jma1009
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Post by jma1009 on Dec 23, 2013 23:29:37 GMT
hi dianne,
you are probably expecting this but 'here goes'!
you cannot soft solder tubes into endplates. soft solder is simply only a caulking medium as previously stated. it has no inherent strength. neither can you try and expand the tubes after soft soldering as the joints will fail. they will crack. some other mechanical sound means of fitting the parts together is required where soft solder caulking is employed eg threaded firebox side stays with nuts added. we moved away from threaded soft soldered caulked in tubes in 1923 in the UK for miniature work in copper. soft solder is ok if not subject to any stress, otherwise it fails after the first few steam ups. hence the mechanical fixing of such parts.
neither can you rely on one central longitudinal stay to avoid any stress on the firetubes.
i previously advised the main firetube containing the fire be of thick copper tube (13 swg). your main firetube looks far too thin to me.
the theoretical thickness of copper tubes is ridiculously thin (eg 8 thou for a 3/8" dia firetube on a boiler with 80 psi working pressure) - experience has shown they must be far thicker than the theoretical thickness.
some extremely experienced model engineers and engineers have also voiced their concerns albeit perhaps in a less forthright way and more diplomatically and politely. where safety is concerned i think directness and forthright advice is quite appropriate!
i have double checked everything ive stated with martin evans' book 'model locomotive boilers' MAP 1969.
i am not an armchair model engineer and have built quite a few boilers to my own design and published designs over 30 years.
cheers, julian
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Post by wdiannes on Dec 24, 2013 0:32:00 GMT
I am in no means a expert boiler maker, but I think the solder you have used will just melt off into a nice puddle on the floor when you steam it. At a pressure of 100 PSI, the water temperature can not rise above 350F. Copper is an excellent conductor of heat therefore the temperature of the surface of the copper can never be more than a few degrees above or below the temperature of the water (since the other side of the copper is in intimate contact with the water). The solder has a melting point a bit above 600F. That's 250F higher. Ain't gonna happen.
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Post by wdiannes on Dec 24, 2013 0:42:53 GMT
I was going to prepare a detailed response to your last post but I realize that would be a complete waste of time. You goal seems to be simply to pi@@ in someone else's corn flakes and you will do it no matter what scientific support I show for my design so I will no longer reply to your posts.
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steam4ian
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Post by steam4ian on Dec 24, 2013 3:55:59 GMT
Dianne
It's a pity that when you took the boiler up to soldering temperature you didn't silver solder in the flue and ignore my comments.
Thankfully for us all Julian does not have to approve your boiler and you appear to have the engineering design background and apparent competence to back your own design and methods and to satisfy your insurance underwriters.
Regarding the non silver solder temperature the highest melting point lead based solder is Comsol or equivalent, it has a melting point at 301 deg. C which is 574 deg. F.
For your comfort I have two letters from the AMSB Committee approving a Comsol equivalent for sealing tubes in firebox tube plates.
I am a little concerned at your firetube because it appears to be standard copper tube. I would advise you to check against the AMBS Code for firetube sizes before finalising you operating pressure. Interestingly it was the tube though the middle of the test vessels which failed long before the main shell ruptured and the butt end connections did not fail, see Australian testing report for subminiature boilers.
Please, please keep reporting even if you cornflakes have a strange taste.
Regards Ian
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Post by runner42 on Dec 24, 2013 6:29:32 GMT
A little diversion www.youtube.com/embed/Gw8P8L1LfDM?rel=0 Again I need to state I am no expert. Silver soldering has been the default method of joining copper plates in boiler construction because of their inherent strength, there are places in a boiler construction where they must be used. So for ease of construction silver soldering continues to be used even where it may not be essential to use it. It's easier to silver solder everything because then there can be no question on the inherent strength, the joints are stronger than the parent metal. If the tubes contributes nothing to the strength of the end plates, then comsol or other types of solders may be acceptable. But this has to be justified by the appropriate strength calculations, which will continue to place a question mark on the integrity of the boiler. It almost requires that the documentation accompanies it to diffuse any future concerns. Martin Evans proposed caulking the stays in my Rob Roy boiler. But if the hydrostatic test yet be conducted show leaks in the silver soldering, then to reheat the boiler to 700deg C is going to make a pretty mess of the comsol soldering and as stated previously once soft solder is used you can't silver solder over it. It would have be simpler to silver solder all the joints, instead of using alternative methods of joining. Brian
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uuu
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Post by uuu on Dec 24, 2013 8:30:25 GMT
The proof of this pudding will be in the heating! I'm looking forward to reports of successful testing. Bold and innovative, yet using a considered and researched use of traditional techniques.
