Live steam engine boilers

[Bernd]

Here's an interesting discussion I picked up on the Live Steam news group. I'm posting an edited version here, mainly peoples names have been eliminated and some extraneous other stuff of no interest.

I'll add more if the discussion has more interesting material or links. Please be reminded that this is only for discussion and in no way means that it's technical and should be followed to the letter.

Regards,
Bernd

If you are running the boiler at 125 psi, this equates to about 350 
degrees water temperature in boiler.  If you have a rupture in the 
boiler, I will guarantee you will have flash steam.

If you do not believe, look at some boiler explosion pictures.  I'm 
sure most boiler explosions started in the crown sheet area because 
of a rupture from low water, etc.

We probably get by in our hobby because of low quantity of water, 
but still could be nasty.

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Apparently there were pictures attached to this post: Bernd
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ALL the pictures are of much larger boilers

the flash steam is not due to the hot crown sheet - but rather the 
crown sheet failure & release of liquid water at 350 or hotter

small model boilers have such a small crown sheet that getting a large 
enough rupture to cause a flash condition is very very unlikely

I've been around two small boilers with low water failures -the most 
prototype had a poor water pump and a useless cheap injector -  its a 
pfft & cloud of steam - the loco stayed down on the tracks, nobody got 
burned the boiler was toast. the crown sheet was failing but the top 
row of flues actually let the water out - it did not flash.

equating pictures of a 60" dia boiler made of 1.4" plate to a 6" dia 
boiler made of 1/4" plate and expecting it to fail the same is simply 
not rational

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I am thinking here that this may be a case of what scales and what doesn't scale.  As has often been observed, you can't scale down the physics of steam generation and action, so even though your model may be accurate to the prototype, and assuming it's 1/8 scale, the size of the crown sheet is not 1/8th of the original...because area does not scale down with dimension.

The crown sheet in my consolidation model will be approximately 6" by 16", for a total area of 96 square inches.  If the same boiler were built full size, the dimensions of the crown sheet would be 48" x 128" for an area of 6144 square inches.  The model boiler's crown sheet is actually 1/64th the area of the full size boiler, not 1/8th, even though that is the scale.  (8 x 8 is 64...so the area reduces by the square of the scale.)  So in a model boiler at 1½" to the foot, you have considerably less area to heat, and considerably less volume in the boiler for water (and in volume the difference would be cubed).

When you take into account the materials used in model boilers are probably similar to those used in full size boilers, a failure in the model boiler is less likely to produce the same result as in the full size version just because the scale of difference in heat energy and water volume is so great.

A local steamer at our club had a boiler that was having trouble keeping steam pressure up when running.  The train master finally started doing his own inspection.  After a successful hydro test, he started poking at the various parts.  He was actually able to penetrate the inner sides of the waterlegs with a pick.  A full size boiler in that condition would likely fail under steam.  This one just wouldn't transfer heat well enough to maintain steam under load.

Now...this is NOT to say that care and attention shouldn't be paid when operating a model boiler.  Quite the contrary.  I haven't heard of a model boiler failure in the crown sheet, but I'm sure there are conditions under which it could happen, and if it did, the results would be unpleasant for anyone nearby.  We perhaps have it fortunate that many states look upon the hobby boiler as something that doesn't need the same level of inspection that the full size or even half-size counterparts do, but that fact really is a call for more vigilance on our part. (I know that some jurisdictions do require full boiler inspections even on hobby boilers.)  Were we to have a model boiler incident that did result in a fatality, or even serious injury, we would all pay the price in government regulations that would likely force many of us out of the hobby just from the sheer expense of complying.  And that is to say nothing of probably losing some valuable experience to the hobby from the victim or victims of such an event, or of the loss to them personally.

I have to say that I have been very impressed over all with the extent of discussions I have seen on this topic on the various forums.  It suggests to me that most everyone in this hobby takes their responsibilities around a steam engine boiler very seriously.  There may be exceptions, but I hope not.  As Red Hadler, our former steam instructor at SVLS, and the motive power foreman for Sacramento's California Railway Museum, is fond of saying, "Steam is a luxury.  Water is a necessity."

In this hobby, words to live by.  Literally.

[sbwhart]

Hi Bernd

That a great bit of information especially about scale effects, I new scale came into it some ware but hadn't quite got my head round it, I'm going to have to think about this one some more.

Some day I'm going to have to bite the bullet and build the boiler for my loco so any info like this is great, thanks for sharing.

Stew

[shred]

FWIW, there's some comments in Kozo's "New Shay" book around boiler failures (he says the most likely low-water condition leads to a firebox tube-joint failing, putting out the fire, trashing the boiler and making a mess, but not a massive explosion) as well as some interesting Australian tests where they hydro-tested to destruction some simple boilers.  Definitely nothing to take lightly, but I think the 'grenade' factor is acceptably low in carefully built and tested designs.

[Bernd]

Your welcom Stew.

The above two comments is kind of what I'm looking for in this disscusion. Boiler building is definalty not for the newbie to steam or a person who cannot solder. Unfortunatly there is a lot of misconceptions about model boilers.

Bernd

[NickG]

Good points raised here guys. I don't know any more background to this thread, was the debate just about running at 125psi? Not sure, our boiler inspectors / testers usually say there is no reason to go above about 80psi with a model boiler, but maybe that's just their own take on things, their own safety factor if you like.

What I do know is, our club is goverened by the southern federation of model engineering societies as far as boiler making and testing is concerned, and they have a strict Code of Practice for Boiler Testing. It's prepared in association with the representatives of various Model Engineering Bodies, the Trade, the HSE and the Insurance Companies, and accepted by them. So it's all down on paper somewhere what should and shouldn't be done.

