Valve Comparison Guide
Bolted Bonnet vs Pressure-Seal Bonnet Gate and Globe Valves
Bolted bonnet vs pressure seal bonnet comparison for gate and globe valves. Pressure class, temperature, maintenance, and when to use each in power plant and refinery service.
Overview
A bolted bonnet valve uses flange bolts to clamp the bonnet to the valve body, with a spiral-wound, soft, or ring-type joint (RTJ) gasket providing the seal at the body-bonnet interface. The standard bonnet design for Class 150 through Class 900 (occasionally Class 1500) gate and globe valves. Widely used in process plant, power generation, and pipelines.
DN15–DN600 | Class 150–900 | WCB, WC6, F316 | API 600, ASME B16.34
A pressure-seal bonnet uses the process pressure itself to create the body-bonnet seal. The bonnet has an internal ring and seating surface that is forced tighter by increasing process pressure — at higher pressure the seal is tighter, not looser (as in a bolted bonnet where bolt load relaxation can occur). Standard for Class 900 and above (Class 1500, 2500) gate and globe valves in high-pressure steam and critical process service.
DN25–DN500 | Class 900–2500 | WC6, WC9, P91, F22 | ASME B16.34, ASME B31.1
Pros & Cons
Bolted Bonnet
Pressure-Seal Bonnet
Bolted Bonnet vs Pressure-Seal Bonnet — Specification Comparison
| Parameter | Bolted Bonnet | Pressure-Seal Bonnet |
|---|---|---|
| Pressure Class Range | Class 150–900 (occasionally Class 1500 at small sizes) | Class 900–2500 (the standard above Class 600) |
| Sealing Mechanism | External bolt preload + gasket (spiral-wound or RTJ) | Process pressure self-energised (pressure-seal ring) |
| Weight at Class 1500 | Very heavy — large bolt flange required | Lighter — compact self-energised design |
| High-Temperature Reliability | Bolt creep can cause relaxation above 400°C | Seal improves with temperature/pressure cycling |
| Maintenance Access | Simple — unbolt and remove bonnet | More complex — pressure ring must be carefully handled |
| Cost | Lower at Class 150–900 | Higher first cost but standard for Class 1500+ |
| Applicable Standard | API 600, API 603, ASME B16.34, MSS SP-45 | ASME B16.34, ASME B31.1, API 600 (pressure-seal option) |
| Typical Industries | Process plant, oil & gas pipelines, general steam | Power plant steam, HP hydrogen service, refinery high-pressure |
When to Use Each
Use Bolted Bonnet when:
Use Pressure-Seal Bonnet when:
Decision Guide
Choose bolted bonnet when: (1) the pressure class is Class 150 through Class 600 — pressure-seal design offers no advantage at these classes; (2) Class 900 service where budget is a priority and bolted bonnet is acceptable per the piping specification; (3) field maintenance simplicity is paramount — bolted bonnets are removed and repaired by virtually any experienced valve technician without specialist tools; (4) replacement gaskets and bolts must be sourced quickly from standard stock. Choose pressure-seal bonnet when: (1) the pressure class is Class 1500 or Class 2500 — pressure-seal is the industry standard for these classes in gate and globe valves and provides a lighter, more reliable design than a bolted bonnet at equivalent class; (2) Class 900 in high-temperature steam service where bolt creep and thermal cycling are concerns; (3) the operating standard (ASME B31.1, ASME B31.3 Category M, refinery specification) specifies pressure-seal for the applicable service conditions. As a rule of thumb: bolted bonnet up to Class 900; pressure-seal from Class 900 and above. Most engineering companies and piping specifications switch to pressure-seal bonnet at Class 1500.
Frequently Asked Questions
What is a pressure-seal bonnet and how does it work?
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