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This is a short edited extract from MAINSTEAM MODELS "The Complete Guide to Miniature Steam" which is available on CDROM or by Internet download.

A lot of newcomers to the hobby of live steam spend too much time worrying about their boiler blowing up. During the many years that I have been involved with miniature steam engines & boilers, both locomotive, marine & stationary types, I have never personally experienced a model boiler explosion.

In the past I have accidently abused the odd boiler or two, but never seen one explode. From speaking to many other model engineers over the years, the general observation with copper boilers is that they split rather than "shrapnelize". With a splitting copper boiler, of course there will be a sudden release of energy so some part may whistle past your ear or worse! The water released will also be very hot, so obviously personal injury to yourself & others could result.

In the past, with full size boilers, the whole principle is different. The scale is far larger, as is the internal surface area of the boiler. The scale & design of a coal fired full sized boiler is very different to the small ones used in models. The volume of boiling water is far higher & the boiler material would "shrapnelize". On a full sized steam locomotive, as on a model, the firebox is the equivalent of a blast furnace, so if you let the water disappear from the crown of the firebox .......... if it doesnt blow the fusible plugs (lead filled hollow boiler stays fitted in the top of the firebox) the boiler is seriously damaged - or worse! A full size boiler explosion was a very serious problem, often caused by operator error - low water level, or internal corrosion coupled with solidified sludge deposits around the throatplate or foundation ring of the boiler, which would then overheat, softening the metal & then "bang". Also the quality of the metal that the old boilers were made from was of a lower grade of metal than we have today. 

Unless the boiler is seriously flawed, ie: severe internal rust on a steel boiler, or a badly made copper boiler, you should have little or no problems.

The rules are simple:

Always watch your water level, if the water disappears from the boiler altogether, the fire will still be there, which will overheat the boiler, unsoldering the components (even on a silver soldered boiler) & probably also turn your nice wood cladding to charcoal. For coal fired boilers there are further problems - please read the guide on coal fired boilers.

Test the principle yourself, using a blowlamp, try soft soldering a piece of copper to a copper water pipe. Block up one end of the pipe only & fill it nearly to the top with water - then empty the water out & try again - you will see the principle.

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