A valve bonnet closes an opening in the valve body and forms part of the pressure boundary. In many gate, globe and sliding-stem control valves, it also contains the stem passage, packing box and gland, and it often provides the mounting point for a yoke or actuator. The bonnet therefore affects both static sealing at the body joint and dynamic sealing around the moving stem.
Two design questions must be kept separate. The first is how the bonnet connects and seals to the body, such as a bolted, welded or pressure-seal joint. The second is whether the bonnet has a standard, extended, cryogenic or bellows-seal form for a specific service condition. A valve may combine one choice from each group, such as a bolted extension bonnet.
What is a valve bonnet, and what does it do?
Pressure boundary and body opening
Where a separate bonnet is used, its first job is to close the access opening through which the stem and internal parts are assembled. The body-bonnet connection must contain line pressure under the valve’s rated pressure and temperature. A gasket, weld, threaded interface or pressure-energised seal completes that joint, depending on the construction.
The bonnet is not a loose protective cover. Damage to its wall, flange, threads, weld or sealing face can affect the pressure boundary. The correct response to a defect therefore depends on the valve design, applicable product standard and approved repair procedure, rather than on a general bonnet repair rule.
Stem seal, guidance and actuator support
The stem normally passes through the bonnet. Packing around the stem limits external leakage while allowing movement, and the gland applies the load needed to keep the packing in contact with the stem and stuffing-box wall. In some designs, bonnet geometry also helps guide the stem, align the trim or retain a cage or seat ring.
On a sliding-stem control valve, the bonnet commonly connects the pressure-containing valve assembly to the actuator structure. The Exida engineering definition of a valve bonnet also identifies the stem passage, stem sealing and common actuator-mounting function. These features are design-dependent, so the sectional drawing and bill of materials remain the final reference for a specific valve.
Readers who need the wider relationship among the body, trim, actuator and accessories can use the MacoTango Valve Basics resources. The rest of this guide stays with bonnet construction, selection and leakage.
Valve bonnet types by body connection
Body connection describes how the bonnet is held to the body and how the joint seals. This classification answers a different question from terms such as extended or cryogenic bonnet.
| Connection type | How the joint is formed | Service access | Main point to verify |
|---|---|---|---|
| Threaded | Threads retain the bonnet and create or load the sealing interface | Removable, but repeated work can damage or gall threads | Product-standard permission and thread condition |
| Union | A union nut draws the bonnet into its sealing seat | Designed for easier removal | Seat, nut and thread condition |
| Bolted | Bolting compresses a gasket between mating faces | Good access for planned disassembly | Gasket, faces, bolting and specified preload procedure |
| Welded | A weld makes the body-bonnet pressure joint | Internal access requires cutting and qualified repair | Weld design, material, inspection and repair controls |
| Pressure-seal | Internal pressure increases load on an internal seal ring | Removable with design-specific steps | Low-pressure preload, seal surfaces and manufacturer range |
The table compares mechanisms, not universal pressure classes. Size, rating, temperature, material and valve family still have to match the applicable standard and the manufacturer’s data.
Screwed or threaded bonnet
A threaded bonnet screws into or over the body neck. The threads retain the pressure load, while the sealing detail may use a shoulder, gasket, seal weld or another product-specific feature. This construction can be compact, but it should not be treated as suitable for every small valve or every pressure class.
Corrosion, galling and thread damage can make later removal difficult. Before reuse, the thread form, engagement, sealing surface and any locking or seal-weld feature need inspection against the product procedure.
Union bonnet
A union bonnet uses a separate retaining nut to draw the bonnet into the body and load its sealing seat. It can provide faster access than a screwed bonnet because the bonnet itself does not have to rotate through the body threads during removal.
The union nut is still part of a pressure-containing assembly. Wear on its threads, distortion at the seat or the wrong replacement seal can prevent correct loading. Its use must follow the valve design rather than a general claim that union bonnets fit all frequently serviced duties.
Bolted bonnet
A bolted bonnet clamps a gasket between the body and bonnet faces. Stud or bolt load must be high enough and even enough to keep the gasket sealed through pressure, temperature and operating cycles. The joint remains removable, which is why this construction is common where planned access to the trim is required.
