As a cover over the valve body, the valve bonnet plays a vital role in the operation of various valves. It withstands the pressure of the medium and, together with the valve body, forms a pressure boundary. It also protects internal components from the external environment and provides a mounting base for the actuator or handwheel. Low-temperature valve bonnets often feature extended bonnets to isolate ambient heat and prevent seal failure due to extreme temperature differences.
This demonstrates the crucial role of the valve bonnet. Some may have questions about the materials used to make the valve bonnet, which valve bonnet is suitable for them, and what to do if the valve bonnet malfunctions. Let’s explore these issues.
Materials for Making Valve Bonnets
| Category | Standard | Applicable Conditions | Key Features |
| Ductile iron | QT450, QT40015, QT5007, etc. (GB/T 12226) | PN≤4.0 MPa, 30～350℃, water, steam, oil | High strength, corrosion resistance, easy processing |
| Carbon steel castings | WCB、WCC（GB/T 12229，ASTM A216） | PN≤32 MPa, 29～425℃, medium and high pressure steam, petroleum | High strength and good weldability |
| Low-temperature carbon steel | LCB、LC1（ASTM A352/GB/T 7248） | PN≤6.4 MPa, as low as 196°C, ethylene, liquid nitrogen | Good low temperature toughness |
| Stainless steel castings | CF8（304）、CF3（304L）、CF8M（316）（ASTM A351） | 196～600℃, corrosive media | Corrosion-resistant and high-temperature-resistant |
| Alloy steel forgings | F11、F22（ASTM A182） | 550～650℃, high temperature and high pressure steam | High temperature strength, anti-oxidation |
Tips for selecting bonnet materials
Pressure and temperature rating: Carbon steel or alloy steel is recommended for high pressure and high temperature applications; ductile iron is recommended for low pressure and normal temperature applications.
Media compatibility: Stainless steel or duplex steel is preferred for corrosive media; low-temperature carbon steel is recommended for low-temperature applications.
Manufacturing standards:
National standards: GB/T 12226 (ductile iron), GB/T 12229 (carbon steel castings)
American standards: ASTM A352 (low-temperature steel), ASTM A351 (stainless steel)
Process requirements: Castings must undergo nondestructive testing (such as radiographic testing); forgings must meet grain size and heat treatment requirements.
Special operating conditions supplement
Cryogenic valves: The bonnet requires cryogenic treatment throughout, and the material should be LCB or LC3.
Nuclear power plants: The bonnet must meet ASME BPVC specifications and use ultra-low carbon stainless steel (such as CF3).
How to Choose the Right Bonnet
I. Identify Media Characteristics
1. Corrosiveness
• For highly corrosive media (such as acids, alkalis, and salt solutions): Choose corrosion-resistant alloys (such as Hastelloy and Monel) or lining materials (such as PTFE and rubber).
• For weakly corrosive media (such as water and steam): Choose carbon steel or stainless steel (such as 304 and 316L).
• For oxidizing media (such as high-temperature oxygen): Use oxidation-resistant materials (such as Inconel 625).
2. Temperature and Pressure
• For low-temperature applications (below -196°C): Use a long-neck bonnet (neck length ≥ 250mm) with low-temperature steel (such as LCB and LCC) to prevent packing from freezing.
• For high-temperature applications (>500°C): Choose heat-resistant steel (such as F91 and F92) or high-temperature alloys, and design a heat dissipation structure (such as heat sinks).
• For high-pressure applications (>100 MPa): Use a self-sealing bonnet or a preload design to ensure sealing reliability.
3. Media Conditions
• Gas/Steam: Flow rate must be controlled to prevent cavitation, and the bonnet design must minimize pressure loss.
• Liquid: Pay attention to flashing conditions to avoid liquid accumulation within the bonnet cavity.
• Solids: Use carbide seats or scraper seals to prevent particle jamming.
II. Analyzing Operating Conditions
1. Installation Environment
• Buried/Deep Well Installation: Use an extended bonnet to accommodate the need for a cold insulation layer or soil cover.
• Flood-Prone Areas: The bonnet must have a waterproof design (e.g., drainage grooves, sealing structure).
• Hazardous Environments (Explosion, Radiation): Comply with explosion-proof standards (e.g., Ex dⅡCT4) or nuclear grade requirements (RCC-M).
