4 to 4 1/2" ceramic will fit? - but what height is there between the floor and the top of the firebox?
I'm a long winded old git so the information is there. Just not the "quick" answers.
Hope this helps? - as per my earlier posts? (#8 onwards).
(It saves me repeating stuff already published).
Anything you don't understand, please ask. I have made burners for my smallest boiler (1 1/2") to a 9" vertical boiler, so have a little experience. I am glad to discuss with anyone as that increases my experience.
I will stress that a key to sizing the boiler is the flue cross-section - but I haven't written much about that. It was the significant constraint sizing the burner for the 9" boiler - and also on my 1 1/2" boiler. Correct sizing is the key to "a happy customer".
But Please can you tell us some dimensions of the boiler, flue tubes, firebox size, etc. that you have made, the planned Normal Working Pressure, and the engine it will power? - Unless we know what you are doing we can't just give any specific answers. ("Engineering" takes Study, working-out, experience, and practice to develop. But "Information" comes first, otherwise we can't do the "study" bit.).
Regards,
K2
 

Emers

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Hi again
I have attached a picture of my firebox there are 43 firetubes in this boiler they are .312 ID copper tube . It has been made to the Australian boiler standards and passed all tests and has a boiler certificate. I am trying to build the best heat source for my boiler. I am using a portable gas stove element I purchased from china it is 4 inch outside diameter. I am using a regulator on a propane gas cylinder and about 10 psi. I have checked the burner temperature and it gets to about 430 C . It takes 19 minutes to reach 95 psi. I am running 2 steam engines of this boiler 1 is a twin Victoria 1 inch bore 2 inch stroke I like to run slow as I think this looks better. 2nd is a 8 inch Southworth duplex pump 15/16 Bore x 1.5 inch stroke. I have attached some pics
 

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Hi Emers,
What a smashing boiler and engines! You must be a very proud owner. I can work with this information. But just one more thing? - What length are the 43 firetubes? - I could make an estimate, but the real numbers are better.
At first glance... I think maybe a ceramic burner may give a bit more heat, but actually your selection of burner is very good! I am not actually sure that a ceramic will be better? - But I shall see what the numbers say!
This may take a few days, as I have a few things on-the-go, but I do like the boiler! - and the challenge!
Of course, as it is a design for coal firing, the best fire will be coal.... but are you powering anything from the engines, or do they just idle? I.E. the water pump... does it pump water with flow but no back pressure on the water, or do you feed the boiler with it? - If so, at what speed does it run? (cycles per minute) - Just like a car engine, there is a big difference between the "fuel input" required for idling or running flat-out!
A friend has a Steam wagon he made: The coal fire to run the wagon was rated around 27kW.... The ceramic burner that fitted in his firebox was rated around 2.7~3.5 kW, but he could raise steam to blow the safety (a good check every steaming) and idle the engine, with the wheels off the ground. Yet when idling the steam dropped to 50psi (NWP = 80psi). The first gas burner he fitted is about 4.5kW, and his latest gas burner is rated close to the 27kW estimate for the coal fire... - so there are gas burners that can do the job... But what do you need? and what do you want to do with the steam? The limiting factor may be the flue tubes, but also may be the size of the safety valve. Having designed the boiler to the regulations, what is the size of the safety? (Orifice diameter will do). I have to limit most second-hand boilers that I re-build because they have tiny bronze bushes for safety and steam take-off, which limits the heat input permitted for a safety valve that will physically fit. Also, I find that any boilers with tubes having "external" pressure can be a limiting factor. The firebox is one such tube: I'll check it if you do not have the calculation, but if you do have the stress calculation, please can you let me know? I don't know the Australian boiler standards.
I'll probably have more questions ... but for now I'll compare your burner with a ceramic.
regards,
K2
 

