Stay material

[Deleted]

Hi folks,

Just wondered what people's thoughts are about the choice of material for boiler stays. Whilst silver soldering the ends always takes place, PB, GM or monel metal, threaded with blind nipples and nutted inside, have been specified before, but there seems now to be a tendency towards simple, unthreaded copper rivets, certainly less work but perhaps not as durable as the other options. Also, how does this lie with current code?

Thanks

Dave

[alanstepney]

I prefer copper.
Monel is out of favour now, PB and GM and expensive, so copper is th eobvious choice.

[Shawki Shlemon]

In my opinion hard drawn copper rods is the best option ( plain silver soldered ) , just my opinion .

[Nigel Bennett]

I don't really understand your concern about copper rivets being "not as durable as the other options". If the stays are correctly silver-soldered and are not over-stressed due to poor stay layout, then they should last the life of the rest of the boiler. They are not weakened by threading, and they would be expected to fail (on a destructive tensile test) somewhere along the plain section away from the plate joint - i.e. the joint should be stronger than the rivet.

Where the problem might be is where a stay is imperfectly silver-soldered, as there would be very little to hold the outer plate to the rivet - assuming the head is on the inside as usual. Such stays could easily be botched with soft solder, and would in all probability pass a hydrailic test without problem. However, external stays can usually be examined very easily to determine their acceptability to the Boiler Inspector.

I find it remarkable how little mechanical strength suffices to hold a boiler together. I still shudder to think how I pulled off a dome from an ancient Juliet-sized boiler with my fingers - it had been held on with some tiny vestiges of brazing. It had been in steam a few years previously and had just sat on the shelf in the warm since then, so I don't think the joint had deteriorated much in the meantime.

Another area of concern might be roof stays, where there is a water level someway along their length, and you may get slight necking here at the steam/water interface after a loooong period of service - but roof stays tend to be PB and made from larger diameter stuff anyway, so their rate of corrosion in a copper boiler would be extremely slow.

How many commercial boilermakers use threaded and nutted stays? In view of the number of loco boilers with rivet stays in service, I don't think there is a history of them failing. I've not failed too many boilers in my time, but ones I have failed had leaks from the foundation ring.

I've never used anything other than plain silver-soldered rivets myself - and if I wasn't happy about them, I wouldn't sit a foot behind them...

[russell]

I suspect that the idea of using threaded bronze stays and nuts arose in the early days of the hobby when all the average model engineer had for soldering and brazing was a parafin blow lamp. The stays were nutted and then sealed with soft solder.

We have it much easier now with fancy propane, oxy-propane or even oxy-acetylene gear so silver soldering the stays is no problem.

Russell.

[pauldenney]

I used dome head rivets (dome inside the fire box) on my Tich which passed it's boiler test with flying colours earlier this year.

Paul

[fostergp6nhp]

Dec 7, 2010 19:31:20 GMT russell said:

I suspect that the idea of using threaded bronze stays and nuts arose in the early days of the hobby when all the average model engineer had for soldering and brazing was a parafin blow lamp. The stays were nutted and then sealed with soft solder.

We have it much easier now with fancy propane, oxy-propane or even oxy-acetylene gear so silver soldering the stays is no problem.

Russell.

Correct.

[Jim Woods]

Hi Folks,

why is monel not "in favour" for stays these days. my first boiler has them, and they silver soldered up really well. the next one was copper stays and was not as easy.

Jim

[Nigel Bennett]

You can go and buy a hundred copper rivets for a fiver or so. Where are you going to get monel rivets at the same price?

[Jim Woods]

so it's not a structural thing then. I make my own rivets. abit time consuming but that's what model building is about I think. I have a simple jig and forge heads on with a small air hammer

Jim

[alanstepney]

There were / are also some technical (as opposed to financial) problems with monel.

I dont recall the details, but there was somethng about stress corrosion, plus (I believe) their use accelerated corrosion in other parts.

(Note to self, I must look up some data and see what and why.)

[weldsol]

Hi Alan it could be down to sulphur content within the fire box gasses as the Nickel in Monel does not like sulphur ( corrosion )

Paul