Boiler Theory

[Artie]

Im on my knees begging for help.......... ;D

Im in the process of thinking about my boiler. My build will use the boiler to mount the engine pairing and therefore has to be (at least mocked up) in place to mount the cylinders and slide carriage pretty soon.

Ive read lots about miniature boilers but must admit to becoming a little confused with the info flow Ive tried to inhale. So... a couple of questions for the boiler-aholics on here...

My boiler is 120mm (approx 5 and 3/4") high and 250mm (approx 9 and 3/4") long. Ive read that it is advisable to move to welded steel at around this size.

No problem, but is there a specific type of tubing recommended?

Im guessing that heavy wall truck exhaust tubing is not advisable? (Too heavy a wall and getting it to temp would take ages considering steel wont absorb heat as readily as brass or copper and these have a defined 'seam')

I am concerned about the inherent dangers of working with and around live steam and I feel I understand the needs of the safety valve but I want redundancy in this area (2 valves, a secondary valve set slightly higher then the primary valve and each capable of venting faster than steam is generated. This unit is intended for a radio controlled paddle wheeler and may well be a distance away should anything go wrong).

Is there a precedent here?

Is there anything I should know? (I have NO experience in this area and am feeling my way forward very slowly, reading is no substitute for experience, hence my questions).

I hope to be working at around 40 to 50 PSI (the engine will dictate this obviously) and will hydraulically test to 100 before commisioning the unit.

Have I missed anything here?

Thanks in advance.

Artie

Just added a PS. Its propane fired with a throttlable flame control and self igniter using little camping disposable propane bottles (recycling a camping cooker... ;D)

 

[PTsideshow]

The groups in OZ have a very good and complete miniature boiler code about building that is followed around the world. MY link to the site no longer works maybe one of the members from OZ can give you the correct info.
Also it isn't a good idea to used, seamed tubing. As most of the truck exhaust tubing is endless welded mandrel bent tubing. Don't know what it is down your way.

Somebody was building a welded boiler meaning the ends and flue tubes were welded together not necessarily the shell. Most I have seen are pipe seamless.
;D

 

[GWRdriver]

My boiler is 120mm high and 250mm long. Ive read that it is advisable to move to welded steel at around this size.

Artie,
Not necessarily, and from what little I know so far I think copper will make more sense for you, but a lot will depend upon the skills, materials, and resources you have available. We need to know more about the boiler, such as the orientation. Is the diameter 120mm the dimesnion? If so I assume it will be a horizontal flue marine boiler.

is there a specific type of tubing recommended?

The choice for model boilers is always seamless hard drawn tube, whether steel or copper. Some off-sized boilers must be rolled to size from sheet and will have a welded or silver soldered seam, and there are prescribed ways of doing that, but whenever possible most people stick to the nearest available seamless tube.

Im guessing that heavy wall truck exhaust tubing is not advisable? (Too heavy a wall . . .

Well a heavy wall (within reason) is GOOD for a steel boiler, in order to provide a corrrosion allowance, but my guess is exhaust tube is a welded (seamed) low grade steel and your guess that it isn't advisable for boilers is correct. Brass should not be used anywhere in the boiler.

Is there a precedent here?

There's lots of precedent, depending upon the technical resources you use, and for your part of the world the Australian Code (AMBSC) makes sense.

 

[Artie]

Thanks Harry and Glen.

Im in the process of chasing a copy of the Aus boiler codes and sent an email to the suggested address a little while ago but as yet havent received a reply. The website only lists the books contents and contains no technical info. This needs to be paid for (no problem) and I guess I will just have to wait.

Im not sure which post I read the recommendation to move to steel at around 6" dia, but I 'think' it was on HMEM. I cant be too sure now as I have read so much about this of late from so many sources.

Horizontal flue marine boiler it is.

I guess I really need to know with the seamless tube is... how thick a wall do I need? Ive read suggestions that in copper 1.6mm (approx 1/8") is the minimum, but nothing on steel.

Keep it coming guys, all offerings gratefully catalogued...

Artie

 

[GWRdriver]

Im not sure which post I read the recommendation to move to steel at around 6" dia, but I 'think' it was on HMEM.

It may very well have been me, I've commented on that question a couple of times recently. In any case what I would have said is this, generally speaking the decision point for copper vs. steel is about 6" in diameter. At 6" diam and below the thickness of steel (including a sufficient corrosion allowance) can force critical spaces, such as the water legs and the grate area, to be smaller than they need to be to perform well. That typically isn't a problem with copper and below 6" diam the cost of copper is merely painful, rather than pure torture. Above 6" diam the effects of the steel corrosion allowance are less problematic, and at the same time the cost of copper becomes considerably more, so for some people steel begins to be a better choice. Overriding all this are your individual skills and resources. For instance, some of us are good silver-solderers but lousy welders, or vice versa. Some of us might have a brother-in-law who is a good welder and has access to certifiable steel. Individual resources almost always play a part in the decision.

