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Bolted Bonnets vs. Pressure Seals
- Aug 24, 2018 -

To better understand the pressure seal design concept, let's contrast the body-to-Bonnet sealing mechanism between bolted Bonnets and pressure seals. FIG. 1 depicts the typical Bolted Bonnet valve. The body flange and Bonnet flange are joined by studs and nuts, with a gasket of suitable design/material inserted between the flange faces to facilitate sealing. Studs/nuts/bolts are tightened to prescribed torques in a pattern defined by the manufacturer to affect optimal sealing. However, as system pressure increases, the potential for leakage through the body/Bonnet joint also increases.

Now let's look at the pressure seal joint detailed in FIG. 2 Note the differences in the respective body/Bonnet joint configurations. Most pressure seal designs incorporate "Bonnet take-up bolts" to pull the Bonnet up and seal against the pressure seal gasket. This in turn creates a seal between the gasket and the inner dia (I.D.) of the valve body.

bolted_bonnet.gifpressure_seal.gif

A segmented thrust ring maintains the load. The beauty of the pressure seal design is that as system pressure builds, so does the load on the Bonnet and, correspondingly, the pressure seal gasket. Therefore, in pressure seal Valves, as system pressure increases, the potential for leakage through the body/Bonnet joint decreases.

This design approach has distinct advantages over bolted Bonnet Valves in main steam, feedwater, turbine bypass, and other power plant systems requiring Valves that can handle the challenges inherent in high-pressure and temperature applications.
But over the years, as operating pressures/temperatures increased, and with the advent of peaking plants, this same transient system pressure that aided in sealing also played havoc with pressure seal joint integrity.