Boiler calculations, end plates

I used BS5500, unfired fusion welded pressure vessels. Why, because I had a copy as a result of having to design a PV in the day job (no longer). I know that boilers are fired, but one of the pressure vessel guys at work advised that as far as stays are concerned there is no difference, at least until you start talking about flexible stays (don't go there). It is mainly aimed at steel, but there is a aluminium supplement, and using a bit of logical thinking you can come up with an acceptable stress for soft copper. My figure agreed well with the Aussy figure. It also agrees reasonably as regards stay pitch.

I think BS5500 Is now known as PD5500. I have no intention of publishing my ruminations in detail, not checked by a SQEP.

Many thanks Duncan. I will look those up. What is an SQEP?

Suitably qualified and experienced person

Many thanks Duncan. When coming up with such a calculation, given we don't make many steam boilers, who would count as suitably qualified and experienced.

As far as I can tell, you would need to confirm by experiment. I am fortunate that I can rig up a test rig to test end plates and buy scrap (unused offcuts) of copper from a scrapyard though.

Experiment is not required. The design codes are there. They are based on a mix of theory and experience gained in making and using many thousands if not millions of pressure vessels. They might need a bit of judgement to suit our particular needs, and this interpretation must then be checked, no-one is infallible.

If you are trying to produce a formula for something not accepted in the literature I can't see it becoming widely accepted without proof. Besides which hydraulic testing end plates is not difficult, you just need a suitably thick walled steel tube to solder test copper plates to to pressure test till it breaks. Be fun too!

Yeah, I'm the other way round - comfy with CAD and hopeless at maths!

The best book in my library for pressure vessel formula is Machinery's Handbook. Mine is the 20th Edition (1976) which is good for US, UK Imperial, and Metric. Highly recommended - most mechanical engineering facts are covered!

The 6th Edition (1924) is available free online, and might get you started. Pressured cylinders are in the Strength of Materials section, try page 404.

To me the most obvious problem is that pressure vessel design has long been decided by Codes Of Practice aimed at large boilers. These codes contain 'best practice' derived from a mix of theory and decades of experience, and aren't obviously useful for small amateur boilers: scale matters. The guides put out by the Clubs and Federations are more relevant, but they don't encourage experimental work. And maybe new ideas aren't needed? I suspect it's difficult to get a modern small boiler to outperform older designs because they're already nearly as good as it gets.

Whatever the reason, model loco boiler construction in the UK hasn't changed much for a century. Could be time for a change!

Dave

Many thanks SillyOldDuffer

I have a Machinery's handbook, can't remember which year. I will have a look in that. Not sure why I didn't look.

I'm not trying to shake anything up, if the books had a rule of thumb that went down to 0.04" I would have just worked from that, but it stops at 1/16, or 0.0625 which is close... but not close enough. I can get some 1/16" if I need to but would rather work from what is to hand and make sure it is safe.

Why, many thousands of perfectly satisfactory model boilers have been made over many years in home workshops Their safety record is first class.

From the point of view of stress analysis, I see very little to be gained by a mathematical approach.Typical boilers have a lot of flat plate in their design, very difficult to model mathematically. Also copper, the most usual material, has in its highly annealled state an extremely low elastic limit and considerable plastic ductility.

Clive, I am not looking to reinvent the wheel, just extend slightly its reach.

The formula in KN Harris's book and others is derived from mathematical models, one of my books goes through the derivation of the maths for the shell. What it does not do is the end plates.

(Another one goes through the maths for valve gear including Joy valve gear).

Can't disagree with that, but one opportunity might be steel welded boilers,. They're potentially much cheaper than copper and silver solder constructions. I think an assured safe design would be popular because lots of people do home welding now.

Dave

The fed code does include a section on steel. There are issues with steel instead of copper though, for example steel is a poor conductor of heat compared to copper and as a consequence a too low water level is an instant disaster to the firebox crown whereas you have time to refill the boiler in a copper one before the crown goes. Also harder to steam.

These are much less of a problem at 7 1/4" and above than 5" and below.

It's the quality home welding that is the issue not the design of plates, tube and suitable welds

This is just going round in circles. The Aus code and BS both have formulae which allow one to calculate the stay circle diameter for any thickness of flat plate once you've decided on a safe working stress. The Aus code has this for copper, and it is possible to derive one from the BS as I have done, coming to a figure remarkably similar to the Ausy one. If not confident of coming up with this stress, just buy the Aus code and have done with it.

Just because our boilers are smaller than big industrial ones doesn't make the calculations invalid. Some time ago I outlined a layout of tubes which would meet the Aussy code with 16g end plates, a couple of bits of copper about 3" diameter isn't going to break the bank.

Going back to the beginning, which is a small copper boiler of under 3'', some of the older construction articles simply instructed the builder to slit and flatten out a piece of the boiler tube material to make the end plates - no calculations presented.

And for a small, low pressure multitube boiler none needed. Properly built, failures did not occur, the staying provide by the tubes being more than adequate.

The AUS code for small (copper) boilers provides a more technical approach for those who feel the need.

Pero

As Pero has said "going back to the begining " and as Duncan has said ( in so many words ) this is destined to disappear up it's own fundamantal orifice. The O/P has a bit of scrap copper water or gas pipe and wants to build a boiler using this ? He has done the calculations for the stress in the shell and it works out safe, the end plate is under the same pressure but is flat - it needs stays, be they solid or tubular. A rough calculation at even 70 psi is that it will be under 2 bar/litre and not much bigger than a Mamod and only requires a safety valve. The calculations are in many books, 2 of which he has, After 3 pages of answers and other comments is this one NOT resolved ? Noel.

Noel, can you say which page the calculation is on in Harris's book as like the OP I can't see one.

As far as I can see it tells us how to calculate stay diameter , gives a table for spacing of stays for various material thicknesses but not the thickness the OP wants to use. No indication of how the table was arrived at.

Thank you Jason. That was roughly the original problem I had faced. Also my machinery's has some odd rules under stress which involves formula but with rules of thumb for some constant whose derivation and units are unclear. (1966 edition)

I do have a "Fowlers Mechanical Engineers Pocket Book 1955" Which does have a formula bit it uses a value called "fibre stress" for which I am unable to find a definition. However my next port of call is to use the rules of thumb in K.N. Harris's book to reverse calculate "fibre stress" of copper for one plate thickness and then see if the formula then produces results at other thicknesses consistent with the rules contained in the K.N. Harris book.

Whilst I am talking about stays and plate thickness, the other problem is that the K.N. Harris rules of thumb make no allowance for pressure which is not satisfactory. I will have to assume that it is on the basis of around 90PSI for the rules of thumb.

Harris book says the stay table is for pressures of 100-120psi, last line above the butt strap drawings