.. how thick a wall do I need? Ive read suggestions that in copper 1.6mm (approx 1/8") is the minimum, but nothing on steel.

For steel certainly not less than 1/4" and that's cutting it close IMHO. Copper is a bit different in that the minimum shell thickness depends only on the diameter and working pressure, which aslo introduces temperature into the equation, and the strength of copper is temperature-dependant. There are formulas to determine what that thickness should be but generally by the time you get to 6" diameter a 1/8" wall is typical. It's possible that the shell of a 5" boiler with a design WP of 50psi could be as little as 2.5mm but that's an educated guess on my part and the shell data thickness should be run through the formula. When I fudge, of find myself between two standard sizes, I always fudge thicker or heavier.

I don't know how complex your boiler will be, although you won't have water legs to worry about, but you could make it a composite boiler, ie, a steel shell and heads with copper flues. This is a common combination in larger scale model locomotives in the USA and works just fine. The only ongoing disagreement is whether 'tis better to silver-solder the flues in, or roll (swage) them in, but I can say that for practical reasons rolling seems to be preferred.

The formulas I refer to for copper are found in books by K. N. Harris and Martin Evans, both long-time contributors to Model Engineer magazine. No doubt the AMBSC will have tables or formulas to develop the same information in generally the same way.

 

[Artie]

Thanks Harry, that all makes sense.

My welding and silver brazing skills are competant so either medium is ok from that respect. I think I glean from your posts that you would go with copper and thanks to your insights, I feel that is probably the direction I will go (unless someone comes up with a compelling reason not to.... :-\).

I think one of my concerns was in using the propane burner. Yes it is throttlable, but I was concerned with the hard solder at the end caps and this intensity of flame (much higher than a spirit burner). I think Im being overly cautious (yep thats me..) as with water in the boiler the joint should never get near melting point of the solder. I think a guard near the ends of the boiler to stop the flame encroaching upon the joint may make me feel better.

I decided upon the propane burner as this is a large model and the engines are comparitively large as well (20mm bore x 40MM stroke x 2) and I wanted this to be able to create the necessary steam with a margin. Is my thinking flawed in some way in your opinion? (anyones opinion... this is an area in which I am totally out of my depth... and I dont like it..... )

6mm (1/4") is an awfully thick wall for a boiler. I understand its to help counter corrosion (im thinking internally plus the effects of the burner?). I think it would take a month of Sundays to heat. But when it did it would stay hot for another month of Sundays.

In my previous post I stated that 1.6mm was 1/8", that should be 1/16" as 1/8" is 3mm (approx), sorry if I confused... (I do it to myself, why not you lot?).

Leaarning heap so far, thanks all.

Artie

 

[GWRdriver]

Artie,
I'm not trying to stear you in one direction or another although the majority of my individual experience is with copper boilers.

 

[shred]

Fire-tube? Water tube? Pot boiler?

All of those decisions are going to affect the design.
Lots of small-scale live steamers run butane burners on silver-soldered copper boilers.

 

[hammers-n-nails]

in the book steam Boilers: their thoery and design by Harry de Berkeley Parsons http://books.google.com/books?id=DO...steam+boiler#v=onepage&q=steam boiler&f=false page 155 the formula that is given for burst strength of a seamless tube is- pressurexdiameter=2xthickness of wall x tensil strength af material. boiler plate here in the states is a516 dont know if its the same there or what the seamless tube equivalant is, interesting fact: they also use this steel for armor plate because it streches more than normal before the bullet breaks through. anyway a factor of safty that was reccomended to me was 6-10. as for sweging the tubes, if thats what you choose to do, a roller can be made fairly easily for copper, i think there pretty expensive to buy. thats all i have, get ahold of me if you need anything else i can try to help, im not an expert but ill sure try.

 

[steamer]

Sandy may pipe up here as he was the Chair of the Au model boiler code panel. What I have read of it, its well thought out and specific...check it out when you can.

Dave

 

[ianjkirby]

Hi Artie,
With all due respect to Dave and Sandy, its me who used to be Chairman of AMBSC, and I can offer the following advice.
There is no hard changeover size between steel and copper, but at around 6-8" one needs to consider both materials before making a choice. Certainly for a 6" marine type boiler I would recommend copper if you can get it, and 2.5-3.0mm would be fine.
My strongest suggestion concerns how you intend to use the boiler. If you wish to use it near any 'public' persons (in Australia, this means anyone other than yourself, including family members!), you should consider insurance cover. The best way, in my opinion, is to join a club, and as you are at Griffith, the nearest is probably at Wagga, or maybe Jerilderie. There you will be able to discuss your project with a club boiler inspector, and get a certificate for your boiler when it is satisfactorily finished, and your club membership will provide a degree of insurance cover for operating near the public. As several have suggested already, a copy of an AMBSC code, steel or copper, is also good advice. You can get them here http://www.ameng.com.au/ame_retail_dvds_boiler_codes_and_miscellaneous.htm.
Regards, Ian Kirby.
Wollongong NSW

 

[Artie]

Sorry to be so slow getting back to this, tis the season.....