Emers

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Hi again
The firetubes are 124mm long. The burner has a rating of 3800 W (3.8KW) I do not know how good this figure is as the burner came from China. I do not run anything off the twin Victoria and I usually run it as slow as it will probably run, under 100 RPM I have never put a taco on it. The duplex water pump runs with no load most of the time just circulating water through a condenser I have recently finished it has about 10 feet of copper tube in a tube into which I put all the exhaust steam which can get my water tank temperature up to about 75C after 20 minutes or so. I use the duplex pump to top up the boiler as needed it pumps a heaps of water very quickly so the pump is on bypass most of the time. The Australian standards allow for Max 100psi ( tested at 200psi for boiler certificate) I have my safety set at 95psi I built the safety valve myself I found several designs online and I picked the one with the largest hole to vent steam 5/16 diameter it easily vents the boiler at blowoff. As the boiler drawings were given to me from another modeler I do not have any calculations for the design. This boiler I think was for coal firing you can see the grate in the drawings I have not tried it on coal only LPG. The firebox and boiler barrel are both 10G (.125) wall thickness.
Yes I am very pleased with efforts to date, I am a semi retire machinist still doing some payed work it covers the cost of the hobby. My interest in steam stems from my apprenticeship which was done on the NSW railways in the days when steam was being replaced by diesel (1966-1970).
Looking forward to any help
Regards
John Emery
 

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Hi John.
This is helpful.
Just a couple of observations:
  • As this is an inherited design - and very close to some "traditional" designs I have seen, is should be OK. If the Boiler tester accepted the design as "proven and appropriate" then that should be OK for your insurance. I take "obsolete" boilers and repair or re-use the materials. But I do the calculations to prove to myself what is safe. Often, I find the "old" thinking was "test it - if it doesn't fail it must be safe". But modern Engineering (E.g. ASME) uses a safety philosophy that effectively has a factor of 8 times built-in to the calculations, as that then gives a lifetime of safety, not just on the day it was tested. (materials do age, fatigue, corrode, get mis-used, etc.). So doing the calculations are a "first check" for me. Putting it simply: If ASME limits have a factor of safety of 8, and my calculations have a suggestion that you are near twice the pressure of the stresses specified by ASME, then in reality (IF my calcs are correct) you have a factor of safety of 4. So testing to Twice the pressure you are using means you know you have proven a factor of safety of 2, but don't know if it is anywhere near the USA regulators limits of a factor of 8. Of course, I assume you have used good new materials, the correct solder grades, fluxes, etc. and followed the design correctly so you will have achieved a boiler with the "estimated" factor of safety of 4. But I don't know if the Australian regs permit that or not. Although I was a professional Engineer in various industries and jobs, Copper Boiler design was not one of them, so I have no background or knowledge other than my own learning from books. - Which means I am "imperfect" to judge this one. Therefore, for your peace of mind I suggest you check with a competent Engineer who can check the firebox tube thickness and safe pressure for the NWP... I would be irresponsible if I didn't advise that.
My wife summons me, so I'll write about safety valves, etc. later.
Regards,
K2
 

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Hi again.
Right: comments on the various items.
Flue tubes - I guessed 125mm long so about right... CSA of flues and central chimney = looks adequate for the size of burner. Some typical "old" designs have flues = 1/9th of the firebox/grate area. Your boiler has flues and chimney (with superheater) approaching 1/5th the fire-box/grate area. = Very good for gas burner application.
The burner: 3800W rating is based on the jet size and standard tables, so comparable with the calculations. But the rating will be for Butane @ 15psi. So running on Propane I recommend you reset the regulator to 20psi. It may be a tad too much, but only 1 or 2 psi if anything. The problem is not the gas, but the faster jet stream from higher pressure entrains more air, and the burner may not like the slightly leaner combustion. (Ceramics don't! - They have to have the air holes adjusted with a change of fuel pressure to maximise the heat output without overheating the ceramic). But using Propane at 20 psi compared to Butane at 15psi there should be less "top-flame" hovering above the burner. This is a good thing.
Australian standards are probably very close to USA ASME regulations. The "100psi max" is an overall limit for silver soldered copper boilers. It doesn't mean that any boiler is made to withstand the pressure. It is due to the long term weakening of the silver solder, fatigue in copper at joints, temperature degrading the UTS of the copper, etc. so they are based on around 3000psi max permissable stress at 400degrees F which aligns with the copper firebox temperature at 100psi steam pressure. (With consideration of temperature drop across the copper boiler wall). So effectively, - assuming my guesses and calculations are right - you will erode the Factor of Safety from 8 to 4 by running at 95psi. So I would redefine the Normal Working Pressure on the documentation, re-mark the pressure gauge with a 50psi limit, and reset the safety valve to prevent >53psi being achieved, in order to comply to my understanding of the Regs. Talk to your tester and see what he says. He signs the certificate, but it is your skin that could be cooked if the firebox fails. I can only advise from this distance.
Safety valve: My quick check says you need a minimum of 7/32" diameter through the safety valve. Your 5/16" is plenty big enough.
The calculations on steam consumption are a bit over-rated, so I am sure you should easily manage the running modes you describe with that burner. Also it should be very controllable with the pressure regulator.
So my conclusion stands, that you have got "the best" burner for your needs. A ceramic (at half the power) would not be as useful for you. (I think that answers the original question).
Finally "Thank you" - for the opportunity to study your boiler. I have enjoyed this one very much. (It's a good way to keep the brain active in retirement!).
Please let me know if you need more "advice"?
Regards,
K2
 

