A pressure-seal bonnet uses internal line pressure to energise its own sealing: the bonnet sits inside the body neck against a soft iron or graphite gasket, and rising pressure wedges the gasket tighter. It replaces heavy bolted flanges on Class 900–2500 valves, sealing best exactly when pressure is highest.
At Class 1500, a bolted bonnet on a 12" gate valve needs a flange thick as a millstone and studs the size of a forearm — and every thermal cycle of a main-steam line works those studs loose. The pressure-seal principle inverts the problem: instead of bolting against pressure, it recruits pressure as the sealing force. The bonnet is a plug inside the body neck; a wedge-sectioned gasket ring sits between bonnet shoulder and body bore; internal pressure pushes the bonnet up, driving the gasket outward against the body wall. More pressure, tighter seal.
The design cuts valve weight by 25–40% at high ratings and eliminates the large flange joint that leaks first on steam cycling. Its trade-offs: sealing is weakest at low pressure (hydrotest procedures account for this), and gasket replacement requires pulling the internals rather than unbolting a flange. Modern flexible-graphite gasket rings have largely cured the old soft-iron ring's sticking and corrosion issues.
Pressure-seal construction is the norm for power-station gate, globe and check valves from Class 900 upward. VTXFLOW manufactures all three types with graphite pressure-seal gaskets in WCB, WC6, WC9 and F-grade forgings.