LBSC's Doris versus the Code

[jma1009]

i think you are being pedantic wilf!! who on earth would not ensure that the superheaters were clean and swept through? its rather like saying for a boiler test no ash or clinker should be left in the grate and the grate should be removed! however this is completely 'off topic' and doesnt help brian who has built a superb working chassis and now wants to build his own boiler to put on it!
cheers,
julian

[runner42]

Hi Julian,

yes you are right that LBSC's Doris did not have 5/8" dia flues in the original design, it was in fact 11/16" dia. However, 11/16" dia is no longer available, so I am forced to utilise 5/8" dia. Blackgates provide 5/8" dia copper tube in their Doris boiler kit. Thanks for the heads up on the need to have space around the superheater elements to clean the flues, that is something I hadn't thought about. The code is silent on the need to clean flues but in the sketches for superheaters it does show the spear head (the connection at firebox end) occupying most of the flue ID making cleaning impossible without removal of the superheater. Concentric Radiant type is even more difficult due to the support spiders required. The flue cleaning brushes I have seen advertised are predicated on the need to have the superheater removed to be effective.

Hi Ed,

you are obviously concerned that 26mm stay spacing will produce problems, even though the formula shows that 3/16" x 26mm is OK. The Romulus boiler example you quoted had a bulge in the firebox, what stay dia and spacing was used and was the bulge the reason for rejection?  A lot of Rob Roy boilers deflected at the top of the firebox at first steaming due to the inadequancy of girder stay in the crown, but it didn't as far as I know cause rejection of the boiler. But that was the UK experience, I don't know if boiler deflections are a routine cause for rejection under the Australian code.

Back to the drawings to make further changes to the design of the boiler required by the boiler inspector.

Brian

[jma1009]

hi brian,

hopefully your above illustration should solve any further arguement re stay spacing/'pitch'...

the girder type crown stays on ROB ROY have a gap of 7/8" between them, yet you quote deflection of the crown with this spacing.

if i were making ROB ROY i'd fit an additional girder stay in the middle as don young and LBSC always did - and i did exactly the same on Stepney's boiler as i considered there was too large an unsupported area on the firebox crown despite it being slightly curved.

(i should add that the inner firebox wrapper is 1/16" on the ROB ROY drawings, but hopefully you get my point)

if i saw a ROB ROY firebox being constructed i would advise the builder to fit an additional middle girder, and if i saw a firebox crown deflect on test that would be a FAIL im afraid.

(i also consider the firebox side stays and throatplate and backhead stays on ROB ROY to be far too few and starting too far from the inside flanges)

for threaded stays i have always used hard drawn phos bronze rod. i do not consider martin evans' 'gunmetal' stays to be a suitable material for stays.

were you planning on silver soldering the firebox stays? if so, you could use 5/32" dia copper snaphead rivets if your club boiler inspector doesnt like 1/8" dia stays.

cheers,
julian

[ejparrott]

Hi Brian,

I'm not concerned that the 26mm spacing will cause problems, so much as it's a full 10mm more than the original design - which works perfectly well - and therefore the 'safety factor' is so much less - can't give you an exact figure without sitting down and running all the calculations. You could argue that more stays produces more heat transfer being copper, the surface area exposed to water would be greater. I'd argue the difference would be insignificantly small. I'm concerned that the 26mm MINIMUM is being taken as 26mm end-of-story, and as a result boilers are being built lighter and being pushed closer to their limits. As I read it there's no reason not to fit them at 24, 22, 18mm spacing etc, as long as it doesn't exceed 26mm. I'd be dropping another row and column of stays in there at least, although to be fair, I fail to see why the original 15.88mm spacing can't be used, it's within the 26mm minimum requirement.

The Romulus boiler just failed is built to the original Marsh design, the stays are 1/2" or bigger, the inner firebox plate is 1/4". I haven't got to the bottom of it yet but we suspect scale build up in the water legs has caused localised heating of the plates, which have then softened and moved under pressure, and then cracked.