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Hi Steamchick
I run this boiler at our club each month (Sydney society of Model Engineers) I will miss this month as I have family things to attend. We have a stationary steam display with about 20 models which are supplied steam from a boiler we have as a permeant thing in our shed and is piped to a stand with the models on display for the public to watch. I set my models up with the others.
One thing I just remembered the friend who gave me the drawings did say that the original design was a 5 inch boiler which he resized to 6 inch as he had that size material and I do not know if any recalculating was ever done would this change things much. I will check with him to see if he has to original drawing before he changed the dimensions. Also I just checked my fire tube dimensions they are .373" OD with .031" (.8mm) wall thickness.
I agree with your comment regards keeping the brain active I am 71 now and still enjoy going into the workshop each day.
Did you ask the mesh diameter of the burner i am using, it measures about 3.75" diameter.
 

Emers

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Hi Ken
As I will miss this months open day at the club it will be a few weeks before I talk to the boiler inspectors about this discussion. In the interim I will record some more information ie Southworth duplex pump strokes per minute, Twin Victoria RPM, Record the pressure I am using with the burner as I now have a adjustable regulator, Try to get update on the original drawing design.
 

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Hi John. Good to hear about you club's activity. Please post a Web address?
My club have 2 shows per year - outside, and always planned well in advance for cold wet days in June and September. Amazing planning required! I sit in a small gazebo tent with 3 tables of club models. I run about 4 members' models on air around 20psi, and one steam engine on a small boiler at around 20 psi. A guy with infernal combustion models usually demonstrates 3 or 4. The club track runs steam and electric rides every weekend in summer. A good crowd of lads!
I'll explain my concerns about firebox wall thicknesses.
I first met this issue when I had sized tubes and made a boiler for use at 40 psi NWP. Then decided with recycling the calculations that I could reset it for 80psi NWP. But at around 130 to 140 psi during an hydraulic test (target 160psi) a tube collapsed. Although the tube had been sized to all the boiler calculations (hoop stress etc.) there were NO standard calcs for tubes with external pressure.... and flue tubes and firebox tubes are tubes stressed with external pressure.
People who do these calculations have some vicious factors for de-valuing the compressive strength of tubes dues to any shape deformations, or stress raisers, that distort the uniformity of stresses on the tube and cause zones that instigate collapse.
Your firebox tube - 5", 10 gauge - has a stress raiser where the fire door connection is soldered. The extra stiffness here creates stress raisers that de-rate the overall pressure of collapse. But I can't find any calculations for that. So all I can assume is that it is a "perfect" tube.
Notwithstanding, the "perfect tube" - with a stress limit of 3000 psi (ASME Limit) - seems good for 53psi NWP boiler pressure....
Now, I know the calculations use 3000psi stress limit because of the degradation in strength of copper at elevated temperature, and allowing an adequate safety factor to account for any imperfections. These imperfections can be as little as 1mm ovality of the tube when in the working condition. I.e. something small enough you won't necessarily notice! The fire hole could do this. I don't know, to be able to advise.
What I do know, but based on my imperfect calculations, is that
1. There are lots of designs that use the same or slightly thinner walled firebox tubes as you have.
2. The factors of safety are there to accommodate variables that may or may not affect an individual boiler.
3. A modern boiler design - for a locomotive - commercially designed, so all the calculations will be professionally certified - has a firebox of 4 inch diameter and wall thickness around 6mm.... !!

