Häger GmbH & Co. KG
ANSI industrial valves

The valve body, also known as the housing, is the primary component of a valve. It serves as the main structure that holds all other parts together. As the first pressure boundary of a valve, the body must withstand the fluid pressure from the connected pipelines. It links the inlet and outlet pipes through threaded, flanged, bolted, or welded connections.

The ends of the valve body are designed to connect the valve to the piping system—commonly via butt weld, socket weld, threaded, or flanged joints. Valve bodies are manufactured in a variety of shapes, typically cast or forged, with each design fulfilling a specific purpose. Every part is made from materials selected to suit its particular function and the demands of the process.

The bonnet is the cover for the opening of the valve body and is the second most important part of a pressure valve. Like valve bodies, bonnets come in various designs and models. It acts as a cover for the valve body, typically cast or forged from the same material as the body. The bonnet is usually attached to the body using threaded, bolted, or welded connections.

During valve assembly, internal components such as the stem, disc, or wedge are inserted into the body. The bonnet is then installed to secure all internal parts in place. In all cases, the connection between the bonnet and the valve body forms part of the valve’s pressure boundary—meaning the welds or bolts securing the bonnet are considered pressure-retaining components.

The removable and replaceable internal parts of a valve that come into contact with the flow medium are collectively referred to as the valve trim. These parts include the valve seats, disc, wedge, fasteners, spacers, guides, bushings, and internal springs. Components like the valve body, bonnet, and gaskets, although also in contact with the medium, are not considered part of the trim.

The trim is defined by the interface between the disc and the seat, as well as the relationship of the disc’s position to the seat. It is the trim that enables flow control and basic movement inside the valve. In rotary motion designs, the disc or wedge moves across the seat to regulate the flow opening. In linear motion trims, the disc lifts vertically from the seat, creating an annular opening.

Trim components are made from different materials depending on the varying forces and conditions they must withstand. For example, bushings and packing don’t experience the same stress as the valve disc and seats. Factors like flow medium properties, chemical composition, pressure, temperature, velocity, flow rate, and viscosity are important considerations when selecting suitable trim materials. The trim may or may not be made from the same material as the valve body or bonnet.

The disc is the component that allows, throttles, or stops flow depending on its position. When stopping the flow, the disc functions as the shut-off element. It represents the third most important primary pressure boundary in a valve. When the valve is closed, the entire system pressure is applied to the disc, which is why it must be considered a pressure-retaining part.

Discs are typically forged or cast and, in some designs, provided with a hard-facing material (such as Stellite) to ensure excellent wear resistance.

The seat or sealing rings provide the seating surface for the disc. A valve can have one or multiple seats. In the case of a check valve, there is usually a single seat that forms a seal with the disc to stop the flow. In the case of a shut-off valve, there are two seats: one on the upstream side and the other on the downstream side. A shut-off valve disc has two seating surfaces that come into contact with the valve seats to form a seal that stops the flow.

To improve the wear resistance of the sealing rings, the surface is often treated by overlay welding. A fine surface finish in the seat area is required to ensure a good seal when the valve is closed. Sealing rings are generally not considered pressure-retaining parts because the valve body has sufficient wall thickness to withstand the design pressure without relying on the thickness of the sealing rings.

The valve stem provides the necessary movement for the disc, plug, wedge, or ball and is responsible for opening or closing the valve, thus ensuring the correct positioning of the shut-off element. One end is connected to the valve handwheel or lever, while the other end is attached to the shut-off element. For gate valves or globe valves, a linear movement of the disc is required to open or close the valve, whereas for butterfly valves, ball valves, and plug valves, the disc is rotated to open or close the valve. Stems are typically forged and connected to the disc by threads or other methods. To prevent leakage, a fine surface finish is necessary in the sealing area of the stem.