2. Operating Frequency
• High-Frequency Operation: Prefer wear-resistant materials (e.g., Stellite overlay) and low-friction sealing structures.
• Long-Term Stationary Operation: Pay attention to the aging resistance of sealing materials (e.g., graphite packing).
3. Matching Valve Types
• For high sealing performance, choose a bellows-sealed bonnet (zero leakage) or a dual-seal structure (bellows + packing).
• Low-temperature globe valves require a long neck design, while high-temperature globe valves require a heat dissipation structure.
Ball Valve
• Soft-seal ball valve: The bonnet must fit precisely with the valve body to prevent media from invading the sealing surface.
• Hard-seal ball valve: The bonnet must withstand high pressures, requiring high material strength.
• For precise flow control, the bonnet design must minimize dead zones to prevent media from remaining.
• High-pressure control valve: Use a self-sealing bonnet or preload design.
4. Determine Sealing Requirements
Zero Leakage Requirements
• Bellows-sealed bonnet: Suitable for highly toxic, corrosive, or high-vacuum media.
• Dual-seal structure: Bellows + packing provides dual protection.
General Sealing Requirements
•Packing-sealed bonnet: Requires regular packing replacement, but is less expensive.
• Metal-sealed bonnet: Suitable for high-temperature and high-pressure conditions, with a long service life.
What to do if a bonnet fails
A bonnet failure is a serious issue, as it can cause leakage of media (water, oil, gas, chemicals, etc.), leading to equipment downtime, reduced efficiency, environmental pollution, and even safety accidents. If a bonnet failure is discovered, the most important thing is to ensure your own safety and the safety of your colleagues. Secondly, contact a professional. Do not attempt to resolve the issue yourself blindly; personal safety is paramount.
Failure Types
The bonnet is the “neck” of the valve, connecting the valve body to the operating mechanism. Common failures include leaks in the bonnet flange, leaks in the stuffing box, and cracks or pinholes in the bonnet body.
Bonnet flange leakage: This is the most common failure. It is caused by aging or damage to gaskets, loose bolts, or corrosion or damage to the flange sealing surface.
Stuffing box leakage: Leaks at the point where the valve stem exits the bonnet (stuffing box). Causes include worn, aging, or drying of the packing (packing); loose packing gland bolts; or scratches or corrosion on the valve stem surface.
Cracks or pinholes in the bonnet body: This is due to a casting defect, long-term corrosion, or stress fatigue.
Troubleshooting Methods
Bonnet Flange Leakage Treatment:
1. Tighten the bolts (applicable only to minor leaks and when system pressure has been relieved). Use the correct tools to tighten the bolts evenly and diagonally. Be careful not to overtighten, as this may break the bolts or crush the gasket.
2. Replace the gasket: Isolate and bleed the upstream and downstream pressures of the valve. Remove the bonnet bolts.
Note: The disc of some valves (such as gate valves) may become detached. Before disassembly, ensure that the valve is fully open or fully closed to avoid problems.
Stuffing Box Leakage Treatment:
1. For minor leaks, try evenly tightening the bolts on both sides of the packing gland (commonly known as the gland). Tighten approximately a quarter turn at a time and observe whether the leak stops. Overtightening can cause the valve stem to seize or increase wear.
2. Add or Replace the Packing: Isolate and relieve system pressure. Loosen the gland bolts, remove all old packing rings, and clean them. Check the valve stem for a smooth surface. If there are any burrs, smooth them with fine sandpaper. Cut new packing rings to a length that wraps around the valve stem, with the cuts at a 45° angle. Place the packing rings into the stuffing box one layer at a time, staggering the cuts and gently pressing them firmly in place with a special tool. Reinstall the gland and tighten evenly, ensuring the valve stem can still rotate freely.
Treatment of Valve Bonnet Body Damage
If the valve bonnet has cracks, pinholes, or severe corrosion, it is not worth repairing and the entire valve or bonnet assembly must be replaced. Welding repairs are generally not recommended unless performed by the original manufacturer’s experts using specialized techniques, as they pose significant safety risks.
In short, the core principles for addressing valve bonnet failures are: safety first, accurate judgment, appropriate action, and professionalism.
Summary
The above is a sharing of knowledge about valve bonnets. If you are interested in purchasing valves, please contact us. Newton is a good choice. Of course, if you purchase valves from Newton, we provide professional after-sales service to ensure your experience.