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Post by Deleted on Dec 24, 2013 10:11:46 GMT
Nicely put, UUU ............Now then, why is DRACULA afraid of Cornflakes ??-------------------------Because it's a SUNSHINE breakfast !!..............
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Post by wdiannes on Dec 24, 2013 12:35:03 GMT
If the tubes contributes nothing to the strength of the end plates, then comsol or other types of solders may be acceptable. But this has to be justified by the appropriate strength calculations, which will continue to place a question mark on the integrity of the boiler. It almost requires that the documentation accompanies it to diffuse any future concerns. Refer to my post of Dec 22, 2013 at 8:01pm for the calculations. What seems to be confusing so many people is that I am not using copper sheeting of sizes normally found in model boiler construction but much heavier material; 0.130" for the barrel wall thickness and 0.250" for the end plates. Heavy material is inherently stronger and requires less support (using the same calculations you would use for thinner material) so the finished boiler will not look the same as a boiler of thinner material which would require more stays and supports. One would not normally use such heavy material for a number of reasons, chief of which is COST. However I had both 1/4" copper plate and 0.130 wall tube on hand from previous (non-steam) projects years ago so cost was not a factor. I am also modelling a boiler that was originally cast iron and as such had no stays or rivets and using heavy material allows me to keep the appearance closer to prototype. In addition, the finished model will be quite small and therefore light and using heavy material helps increase the weight and also tractive effort without having to add lead weights which would be difficult to hide on this particular locomotive. A second disadvantage of heavy material is the sheer mass of the boiler and the amount of heat that will be required to heat the boiler to steam temperature and maintain it there while operating. The Trevithick engine had no lagging so I am at a disadvantage there as well. Being such a small boiler (approx. 4" diameter and 6" long) getting enough heating surface to steam well is a challenge. I fit as much heating surface in the available space as practical and have kept the flue and tubes relatively thin to maximize the rate of heat transfer but if there is a "weak point" in this boiler it will be the combustion flue. I will not know for sure if the combustion flue will hold up when the fire is pushed hard or if it will soften and crush. "The numbers" say it is okay but there are more vagueries in calculating temperature differentials and heat transfer than there are in mechanical design so the performance of the combustion flue will have to be verified by a steam test. By all other (mechanical) calculations this boiler is significantly over designed with a minimum Safety Factor above 20 (where 8 is normally considered sufficient). It would have been much more simple to stay with a conventional design of thinner sheets and flanged and riveted tube plates but the it wouldn't have looked like Trevithick's Coalbrookdale locomotive and that is, after all, the objective. (Not to mention that such methods of construction would have attracted far less criticism!) I could also have built in steel where this style of construction is well documented and supported by ASME Boiler Code but it is difficult to find certified boiler plate in thinner sections - the "big boys" (certified boiler shops) think anything below 3/8" thick is flimsy LOL! Still we will soldier on putting our faith in sound mechanical engineering and see where we end up. Like the engineers of old, I will be sitting under the bridge in a boat when it is tested to capacity - well sort of - I will be right there when it is pushed to its design limit!
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Post by Deleted on Dec 24, 2013 15:42:57 GMT
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Post by ejparrott on Dec 24, 2013 15:43:47 GMT
Sorry but 0.130" thick copper is very well known and normally found in Model Engineering. 10swg is the most common size for boiler barrel tube for 5" gauge models. Those of us modelling in 7.25", are very familiar with 3/16" copper plate and tube, and some are even well acquainted with 1/4".
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Post by wdiannes on Dec 24, 2013 17:36:57 GMT
All buttoned up and passed her leak test - tight as a drum. So far, so good. Next step is to build a hand pump to do the hydrostatic test. I don't have a spare one and will need one eventually so I may as well build it now.
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Post by runner42 on Dec 25, 2013 0:15:25 GMT
Let me preface this by stating that I have spent hours reading your website (a.k.a rusty blades), you are a formidable lady eminently qualified in many fields of engineering, so I have absolutely no doubt that you know what you are talking about. The only thing that I can eclipse you is I am much older and maybe run farther (but probably not faster).