Leak resistance depends on the complete joint. Gasket type and condition, flange stiffness, surface finish, alignment, bolting material, lubrication and the specified tightening method all affect the result. A generic torque value or one fixed tightening sequence cannot be transferred safely between valve designs.
Welded bonnet
A welded bonnet removes the removable gasketed body joint and joins the bonnet to the body by welding. This can reduce one routine joint-leak path, but it does not make the valve leak-proof. The weld and adjacent base metal become part of the pressure boundary and can still be affected by defects, thermal stress, corrosion or an unsuitable repair.
Internal access normally requires the joint to be cut and later restored under a qualified welding, heat-treatment and inspection procedure where applicable. That loss of easy access is the main trade-off, so a welded bonnet should be selected with the expected maintenance strategy in mind.
Pressure-seal bonnet
A pressure-seal bonnet places the sealing ring inside the body opening. Internal pressure pushes the bonnet towards its retaining structure and increases contact load at the seal. In contrast, a conventional bolted bonnet relies mainly on external bolting to maintain gasket compression.
The design still needs an initial seal before full system pressure develops. Seal-ring geometry, surface condition, preload parts and assembly steps therefore matter at low pressure as well as at operating pressure. The Flowserve pressure-seal bonnet example describes the pressure-assisted mechanism in a high-pressure gate-valve product family. Its pressure range is product evidence, not a universal threshold for all pressure-seal valves.
Special bonnet designs for temperature and stem sealing
Terms such as standard, extension, cryogenic and bellows-seal describe what the bonnet assembly does for a service condition. They do not, by themselves, identify whether the body joint is bolted, welded or pressure-sealed.
Standard bonnet
A standard bonnet keeps the packing box relatively close to the valve body. It is compact and can suit service where the packing, gasket, bolting and actuator environment remain within their rated limits. The word standard does not define one shape or temperature range across all manufacturers.
Selection should check the actual packing temperature, access around the gland and the effect of nearby insulation or heat sources. Process temperature alone does not show the temperature that the stem seal will reach.
Extension bonnet for high- or low-temperature service
An extension bonnet increases the distance between the process fluid and the packing box. That extra heat path can reduce heat transfer to the packing in hot service or help keep the packing away from the cold zone in low-temperature service. It may also place the gland above insulation so it stays visible and accessible.
Fins increase the exposed surface area and can change heat transfer between the bonnet and surrounding air. Their effect depends on orientation, ambient conditions, insulation, stem conduction, process temperature and valve geometry. The packing temperature still has to be checked for the complete valve assembly.
An extension is not a pressure upgrade by itself. The body, bonnet, joint, gasket, bolting, packing and other pressure-retaining parts must still meet the required rating at the actual temperature.
Cryogenic extension and the insulation boundary
A cryogenic bonnet extension moves the packing and actuator connection away from the cold fluid. The long neck, internal gas space and installation orientation can help keep the stem-sealing area warmer than the valve body. The aim is to protect sealing performance and keep the gland accessible, not to prevent every internal part from becoming cold.
Extension length and orientation depend on the fluid temperature, valve type, insulation thickness, heat input and manufacturer design. The insulation boundary is especially important because covering the wrong part of the extension can change the intended heat path. The MacoTango guide to cryogenic control valve selection covers those wider service conditions.
Bellows-seal bonnet assembly
A bellows-seal assembly uses a welded metal bellows as a barrier between the process fluid and the atmosphere along the moving stem. One end is joined to the stem and the other to a fixed part of the bonnet assembly. Conventional packing may remain as a secondary seal and as protection if the bellows develops a leak.
Bellows design must match pressure, temperature, fluid chemistry, stroke and expected cycle duty. Weld quality, bellows material, torsion control and fatigue life all affect the result. A bellows can reduce stem emissions in the approved service envelope, but the phrase zero leakage should not be used as an unconditional guarantee.
Why “extended bonnet” can mean something different in waterworks service
In municipal and waterworks products, an extended bonnet may move the operator away from a buried, submerged, insulated or hard-to-reach valve body. The design is then solving an access or operator-location problem rather than controlling packing temperature.
Gate, globe, control and rotary valves do not use bonnets in the same way
Bonnet terminology developed mainly around valves with a stem moving through a pressure-boundary opening. Applying that same parts list to every valve family creates errors in drawings, spare-parts requests and maintenance plans.
