Figure 24 shows the gaskets that are included in a standard cage-guided valve.
Spiral-Wound Gasket – Temperature gradients can cause the combined height of the seat ring and the cage to change because of thermal expansion and contraction. For this reason, a spiral wound gasket is included in many valve designs. The spiral wound gasket is actually a spring that can absorb a slight amount of cage expansion while maintaining a tight seal. The spiral wound gasket is a coil of a metal alloy that has been formed into a V-shape. Each V-shape in the finished gasket is separated from the next V-shape by a filler material.
Bonnet Gasket – The bonnet gasket is a flat sheet gasket that creates a tight seal between the bonnet gasket surface and the valve body gasket surface.
Shim – The shim prevents the sharp edge of the spiral wound gasket from cutting the flat sheet bonnet gasket.
Seat Ring Gasket – The seat ring gasket is a flat sheet gasket that prevents leakage between the seat ring and the valve body.
Gasket Options – The selection of a particular gasket material is determined by the temperature limits of the application and by the corrosion resistance that is required. Various options are described in Figure 25.
Spiral Wound Gasket Options – A standard spiral wound gasket is made of stainless steel that is filled with a composition material. As indicated in Figure 25, the standard gasket material is limited to 450 degrees F and it must be derated to 300 degrees F if the temperature cycles repeatedly. For high temperature (up to 1 100 degrees F) and for temperature cycling applications, an Inconel or other alloy that is filled with a graphite material is specified.
Flat Sheet Gasket Options – A common standard for flat sheet gaskets is a composition material. Manufacturers may use proprietary materials such as Fisher Controls’ FGM (Fisher Gasket Material). Most composition materials are suitable for temperatures up to 1 100 degrees F; however, they may not provide the required corrosion resistance. Options such as PTFE coated Monel provide corrosion resistance, but at reduced temperature ratings, as shown in the table below.