Bar None

IainM · 23-01-2007

I'm not sure whether this is in the right place ..... perhaps it should be better titled "Learning from your mistakes".
A few pikkies of how building model steam boilers can go wrong!
In this case I remember thinking that the fire tube material I had was a wee bit thinner than it should be, but decided that it would be OK for 60 psi (approx 4 bar).
I also remember thinking just how wrong I'd been when the test pressure gauge suddenly dropped to zero without the gurgle of water!!
My tutors on the pressure vessel design course would have been black affronted had they got to hear of it. My only excuse - it was a long time ago!!
I'm not all that proud of the brazing either ...............

If anyone doubts why cold pressure testing is necessary, imagine the result had the boiler been steaming.

**Bunkerbarge** · 23-01-2007

We do play around with some potentially very dangerous things with steam. The boilers and the gas tanks can cause serious injury if not used carefully and, as you say, we have to ensure that things are tested properly. That could have been very nasty if it had been full of water as the water would instantly flash off at a rate of one unit of water to 1,700 units of steam. Thats why steam plant explosions are so violent.

kevingambrell · 24-01-2007

Hi Richard

Great message! i hope this will be read by everyone thinking of making a boiler. Have to say the soldering looks ok on the boiler maybe a bit too much but the fillets look deep and even and thats what counts.
Can I ask was it silver solder or braze? Looks like it was rodded on rather than letting it flow out from cut bits of solder.
It looks like the boiler would have been OK if the tubes had been a little heavier wall, maybe two three gauges up on the one you have.

I have a feeling you did a service posting this one, it may serve as a lesson to new boiler makers.

Kevin

kevingambrell · 24-01-2007

Just in passing,

there was an article in Old Glory a month or so back about a Fowler that blew up in a field. If memory serves it was a failed stay or crown stay that caused it. Any way most of the 16 tonn engine looked to have taken off. The driver was taken away in a wheel barrow, bugger about with a bad boiler and its a half day out with the undertaker every time.

good stuff this steam! i love it

alan2525 · 24-01-2007

There are a lot of posts online about people wanting to use old pressure vessels, half rotted out compressor receivers found in the back of old barns etc. Lorry tyres pumped up to crazy pressures to act as airbrush resevoirs...

Best to do your homework prior to messing around with any kind of high pressure vessels.

**Bunkerbarge** · 24-01-2007

I think by looking at the failure the brazing was the strongest part of the job! It looks like a failure of the internal flue due to the wall thickness being too weak.

Looks like Ian got his 'Hoop Stress' calculations wrong!

kevingambrell · 24-01-2007

Hi Allen,
Your not wrong I wont have a compressor in the workshop it lives outside in a brick alcove. never using the cursed spray guns tyres are not a concern but I have seen a mattador tyre burst, made my ears ring!

I am now on my seventh boiler, four in copper and three in steel. One all rivet. And I still dont claim to have all the answers. The only word I do know is if it seems too thin then it certainly is. I go one gauge over the calculated thicknes as long as its not going to cause shedding in the firebox.

One location that I feel is always missed on model boilers is the firebox crown. I have seen a number of designs that used girder stays that just didnt give anough surface area to silver solder on. Easy to remedy but possibly a huge problem if the builder isnt aware of it.

the other area that causes me concern is the choice of copper used on some boilers. Water pipe and the like is just not safe to use, it can loose strength very quickly especialy in hard water areas. I think its now covered in the boiler regs (not sure).

All said id like to see Bunkers boiler built again looks like it could be a good steamer.

kevin

IainM · 24-01-2007

Yes .. you're quite correct! I did get the calculation wrong. In hindsight the furnace tube should have been 12 gauge (0.104") wall as compared to a minimum wall for the shell of 0.05" assuming 100 psi DP.
I had assumed that the stiffening effect of the cross tubes would prevent collapse.
I guess the lesson I learned was.... calculate first, re-calculate and only then buy the materials!!

In answer to Kevins query, if my memory serves me right it was stick brazed and I do recollect just how difficult it was to get adequate heat in the right place with a large paraffin blowlamp. Also lost a few sets of eyebrows into the bargain!

kevingambrell · 24-01-2007

On copper I use oxyacetalin and a large propane torch to keep the barrel hot.
I never use braze only easyflow 2 silver solder. Still have to work to keep the heat in especialy on a 5.5 inch diameter traction engine boiler.

Rodding in on boilers isnt a great idea its always better to cut and lay bits of solder on the joints and let the heat flow into the joint.

Mind you its a real pleasure welding a boiler after soldering the last one.

Kevin