Boiler

[Mick HenshallParticipant @mickhenshall99321]

Wow! The breadth of knowledge on this forum never fails to leave me in awe, I'm not experienced enough to give advice just try and learn as I go along , good point Phil it is possible to get a copy of the MOT Testers bible so it would be good to see the rules the inspectors use as long as there arn't too many big words in it, perhaps one testers interpretation is different to another, I seem to remember a post recently about a tester wanting to see receipts of materials showing the specs of those materials

My thanks to all especially Jason &Phil for taking the time 

Mick

[JasonBModerator @jasonb]

The barrel comes well within the safety factor for both drawn and rolled, a drawn one would be OK for 160psi (SF10) and the rolled one 128psi (SF8) It was only the flat surfaces that are near the mark.

Having said that the spreadsheet works on 120psi max temp of 350deg C, 20000UTS. This gives a working stress of 4000psi when divided by the factor of safety which is similar to Evan's 25000/6

The spreadsheet that I am using is used by at least one of the established professional boiler makers and member of APCBM (ME).

I think when the subject of what calcs are acceptable has come up in the past it was generally felt that Evans' ones would be OK and more so than Harris's.

[Phil H1Participant @philh196021]

Jason,

Thanks for that. As far as you know, do the agreed or acceptable methods given by Evans apply to all features including the stay pitches on the firebox sides?

Are the APCBM (ME) Australian?

Mick,

For some reason, I am strangely interested by the boiler stuff. One major reason is that back in about 1983ish, I tried to build a Rob Roy boiler. I recently blew the dust off it and Julian was kind enough to give his opinion on a few of the pictures I took of it. A major headache with my attempt is that it is a sort of mongrel i.e., some features are actually quite modern but some parts are considered not acceptable current practice – even though they were taken from the drawings. I think it might be worth posting the pictures in a separate thread. Even if I can't retrieve the boiler, it might help to prevent others from making the same mistakes.

Phil H

[JasonBModerator @jasonb]

I've not seen what Evans suggests for stays so could not say for sure (I'm not a loco man) but I would have thought if his other calcs are considered OK then the stay ones would be too.

APCBM = Assoc of Professional copper boiler makers. UK assoc

[duncan webster 1Participant @duncanwebster1]

It would be interesting to know what the APCBM spreadsheet is based on, as 3/4" pitch for 80 psi and 16g plate seems big to me. Can anyone let me have a copy?

[JasonBModerator @jasonb]

Duncan, I just entered the figures given earlier by Mick and said that 3/4 spacing only just made the safety factor. The spreadsheet does not give the spacing, you have to enter it. It gives the max working pressure for the stayed areas and what safety factor that is.

5 x 5 stays at 0.625" spacing would be better and upping the thread to 4BA would not go amiss

 

Edited By JasonB on 09/05/2017 19:45:37

[Mick HenshallParticipant @mickhenshall99321]

Good suggestion Jason, I shall do that to be on the safe side also pics of other boilers would be interesting Phil

Mick

[Phil H1Participant @philh196021]

Jason,

Thanks for the APCBM definition.

Mick,

Ill create a parallel thread but while we are on the subject, my mongrel has a 4 x 4 pattern for the side stays using 1/8" silver soldered snap head copper rivets (snap head on the inside of the firebox).

Phil H

[Mick HenshallParticipant @mickhenshall99321]

Thanks Phil,  it was suggested monel,gunmetal or copper could be used for stays, copper being least suitable for making the thread I was thinking gunmetal might be a bit more tensile, 

Mick

[Mick HenshallParticipant @mickhenshall99321]

I keep getting "A" appearing in my text and I don't know why  

Mick

[Phil H1Participant @philh196021]

Mick,

Quite a few of the builds that I have seen now use silver soldered snap head copper rivets or bronze turnings for the side stays and even the crown stays. There are several build threads, with pictures on this and other forums to show the sequence. It might be easier to search for them rather than following my description. If you manage to visit your boiler inspector, he and or other club members will probably go through the sequence.

Phil H

[JasonBModerator @jasonb]

The use of rivits is quite common now but as I said earlier they would be classeed as "headless" as it is just the soldered joint on the outside that provides the strength. On thin material there is less area for the solder so a weaker joint results. To compensate for this the spacing may need reducing or larger dia stays used.

[Roderick JenkinsParticipant @roderickjenkins93242]

Mick,

I'm currently building a Rob Roy boiler (mostly) according to the drawings. I bought a kit from Blackgates with flanged plates for slightly over £200. I took my latest efforts to my inspector last night.

Currently, the firebox and foundation ring pieces are held together with 10 BA nuts and screws – these will be replaced by 1/16" rivets before silver soldering. I'm proposing to to use 1/8" dome head rivets for the side stays, head on the inside and the outer just poking through and silver soldered. The inspector is happy with that, I think it is generally preferred to nuts and caulking these days. As you can see the, I extended the sides after opening up the tube. In the interests of symmetry I did both sides the same. This was a mistake as the butt strap interferes with the foundation ring and I had to mill a tapered recess in the foundation bar to make it fit.

It would have been much easier to leave one side long and to have a butt strap on the other side much higher up and out of the way.

Making the crown stays to fit is also quite awkward to get a good fit.

Long 1/8" rivets would be much easier and my boiler inspector suggested using these – but as I've made the sheet crown stays now I'll use these and they will be soldered to the firebox crown before the firebox and tube assembly is finally inserted into the boiler tube.

Not he world's neatest boiler but, so far, it looks like it might be OK.

