Pressure Seal Bonnet Valves for High Pressure

Pressure seal bonnets are a design feature used in high-pressure valves, typically above ASME Class 900, to create a self-energizing seal that becomes tighter as process pressure increases. This design is preferred over bolted bonnets for high-pressure applications because it reduces bolting requirements and improves reliability.

Pressure Seal Principle

In a pressure seal bonnet, the sealing ring is trapped between the valve body and bonnet in a geometry that allows line pressure to act against the seal, forcing it outward against the body bore. As pressure increases, the sealing force increases proportionally, making pressure seal bonnet valves inherently reliable in high-pressure service where bolted bonnets might relax over time.

Design Configurations

• Segmented retainer ring holds bonnet against upward pressure force

• Pressure seal ring manufactured from soft iron, copper, or stainless

• Ring cross-section designed to deform slightly under pressure for sealing

• Bonnet guide ensures concentricity with body bore during assembly

• Jacking screws facilitate bonnet removal during maintenance

Material and Pressure-Temperature Ratings

• Class 900 to Class 4500 pressure ratings in ASME B16.34

• Carbon steel (WCB, WCC) for standard hydrocarbon service

• Chrome-moly (WC6, WC9, C5) for high-temperature steam service

• Stainless steel (CF8M, CF3M) for corrosive service

• Alloy steels for hydrogen, sour gas, and special services

Applications

Pressure seal bonnet gate, globe, and check valves are extensively used in high-pressure steam systems in power plants, high-pressure hydrocarbon injection systems, main steam isolation valves, and boiler feedwater service. The pressure seal design is particularly advantageous in cyclic high-pressure service where bolted bonnet flanges might experience fatigue-related relaxation.

Maintenance Considerations

Pressure seal bonnets require careful maintenance procedures. The seal ring is a sacrificial component that deforms to create the seal and must be replaced during maintenance. Disassembly requires depressurizing and cooling the valve before the retainer ring is removed using jacking screws. Reassembly requires proper ring seating and torquing of retainer components to manufacturer specifications.