Gate and globe valves
Gate and globe valves commonly use a distinct bonnet above the body. It contains the stem passage and packing box, supports the yoke structure and provides access to internal parts. A backseat may also be formed in the bonnet or related assembly, depending on the design.
The bonnet’s internal function differs between products. A globe-valve bonnet may guide the stem or help retain a cage or seat assembly, while a gate-valve bonnet may mainly house and support the rising-stem arrangement. These details must come from the sectional drawing.
Sliding-stem control valves
In a sliding-stem control valve, the bonnet normally contains the packing system and provides the structural path between the valve body and actuator. Its bore and guiding parts can affect stem alignment and packing wear. In cage-guided designs, the bonnet may also load the cage and related internal seals when the joint is assembled.
Those roles link bonnet condition to control performance. Misalignment, damaged guides or an unsuitable packing arrangement can increase friction, disturb stem movement or create external leakage even when the body-bonnet gasket remains tight.
Ball, butterfly and other rotary valves
Many ball and butterfly valves do not have a separate bonnet in the same sense as a gate or globe valve. Stem packing may sit in an integral body neck, gland housing or cover, while internal access comes through a split body, end cap or separate cover.
The term ball valve bonnet can still appear in a product catalogue, but its meaning is tied to that product’s construction. Long bonnet ball valve can also refer to an extended stem or operator arrangement in plumbing and utility markets. For an industrial rotary valve, use the drawing and parts list to identify the pressure-boundary component rather than assuming a globe-valve layout.
How to select a valve bonnet design
Bonnet selection starts with the complete valve design. A connection that works for one valve family may be unavailable or unsuitable in another, even when nominal size and pressure class appear similar.
Start with the valve family and body-bonnet joint
First identify whether the valve uses a separate bonnet and what the applicable product standard and manufacturer design allow. The choice between bolted, welded, threaded or pressure-seal construction changes maintenance access, joint-sealing behaviour and the parts that must be inspected.
Frequent internal access may favour a removable joint, but maintainability cannot override the required pressure boundary. A welded or pressure-seal design may fit a severe duty, yet it also creates different tooling, inspection and repair needs.
Check the complete assembly’s pressure-temperature rating
The bonnet cannot be rated from its shape alone. Body and bonnet material, wall design, bolting, gasket or seal ring, packing, end connection and product standard all affect the permitted pressure at a given temperature.
Use the manufacturer’s current pressure-temperature data for the exact construction. Do not transfer a class threshold from another valve type, and do not assume that an extension or bellows raises the pressure rating.
Control the temperature at the packing, not only the process temperature
Packing performance depends on the temperature at the stuffing box. Process temperature is one input, but conduction through the stem and bonnet, ambient air, insulation, valve orientation, cycling and nearby heat sources can move the packing temperature in either direction.
An extension is useful only when its geometry keeps the packing and actuator interface inside their limits under the real installation conditions. This is why bonnet length and fin arrangement are manufacturer design variables rather than universal dimensions.
Match fluid, gasket, packing and pressure-boundary materials
The body-bonnet joint is a static seal, while stem packing or a bellows controls a moving leak path. They face different loads and may need different materials. Fluid chemistry, temperature, external corrosion, cleaning chemicals and fire or emissions requirements can rule out an otherwise workable construction.
Material names alone are not enough. Grade, heat treatment, hardness, gasket type, packing set, bolting combination and any weld procedure must agree with the valve design and service. A generic carbon-steel or stainless-steel temperature range cannot replace the complete rating and compatibility check.
Account for emissions target, cycle duty and maintenance access
A low-emissions packing system, live loading or bellows seal may be considered when stem emissions drive the decision. Cycle count and stroke affect packing wear and bellows fatigue, so the tightest-looking design is not automatically the best choice for a frequently operated valve.
Access also matters. The gland must remain reachable, insulation must stop at the correct point and there must be enough room to remove the actuator, yoke or bonnet when the approved maintenance method requires it. A design that meets the pressure rating but cannot be inspected safely is incomplete for the installation.
