Hollow Stay - Blower Feed

[Tony K]

All of the locomotive boilers I have seen have a hollow stay to feed steam from the backhead blower valve down to the other end of the boiler, then piped to the blower nozzle. To me this is an unnecessary complication to the boiler and a more reliable arrangement would be just to pipe the steam down there and into the smokebox, probably underneath.

However, the hollow stay being the common practice, there seems to be 2 ways of doing it.

Method 1 seems better in that if soldered properly it should be trouble-free. However, if it does fail it will be fundamental to boiler construction and difficult to fix.
Method 2 seems more likely to fail, but it should be easier to fix, having simple screwed joints.

What do you think?

[havoc]

I'm partial to your first opinion: it is a complication and a better arrangement can be found.

[baggo]

I suppose the advantage of the hollow stay is that the steam is kept hot on it's way to the blower jet and doesn't get chance to condense in the pipe, although an external pipe will obviously heat up quickly.

The disadvantage of the soldered in stay is the great difficulty of getting the stay and the boiler at the same temperature when the soldering is done. Usually, the boiler gets a lot hotter than the stay and so expands more. When the whole lot cools, the boiler contracts more and puts the stay into compression causing it to sag, rendering it pretty ineffective.

Martin Evans was of the opinion that longitudinal stays in most boilers do little, if anything at all, to strengthen the tubeplate and backhead and thought them unnecessary, although he still fitted the hollow stay for the blower.

So far, I've used the threaded stay into bushes method but they can be a pig to seal sometimes Martin used to recommend running them over with Comsol after fitting which solves that problem. I can't see any obvious reason why the stays would ever want to be removed again.

John

[russell]

The threaded stay and bush doesn't seem to me to be a good engineering solution. Depending on the positions of the thread starts there can be up to half a pitch dimensional error - no wonder they are prone to leak! Comsol on the outside will fix the boiler to outside world leaks but there is also the possibility of a leak between the boiler and the inside of the blower tube. Is there a high temperature solder paste like Comsol that could be used?

Russell.

[Shawki Shlemon]

Depending on what code one is using to build the boiler . If longitudinal stays are required then they must be fitted and therefore using one of them to be a hollow will not complicate the job at all , it will eliminate one external pipe as well as keep the steam hot.In the past here in OZ was permitted to silver solder these stays but in the current code they must be screwed unless the length of stay is no more than 40 x D . I have used both types and see no problem with them and as baggo says they should not need any maintenance at all .

[Tony K]

Thanks for all replies. As always - very informed and comprehensive.

[drumkilbo]

Anyone any further thoughts on an external pipe for the blower ? I read somewhere that they were prone to sneezing and spluttering, I assume because any steam sitting in the pipe for a while will revert back to hot water and you'll get a geyser effect initially when you open the blower valve.

Ian..

[havoc]

As you say they are prone to condensation, certainly in the sizes we use. But you could run it inside the boiler wrapper and keep it against the boiler. Or maybe some insulation.

[drumkilbo]

Yes, had wondered about that Havoc, but clearances are usually very tight between the boiler and the cladding.

Ian.

[arch1947]

G'Day,
I am not an expert on copper boilers but I understand that longitudinal boiler stays have to remain in the hard drawn state according to our AMBSC standard. The rationale is that if the stay is silver soldered it will become annealed and the strength affected. I now the rest of the boiler is also annealled but the long stays would proabably elongate too much and not provide the support to the backhead and smoke box tube plate. The stress would be quite high unlike the firebox stays by virtue of the fact that there 'aint many of them, again unlike the firebox.
I can check with our boiler person if anyone wishes.
Cheers,

Arch

[russell]

I wonder how the strength of an annealed blower tube/stay compares to the strength of a 40 tpi thread on copper? Especially when it has to be a relatively slack fit to take up positional errors between the inner and outer threads in the union.

Does anyone have any figures?

Russell.

[simonwass]

Apr 21, 2009 10:04:01 GMT russell said:

I wonder how the strength of an annealed blower tube/stay compares to the strength of a 40 tpi thread on copper? Especially when it has to be a relatively slack fit to take up positional errors between the inner and outer threads in the union.
Does anyone have any figures?
Russell.

Never make threads small to fit a missmatch, especially in boilerwork, positively dangerous.
To give the blower pipe a bit of pretension, if the threads dont match up tap the smokebox tubeplate bush in a touch and try again, sooner or later you will find that few thou missmatch and it'll screw straight in even with full form threads. I make mine in one piece by silver soldering the pipe into both bushes, the frontend one has no shoulder so it goes through the backplate and then in from the rear of the front tubeplate.
Dont forget all copper stays are annealed if silver soldered in, the middle parts are not but weakest link & all that. Makes comsol sealed stays look safer by comparison, that are bound to be stronger!

[alanstepney]

By the time a (copper) boiler has heated and cooled a few times, I doubt that any of it will still be in the soft state.

Using one of the stays to feed the blower is common practice in both models and full size.

Whilst there are alternatives, I dont really see much point in re-inventing the wheel.

[Tony K]

Alan - why not read the question? ;D

[alanstepney]

Apr 21, 2009 15:45:51 GMT Tony K said:

Alan - why not read the question? ;D

Which question?
If you read the thread there are several.

[simonwass]

Apr 21, 2009 14:35:32 GMT alanstepney said:

By the time a (copper) boiler has heated and cooled a few times, I doubt that any of it will still be in the soft state.
Using one of the stays to feed the blower is common practice in both models and full size.
Whilst there are alternatives, I dont really see much point in re-inventing the wheel.

Just doing the 2x pressure test work hardens the copper to a degree. Small I guess but even a semisoft boiler is very strong given they are designed with 8x safety factors.
Full size doesnt actually have a hollow stay in the way we do, the longitudinal stays are big and solid steel! They simply have an internal copper pipe which is expanded into both back & frontplate mounting pads, the pipe is usually jointed somewhere with a single union nut & fitting to aid assembly.

[simonwass]

Apr 21, 2009 15:45:51 GMT Tony K said:

Alan - why not read the question? ;D

I believe you actually answered your own question
Option 1 is leak free if made leak free in the 1st place.
Option 2 is easy to make (so is opt.1) but is not so easy to assemble leak free, I've had boilers were the blower weaped due to the multitude of internal threads which need perfect sealing.
I now only do fully silversoldered assemblies where possible

[russell]

Thanks Simon for your reply. I hadn't thought of distorting the backhead a little to get the threads to line up. I still wonder about the strength of the threads compared to the pipe itself. I think people are worrying unduly over possibly annealing the pipe as its cross sectional area is so much greater than that of the threads.

Russell.

[alanstepney]

You can calculate the strength of a thread, if you have the time and inclination.

As it has been a "standard" for so long, and without any history of serious failures, I wont bother to do so, but suffice to say, I would, and do, happily use that thread and method, without any concern.

[havoc]

Full size doesnt actually have a hollow stay in the way we do, the longitudinal stays are big and solid steel! They simply have an internal copper pipe which is expanded into both back & frontplate mounting pads, the pipe is usually jointed somewhere with a single union nut & fitting to aid assembly.

Thanks for confirming this. I had been looking through a few full size plans and never found a hollow stay, let alone being used as blower piping. And not much longitudinal stays either. Mostly the smokebox end is stayed with ribs.