[jackrae]

Sept 4, 2014 8:17:05 GMT ejparrott said:

Hi Brian,

I'm not concerned that the 26mm spacing will cause problems, so much as it's a full 10mm more than the original design - which works perfectly well - and therefore the 'safety factor' is so much less - can't give you an exact figure without sitting down and running all the calculations. You could argue that more stays produces more heat transfer being copper, the surface area exposed to water would be greater. I'd argue the difference would be insignificantly small. I'm concerned that the 26mm MINIMUM is being taken as 26mm end-of-story, and as a result boilers are being built lighter and being pushed closer to their limits. As I read it there's no reason not to fit them at 24, 22, 18mm spacing etc, as long as it doesn't exceed 26mm. I'd be dropping another row and column of stays in there at least, although to be fair, I fail to see why the original 15.88mm spacing can't be used, it's within the 26mm minimum requirement.

The Romulus boiler just failed is built to the original Marsh design, the stays are 1/2" or bigger, the inner firebox plate is 1/4". I haven't got to the bottom of it yet but we suspect scale build up in the water legs has caused localised heating of the plates, which have then softened and moved under pressure, and then cracked.

I know it's only semantics but the26mm stay spacing in the table is the MAXIMUM permitted, not the "minimum" as might be interpreted from the above.

[Shawki Shlemon]

All this argument boils down to
1- If one builds the boiler to the code it will be safe , it has a conservative safety factor , no need to over engineer .
2- The requirements are minimum standards that must be met , one can exceeds the standards and over engineer the boiler ( no real benefits but self satisfaction ) , I don't think any boiler inspector will oppose that .
3- After all the builder must be happy with his/her new toy .

[steam4ian]

Brian

I don't have my AMSBC Code handy but recall that the code recognises that there may be some deflection under test. To be expected when you consider that the boiler just taken out of the pickle and sealed up has annealed copper which is soft and has not work hardened.

While our English and other colleagues have very sound experience they are not building to the Australian code to which your boiler has to comply. The Code is a minimum but it is a conservative minimum.

One of the local experts to which you referred built his first boiler aged 16 years and it still steams regularly and pulls the public. (Irony, an LBSC design.) His latest boiler was finished in what must be record time for a 7.25" Black 5, his design to the AMSBC code.

Don't confuse yourself with too much counsel.

[Jim]

Very good advice Ian, I couldn't agree more.

Jim

[jma1009]

iam sorry ian, but if for 2.5mm plate the 'aussie' code allows a spacing of 26mm for stays that is hardly 'conservative' in my opinion!
cheers,
julian

[ejparrott]

It's your boiler, you build it as you wish. I'm afraid there's no way I'd build a boiler with a 2.5mm inner wrapper with stays that far apart, whatever diameter they were. In all the years I've been building and testing boilers, I've seen 1/8" wrappers move with stays that far apart, and while movement is allowed in the UK code, it's something I prefer not to see.

[runner42]

Sept 4, 2014 8:17:05 GMT ejparrott said:

Hi Brian,

The Romulus boiler just failed is built to the original Marsh design, the stays are 1/2" or bigger, the inner firebox plate is 1/4". I haven't got to the bottom of it yet but we suspect scale build up in the water legs has caused localised heating of the plates, which have then softened and moved under pressure, and then cracked.

Hi Ed,

the Australian code addresses the problem referred to above by requiring washout plugs be fitted to all outer plates of the firebox and that boilers are periodically washed out to remove any scale or sediment. So on the whole the code is very comprehensive in scope and detailed in it's requirements.

Brian

[ejparrott]

This boiler had washout plugs, doesn't mean you can get everywhere though.

