Garden Railway Forum

Thanks for sharing that Tony, so nothing to worry about then really given the results!

If my boiler bulged this much, I might think there was something wrong.

TonyW wrote: ↑Tue Jun 06, 2017 1:11 pm The Australian test document is now available on my web site: http://www.rhoshelyg.me.uk/RITG%20Boiler%20Tests.pdf

Note that none of the boilers tested had a pressure gauge, which would surely be the first component to fail if pressurised beyond its design capabilities.

So its not a boiler certificate we need, it's at what pressure does the pressure gauge fail. Having opened one of these up and seen what's inside, I can't think that it is going to be much beyond its max pressure reading of 80psi. Anybody with an air compressor fancy doing a test on one?

TonyW wrote: ↑Mon Jun 05, 2017 2:56 pm That link unfortunately says "The site you tried to access does not exist". Do you know if it has been posted elsewhere? I think I remember this from a few years ago and, if correct, I recall that the lowest failure pressure was around 1000psi.

Sorry, I forgot to test the link before posting. There are other copies out there, but I see Grant sent you one.

The boiler shell will stand 1000psi or more of hydraulic pressure, but the flue fails long before that, as metals are weaker in compression than in tension and flues usually have thinner tube walls. If a centre flue gas fired boiler was run without a working safety valve, the flue would collapse onto the burner and put it out before the shell failed. Although if a pressure gauge was fitted it would likely fail first, as you say.

The heat conductance of copper is so high that the boiler temperature won't rise higher than the steam saturation temperature, as long as there is some water in the boiler. As you also say, when the water runs out, no water means no steam and no pressure, so no risk of 'explosion'.

Unless of course someone decides to try for a Darwin Award and pumps water into a dry and very hot boiler, instead of just turning off the gas and waiting for everything to cool down. It would depend on the temperature of the boiler at that point, but a quick calculation suggests the flash steam pressure could easily exceed 2500psi for a very short period........

TonyW wrote: ↑Mon Jun 05, 2017 2:56 pm Graeme, could you give us a precis of what was written, please? A feature of coal-fired boilers is that once they run dry there is no steam to provide a forced draught from exhaust or blower and the fire will then die fairly quickly, and certainly before paintwork or radio control damage could occur. And no water means no steam so no explosion.

In a nutshell, they investigated the scenario you describe.

The first article quoted measurements on a real coal fired boiler, which were taken on the outside shell as it was allowed to run dry. Temperatures peaked within a minute of the blower stopping (i.e. no pressure), then within another minute started to reduce, even without the fire being dropped. The highest peak temperature they found was around 240degC on the outside of the firebox at about crown level.

They also tested an alcohol fired pot boiler. After the water ran out, it had risen steadily to 300degC when they abandoned the test.

The second article on thermodynamic modelling showed internal temperatures on the firebox crown at the front could peak at 360degC after the boiler ran dry, which is well within the limits for silver brazed joints with no pressure in the boiler.

It would depend on the paint type and where any electronics were located, as to whether they'd escape damage at the sort of temperatures reached in these coal fired boiler tests.

Regards,
Graeme

but the flue fails long before that,

As Graeme has mentioned, I've heard of two instances from a very reliable repairer, of Roundhouse locos that could no longer be lit up. No apparent outside damage, but boiler removal revealed a collapsed flue in both cases, due to a lack of water. Damaged flue was removed by trepanning and replaced.
Grant.

TonyW wrote: ↑Mon Jun 05, 2017 2:56 pm
Graeme, could you give us a precis of what was written, please? A feature of coal-fired boilers is that once they run dry there is no steam to provide a forced draught from exhaust or blower and the fire will then die fairly quickly, and certainly before paintwork or radio control damage could occur. And no water means no steam so no explosion.

Tony,
not to further 'stir the pot' on this topic, or to put into question the excellent work put into part 3 of the Boiler Code, but I think it is worth recording that one of the minor conclusions of the second AME article is that the need for two separate water supply systems in the Code should be reviewed.

Tim

11thHour wrote: ↑Thu Jun 08, 2017 1:29 amTony,
not to further 'stir the pot' on this topic, or to put into question the excellent work put into part 3 of the Boiler Code, but I think it is worth recording that one of the minor conclusions of the second AME article is that the need for two separate water supply systems in the Code should be reviewed.

I've not seen the article Tim, so can I ask what is suggested?

From my brief time of playing with 3.5" gauge the two sources of water into the boiler is a must but they aren't required for our little ones although many coal burners tend to have more than one water feed...
Usually via a Axle Pump, hand pump, steam pump and/or injector(s) for the bigger stuff.

TonyW wrote: ↑Thu Jun 08, 2017 9:14 am
I've not seen the article Tim, so can I ask what is suggested?

Rather than paraphrase, I will quote directly from the article. I am at work, its friday, monday is Queens birthday weekend ( go figure - we love the monarchy they give us holidays) so a reply will be a little time.

Tim

The test document is here: http://www.homemodelenginemachinist.com ... 1342623685

The copy on my web site referred to above has now been removed.