So in conclusion (and having been a professional Engineer) if I identify something that suggests an increased risk, I must inform the user of reasonable and sensible action to take.
Hence I feel responsible in suggesting the boiler should have "a professional" decide if the firebox tube is adequately safe for the Regulations, for the NWP you have. In most clubs I think there is a professional charged with boiler certification, so I suggest you discuss it with him? Possibly the club insurance covers you if you steam the boiler at your displays? Mine does, providing the club's Engineer has issued or witnessed the current boiler certificate.
The only "moral" issue I have - please forgive me for fussing about this - is that there is a huge difference of risk between a factor of safety of 8 (design requirement) and the proven factor of safety of 1.5 or 2 - that is the proof test by hydraulic test.
The hydraulic test is not intended to supercede the design FOS.
Recently I saw a project where someone proved their design for a 100 psi NWP boiler with an hydraulic test over 800psi. But Regulations only need the boiler made to that design to be hydraulically tested to 200psi...
Now a simple solution to stiffen and strengthen tubes subjected to external pressure, is a series of strengthening rings fitted internally. I would need to do some calculations before I could suggest sizes.
Meanwhile, to answer your question about a design scaled-up from 4" to 5" for the firebox tube: I'll have to check, but it may be that the 5" tube can only take 16/25th of the pressure of the 4" tube before collapse. That is 64% of the pressure. So a 4" firebox tube for 95 psi NWP would only be good for about 60psi NWP at 5" diameter.
Just think of a plastic Coke bottle. Or a beer tin. Or aerosol can... It will take 90 psi internal pressure... but hardly any external pressure without collapsing. Put some boiling water into one, fit the top and watch a few psi of atmospheric pressure collapse the bottle when the steam condenses! Have you flown? And seen a half-full water bottle collapse when the cabin pressure drops just 3 or 4 psi?
I hope this helps? - and doesn't worry you!
Incidentally, if there are Engineers on the thread that can correct anything I have stated, I would appreciate your help? I have made mistakes before, and don't want to wrongly advise anyone!
Regards,
K2
 

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Hi Steamchick
I run this boiler at our club each month (Sydney society of Model Engineers) I will miss this month as I have family things to attend. We have a stationary steam display with about 20 models which are supplied steam from a boiler we have as a permeant thing in our shed and is piped to a stand with the models on display for the public to watch. I set my models up with the others.
One thing I just remembered the friend who gave me the drawings did say that the original design was a 5 inch boiler which he resized to 6 inch as he had that size material and I do not know if any recalculating was ever done would this change things much. I will check with him to see if he has to original drawing before he changed the dimensions. Also I just checked my fire tube dimensions they are .373" OD with .031" (.8mm) wall thickness.
I agree with your comment regards keeping the brain active I am 71 now and still enjoy going into the workshop each day.
Did you ask the mesh diameter of the burner i am using, it measures about 3.75" diameter.
Thanks.
I didn't comment on flue tubes, but that is the same as sizes I used (from standard material tables).
I still think you have sized the burner correctly. I can' t design a ceramic of more than half this power, as ceramic won't take the temperature that you stainless steel mesh will manage! You have an excellent burner for this boiler.
My 99 year old Mother says "after 70" you say you are "71 years young". I am 66 - and still 37 in my head!
Keep well and happy!
K2
 

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Hi Ken
You have raised several things for me to consider I will set the safety valve to 50 psi until I can confirm the boiler design to be on the safe side. It will be several weeks until I can get the club to evaluate your suggestions.
Our club address SSME - The Model Park as you will see we have many types of models for people to see. Check it out and let me know your thoughts
Regards
John Emery
 

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Thanks John. I am concerned about Safety for all when running live steam. My mate's Brother had 3 months off work following a steam accident. A seal failure caused steam to blast an arm. It cooks flesh that never grows back. Even if skin grafts can cover the wound. And it extremely painful. (But he ended up marrying the nurse who gave him care and special attention!). So awareness of risks and due diligence to good practice can ensure we all enjoy the hobby safely. Hopefully I am wrong and you can return to your previous NWP after getting professional confirmation?
Regards,
Ken
 

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Hi John. An excellent website. Clearly shows what you are about. But not so much of members models. I was hoping to see a bit more? But I haven't studied all the sub-videos yet. The Sunderland website is on www.csmes.co.uk but it has hardly changed with the lack of club activity in the last couple of years. However, the "Oily rag" gets a monthly update.
Regards, K2
 

