Main Steam Stop Valve

stop valve
n: a valve used to shut off the flow of ammonia.
A globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body.
The shutoff valve under sinks and toilets. Allows water supply to be cut off to one fixture without affecting the water supply to other fixtures.
A valve used to stop the flow of liquid in a pipe

steam
The vapor into which water is converted when heated, forming a white mist of minute water droplets in the air
The invisible gaseous form of water, formed by boiling, from which this vapor condenses
emit steam; "The rain forest was literally steaming"
The expansive force of this vapor used as a source of power for machines
steamer: travel by means of steam power; "The ship steamed off into the Pacific"
water at boiling temperature diffused in the atmosphere

main
Denoting the center of a network, from which other parts branch out
chief(a): most important element; "the chief aim of living"; "the main doors were of solid glass"; "the principal rivers of America"; "the principal example"; "policemen were primary targets"; "the master bedroom"; "a master switch"
any very large body of (salt) water
a principal pipe in a system that distributes water or gas or electricity or that collects sewage
Chief in size or importance