Gate valves are the workhorses of isolation service in industrial pipelines — used wherever you need fully open or fully closed flow with minimum pressure drop. But not all gate valves are built the same. The two primary design variants — bolted bonnet and pressure seal bonnet — serve very different pressure and temperature ranges, and choosing the wrong type leads to premature failure or unnecessary cost.
This guide explains both designs, how they differ structurally, and when each is the right choice.
1. Gate Valve Basics
A gate valve uses a sliding wedge or parallel disc (the "gate") that moves perpendicular to the flow path. In the open position, the gate is fully retracted into the bonnet — leaving an unobstructed full-bore flow path with near-zero pressure drop. In the closed position, the gate seats against two seat rings to block flow completely.
Gate valves are designed for on/off isolation service only — not for throttling. Partial opening causes vibration and seat erosion.
2. Bolted Bonnet Gate Valves
Design
In a bolted bonnet gate valve, the bonnet is attached to the body using a ring of flange bolts (studs + nuts). A spiral-wound or ring-joint gasket provides the body-to-bonnet seal. The bonnet contains the stem, packing gland, and yoke.
- Standard design per API 600 and API 602
- Pressure ratings: Class 150# to Class 600# (PN 20 to PN 100)
- Temperature ratings: up to ~540°C depending on material
- Most economical for the majority of industrial applications
- Bonnet gasket is accessible for maintenance and replacement
When to Use Bolted Bonnet
- General isolation in refineries, chemical plants, and water treatment
- Steam and hydrocarbon service up to Class 600#
- Where regular bonnet maintenance or gasket replacement is anticipated
- Cost-sensitive projects — bolted bonnet is significantly cheaper per unit
3. Pressure Seal Bonnet Gate Valves
Design
The pressure seal bonnet uses a fundamentally different sealing mechanism. Instead of external bolts, the bonnet is held in place by a retaining ring or lock ring, and the seal between bonnet and body is achieved by a self-energizing wedge-shaped pressure seal ring. As system pressure increases, it forces the seal ring outward against the bonnet bore — creating a tighter seal at higher pressure.
- Standard per API 600 (high-pressure variants) and ASME B16.34
- Pressure ratings: Class 900# to Class 2500# (PN 150 to PN 420)
- Temperature ratings: up to 600°C
- Lighter and more compact than equivalent bolted bonnet at high pressure
- Self-energizing seal means performance improves with pressure — not degrades
When to Use Pressure Seal Bonnet
- High-pressure steam service in thermal and nuclear power plants
- High-pressure hydrocarbon pipelines (Class 900# and above)
- Refinery applications where Class 1500# or 2500# is specified
- Where weight is critical — pressure seal bonnet valves are lighter than bolted bonnet at equivalent high pressure
4. Side-by-Side Comparison
| Parameter | Bolted Bonnet | Pressure Seal Bonnet |
|---|---|---|
| Applicable Standard | API 600, API 602 | API 600 (HP), ASME B16.34 |
| Pressure Class | Class 150# to 600# | Class 900# to 2500# |
| Seal Mechanism | Gasket + bolts (external compression) | Self-energizing seal ring (pressure-assisted) |
| Sealing reliability at HP | Degrades at very high pressure | Improves with pressure |
| Weight (at Class 900+) | Heavier | Lighter |
| Maintenance | External — easy gasket access | Internal — requires depressurizing |
| Cost | Lower | Higher |
| Typical applications | Refineries, chemical, water, steam ≤ Class 600 | Power plants, HP pipelines, refineries ≥ Class 900 |
5. Body Materials for Gate Valves
| Material | Grade | Application |
|---|---|---|
| Carbon Steel | WCB / A216 | General service, hydrocarbons, steam up to 425°C |
| Alloy Steel (Cr-Mo) | WC6, WC9 | High-temperature steam, hydrogen service |
| Stainless Steel | CF8M (SS316), CF8 (SS304) | Corrosive media, chemical service |
| Low Temp. Carbon Steel | LCB, LCC | Cryogenic and sub-zero service |
| Duplex SS | CD3MN | Highly corrosive offshore service |
6. IBR Requirement for Gate Valves
In India, gate valves used in boiler, steam piping, and pressure vessels governed by the Indian Boiler Regulations (IBR) must be sourced from IBR-approved manufacturers. Novel Valves holds IBR approval covering gate valves in applicable sizes and pressure classes.
Need gate valves for your plant or pipeline project? Novel Valves manufactures API 600, API 602, and IBR-approved bolted bonnet and pressure seal bonnet gate valves in carbon steel, alloy steel, and stainless steel.
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