As steam pressure builds in a more-or-less sealed vessel like a boiler, it begins to seek weak places in the metallic structure (microscopic cracks, fractures, metal fatigue spots, loose rivets, etc.), which become points of escape. If sufficiently high pressure, there can be an explosion—injuring workers and bystanders. The automatic safety valve was invented to provide relief when pressure rises to pre-measured levels, lifting a spring-loaded cap or plug--and venting steam until pressure is reduced.
Quick facts
First boiler explosion
Safety valves in England
Don't do that