Philip wrote:One day they invited some girls up on the footplate, and when the safety popped off, the young ladies wet themselves
Yes, when a fireman is taking care of a loco which is booked to stand in a station or is on display for a wedding, or whatever, he has to keep the fire well under control to avoid blowing off. It is relatively easy to do so under those controlled circumstances.
It's different when you're working a train. The rule book says you should only blow off twice to test the safety valves - once before you go off shed, and once early in the journey. However it is almost impossible to avoid blowing off occasionally in real life. You've just built the fire up for a steep gradient and you suddenly get a red signal. You've just been told by the central train control that you are about to depart from a station, so you build up the fire, and then five minutes later they call you back and tell you they're delaying you for another thirty minutes. You can delay blowing off by putting on the injectors and filling the boiler, but once the boiler is full you can't do that any more as an overfilled boiler can lead to priming. You could also open the blowdown cock, but that is discouraged, and anyway it is completely forbidden in stations.
I've had a few very surprised punters when the safety blew, but I can't say I've ever made a young lady wet herself!