Valve Bonnet Designs: Bolted, Pressure Seal, and Welded Bonnets

The valve bonnet is the pressure-containing cover over the valve body that houses the stem seal, provides access to the valve internals for maintenance, and in some designs contributes to the structural integrity of the valve assembly. Bonnet design is closely tied to the pressure class of the valve: low-pressure valves use simple bolted bonnets, while extreme-high-pressure valves use pressure-seal bonnets that actually increase their seating load as pressure increases. The bonnet-to-body joint is one of the most critical pressure boundaries in the valve, and its design determines the valve's suitability for high-pressure and high-temperature service.

Bolted Bonnet Design

Bolted bonnets are the most common bonnet design for valves in ASME Class 150 through Class 600. The bonnet flange is bolted to the body flange through a ring gasket (spiral-wound, ring joint, or flat gasket depending on pressure class). The bolting preload compresses the gasket to create a pressure-tight seal. Bolted bonnets can be removed for access to valve trim and internals during maintenance. The bolting design must generate sufficient gasket stress to maintain sealing under the hydrostatic end force created by internal pressure, which acts to separate the bonnet from the body. At higher pressure classes, the required bolt size and number increases, making the bolted assembly larger and more expensive.

• Class 150-600: standard bolted bonnet with spiral-wound or ring gasket

• Pressure-seal bonnet: Class 900 and above, bonnet seal loads increase with pressure

• Welded bonnet: permanent, no disassembly access, used for small instrument valves

• Union bonnet: screwed union nut, removable for cleaning in small valves

• Extended bonnet: lengthened bonnet for cryogenic or insulated service to locate packing at ambient temperature

Pressure-Seal Bonnet Design

Pressure-seal bonnets, used for Class 900 and higher pressure ratings, use a fundamentally different sealing mechanism that becomes more effective as pressure increases. The bonnet is inserted into the valve body bore and retained by a snap ring or retaining ring. A C-shaped metallic seal ring is seated in a groove between the bonnet and body. When internal pressure acts on the seal ring, it forces the ring outward against the body bore and inward against the bonnet, creating a wedging action that increases seal contact pressure proportionally with internal pressure. This self-energizing characteristic means the seal actually gets tighter as pressure increases, making pressure-seal bonnets the preferred design for high-pressure gate and globe valves where Class 900 to Class 4500 ratings are required.

Welded and Special Bonnets

Welded bonnets provide the most compact and lightweight bonnet design by welding the bonnet directly to the body without any bolted connection. This eliminates the bolted joint as a potential leak path, making welded bonnets suitable for extremely clean, toxic, or lethal services where any external leakage is unacceptable. The primary disadvantage is that the valve cannot be opened for maintenance without cutting the weld, so welded-bonnet valves are typically used in non-maintainable services or are replaced as complete units when they fail. Extended bonnets are used in cryogenic and insulated piping service to position the packing gland away from the cold pipe, keeping the packing at near-ambient temperature where it provides better sealing performance and where the gland bolting is accessible without disturbing the thermal insulation.