Valve Bonnet Types Explained: How the Top End Is Sealed and Serviced

Most valve discussions focus on the body and the trim, but the bonnet — the top cover of a valve body — carries the stem, seals the pressure boundary, and determines how the valve is assembled and serviced. Bonnet types matter because they affect pressure rating, temperature capability, leak tightness, and maintenance effort. Reading a valve spec becomes much easier once the bonnet design is part of the picture.

The Short Answer

The bonnet is the pressure-containing cover on top of a valve body, bolted or threaded to it, through which the stem passes. Common types include the bolted bonnet, the threaded (screw-in) bonnet, the welded bonnet, and the pressure-seal bonnet. Each trades off assembly simplicity, pressure capability, temperature range, and ease of maintenance. The bolted bonnet is the most common general-purpose design; pressure-seal bonnets are used for high-pressure, high-temperature service.

What Each Bonnet Type Is

Bolted bonnet

The bonnet is a separate casting or forging bolted to the body with a gasket between them. It is easy to assemble, disassemble, and repair, and it suits a wide range of pressures and temperatures. It is the standard for many gate, globe, and check valves in refinery and general industrial service.

Threaded (screw-in) bonnet

The bonnet screws directly into the body, eliminating body-bonnet bolts. It is common in smaller valves — bronze and small steel gate and globe valves — and keeps the design compact. The joint relies on the thread seal and is generally limited to lower and moderate pressures.

Welded bonnet

The bonnet is welded to the body, creating a permanent, leak-free joint. It suits high-integrity services where leakage must be minimized, but it cannot be disassembled for maintenance, which matters for cleanout and repair access.

Pressure-seal bonnet

The bonnet is sealed by internal pressure pushing a seal ring into the body; a small bolted clamp holds it in place. As pressure rises, the seal tightens. This design is used on high-pressure, high-temperature valves, such as large gate and globe valves in power and high-pressure process service.

Key Differences

Type	Joint	Pressure capability	Maintenance access	Typical service
Bolted	Bolts + gasket	Broad range	Easy	General industrial
Threaded	Screw threads	Lower to moderate	Easy on small valves	Small bronze/steel valves
Welded	Weld	High integrity	None — permanent	High-leakage-integrity lines
Pressure-seal	Internal pressure + clamp	Very high	Requires special procedure	High-pressure/high-temperature

Which Bonnet to Specify

Use a bolted bonnet as the general-purpose default where maintenance access matters. Consider a welded bonnet where fugitive emissions or leakage risk is the top concern and disassembly is not expected. Use a pressure-seal bonnet for high-pressure, high-temperature service where the bolted design would be impractical. Threaded bonnets are a compact, economical choice for small, moderate-pressure valves.

Common Pitfalls

• Choosing a welded bonnet for a service that will need regular trim access; every maintenance job then becomes a cut-and-reweld operation.
• Using a threaded bonnet at pressures above its intended range and finding the thread joint leaking.
• Assuming the bonnet gasket material matches the body rating; gaskets and bonnet bolts must be rated for the full pressure-temperature envelope.
• Ignoring bonnet orientation when space is tight; stem clearance and actuator fit depend on the top-end design.

Practical Recommendation

Match the bonnet to the service pressure and the expected maintenance cycle. For most plant valves, a bolted bonnet balances integrity with serviceability. For high-pressure steam or high-pressure process, consider pressure-seal. Confirm that the gasket and bolting are rated for the full envelope, and verify maintenance access in the installed layout.

Conclusion

The bonnet seals the valve’s top end and defines how the valve is serviced. Bolted, threaded, welded, and pressure-seal designs each fit a different balance of pressure capability and access. Read the bonnet type on the spec, and you immediately know the pressure class intent and the maintenance story.

This article provides general educational knowledge only and is not professional engineering advice.

Tags: Valve Selection