Though I invite the spotlight onto my ignorance with the question... what does 'frangible' mean in this context? Why would safety valves made of fragile materials be used in the first place?
"Frangible" here means what it always means. Think of the safety valve relieving pressure, Frank: the disk breaks. But presumably it can't leak, short of bursting.
That does make sense. My confusion I think came from picturing one of these broken by accident and sending the tank's contents spilling all over the place. I take it this is a rather earlier method of a pressure valve.
