"Square end" welding method?

[wdiannes]

I am starting the design of Trevithick's Coalbrookdale engine in 1" scale. The original boiler is simply a cylinder with flat end plates bolted on.



The model boiler will be 4.5" diameter but "bolt-on" end plates don't comply with anybody's Boiler Code. The closest I can come to Code in this style of (steel) boiler is one full-penetration weld to affix the end plates and then machine the weld flush to preserve the appearance of the original. The two bolted flanges would then be for  appearance only.



Any other ideas on how this can be done safely?

Thanks!

[alanstepney]

I would make the boiler out of copper, and round it have the replica of the original, with bolt-on ends etc.
Effectively, it would be the "lagging cladding", and could be made to perform the structural parts (supporting brackets etc) leaving the actual copper boiler just to heat water.

[wdiannes]

I am sorry but I don't comprehend your description.

[jackrae]

I am at a loss to understand why you cannot build a "bolted flanged" boiler shell., but I'm sure someone will be able to advise.

If you cannot bolt an end-plate to a boiler flange, how come you are permitted to bolt a dome assembly to a boiler flange ring. Yes the relative areas of the dome items are smaller but proper design and specification of both flange and plate should take account of increased stress loads.

The structural "weak point", for want of a descriptor, is the connection of the flange ring to the boiler shell, so a full penetration weld, properly inspected, should be adequate.

The boiler codes concern themselves with, amongst other things, welded joints, which are subject to human failure if incorrectly made. Flange bolts, end plate and ring strengths are defined objective items so can be taken into account in design calculations.

Your proposed full depth welded end plate is no superior, in terms of subjectivity, to a full depth welded flange ring. In fact there is a possible argument that the sharp internal welded corner produces a high stress point which will be subject to failure.

[wdiannes]

Well that's an interesting thought Jack though I am not sure there is an advantage to having the ends removable.

[ettingtonliam]

I've been contemplating building a Garrett semi portable engine in 3 or 4" scale, using their patent boiler which had flanges either end of the barrel, outwards at the firebox end, inwards at the smokebox end. The cylindrical firebox had a flange at one end to fit to the barrel flange, and the tubeplate at the smokebox end bolted to the inwards facing flange, the whole thing arranged so that when the numerous flange bolts were removed, the whole firebox and tube assembly could be removed for cleaning, inspection and repair. The advantage in a model boiler would be that all joints are visible on both sides, and there isn't a huge mass of metal to heat as the boiler approaches completion. I'm interested to know if a boiler on this arrangement contravenes the model boiler codes, or if the codes simply don't contemplate a boiler of this type.

Richard

[Deleted]

You won't be able to produce your full penetration weld without burning the edge of the tube away. Jackrae's idea is much better. If there's no advantage in having removable end plates - just don't take them off!

[andyhigham]

An advantage of the flanged design with removable ends is the ability to weld both sides of the flange.
I don't see any reason why this design should be refused, its really just a short length of pipeline

[nonort]

The remark about a short piece of pipeline is so right. The main problem is the number and tensile strength of the bolts holding the end covers on. And then the strength of the end plate which my require strong backs to be attached on the inside. This would not matter as they would be out of sight.

[jackrae]

May I refer you to the following text from wikipedia :

""Coalbrookdale Locomotive

A drawing of the Coalbrookdale locomotive from the Science Museum.
The Coalbrookdale company then built a rail locomotive for him, but little is known about it, including whether or not it actually ran. ""

I don't wish to be the damp squib on your project, but I feel building a scale model of this loco, with sufficient material thickness to guarantee structural strength at a reasonable working pressure could result in a metal mass with so much demand on heat input that it might be nigh on impossible to raise steam. There are aspects of scale locomotive design that do not scale linearly, steam pressure and heat transfer rates being but two of them

[wdiannes]

Thanks Jack but the calculations show that there wont be any problem getting enough evaporative surface to raise steam and power the cylinder up to 3 MPH (which is faster than I would want to run something with a "whack-whack" valve gear LOL!)