Major Valve Parts

Pressure Retaining Parts

Valve body, bonnet or cover, disc, and body-bonnet bolting are classified as pressure retaining parts of a valve. The following provides a brief description of these parts: Body. The valve body houses the internal valve parts and provides the passage for fluid flow. The valve body may be cast, forged, fabricated, or made by a combination of cast, forged, or fabricated portions. Valve bodies can be and are made from a variety of metals and alloys. Also, valve bodies can be and are made of nonmetals; however, these must be within certain size and pressure-rating limits.

The valve-body ends are designed to connect the valve to the piping or equipment nozzle by different types of end connections, such as butt or socket-welded, threaded, flanged or bolted, soldered, brazed, solvent cement joint, mechanical joint, or coupling.

Bonnet or Cover.

The bonnet or cover is fastened to the valve body to complete the pressure-retaining shell. In case of gate, globe, stop check, and diaphragm valves, it contains an opening for the valve stem to pass through. Usually, it contains a stuffing box. It provides access to valve internals, especially when the valve is installed. The top works of valves include bonnet, yoke, and operating mechanism. The bonnet is the base that supports the valve top works.

The valve bonnet is attached to the valve body by many different types of joints: bolted, pressure-seal joints—breach lock and standard pressure seal joints— threaded, welded, union joint, and clamp seal. Some valves have a bonnetless design in which valve body and bonnet are combined into one. This is also known as an integral bonnet.

The bolted bonnet design is commonly used with cast or ductile iron, cast or forged steel, and alloy valves, which are rated NPS 2¹⁄₂ (DN 65) and larger, and Class 600 and below. Valves with a pressure seal–bonnet design are usually manufactured in Class 600 and higher rating classifications and are considered superior in regard to the leak tightness of the body bonnet joint. Bolted bonnet designs are also produced in valves rated Class 900 and higher. The user needs to consider the potential consequences of flow-medium leakage through body bonnet joints and take the necessary measures to prevent or contain the leakage.