Steam Boilers, Engines, Nozzles and Turbines
The principal function of a stop valve is to

Control the flow of steam from the boiler to the main pipe and to shut off the steam completely when required
Increase the temperature of saturated steam without raising its pressure
Put off fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit
Empty the boiler when required and to discharge the mud, scale or sediments which are accumulated at the bottom of the boiler