I was at a rally a week ago when the gauge glass on my 2 inch engine blew out explosively. Luckily I was attending to something else at that moment, or I would have received a faceful of glass fragments.On this size of engine I usually have to lean down closer to the glass to check the level.
I fitted a spare glass, re-filled, re-lit the fire and carried on with running, having a successful afternoon. However, when I got home I decided to heed the warning from Lord Murphy. I got out the gauge glass protector, which I had made in 2001 but never bothered to fit, and cut a piece of clear plastic from an old tape cassette box to fit in front of the glass. So the protector now protects me from the glass instead of the original opposite function. I hope it will withstand the temperature.
I have had glass tubes break before, but with a neat quiet crack, not an explosive blowout. I mention this for others to think about, though with larger scale engines there is perhaps less hazard due to more distance from the glass (but also a larger steam escape)..
Paul