However, I could not find the post 22 Dec 2013 8:01 AM where you state that you provided your design calculations. It might be a senior's moment on my part but there is a post from you on that day at 7:58 AM and the next was by Steam4Ian at 8:09 AM. Not that I have the ability to confirm or otherwise your calculations, but I was looking for them just to be impressed. But it does raise a question with me that you may care to comment on, is it also good engineering practice to have peer reviews of design calculations and if so do you intend to submit these to such a review?
It's good that you are nearing the completion of the boiler and that you can move onto areas of the locomotive build that is not going to solicit as much critical comment.
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Post by wdiannes on Dec 25, 2013 1:20:29 GMT
..... Barrel O.D. = 4.130", area = 13.396 inches squared Barrel I.D. = 3.834, area = 11.545 inches squared Area of barrel edge = 13.396-11.545 = 1.851 inches squared. Using the tensile strength of copper @ 23,600 pounds/inch squared (which is lower than the tensile strength of silver solder), the failure stress of the joint will be = 1.851 inches squared x 23,600 = 43683 pounds. (The tensile strength of silver solder is such that deflection of the end plate will cause the copper to bend without the joint 'ripping apart'.) Using a centre stay of 1/4" dia copper, the tensile strength of the stay is (0.250/2)squared x Pi x 23,600 = 11579 pounds Strength of circumference joint + stay = 43,683 + 11,579 = 55,262 pounds At a pressure of 150 PSI, the load on the end plates (ignoring the area reductions for flues and tubes) is 11.545 inches squared x 150 pounds = 1732 pound. Safety Factor = strength / load = 55,262 / 1732 = 32 (at 150 PSIG) If the area of the end plates is reduced by the area of tubes and flue, the Safety Factor is even higher. ...... Yes, "peer review" is common when a design is complex or outside of normal engineering practice but these calculations are NOT outside normal engineering practice (just a bit different than covered in the UK model boiler practice apparently). This type of calculation could be found in any first-year college or university mechanical engineering course text.
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jma1009
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Post by jma1009 on Dec 25, 2013 23:56:34 GMT
hi dianne,
ok as far as it goes, but what is the SWG/thickness of the main flue, and the SWG/thickness of the smaller flues and dome tube material?
is the longitudinal stay threaded into the end plates, or in a clearance hole caulked by soft solder? and what is that longitudinal stay made of? doesnt look like phos bronze or copper to me. and the nuts on the ends dont look like same either.
longitudinal stays should be threaded into the end plates if they arent silver soldered in position.
have you used the correct grade copper for the tubes?
none of this info has been provided so far, yet is standard stuff on our drawings.
plumbing grade copper is quite unsuitable for boiler work, particularly as you cant tell whether it has lead added or will laminate.
cheers, julian
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Post by wdiannes on Dec 26, 2013 3:05:40 GMT
I am done discussing the boiler design.
Merry Christmas!
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Post by runner42 on Dec 26, 2013 8:11:26 GMT
In your opening post you stated: Still, after silver soldering the end plates in place, I will drill and pin them through the shell with stainless pins just to ensure any failure is progressive rather than catastrophic.
Is this still proposed? I was surprised to read that catastrophic failure was even a possibility.
The pressure you selected for your design calculations was 150 PSI, is it not usual to use 2 x the working pressure i.e 2 x 100 PSI?
A question to anyone having to meet safety boiler codes in the construction of small copper boilers, why is flanged plates invariably required when a butt joint can suffice if of sufficient cross sectional area?
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Post by wdiannes on Dec 26, 2013 12:56:56 GMT
I had proposed pinning the end plates for extra safety as I had not yet purchased the silver solder I was going to use and did not have the actual numbers of the tensile strength at temperature. Once I had the manufacturer's specifications and realized that the strength of the silver solder far exceeded that of the copper, I realized I could change to the "tongue-and-groove" to make assembly easier, to retain the prototype appearance, and still have a stronger joint than a butt joint. Then pins would serve no function since the copper would tear next to the silver solder and the pins accomplished nothing.