HTH,

Rod

[Mick HenshallParticipant @mickhenshall99321]

Looks good Rod, did you use 3/32"thick copper for firebox in my book it is shown the same as the boiler at 16g,interesting using rivets for the stays saves a lot of threading for nuts, I see Kennions do a kit ( with unflanged plates)

for around £140 plus? but I'm not sure what is in the kit, having made frames , buffers horns, axleboxes stretchers and erected it all I am at an early stage wheels and axles next

Regards Mick

[Phil H1Participant @philh196021]

Jason,

I will have a look back through the book I mentioned earlier and see what the sums suggest regarding the side stays.

Rod,

I understand that you have only shown part of the crown stay in your picture. I did read (somewhere) that the some builders had observed distortion to the top of their Rob Roy firebox. One proposed solution is to add a second 'L' shape riveted to the piece shown in the book. Its flange helps to strengthen the inner wrapper.

This is a view of my firebox stays.

[Phil H1Participant @philh196021]

The side stay spacings from the book don't seem to make much sense when compared to the Rob Roy drawings – as follows;

The book gives a table for the minimum diameter for the stay per thickness of plate and for 1/16" it states 5BA.

It then gives Stay pitch = Sq' root of D^2 x T x 3/ P x F x 4

where D min stay thread diameter, T is UTS for copper (25000), F is safety factor and again suggests 8 and P is the working pressure (psi).

this gives sq root of 0.096" ^2 x 25000 x 3/ 80 x 8 x 4 = 0.52" – half an inch!!!! or about 7/16" if 4BA is used!!!

Assuming that I have pressed the numbers on my calculator correctly, neither can really be right because his own Rob Roy boiler drawing shown in the same book, a few pages earlier shows 5BA with a pitch of 3/4" x 5/8"

Interesting.

[Phil H1Participant @philh196021]

Sorry, I meant to type 9/16" for 4BA – not 7/16". It also gives just over 5/8" pitch if 1/8" is used.

[Bob YouldonParticipant @bobyouldon45599]

Good evening Mick,

On the subject of stay material may I suggest you avoid monel at all costs, I have in the past been involved with several boilers using monel stays around the time Martin Evans was suggesting their use as a practical alternative to the more usual copper, quite where Martin got the idea, I cannot say but I'm pretty sure he never attempted to build a boiler incorporating monel stays! If i the stay is to be threaded into the inner fire box with nuts and silver soldered through a plain hole in the outer wrapper then I would suggest the use of drawn phosphor bronze, although my personal preference now is the use of copper rivets silver soldered on both inner and outer fireboxes much as Phil Hale has shewn above.

Regards,

Bob

[Mick HenshallParticipant @mickhenshall99321]

Hi Bob, 

I think Martin was suggesting that monel was more tensile than copper or pb, I was considering pb but copper rivets as you suggest seems the better easier option

Mick

[John BaguleyParticipant @johnbaguley78655]

I opted to use 1/8" rivets when I built the boiler for my Helen Long. These were threaded 5BA and put in with the heads on the outside and brass nuts on the inside of the firebox. I didn't fancy trying to silver solder the stays inside the very narrow firebox so opted for the brass nuts caulked with Comsol. It's a method that I personally as a boiler inspector don't have a problem with and I think it's a much easier way for a beginner to get a good boiler until they gain more experience. The heads of the stays were siver soldered on the outside as this is relatively easy to do.

The outer wrapper and the firebox wrapper are both 1/16" (1.6mm) copper and the original stay spacing by LBSC was given as 3/4 x 3/4. I consider this too far apart for 1/16" copper sheet and reduced it to 5/8 x 5/8.

John

[duncan webster 1Participant @duncanwebster1]

Stays are there to support the flat plates, so stay spacing should logically be calculated to suit plate thickness and pressure. The diameter of the stays is then calculated to get acceptable stress in the stays. If stays are at the correct pitch to suit the plates, then increasing stay diameter does not allow wider spacing.

Neither KNHarris nor Keith Wilson methods would accept 3/4*5/8 pitching for 16g and 80 psi. Just for interest, neither would the Aussy code, or the nearest relevant BS (interpreted for copper)

Monel stays were used in some full size locos, but they were screwed and nutted not silver soldered. In both my boilers I stuck to silver soldered rivets

Edited By duncan webster on 12/05/2017 08:37:21

[Phil H1Participant @philh196021]

Duncan,

Mine are 3/4" x 5/8" spacing – to the drawings, using 1/16" inside and outside sheet. It has not been tested so it will be very interesting to see what happens. There are quite a few other issues with my boiler and it will be interesting to gather other views on possible solutions. As I stated in an earlier response, I will create a separate thread.

Phil H

[Phil H1Participant @philh196021]

Posted by duncan webster on 12/05/2017 08:34:17:

Neither KNHarris nor Keith Wilson methods would accept 3/4*5/8 pitching for 16g and 80 psi. Just for interest, neither would the Aussy code, or the nearest relevant BS (interpreted for copper)

Edited By duncan webster on 12/05/2017 08:37:21

Duncan,

From what I can gather, the Australians appear to have a reasonable method for dealing with model boilers but I am in the UK. Our system appears to be to try and muddle through some out of date designs then dig around for methods on forums or the model engineer to try and find the correct design data to modify the out of date designs. We then use an inspector as the design authority. From where I come from, relying on an inspector is completely wrong. It would be the duty of the designer/ builder to present the correct information to the inspector for his confirmation.

So, I am trying to gather the correct information. For example, would you be kind enough to say which BS you referring to and or the Harris/ Wilson references please?

Phil H

[duncan webster 1Participant @duncanwebster1]

Phil, I've sent you a PM

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