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Hi John. Good to hear about you club's activity. Please post a Web address?
My club have 2 shows per year - outside, and always planned well in advance for cold wet days in June and September. Amazing planning required! I sit in a small gazebo tent with 3 tables of club models. I run about 4 members' models on air around 20psi, and one steam engine on a small boiler at around 20 psi. A guy with infernal combustion models usually demonstrates 3 or 4. The club track runs steam and electric rides every weekend in summer. A good crowd of lads!
I'll explain my concerns about firebox wall thicknesses.
I first met this issue when I had sized tubes and made a boiler for use at 40 psi NWP. Then decided with recycling the calculations that I could reset it for 80psi NWP. But at around 130 to 140 psi during an hydraulic test (target 160psi) a tube collapsed. Although the tube had been sized to all the boiler calculations (hoop stress etc.) there were NO standard calcs for tubes with external pressure.... and flue tubes and firebox tubes are tubes stressed with external pressure.
People who do these calculations have some vicious factors for de-valuing the compressive strength of tubes dues to any shape deformations, or stress raisers, that distort the uniformity of stresses on the tube and cause zones that instigate collapse.
Your firebox tube - 5", 10 gauge - has a stress raiser where the fire door connection is soldered. The extra stiffness here creates stress raisers that de-rate the overall pressure of collapse. But I can't find any calculations for that. So all I can assume is that it is a "perfect" tube.
Notwithstanding, the "perfect tube" - with a stress limit of 3000 psi (ASME Limit) - seems good for 53psi NWP boiler pressure....
Now, I know the calculations use 3000psi stress limit because of the degradation in strength of copper at elevated temperature, and allowing an adequate safety factor to account for any imperfections. These imperfections can be as little as 1mm ovality of the tube when in the working condition. I.e. something small enough you won't necessarily notice! The fire hole could do this. I don't know, to be able to advise.
What I do know, but based on my imperfect calculations, is that
1. There are lots of designs that use the same or slightly thinner walled firebox tubes as you have.
2. The factors of safety are there to accommodate variables that may or may not affect an individual boiler.
3. A modern boiler design - for a locomotive - commercially designed, so all the calculations will be professionally certified - has a firebox of 4 inch diameter and wall thickness around 6mm.... !!

So in conclusion (and having been a professional Engineer) if I identify something that suggests an increased risk, I must inform the user of reasonable and sensible action to take.
Hence I feel responsible in suggesting the boiler should have "a professional" decide if the firebox tube is adequately safe for the Regulations, for the NWP you have. In most clubs I think there is a professional charged with boiler certification, so I suggest you discuss it with him? Possibly the club insurance covers you if you steam the boiler at your displays? Mine does, providing the club's Engineer has issued or witnessed the current boiler certificate.
The only "moral" issue I have - please forgive me for fussing about this - is that there is a huge difference of risk between a factor of safety of 8 (design requirement) and the proven factor of safety of 1.5 or 2 - that is the proof test by hydraulic test.
The hydraulic test is not intended to supercede the design FOS.
Recently I saw a project where someone proved their design for a 100 psi NWP boiler with an hydraulic test over 800psi. But Regulations only need the boiler made to that design to be hydraulically tested to 200psi...
Now a simple solution to stiffen and strengthen tubes subjected to external pressure, is a series of strengthening rings fitted internally. I would need to do some calculations before I could suggest sizes.
Meanwhile, to answer your question about a design scaled-up from 4" to 5" for the firebox tube: I'll have to check, but it may be that the 5" tube can only take 16/25th of the pressure of the 4" tube before collapse. That is 64% of the pressure. So a 4" firebox tube for 95 psi NWP would only be good for about 60psi NWP at 5" diameter.
Just think of a plastic Coke bottle. Or a beer tin. Or aerosol can... It will take 90 psi internal pressure... but hardly any external pressure without collapsing. Put some boiling water into one, fit the top and watch a few psi of atmospheric pressure collapse the bottle when the steam condenses! Have you flown? And seen a half-full water bottle collapse when the cabin pressure drops just 3 or 4 psi?
I hope this helps? - and doesn't worry you!
Incidentally, if there are Engineers on the thread that can correct anything I have stated, I would appreciate your help? I have made mistakes before, and don't want to wrongly advise anyone!
Regards,
K2
There are two major things you need to check when a boiler is returned to operation after an inspection or put in service for the first time. Did the engineer or designer do a good job in the design calculations and was the boiler properly built or was something missed in the repair. Each boiler is certified for a design pressure and operated at something less then that. (usually 75% of design). There are several tools used to inspect boilers but the ultimate test is the hydro. Always with water never air. The hydro test will identify leaks and inadequate design as the parts will fail under load. The test pressure is usually established by code and might be as high as 150% of the design number. But at the very least it should be tested a pressure slightly above your intended operation pressure. Model builders do not normally have access to the tools necessary but in general their boilers are often operated at low pressures. A hydro is a good simple method of establishing a first criteria that the boiler is able to hold pressure at ambient temperature. Things change when heat is applied and its possible to have a good hydro only to have the boiler fail due to damage from thermal expansion. So often in these types of conversations the testing of the safety valves is ignored. On large equipment they are required to be pulled and inspected separately. I also know that models will be built and operated by many different skill levels. Not so with larger units. They not only need a boiler certification but a authorized certification to do the work. So about the only advice we can offer to a model builder is make sure your design is well thought out. Test it if you can and if you are going to operate it around others you might need a certificate from a jurisdictional authority to do so.
 

