Valve Bonnet Design: Full Bolted, Pressure Seal, and Welded Bonnets

The valve bonnet provides the removable closure of the valve body that allows access to the internal trim and packing for maintenance. Bonnet design is a critical structural element that must provide a reliable pressure boundary at operating conditions, allow practical disassembly and reassembly during maintenance, and seal reliably against the body without distortion or leakage. Different bonnet designs—full bolted, pressure seal, and welded—are suited to different pressure classes and service requirements.

Bolted Bonnet Design and Gasket Types

Bolted bonnet construction secures the bonnet to the body with a ring of studs and nuts compressing a gasket between the bonnet flange face and the body face. This is the most common bonnet design for Class 150 through Class 600 valves, providing straightforward assembly and disassembly for maintenance with standard tools. Body-bonnet joint gasket materials match the service conditions: spiral-wound metallic gaskets for high-temperature steam and hydrocarbon service, PTFE-envelope gaskets for corrosive chemical service, and ring-type metallic gaskets for higher pressure classes. Stud torque must be applied uniformly in a cross-bolting sequence to prevent gasket distortion. Bolted bonnet joints are subject to bolt relaxation over time and temperature cycling, requiring periodic re-torquing checks during commissioning and early service.

• Bolted bonnet: standard for Class 150-600—easy maintenance access, broad application

• Gasket types: spiral wound (steam/HC), PTFE (chemical), RTJ ring (Class 600+)

• Stud materials: B7 alloy steel standard; B8M stainless for corrosive/low-temp service

• Torque sequence: cross-bolting pattern essential for uniform gasket compression

• Re-torquing: required after initial heatup to compensate for gasket relaxation

Pressure Seal Bonnet for High-Pressure Service

For Class 900 and higher pressure classes, pressure seal bonnets provide a more reliable and compact high-pressure closure than bolted bonnets. In a pressure seal design, the bonnet slides into the valve body from the top and is held in place by a seal ring at the junction between the bonnet and body bore. As internal pressure increases, it acts on the seal ring to increase the sealing force—the valve self-energizes its own seal. This means the higher the process pressure, the tighter the pressure seal becomes. Pressure seal bonnets require no external bolting to maintain the body-bonnet seal under operating pressure. Maintenance access requires a bonnet jack (special tool) to overcome the pressure seal's retention force when depressurized, and the pressure seal ring is typically replaced each time the bonnet is removed.

Welded Bonnet and Hermetically Sealed Valves

Welded bonnet construction provides a permanent, hermetically sealed valve body that cannot be disassembled without cutting the weld. This design offers the highest structural integrity and zero leakage potential at the body-bonnet joint, making it the preferred choice for highly toxic, pyrophoric, or radioactive fluids where any flange leakage is unacceptable. However, welded bonnet valves cannot be maintained in the field without specialized welding and re-qualification, and are typically replaced as a unit when trim replacement is needed. Hermetically sealed valve designs for nuclear, semiconductor, and ultra-high-purity applications use welded bodies with magnetic or bellows-sealed stem designs to eliminate all paths for fluid migration to atmosphere.