Boiler construction question

[andrewtoplis]

Evening all

Station Road have a part-built Hewson standard four tank.

The description states "construction was by silver soldering, with threaded firebox stays fitted with hydraulic sealant. Not a new idea - it's been around for twenty years or more and is well-documented in various articles“

Could anyone point me to more information on this please?

Thanks in advance, Andy

[millman]

Just had a look at it on SRS site and can honestly say that I have never heard of or seen articles on this method of stay fixing before. What exactly is hydraulic sealant and what is it’s temperature range also what does the boiler inspection code books have to say about this kind of construction. Off now to search for these various articles.

[andrewtoplis]

Thanks millman, if you find anything please let me know. Andy

[John Baguley]

I have heard of threaded stays being sealed with some sort of Loctite - possibly thread sealant. I think there were a couple of boilers built like this in the Chesterfield club? No reason why it should not be satisfactory. It would certainly be easier than using Comsol.

John

[davewoo]

I know of a Don Young Mountaineer built at the same time as, and featured in Don's articles in either M.E or LLAS by the late Ted Benn using loctite caulked threaded stays, it passed boiler tests for many years to my certain knowledge, possibly still is. Ted's son was in the aircraft industry and had a vast knowledge and free supplies of the various Loctite grades available at the time. Agree with John this engine proved very satisfacory in service, and after all it is only a sealant all the forces on the stays are taken by the threads.

[jma1009]

From what I recall there is no literature from Loctite that would substantiate using any of their products to seal firebox stays.

That alone is significant.

And I am not aware of the Feds in the boiler regulations approving of such a method.

So I myself would be exceptionally cautious in respect of firebox stays secured with Loctite.

[davewoo]

As a result of my post above I had a call last night from a friend who was a member of the same club at the time (a long time ago!) who remembers very clearly that there was great reluctance from the boiler testers at the time over this boiler. Basically the boiler was accepted but after that no others would be tested until clarification was obtained from the Southern Fed, whether this happened or not neither of us can remember. Ted did leave the club after a disagreement but we are not sure what over, he could be a little prickly! So probably a good idea for anyone considering this loco to get clarification from their club boiler inspectors or one of the federations.

[uuu]

Back in the "good old days", threaded stays would have nuts on, and be caulked. I can see that being very effective, mechanically. A failure of the caulking would be inconvenient, but not catastrophic. Boiler seams would be rivetted - same applies.

I don't feel so good about just threading something into the boiler shell without nuts. Not much thread to rely on, and quite soft materials.

Wilf

[johnd]

I remember that the Model Engineer carried a number of articles in the 1970’s/1980’s regarding the use of Loctite with boiler stays.

[John Baguley]

I don't really see why any boiler inspector should have a problem with this. So long as the stays are properly threaded and nutted then the method of sealing them should be irrelevant. In the early days LBSC advocated using plumbers jointing on them or even just making the threads a tight fit so they sealed themselves. Hopefully, we have moved on a bit since then though.

Loctite produce several adhesives/sealants rated to 200° C that would be suitable. They even do one rated to 700° C and 200bar pressure. One of the examples they show for this is steam at 350° C. Permatex also have a range of high temperature sealants as do other manufacturers. I think I've got some Rocol stuff somewhere based on PTFE that is rated to 200° C or more. The fact that Loctite don't mention sealing stays in the data sheets is hardly surprising. It's not exactly a common use.

I'm sure many of us are using sealants such as Loctite 542 for sealing fittings on boilers with every success. That is really no different to sealing threaded stays.

A quick search through the ME index comes up with three articles/letters on this method of sealing

Vol. 146 Issue 3646 I.Marsh

Vol. 147 Issue 3660 J W Aitken

Vol. 147 Issue 3660 M. Smart

There are probably others but it would mean digging out the relevant magazines and I can't be bothered.

I think Dave's example of the Mountaineer boiler shows that it can be a practical solution. Inside the firebox would be the real test. If any stays did eventually leak though it would be a relatively easy job to remove the nuts and apply fresh sealant.

I did have a few telephone conversations with Ted Ben and he was quite a forward thinking chap. I did hear that he could be a bit temperimental though!

John

 

[andrewtoplis]

Thanks very much for the replies.

Can I just check my understanding then; the boiler plates have a thread cut into them, and a screwed stay (like standard gauge practice) is used. A nut is then placed on the outer and inner ends of the stay, and a hydraulic sealant compound (possibly a type of loctite as mentioned above, is used to seal up any small leaks.

The strength to prevent the plates distorting is provided by the stay itself threading into the boiler plates, and by the nut. The sealant is just to stop any minor leaks.

Is that correct?

What would be the advantage of doing this instead of a silver soldered stay as per 'normal' please? Would it save that much time and effort?

