Boiler Safety? With a steam boiler you can run out of steam, run out of fuel, but you don't run out of water. At least if you want to stay healthy.It is experience that lead to the rules about washing boilers and keeping water glasses and try-cocks clean. As someone once said, "Safety rules are written in blood."
A neglected water glass will lie to you and get you killed if you trust it. The small holes in the gauge cocks plug easily. If the hole in the top gauge plugs, condensation of the steam in the upper part of the tube will pull water up in the glass leading you to believe you have plenty of water in the boiler while the crown sheet may actually be nearly dry.
Keep 'em clean! Jim