Everything has a point of ultimate or catastrophic failure if pushed far enough beyond its design limits. Take for example The World Trade Center, a high-rise building that was designed to withstand fire, flood, earthquake, bombing and every other conceivable event but the designers never considered the possibility of an intense fire fed by thousands of gallons of jet fuel and burning for hours - it was unthinkable and unprecedented. Some forms of catastrophic failure are sudden and violent, like a balloon over inflated until it bursts, others are simply a progressive failure, like a row of Dominoes toppling when one is hit by the next. Some designers take the time to determine the ultimate failure mode but many do not - they simply accept that it will fail at some point. I don't know if the designers of WTC looked at ultimate failure or not but if they didn't, they were very fortunate that the buildings "telescoped" down rather than went over sideways - it was the least amount of damage possible when the unthinkable happened.
(Sorry for the dissertation on "failure engineering" LOL!)
Exactly. Using a design pressure greater than the MAWP is simply adding another 50% to the safety factors.
I can not speak to the restriction of flanged joints in model boiler codes as I have never seen an explanation for that style of joint beyond "tradition" but suspect it was a carry-over from the days when the rivets were to carry the load without relying on the mechanical properties of the solder.
In deviating from the flanged joint construction I used the ASME Boiler Code as a guide. In ASME, with steel boilers, flanged joints are not allowed and all joints must be full-penetration weld. The philosophy is that a welded joint has the same tensile strength as the plates it joins and by virtue of the fillets, it has a larger cross-sectional area thereby making the joint stronger than the surrounding material. Since the tensile strength of silver solder is more than twice that of copper it seemed logical (to me) that the same principals apply to silver solder and copper.
If there IS a justification for the restriction to flanged joints in model boilers, I would sure like to see a reference to the calculations behind it. Even a reference to the reasoning would be welcome!
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jackrae
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Post by jackrae on Dec 26, 2013 15:08:55 GMT
In support of Dianne's argument regarding the suitability of non-flanged joints if properly designed and assembled:-
Published documentation on the assembly of fabrications using brazed joints generally states that butt joints should not be used and that lap style joints are the order of the day. Lap and flange joints are pretty similar, especially if the joint is in shear, rather than tension.
In a fabrication I was assembling (stress frame, not pressure vessel) it was necessary, because of the structure design, to use butt joints so I conducted a series of test joints and attempted to destroy them by shear and bending using a vice and the proverbial club hammer. Not one brazed joints failed before the steel itself tore apart. In that respect I satisfied myself that brazed butt jointing of steel fabrication was more than adequate for my needs.
The classic castellated joint of a rolled boiler barrel is in fact an extended butt joint, so why is it considered acceptable in that instance if not in others. I acknowledge the joint length in a square castellation is up to 3 times longer than a straight butt joint but if the tensile strength of the silver solder is greater than that of the copper, there certainly appears to a bit of over-engineering being applied.
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Post by runner42 on Dec 26, 2013 21:27:49 GMT
Thank you for the replies to my questions. I had a nagging thought that your design was not in accordance with conventional wisdom, but not really knowing what that conventional wisdom was. I have never read a boiler safety code, my only knowledge coming from studying the drawings of a number of model boilers and the construction of one boiler for an 0-6-0 Tank Locomotive Rob Roy. If I had thought more about the issue before posing the question of flanged plates versus butt joints, I would have realised that the throat plate/barrel joint on that model boiler was a butt joint. That I assume is the weakest point, so it could well be that flanged joints that are used everywhere else are more for produceability than safety, i.e holding plates in position (with rivets) for silver soldering. I can now sit back and enjoy the rest of the journey.
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steam4ian
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One good turn deserves another
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Post by steam4ian on Dec 26, 2013 22:40:32 GMT
Dianne et al
Some interesting points have been raised here regarding butt and flanged joints.
Flanged joints are out for steel boilers due to the risk of crevice corrosion. This problem can occur with riveted joins except that the compression of the rivets closes the space between plates; the plate edges are then caulked.
The AMSB Code allows braze welded butt joints but only flanged joint for silver soldering.
The code may be conservative because the silver solder joint can be done by anybody and that penetration of the solder can be seen by it appearing as a small fillet on the side away from the application of solder.
Baggo and I referred to a paper which demonstrated the suitability of silver soldered butt joints which suggests the Codes and Practices are overly conservative. But (pun) that said, on these forums a well crafted Britannia boiler built to "words and music" was scrapped when the silver soldered butt joint between barrel and throat plate opened up late in construction. Had the joint been brazed or been flanged the boiler would be steaming away today and AUD4000 not wasted.
Apart form my worries about the fire tube collapsing I applaud Dianna for her work and methodically engineered approach.
Regards Ian
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