[John Baguley]

Yes, you are quite correct. The strength comes from the threads and the nuts. The stays can be either a length of threaded bar with nuts on both ends or like a screw with a head. The head normally goes on the outside of the boiler and the nuts on the inside of the firebox. The nuts can be brass as they are not exposed to any water. When I built my Helen Long boiler I used copper rivets that I threaded and screwed them in from the outside. Brass nuts were fitted on the firebox end. I did silver solder the outside heads as that was easy to do but I sealed the firebox side with Comsol. The firebox is very long and narrow and I wasn't confident enough at the time to try silver soldering the stays inside the firebox. I only had propane so would have struggled to keep the torch alight in such a confined space.

The advantage is that it's much easier to use threaded stays and caulk them than silver solder the stays. Using silver soldered stays seems to be the downfall of many an amateur boiler maker. The disadvantage is that the water level must be closely monitored to make sure that the stays don't become uncovered and possibly melt the soft solder or damage the sealant if used. If you have the equipment and the experience then silver soldering is the better option.

John

[davewoo]

Ted's mountaineer boiler definitely had screwed stays nutted in the firebox, and either thin heads or oversize bar fitted into a small countersink on the outside of the boiler. I remember distinctly borrowing a coventry die head from work over a weekend so he could make the stays. Ted was an excellent boiler maker with many successful boilers to his credit, although I can't remember if he used the loctite method again, I've lost count of the number of 2 1/2" gauge boilers he built over the years. Whilst Ted could be prickly to say the least he was also extremely helpful to me when I started model engineering, I was a friend of his son from teenage motorbike days (still am we're meeting for coffee tomorrow!) when Ted and his wife Jean tolerated visiting motorbike oiks and with Ted's knowledge of engines a great help keeping our old British twins on the road.

[pault]

Hi
I know someone who many years ago built a 7 1/4" Scott and did exactly this, screwing the stays in and sealing them with Loctite. They were nutted on the inside of the firebox and just screwed in on the outside, the loctite was purely a seal and provided no structural strength.. Jump forward a number of years possibly 10 and stays started leaking on a regular basis. When you looked up the Loctite, the strength graph showed a progressive reduction in strength when used at elevated temperatures, so no great surprise. He then puddled soft solder over the stays, which held for some years before bubbles formed in the solder, which then burst and it was leaking again. There probably is a sealant in this day and age that will do the job without breaking a sweat, its just a case of finding it.

[JonL]

If anyone is looking for adhesives etc, I found the people at Permabond went utterly out of their way to find one for me, even though I was only buying one bottle. I mean, the chap went away with all the specifications and rang me back now and then saying "how about this" and gave me specs. They really, really tried hard to meet my needs, and they did. In my case it was retaining a cast iron sleeve in a pitted cylinder casting. They provided me with an adhesive that ignores oil and deals with superheated temperatures. Amazing.

[ettingtonliam]

Aug 17, 2024 11:57:20 GMT pault said:

Hi
I know someone who many years ago built a 7 1/4" Scott and did exactly this, screwing the stays in and sealing them with Loctite. They were nutted on the inside of the firebox and just screwed in on the outside, the loctite was purely a seal and provided no structural strength.. Jump forward a number of years possibly 10 and stays started leaking on a regular basis. When you looked up the Loctite, the strength graph showed a progressive reduction in strength when used at elevated temperatures, so no great surprise. He then puddled soft solder over the stays, which held for some years before bubbles formed in the solder, which then burst and it was leaking again. There probably is a sealant in this day and age that will do the job without breaking a sweat, its just a case of finding it.

Radweld?

Just curious, but what did the club boiler inspectors make of the 'Scot' saga? What did he eventually do?

[runner42]

I can't add much to this question only that BIs should only be concerned with the mechanical strength of the boiler and not with whether or not it is absolutely free of weeps or small leaks. Obviously if the latter is significant it will be more difficult to maintain 2 x WP for the specified time. Loctite is a suitable kaulking method and mentioned in the boiler code. My experience is at firebox operating temperatures lower rated Loctite may be affected by this temperature and not have the mechanical strength but still retains its kaulking ability.

Brian

[JonL]

I agree to a point, but doesn't the presence of a leak demonstrate that the stay or whatever is no longer fully attached 360 around the point of attachment? Therefore the integrity at that point has been compromised?

[runner42]

The initial question related to threaded firebox stays. This fixing has more than 360 degrees attachment (depending upon the number of threads) so the mechanical integrity of the stay is not a problem.

Brian  

[dhamblin]

Depends on where the joint derives its strength from - is it the threads in the outer(?) and inner firebox copper plate or from the head on the outside and nut on the inside and a reamed fit hole? To be honest in full size practice I have heard it stated that the strength of a threaded joint is normally in the first two threads, so the more threads you have engaged and properly tightened the more safety factor you have.

No threaded joint is perfect from a leak point of view as you have to compromise slightly with fits and there is always a small void where the screw thread crown doesn't fully engage in the nut thread root and vice versa. That's where soft solder caulking or this Loctite method is needed to seal it at high pressure.

Regards,

Dan