Häger GmbH & Co. KG
DIN Industrial Valves

The valve body, also referred to as the housing, is a fundamental component of a valve. It serves as the main structure of the valve assembly, as it holds all other parts together. As the primary pressure boundary of the valve, the body is designed to withstand the internal pressure exerted by the process media from connected pipelines.

It connects the inlet and outlet piping through threaded, flanged, bolted, or welded connections. The ends of the valve body are configured to match the pipeline connection requirements—such as butt weld, socket weld, threaded, or flanged ends. Valve bodies are produced in various shapes and configurations, typically through casting or forging. Each component of the body is designed with a specific function in mind and is made from materials suitable for its intended application.

The cover for the opening in the valve body is called the bonnet. It is the second most important part of a pressure valve. Like valve bodies, bonnets are available in many designs and models. The bonnet serves as a cover on the valve body, is either cast or forged, and made from the same material as the body. It is typically connected to the body by threaded, bolted, or welded joints. During valve assembly, internal components such as the stem, plug, wedge, etc., are inserted into the body. The bonnet is then installed to hold all internal parts together. In all cases, the connection of the bonnet to the body is considered a pressure boundary. This means that the welds or bolts attaching the bonnet to the body are pressure-retaining parts.

The removable and replaceable internal valve components that come into contact with the flow medium are collectively referred to as the valve trim. These parts include valve seats, plugs, wedges, fasteners, spacers, guides, bushings, and internal springs. The valve body, bonnet, gasket, and similar components—although also in contact with the flow medium—are not considered part of the valve trim.

The trim is defined by the interface between the disc and seat, and the disc’s position relative to the seat. The movement and flow characteristics of a valve are determined by the trim design. In rotary motion designs, the plug or wedge moves closely past the seat to alter the flow opening. In linear motion trims, the disc lifts perpendicularly away from the seat, creating an annular flow opening.

Trim components may be made from various materials, depending on the different forces and conditions they must withstand. For example, bushings and packing glands experience different stresses than valve discs and seats. Flow media properties—such as chemical composition, pressure, temperature, velocity, flow rate, and viscosity—are critical factors in selecting the appropriate trim materials. Valve 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 the flow depending on its position. When stopping the flow, the disc functions as the shut-off element. It is the third most important primary pressure boundary. When the valve is closed, the full system pressure is applied to the disc, which is why it is considered a pressure-retaining part.

Discs are typically forged or cast and, in some designs, feature a hard-faced surface (e.g., Stellite) to provide excellent wear resistance.

The seat or sealing rings provide the seating surface for the disc. A valve may have one or more seats. In the case of a check valve, there is typically one seat that forms a seal with the disc to stop the flow. In a gate valve, there are two seats—one on the upstream side and one on the downstream side. A gate valve disc has two seating surfaces that contact the valve seats to form a seal that stops flow.

To improve the wear resistance of the seating rings, the surface is often hard-faced using weld overlay. A fine surface finish in the seat area is essential for a good seal when the valve is closed. Seating rings are generally not considered pressure-retaining parts, since the valve body has sufficient wall thickness to withstand the design pressure independently of the seating ring thickness.

The valve stem provides the necessary movement of the disc, plug, wedge, or ball and serves to open or close the valve, thus being responsible for the correct positioning of the shut-off element. One end is connected to the valve handwheel or lever, and the other end to the shut-off element.

In gate or globe valves, a linear movement of the disc is required to open or close the valve, while in plug valves, ball valves, and butterfly valves, the disc is rotated to perform the same function. Stems are typically forged and connected to the disc using threads or other methods. To prevent leakage, a fine surface finish on the stem is required in the sealing area.