Steamchick

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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?
Boiler Firebox_.jpg

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
 

fcheslop

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Sandy made some calcs available for fire tubes. He is a retired marine engineer and was a manufacturer of model steam boilers.
Forgive me if this has already been covered as I struggle to read long posts due to dyslexia
But here is the info he was kind enough to provide
The crush strength of annealed Copper tube is a little less than 1/3rd of it's burst (Tensile) strength so it is necessary to adjust the strength figure used. The burst (Tensile) strength is 3125psi with a safety factor of x 8... we must, therefore, reduce this by a further factor of 3.5... which gives a figure of 892.857psi. to be used for external pressure calculations. (Crush Strength) The alternative way of calculating this is to take the 25000psi Tensile strength value and divide it by 3.5 giving 7142.857psi... then divide this value by the 8 times safety factor giving 892.857psi. The formulae for tubes subject to external pressure are: - For tube thickness: - T = (PD / (2S +P)) + 0.005D And for maximum safe working pressure: - P = S[(2T - 0.01D) / (D - (T - 0.005D))] Where: - S = The maximum allowable safe stress for the material (892.857psi in this case). T = Thickness. Copyright ACS Engineering Not to be used for commercial productD = Outside Diameter. P = Pressure. The OD for the centre flu is 1.125". The minimum thickness for operation at 90psi will be : - T = (90 x 1.125 / (2 x 892.857 + 90)) + 0.005 x 1.125 : - T = (101.25 / 1875.714) + 0.005625 : - T = 0.053879 + 0.005625 : - T = 0.0595" The centre flu is specified as 16swg wall thickness which = 0.064". The maximum safe external pressure for this is: - P = S[(2T - 0.01D) / (D - (T - 0.005D))] : - P = S[(2 x 0.064 - 0.01x 1.125) / (1.125 - (0.064 - 0.005 x 1.125))] : - P = S[(0.128 - 0.01125) / (1.125 - (0.064 - 0.005625))] : - P = S[0.11675 / (1.125 - 0.058375)] : - P = S[0.11675 / 1.066625] : - P = S x 0.1094574 : - P = 892.857 x 0.1094574 : - P = 97.73psi. This would be for a tube of 1.125" OD x 0.064" wall thickness which is unsupported between the end plates. Since there are cross tubes within the flu these will have the effect of adding considerable extra support in the same way normal stays would have. (See 'Loading on Flu' Below) If you also take into account the x8 safety factor involved, the actual unsupported crush pressure would be closer to 780psi.
 

Steamchick

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Thankyou Mr. Heslop! Excellent! I have been about 25 years since making my first boiler - and curiously a tube flattened when I considered raising the NWP and performed an hydraulic test "to see how strong it really was"! The 45 psi NWP boiler was made mostly with heavier materials than required by the calculations I could find.... (K.N.Harris). It was good at 120psi, but around 130 psi a single flue tube collapsed... After which my pump limit was reached at 160psi. So I keep it at 45 psi NWP! It doesn't leak... But I'll re-do all my boiler calcs now with your new information.
Thanks!
K2
 

fcheslop

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As far as I know Sandy is the only builder who used to show the calcs for firetubes others seem to brush round the prob. I think Alex Wise also uses the formulae in his book on small gas fired boilers
There was some info in the Model Engineer a few years past using a different calc
Alex Wise also covers staying for endcaps and a little more info for calculating endcap thickness from memory. I cannot get to the books at the mo as the house is totally in bits
A few Saito owners have found to there cost that the fire tube is running close to its limits
Iv re tubed several over the years
With a typical marine centre flue the cross tubes would act like stays but I dont take it into account
when calculating the materials thickness
Keep well
cheers
 

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