[Shawki Shlemon]

I am surprised that some insist that in their "opinion" the figures in the AMBSC code are not good enough , the code is designed by engineers using scientific formulas to give a conservative factor of safety and these are approved by government authorities before publishing . These are minimum standards and are to be met by the builders , however if someone has an opinion to increase the standards for his/ her boiler , that still will meet code standards as going above the minimum but can NOT go under the minimum . building a heavier boiler does not mean it is a better boiler , just will make its owner happy . As for deflection , slight deflection during the first hydro generally is considered by the inspector and a decision is made if that affect boiler safety or not , in most cases it is very minor and insignificant , I have tested many boiler in the last 25 years that I have been boiler inspector I have not seen any serious deflection or problems of deflection arising from the code staying .

[uuu]

I found this article quite interesting - although it relates to smaller boilers, it shows some of the thoroughness of the code. And it shows some real deflection picures!

AMBSC small boiler testing.

And I quite like the bit "a 3500psi pressure guage was fitted... dimensional readings taken at 100 psi intervals"

Wilf

[Jim]

Very interesting article Wilf. Somewhere I have an article on a similar attempt to test a boiler to destruction. While the boiler ended up looking like a rugby ball the thing that failed was the pump on the boiler testing gear.

Jim

[steam4ian]

My learned model makers in discussing stay diameter and spacing have overlooked one important issue.

No doubt 1/8" or 3mm stays have the tensile strength. No doubt 26mm seems a large spacing to some.
These are only two of the issues.
A third and important issue is the joint between the stay and the sheet. The AMSBC Code allows silver soldering of stays. This joint will be under sheer stress as will the copper sheet adjacent it. The weakest part will be the sheet just away from the joint, the failure of the test piece in the report above also indicates this. Having larger diameter stays increases the joint area hence the 3/16 not 1/8; hence the larger spacing.

Too many stays and the result is an increase in the chances of poor circulation and fouling of water spaces. The only instances of firebox failure I have seen reported in the model press have been stay/sheet joint failures.

Other boiler condemnations have been on account of water space fouling due to hard water; one reported here. Maybe model engineers need to apply some engineering thought to the aspect of water treatment also.

Ian

[runner42]

Hi Ian,

I should be gracious enough to allow your words to be the final input on this emotive issue because you have summed it up most admirably. Shawki also has shared his vast experience as a boiler inspector in supporting the code. It is understandable that MEs have over many years build up experience on in their opinion what constitutes acceptable standards for stay dia and spacing and because these criteria have found to be sound, these criteria are re-enforced so past practices have become the norm. It was easy to fall into a mindset that it is the unsupported distance between the stays that affect the strength of the boiler and that it is independent of stay diameter. I had to give it a second thought myself. However the formulae in the code showed me that these two aspects are mutually inclusive and because no-one has challenge the formulae as being incorrect then I was convinced that if I built a boiler that meets the code's requirements that it will be safe.

Brian

[suctionhose]

Brian,
I'm pleased your confidence is restored!

[runner42]

Yes Ross my confidence was shaken but not stirred or was that 007's Martini.

Today I had an OMG moment, having spent two days making the smokebox tubeplate I thought I would check to see that it meets the code's requirements. The drawings are on the computer so it was easier to reach for the AMBSC Code Part1 document and look up the ligament distance requirements. My eyes settled on 3.12.2 which specified that the minimum ligament distance is 4mm with a manufacturing tolerance of minus 0.5mm, giving a not less than 3.5mm or 0.138" distance. My Doris smokebox tubeplate had a ligament distance varying from 0.121" to 0.128", and doing a bit of mental arithmetic 0.5" spacing between 0.375" dia holes is only going to provide 0.125" ligament distance. HOW COULD I GET IT SO WRONG, Doris's boiler design isn't capable of meeting the code's requirements. But fortunately sanity was restored I should have referred to the previous para 3.12.1 which specified 3mm with a manufacturing tolerance of minus 0.5mm. Phew.

Brian

[jma1009]

hi brian,
the smokebox tubeplate looks very good.
for a tapered barrel there is something to be said for flanging the smokebox tubeplate after making the barrel. getting a tapered barrel dead to size with the sort of precision we aim for in chassis cylinder and valvegear construction is asking quite a lot! by the same token i dont flange the backhead till the boiler is nearly completed.
cheers,
julian