Where valve bonnet leaks actually occur
Fluid seen near the bonnet does not identify the failed part. It may have travelled down the stem, around the joint or across the outside surface before becoming visible. Locate the first wet or stained point before choosing an action.
| Leak path | Typical origin to check | Evidence that changes the diagnosis | Action boundary |
|---|---|---|---|
| Stem or packing | Around the moving stem above the packing box | Stem condition, packing load, motion, temperature and alignment | Follow the packing and manufacturer procedure |
| Body-bonnet joint | The circumference of the joint or gasket line | Gasket, seal ring, faces, preload, alignment and thermal history | Do not tighten a pressurised joint from generic advice |
| Pressure-boundary wall | Bonnet wall, flange, thread area, weld or body neck | Crack, porosity, corrosion, erosion, distortion or prior repair | Isolate and obtain engineering disposition |
Stem or packing leakage
Packing leakage normally appears where the stem leaves the stuffing box. Possible causes include packing relaxation or wear, incorrect gland load, stem scoring, deposits, misalignment, high packing temperature or the wrong packing for the fluid and motion.
Tightening the gland may reduce some leaks, but excess load can raise friction, damage packing and restrict stem travel. The correct check depends on the packing design and valve condition. The MacoTango guide to control valve stem leakage covers that dynamic sealing path in more detail.
Body-bonnet joint leakage
Joint leakage begins at the seam between body and bonnet. On a bolted design, the cause may involve gasket damage, loss of preload, uneven loading, flange distortion, corrosion or unsuitable sealing surfaces. On a pressure-seal design, the seal ring, contact surfaces and initial loading parts require a different inspection path.
Do not treat visible leakage as permission to tighten an operating valve. Temperature, trapped pressure, bolt condition and the original assembly method can make that action unsafe or can worsen the joint. Preserve the leak pattern and operating conditions, then use the site procedure and manufacturer instructions after isolation.
Bonnet or body pressure-boundary damage
Seepage from the wall, weld, threaded region or flange outside the normal seal line points to a pressure-boundary problem. Cracking, casting or forging defects, corrosion, erosion, wall loss, thermal fatigue and a poor previous repair are possible causes.
A new gasket or more bolt load cannot correct a crack or lost wall thickness. The valve must be isolated and the defect assessed with the inspection method, acceptance criteria and repair route required for that equipment. Surface appearance alone cannot show whether the part remains fit for pressure service.
Inspection, maintenance and repair boundaries
A generic bonnet guide can define what must be controlled, but it cannot supply one safe maintenance procedure for every valve. Energy isolation, fluid hazards, temperature, valve construction and site rules change the work method.
Isolate, relieve stored energy and verify depressurisation
Before loosening packing, bolting, a union nut, threaded bonnet or pressure-seal retaining parts, isolate the valve from every pressure and energy source. Drain or vent trapped fluid by the approved method, allow temperature to reach a safe range, and verify that isolation is effective. A closed upstream valve or a zero reading at one point does not by itself prove that the bonnet cavity is safe.
Preserve evidence before tightening or dismantling
Record the first visible leak point, process pressure and temperature, valve position, operating cycle and whether the leak changes during movement or thermal transition. Photograph deposits, stain paths, joint gaps and the as-found position of gland or body-bonnet fasteners when site rules allow.
This evidence separates packing leakage from joint leakage and may show whether thermal cycling, vibration or movement is involved. Cleaning, tightening or dismantling too early can erase the pattern and leave the real cause unresolved.
When repair needs an approved procedure or replacement decision
Consumable seals such as packing and gaskets may be replaced when the pressure-boundary parts and sealing surfaces remain within the manufacturer’s acceptance limits. Reassembly must use the specified parts, surface condition, alignment, lubrication and loading procedure for that valve.
Cracks, severe corrosion, wall loss, distorted flanges, damaged pressure-seal contact areas or an unqualified weld repair need engineering review. Machining, welding, heat treatment and non-destructive examination may be controlled by the applicable code, owner procedure and manufacturer limits. Generic abrasive lapping is not an acceptable default for a damaged body-bonnet sealing face.
Repair versus replacement should be based on structural condition, permitted repair route, material traceability, inspection results, future service and total outage risk. A fixed percentage of replacement cost cannot decide whether a pressure-boundary part is safe to return to service.
The bonnet decision is a pressure-boundary decision
A suitable bonnet combines the correct body joint, service geometry and stem-sealing arrangement for one valve and one operating envelope. Confirm those features on the sectional drawing and current product data, then keep maintenance inside the approved pressure-boundary procedure. For product-level construction options, review the MacoTango control valve series.