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Pressure Seal Gaskets
- Aug 26, 2018 -

One of the primary components involved in sealing the pressure seal Valve is the gasket itself. Early pressure seal gaskets were manufactured from iron or soft steel. These gaskets were subsequently silver-plated to take advantage of the softer plating material's ability to provide a tighter seal. Due to the pressure applied during the Valve's hydrotest, a "set" (or deformation of the gasket profile) between the Bonnet and gasket was taken. Because of the inherent Bonnet take-up bolt and pressure seal joint elasticity, the potential for the Bonnet to move and break that "set" when subjected to system pressure increases/ decreases existed, with body/Bonnet joint leakage the result.

This problem could be effectively negated by utilizing the practice of "hot torquing" the Bonnet take-up bolts after system pressure and temperature equalization, but it required owner/user maintenance personnel to do so after plant startup. If this practice was not adhered to, the potential for leakage through the body/Bonnet joint existed, which could damage the pressure seal gasket, the Bonnet and/or the I.D. of the Valve body, as well as creating compounding problems and inefficiencies that the steam leakage could have on plant operations. As a result, Valve designers took several steps to address this problem.

Figure 2 shows a combination of live-loaded Bonnet take-up bolts (thus maintaining a constant load on the gasket, minimizing the potential for leakage) and the replacement of the iron/soft steel, silverplated pressure seal gasket with one made of die-formed graphite. The gasket design shown in Figure 3 can be installed in pressure seal Valves previously supplied with the traditional type gasket. The advent of graphite gaskets has further solidified the dependability and performance of the pressure seal Valve in most applications and for even daily start/stop operating cycles.

Although many manufacturers still recommend "hot torquing," the potential for leakage when this is not done is greatly diminished. The seating surfaces in pressure seal Valves, as in many power plant Valves, are subjected to, comparatively speaking, very high seating loads. Seat integrity is maintained as a function of tight machining tolerances on component parts, means of providing the requisite torque to open/close as a function of gears or actuation, and selection/ application of proper materials for seating surfaces.

Cobalt, nickel, and iron-based hardfacing alloys are utilized for optimal wear resistance of the wedge/disc and seat ring seating surfaces. Most commonly used are the CoCr-A (e.g., Stellite) materials. These materials are applied with a variety of processes, including shielded metal arc, gas metal arc, gas tungsten arc, and plasma (transferred) arc. Many pressure seal Globe Valves are designed having integral hardfaced seats, while the Gate Valve and Check Valves typically have hardfaced seat rings that are welded into the Valve body.