Valve bonnet is basically the cover for the opening in the valve body. In some designs, the body itself is split into two sections that bolt together. Like valve bodies, bonnets vary in design. Some bonnets function simply as valve covers, while others support valve internals and accessories such as the steam, disk, and actuator.
The bonnet of valve is second principal pressure boundary of a valve. It is cast or forged of the same material as the body and is connected to the body by a threaded, bolted, or welded joint.
In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the welded joint or bolts that connect the bonnet to the body are pressure-retaining parts.
Bonnets of valves can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost.Bonnets of valves are a source for potential leakage.