Harry, have no fear, my choice of material will be mine alone but I do thank you for your input.

Ian, great to hear from you and thanks for the link. 3mm wall thickness 120mm tubing if I can source it is the direction I would like to go, but, I will wait until I receive my copy of the Copper Boiler codes and (taking your advice) talk to the nearest club I can find.

In the interim I will construct a mock up boiler so I can mount the engine components and get this thing started.

My thanks all who have input into this discussion.

Artie

 

[bob ward]

6mm (1/4") is an awfully thick wall for a boiler. I understand its to help counter corrosion (im thinking internally plus the effects of the burner?). I think it would take a month of Sundays to heat. But when it did it would stay hot for another month of Sundays.

Artie I have a copy of the AMBSC steel boiler code, part 2, issue 4, 1995. The minimum thickness for steel is 6mm.

Without wanting to derail Artie's thread, there seems to be an anomaly between the skills certification required for steel and copper boilers.

Welding on a steel boiler in accordance with AMBSC must be carried out by someone certified to AS 1796 or by a person working under the direct supervision of someone so qualified.

As I understand it, anyone can silver solder a copper boiler together. Have I got that right, any comments?

 

[ianjkirby]

Hi Bob,
I'll try to keep this is short as I can.
Yes, you have correctly interpreted the requirements regarding the joining of these materials under AMBSC regulations.
Copper boilers are easily soldered (provided certain simple preparations are done), and the results are easily inspected by amateur club boiler inspectors. One only has to see that penetration has occurred, and you can be very confident that the joint is full-strength, and will not fail.
Steel is a different case. There are welders, and there are welders. Proper penetration is less obvious, and amateur club boiler inspectors are, in the main, not qualified to inspect electric arc welding. The requirement to have the welding done by a professional welder in a commercial shop has three distinct advantages; (a) the shop guarantees the outcome, and that it complies with current regulations; (b) the shop's product liability insurance helps to "spread the risk" for insurance purposes, and (c), the club boiler inspector is not putting his neck on the line in an area in which he has little skill.
As I have stated many times in the past on several forums, AMBSC codes only apply in Australia and NZ. Elsewhere, use them as a guide by all means, but it is up to the individual to ensure that their boilers comply with their local pressure vessel requirements.
Regards, Ian.
ex-AMBSC Chairman.

 

[steamer]

My apologies Ian,

My intent was to, for the sake of safety, have Artie speak to a knowledgeable member of the board of AMBSC. It was Clearly my gaff to put forth the incorrect name. Please accept my apologies Ian. I meant no slight. I believe the other name has an affiliation with the UK society, not the Australian. Again, my confusion.

Artie, my apologies for the digression, please continue.....

Sincerely,

Dave

 

[ianjkirby]

Hi Dave,
Please don't feel the need to apologise for such a trivial mistake. I am not the slightest bit offended. I only pointed it out so that it would not be perpetuated!
Regards, Ian.

 

[steamer]

Ian,

Your a Gentlemen....Thankyou! Artie...where ever we left off..

Dave

 

[Artie]

Gentlemen, thank you all. No digression has occurred. The information that has come from the discussion is EXACTLY that which I (and Im certain, others as well) required...guidance in an area I had no knowledge of.

Thanks

Artie

 

[steamer]

Gentlemen, thank you all. No digression has occurred. The information that has come from the discussion is EXACTLY that which I (and Im certain, others as well) required...guidance in an area I had no knowledge of.

Thanks

Artie

Ahhh another Gentlemen, this is why I love this Forum!

Dave

 

[Artie]

I have yet to recieve the boiler codes.........

BUT I was talking to a mate who manages an engineering shop about the pro's and con's of steel vs copper. He asked about what the steel had to be and then asked what size did I have in mind.

He walked away and came back with this 'offcut'. Its 5" OD X 4 7/16" ID (been bored and honed for an hydraulic ram housing) and 9 1/2" long . Converting that to 'my' language, (I wanted 120 OD with at least a 6mm wall thickness and 250mm long) this is 127mm OD, 7mm wall thickness and 240mm long. So its 7mm wider and 10mm shorter than I wanted but is 1mm thicker in wall thickness. So its all good.

This is hydraulic tubing (seamless) with a 2600PSI working rating. Best part, it was donated! Next best part, he has a welder with pressure vessel certification who will weld the ends in for me.

I havent yet decided to go with steel but this is certainly pushing me that way... ;D Still I will wait until the codes arrive and then make up my mind...