Three pressure-boundary arrangements
A bolted bonnet clamps a gasketed body-to-bonnet joint with bolting. A welded bonnet closes that joint by welding the bonnet to the body, though the exact weld form and penetration depend on the design. A pressure-seal bonnet uses internal pressure to increase the load on a pressure-seal gasket and retainer system.
Those descriptions do not establish suitability. The selected product standard, manufacturer design, material, design conditions, thermal cycles and project maintenance philosophy must support the construction.
| Point | Bolted bonnet | Welded bonnet | Pressure-seal bonnet |
|---|---|---|---|
| Body-to-bonnet joint | Bolting plus gasket | Specified weld joint | Internal pressure acting on seal and retainer |
| Routine access | Disassembly after safe isolation and procedure controls | Requires weld removal or design-specific access | Requires design-specific unloading and disassembly procedure |
| Key purchase data | Gasket, bolting and joint design | Weld type, procedure, material and examination | Seal material, retainer design and minimum/operating pressure behaviour |
| Typical reason to review | Gasketed-joint leakage and maintenance | Permanent pressure-boundary joint and repair method | High-temperature/high-pressure plant practice and specialist maintenance |
Bolted bonnet
The bolted arrangement gives direct mechanical access to internal parts once the valve is isolated, depressurised, drained and handled under an approved maintenance procedure. Joint performance depends on flange stiffness, gasket selection, surface condition, bolting, assembly load and temperature cycling.
State body-to-bonnet gasket material and bolting materials. If fugitive-emission or fire-safe qualification is required, name the test standard and accepted design qualification separately; a bolted bonnet does not establish either claim.
Welded bonnet
A welded bonnet removes the demountable gasketed joint, but it introduces a weld into the pressure boundary. The purchase documents should identify weld construction, base materials, filler, procedure qualification, heat treatment and NDE. “Seal welded” and “fully welded bonnet” are not equivalent descriptions.
Internal maintenance can require cutting and qualified rewelding. That work may be impractical in an operating plant or may trigger new inspection and pressure testing. Review the entire maintenance route before selecting the construction for a perceived leak advantage.
Pressure-seal bonnet
In a pressure-seal construction, line pressure loads the seal system. The arrangement is associated with high-pressure, high-temperature service, but the exact operating range belongs to the specific valve design. Low-pressure sealing behaviour, start-up, shutdown and thermal cycling need the manufacturer’s procedure.
Maintenance teams need the correct sequence and tooling to release the retainer and extract the seal safely. Replacement seal material and dimensions are design-specific. Do not use pressure Class as proof that a pressure-seal bonnet is installed or required.
A bonnet change is a design deviation
Changing bonnet type alters the pressure-boundary joint, bill of materials, inspection plan and future maintenance. It may also alter overall height, stem access and available spare parts. The proposed valve therefore needs a new datasheet and drawing review.
FerraForge’s published gate, T-pattern globe, Y-pattern globe, piston-check and swing-check profiles use bolted bonnet or bolted cover descriptions. Any other construction must be handled as a separately reviewed RFQ and must not be inferred from those catalogue routes.
Specification checks
- Name bolted, welded or pressure-seal construction on the datasheet.
- For bolted bonnets, specify gasket and bolting materials.
- For welded bonnets, specify weld form, procedure, heat treatment and NDE.
- For pressure-seal bonnets, request operating and maintenance instructions for the exact design.
- Record any bonnet substitution as a pressure-boundary deviation.
Catalogue and RFQ
Standards publisher references
Published by Nordic Valves Ab Oy, the operator of FerraForge. These links describe the publishers’ official scope. Purchase documents must state the edition required by the project and include any amendments or owner specifications.
- API Monogram and APIQR programme documentsAmerican Petroleum Institute · Official standard titles and the API Monogram programme document register.
- ASME B16.34 - Valves - Flanged, Threaded, and Welding EndASME · Official scope covering valve pressure-temperature ratings, materials, dimensions, testing and marking.