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Basic Components - continued.
Bonnet Fittings
The bonnet is connected to the valve body by bonnet nuts and bolts. To provide a leak-free joint, a gasket is placed between the bonnet and the valve body surfaces.
Seat Ring
The inner space of a valve body is provided with two seat rings. These seat rings are designed for two functions. First, they create a mating surface for the valve disc. When the disc is in full closed position, it mates against the rings to provide a good seal. Second, over time, action of the disc against the rings damages the rings, and the valve loses its ability to provide a good shutoff. Under such conditions, the rings can be replaced with new rings and the valve is almost as good as new.
Disc
This is the moveable component which the stem moves up or down. In the graphic shown here, the disc is of solid construction (in other sections we discuss some other designs.) The disc is a replaceable item. If the disc is damaged, it can be replaced.