A bonnet gasket is a sealing component that is located between the body and the bonnet of a valve and is used to seal the body-to-bonnet joint. The body of a valve is sub-divided into flow path and seats, and is trimmed with various components. The bonnet provides housing to components like stem, packing box and actuator and mates to the body to complete the valve assembly. The gasket is placed between the body and bonnet and seals the joint.

During assembly, the gasket is compressed between the body and bonnet of a valve. If the design is a pressure-seal design, system process fluid pressure acts on the gasket to seal. The function of the controlled compression is to close the leak path.

A bonnet gasket seals the body-to-bonnet joint which is a part of the valve pressure boundary. In service condition, the gasket is exposed to process fluid under pressure and varying temperature. It is required that the gasket maintains the valve pressure seal.

During operation of a valve, the bonnet gasket is exposed to process fluid, and high temperature and pressure which causes it to degenerate and swell. For safe operation of a valve it is crucial that the gasket used maintains the seal of the valve and is appropriate for service conditions. Improper selection of a gasket can result in a compromised pressure seal of the valve and fail to contain process fluid.