Pressure Seal Bonnet Valves for Class 900 and Above: Design, Use and Maintenance
Technical guide · Supreme Valves India · Updated 2026-10-25
At pressure classes of 900 and above, especially in high-temperature steam, most gate, globe and check valves change from a bolted bonnet to a pressure seal bonnet. The design uses line pressure to make the body-bonnet seal tighter, which keeps the valve compact where a bolted bonnet would need a very heavy flange and many large bolts.
How a pressure seal bonnet works
The bonnet sits inside the body neck rather than bolted on top. A tapered seal ring, usually graphite or soft iron with silver plating, sits between the bonnet and the body bore. Above it, a segmented thrust ring locks into a groove machined in the body. Line pressure pushes the bonnet upward, which wedges the seal ring between the bonnet, the body bore and the thrust ring. The higher the pressure, the harder the seal is compressed.
Bonnet take-up bolts pull the bonnet up against the seal to give initial preload, so the valve seals before full pressure is reached.
Why it is used at high classes
- Weight and size: no large body-bonnet flange or heavy bolting, so the valve is lighter and more compact.
- Sealing improves with pressure: unlike a bolted joint, where pressure tries to separate the flanges.
- Fewer leak paths in thermal cycling: no long bolts that relax at high temperature.
Bolted bonnets remain common up to Class 600, and some specifications allow them at Class 900 in smaller sizes. Above that, pressure seal is the norm for cast steel valves.
Typical applications
- Main steam, hot and cold reheat, and feedwater lines in power plants.
- Boiler isolation, drains, vents and blowdown at high pressure.
- High-pressure hydrocarbon and hydrogen service in refineries.
Materials
| Cast (ASTM A216 / A217) | Forged (ASTM A105 / A182) | Typical use |
|---|---|---|
| WCB / WCC | A105 | Feedwater and moderate temperature |
| WC6 (1¼Cr-½Mo) | F11 | Steam above the carbon steel range |
| WC9 (2¼Cr-1Mo) | F22 | Higher temperature steam |
| C12A (9Cr-1Mo-V) | F91 | Supercritical and ultra-supercritical main steam |
Seats are normally hard-faced with stellite. Gate valves for steam may be flexible wedge or parallel slide.
Installation and maintenance
- Hot re-torque: after the first heat-up, re-tighten the bonnet take-up bolts to the manufacturer's torque, and again after any reassembly.
- Disassembly: depressurise and cool fully; the thrust ring segments must be removed after the bonnet is pushed down. Use the manufacturer's procedure and tools.
- Seal ring: replace at every reassembly; graphite and soft iron rings deform in service and aren't reusable.
- Welding in line: butt weld ends in chrome-moly materials need preheat and post-weld heat treatment per the code; protect seats and packing as instructed.
What to include in an enquiry
Valve type, size, class, end connection (usually butt weld with pipe schedule), body and trim material, design pressure and temperature, applicable code (ASME B16.34, ASME B31.1, IBR in India), bypass and drain requirements, and operator.
Frequently asked questions
Why are pressure seal bonnets used at high pressure classes?
In a bolted bonnet, higher pressure needs more and larger bolts and a heavier flange, and bolt load must hold the gasket against pressure. In a pressure seal bonnet, line pressure itself pushes the bonnet against the seal, so the seal gets tighter as pressure rises. That makes the valve lighter and more compact at Class 900 and above.
What is hot re-torquing on a pressure seal valve?
After the valve first reaches operating temperature and pressure, the bonnet take-up bolts are re-tightened to the manufacturer's value to compensate for seal ring settling. It is done once after commissioning and after any reassembly, following the manufacturer's procedure.
Can pressure seal valves be used at low pressure?
They rely on pressure for sealing, so at very low pressure the seal depends on the preload from the bonnet take-up bolts. They are designed and used for high-pressure service; for low-pressure duties a bolted bonnet valve is normally the better choice.
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