Stew,

On a bit of a side note, I would recommend before you even start your boiler to join a society with a 3.5" gauge track, boiler inspectors and testers if you haven't already done so, and discuss it at length with them first. I say this because I believe some of LBSCs' designs are a bit 'out of date' compared to some of the current practices these days and some of the stuff he did is frowned upon. When I took my boiler in for our guys to have a look at, they were surprised that some of the joints are brazed with sif bronze .. this is what it calls for on the drawings, nothing too bad about that, but it's a difficult art due to the melting point of the bronze / copper being closer. Also, there are 2 longitudinal stays, one is hollow for the blower tube and it doesn't call for it to be soldered in place, our guys insisted it was, I think the drawing showed it just screwed straight into the back / front plate. Also, apparently curly thought nothing of plugging joints with soft solder, however, I think this is frowned upon these days. So it will definitely be worth just going through the drawings with some boiler inspectors / testers first.

Nick

[sbwhart]

Hi Nick

Good advice thats one reason I've stayed a member of our local club even though they P*****d me off by not being very welcoming.

And your quit right about Old Curly's methods being a bit out of date it took me a while to get to grips with design / method I,ve read quiet a few book on boiler making:- Curly's, Martin Evans, Model Engineer mag etc , and they all seam to have their own methods, Curly calls for screwed stays sealed with soft solder which is a no no by todays standards, the prefered method of fixing fire box stays nowadays seems to be rivets with the heads on the inside with the diameter sticking out that are simply silver soldered in place. I supose whats happend is that technology as moved on with new/better solder and fluxes and a better understanding of requirements. Talking to the boiler inspector at the club an Ex Crewe works copper smith who swears by easy flow flux but I know that there are now available tenasity fluxes that are more tolerant of sustained periods at temperature.

Interesting point about the blower needing to be fixed hadn't picked that one up, its filed away.

With the supper heaters I want to make the tubes from stainless steel and extend them back into the fire box, have any of your guys at your club any exparience of this.

On a similar point with Curleys designs, his water level gauges, the screw valves can be screwed right out alowing the steam to escape, I,ve heard that this is a No No as well, you have to use screw cock type valves fitted now, is this correct ?.

Up to now I've slavishly followed the drawings but from now on I'll start to deviate, boiler feed pump design I'm going to change as well as the cylinder lubricator pump

Thanks for your imput much apreciated

Stew

[Bernd]

Nick,

It was a disscusion that I just picked off the Live Steam news site. If I remember correctly the 1 1/2" scale engines run around 100 to 125 psi. The disscusion was based around boiler explosions between the real ones and the models.

I just thought it would make for some good disscusion here.

I read a while ago about hdro testing of pressure vessels, such as boilers. There are acutally two methods used. One is were the said vessel is placed in a tank of water and is then pressurized. The volume of water is messaured to see how far the vessel has acutally expanded. The other test is what is done on model boilers were the boiler is put under pressure while being totaly full of water at 2 or 2.5 times the normal operating pressure.

Some have said that such checks will actually weaken the boiler as time goes on because you are stressing the material at 2 to 2.5 times it's operating pressure every time you put the boiler through this check.


Bernd
 

[NickG]

Bernd,

We do it as per southern federation code of practice, for copper boilers that is water pumped in to 1.5 x working pressure to be held for 20mins. So much drop is allowed, I forget how much but can get the info at our next meeting. This happens every 2 years I think, the boiler has to be stripped right down out of frames, lagging removed every 4 years. Steam test is required every year to ensure no leaks from fittings and safety valves do not let pressure rise so much above working pressure with the blower fully on and a large fire.

I would imagine the copper stretches and work hardens the first time due to being annealed during soldering.

I think steel boilers call for 2 x working pressure.

Stew,

Sounds like you've already done a lot of research on this, you need to these days, it could end up being the most expensive part of your engine whether you build it yourself or buy it!

Not sure about the stainless superheaters, if there is anybody that's done it I haven't heard about or seen them but will ask around. I think stainless welding is one of those things people are probably fearful of, it's out of their comfort zone!

I can't quite remember what the water level gauge is like on mine. I thought you just needed a water drain, but you might be right, I think i'd heard somethign about being able to shut off top and bottom incase the glass breaks. Will find out these details next time I'm down and post an update.

Nick

[bogstandard]

Nick & Stew,

There are so many federations this and society of that's, it causes a lot of confusion. Each one seems to have their own regulations, and none of them like to think that anyone else is better than them.

If only all these people could come together and agree on a solid set of rules and regulations, then there would be no questioning then. Everyone would be happy because a standard set of rules would be in force. Comply with those rules, and you can steam anywhere.

I still think, at the moment, the steam fraternity is mainly self governing, but it only has to show one tiny bit of weakness or tragic mistake, and the H&SE will be in like a shot, and get more involved than they already are. Then everyone will be in deep trouble.

What has happened in the model boat fraternity is everyone accepted the 3 bar litre rule for boilers in model boats. That made everything clear to everyone what was expected of them. The government were happy, the insurers were happy, and the steam lads now had a set of rules they could work to.
There is just one fly in the ointment, the MPBA (Model Power Boat Assoc.). They have not accepted the new rules (all because I think they were asked to contribute, but declined, saying that they want to make up their own). So a bunch of fuddy duddy old farts, still living in the Victorian era, has thrown the whole lot into chaos again. So if you want to sail in any of their organised events, you have to have a boiler certified to their specs, and any other sailing event, the great 3 bar litre rule is in force. As you can guess, the MPBA is now losing membership at a high rate of knots.

John