Pressure Seal Bonnet Gate Valves: Class 900–2500 Selection Guide | API 600 | Vajra

What Is a Pressure Seal Bonnet?

In a bolted bonnet gate valve, the bonnet is held against the body by external fasteners (bolts and nuts) — the joint is sealed by a gasket compressed by bolt pre-load. At very high pressures (above approximately 1,500–2,000 psi / 100–140 bar), the bolt forces required to resist the pressure thrust on the bonnet become enormous, requiring very large and heavy bolts and flanges. A pressure seal bonnet uses the opposite principle: the internal pressure acts to push the bonnet seal ring harder against the body seat, so the sealing force automatically increases with pressure — the valve becomes harder to leak as pressure increases, rather than harder to seal.

How Does a Pressure Seal Work?

The pressure seal bonnet uses a metallic ring (the pressure seal ring) placed in the annular space between the bonnet and body bore. The ring material is slightly softer than the body (typically soft iron, Incoloy 825, or graphite composite). When internal pressure acts on the bonnet, it drives the bonnet upward against the ring, plastically deforming the ring into the body bore — creating a pressure-energised metal-to-metal seal. The seal tightens as pressure increases. The bonnet is retained in the body bore by a bolted retaining ring that resists the upward pressure thrust — these bolts carry much lower loads than bolted-bonnet fasteners because they only hold the bonnet in position, not provide gasket seating force. This fundamental inversion — internal pressure working for the seal, not against it — makes pressure seal the preferred design for all high-pressure applications above Class 600.

API 600 Requirements for Pressure Seal

• Pressure seal design is standard practice above Class 600 per API 600 — although API 600 technically permits bolted bonnet at Class 900, most manufacturer design standards and engineering specifications specify pressure seal at Class 900 and above.
• Pressure seal ring must be replaced when the valve is disassembled for maintenance — the ring is plastically deformed during initial pressurisation and cannot be reused.
• Pressure seal rings are manufactured in soft iron (standard for carbon steel valves), Incoloy 825 (for alloy steel and stainless steel valves), or spiral-wound graphite with metallic filler.
• Face-to-face dimensions per ASME B16.10 are different for pressure seal vs bolted bonnet valves — pressure seal valves are typically shorter and lighter than equivalent bolted bonnet designs at the same class.
• Hydrostatic shell test pressure per API 598 is 1.5× MAWP at the specified testing temperature — pressure seal design must hold this test without bonnet distortion.

Pressure Seal vs Bolted Bonnet — When to Use Each

Materials for Pressure Seal Bonnet Gate Valves

• WCB (carbon steel cast) — Class 900–1500 up to 480°C; standard for general process service and lower-temperature HP steam.
• WC6 (1.25% Cr-0.5% Mo alloy steel) — Class 900–1500 up to 565°C; standard for power plant high-pressure steam service (main steam and hot reheat).
• WC9 (2.25% Cr-1% Mo alloy steel) — Class 1500–2500 up to 593°C; used for main steam and high-temperature refinery service (hydrocracker, delayed coker).
• F91/P91 (9Cr-1Mo-V forged) — Class 2500; ultra-supercritical steam applications above 600°C where WC9 creep strength is insufficient.
• F316 / F316L (austenitic stainless steel forged) — Class 900–1500; used for high-pressure hydrogen, chlorine, and corrosive service where carbon steel is unsuitable.

Applications of Pressure Seal Gate Valves

• Main steam isolation valves (MSIV) — Class 900–2500 at power stations; typically butt-weld ends to match P91 or P22 steam pipe.
• Boiler feedwater isolation — Class 1500 at high-pressure drum boilers; WC6 or WC9 body with 13Cr hardened seats.
• HP turbine bypass isolation and throttling — Class 900–2500; requires stellite-faced seats for velocity throttling resistance.
• Hydrogen service isolation — Class 900–2500; hydrogen's low molecular weight requires pressure seal's metal-to-metal sealing to prevent hydrogen permeation through standard gaskets.
• High-pressure refinery service — Class 1500–2500 for hydrocracker feed/effluent circuits, decoking isolation on delayed coker units.
• Ultra-supercritical power plants (USC/AUSC) — Class 2500 with P91/P92 material; steam conditions above 600°C and 300 bar.

Ordering a Pressure Seal Gate Valve — Key Specifications

• Class rating per ASME B16.34 (Class 900, 1500, or 2500).
• Body material and grade (WCB, WC6, WC9, A217 C12A, F91).
• End connection — butt-weld per ASME B16.25 (most common for HP steam and refinery) or RTJ flanged per ASME B16.5.
• Trim material — 13Cr for standard steam service; SS 316 for corrosive service; Stellite 6 hard-faced seats for high-velocity and erosive steam.
• Pressure seal ring material — soft iron (standard for CS valves), Incoloy 825 (for SS and alloy valves).
• Face-to-face per ASME B16.10 — specify 'pressure seal type' to avoid being sent bolted bonnet dimensions.
• Testing — API 598 or IBR hydrostatic shell and seat test; specify if EN 10204 3.1 or 3.2 material certificates are required.