I read on the Wikipedia article about sight glasses that some were reinforced with metal, especially in high pressure applications. I'm going to be building my own sight glass to go onto my boiler and I thought why not put the glass tube in some sort of protective housing? So I came up with this
It would be a length of copper or brass tube with an ID a little bigger than the OD of the actual glass. Such a shroud would protect the glass from breakage while allowing adequate monitoring of the water level. One could even paint the inside surface of the shroud to allow better distinguishing between water and steam. You could also mount a plate onto the side of it with measurements from the bottom tube plate so you know exactly how much water you have in your boiler.
There is a slight problem with that type of sight-glass protector in a high pressure situation. If the glass does give out then the ensuing steam and boiling water is projected outwards towards the operator. With the traditional three sided protector, it is projected (mostly) away from him, giving him a chance to throw the large cloth, that he always has to hand, over the thing, for further protection.
I would suggest looking into a reflex style sight glass like can found on steam locomotives. Much safer design and less prone to breaking, also much easier to distinguish between water and steam (water almost appears black). The glass does not need replacement as often as with a conventional glass either.
Conventional glass tubes should be replaced every couple years- take one apart that has been in service for a while and you'll be amazed at how thin they get.
If you go the conventional route, I would suggest a lexan shroud around the outside of the glass, in between the gas cocks, to prevent injury in the event of failure.
I was just thinking about using something like this to help prevent some accidental bump into the glass from breaking it. You could even use some heavy wall steel pipe with a slot milled instead of the brass or copper I mentioned above. The sturdy steel, especially if it had a heavy wall, wouldn't flex in the short lengths that are used in sight glasses. You have peaked my curiosity though as to why the glass wall would thin so much over time.
The Ph number represents if water is acid or alkaline. Above 7 is alkaline, and the recommended Ph for our boilers is in the Ph 9 - 11 range, to inhibit corrosion of the boiler metal. If the water is not completely free of CO2, carbonic acid is constantly formed. Boiler treatment chemicals, such as tri-sodium phosphate keeps the water alkaline, neutralizing the acid.
Pure water is especially agressive at dissloving nearly everything, and when pure steam leaves the boiler (along with some CO2), the chemicals that maintain corrosion resisting properties are left behind. The sight glass constantly condenses this pure steam, and the resulting pure untreated water trickles down the sight glass, and the glass is slowly dissolved by this water. The lower half of the glass is in contact with treated water, but the condensate forming inside the glass constantly washes the lower glass area.
