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
• empty the boiler when required and to discharge the mud, scale or sediments which are accumulated at the bottom of the boiler
• put off fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit
• increase the temperature of saturated steam without raising its pressure