What is the difference between a pressure-seal bonnet Globe Valve and a bolted bonnet Globe Valve in terms of material requirements and pressure ratings?

The core difference is straightforward: a pressure-seal bonnet Globe Valve is designed for high-pressure, high-temperature service (typically ASME Class 600 and above), where internal pressure actually improves the seal, while a bolted bonnet Globe Valve is suited for moderate service conditions (Class 150 to 600) and relies on external bolt load to maintain joint integrity. Material requirements differ significantly between the two designs, driven by pressure ratings, thermal cycling, and mechanical stress.

How Each Bonnet Design Works

In a bolted bonnet Globe Valve, the bonnet is fastened to the valve body using a series of studs and nuts, with a gasket compressed between the two flanged faces. The sealing force is entirely dependent on bolt preload. As pressure or temperature rises, the gasket may relax or creep, requiring periodic re-torquing.

In a pressure-seal bonnet Globe Valve, the bonnet is inserted into the body from above and held in place by a retaining ring. A wedge-shaped, self-energizing seal ring sits between the bonnet and body. As internal pressure increases, it pushes the bonnet upward, forcing the seal ring tighter against the body bore — meaning higher pressure produces a better seal, not a weaker one.

Pressure Rating Comparison

Pressure rating is the most defining factor when choosing between these two bonnet types. The table below summarizes typical ASME pressure class coverage for each design:

For power plant main steam lines operating at Class 1500 or Class 2500, pressure-seal bonnet Globe Valves are essentially the only practical choice. Bolted bonnet designs at these classes would require impractically large and heavy flanges and bolt assemblies.

Material Requirements for Bolted Bonnet Globe Valves

Because bolted bonnet Globe Valves operate at lower pressures and temperatures, a wider range of body and bonnet materials is acceptable:

• Cast Carbon Steel (ASTM A216 WCB): The most common choice for general service up to 425°C and Class 300/600. Cost-effective and widely available.
• Cast Iron (ASTM A126 Class B): Used in low-pressure water, HVAC, and non-critical utility services at Class 125/250. Not suitable for thermal shock.
• Stainless Steel (ASTM A351 CF8M / CF3M): Selected for corrosive services such as chemical processing or offshore, applicable in Class 150 to 600.
• Bronze / Brass (ASTM B61/B62): Common for small-bore (≤2") low-pressure water and steam services in building utilities.
• Gasket Material: Spiral-wound stainless/graphite or PTFE-filled gaskets are standard. Gasket selection is critical, as it is the primary sealing element.

Stud bolt material is typically ASTM A193 B7 (chrome-moly alloy steel) with A194 2H heavy hex nuts — a pairing that provides sufficient clamping force for Class 150–600 service.

Material Requirements for Pressure-Seal Bonnet Globe Valves

Pressure-seal bonnet Globe Valves face far more demanding mechanical and thermal conditions, and material selection must reflect this:

• Body & Bonnet — Forged Steel (ASTM A105 / A182 F11 / F22): Forgings are preferred over castings at high pressures due to superior grain structure and mechanical strength. F11 (1.25Cr-0.5Mo) and F22 (2.25Cr-1Mo) are the standard choices for high-temperature steam service above 500°C.
• Body & Bonnet — Alloy Steel for Ultra-High Temp (ASTM A182 F91): For advanced power plants running above 565°C (1050°F), F91 (9Cr-1Mo-V) is specified. It offers creep resistance far superior to F22.
• Seal Ring Material: The pressure-seal ring is typically made of soft iron, low-carbon steel, or stainless steel. It must be softer than the body bore to deform and seat properly without galling. Stainless steel rings are used when corrosion is a concern.
• Trim Material (Seat & Disc): Stellite (Cobalt-Chromium) hard-facing is standard for high-pressure, high-temperature Globe Valve trim to resist erosion and wire-drawing under throttling conditions.
• Retaining Ring: Must withstand the full system pressure transferred from the bonnet. Typically manufactured from the same alloy steel as the body (e.g., F11, F22, or F91).

A critical point: because the pressure-seal design eliminates the need for a large bolted flange, the overall valve weight and body wall thickness can be reduced compared to an equivalent bolted bonnet design at Class 900 and above — despite handling higher pressures.

Side-by-Side Material and Design Comparison

Maintenance and In-Service Considerations

From a maintenance standpoint, bolted bonnet Globe Valves offer a significant advantage in accessibility. If a bonnet joint begins to weep, an operator can often re-torque the studs while the line is in service (within safe limits). Gasket replacement is straightforward and requires no special tooling.

Pressure-seal bonnet Globe Valves require the line to be depressurized and cooled before the retaining ring can be removed and the seal ring inspected or replaced. However, in high-pressure steam systems, this type of maintenance is already scheduled as part of planned shutdowns — and the reduced frequency of seal failures more than compensates for the more involved procedure.

One practical note: when reassembling a pressure-seal bonnet Globe Valve, the seal ring must always be replaced with a new one. Reusing a deformed seal ring is a common field error that leads to immediate leakage upon pressurization.

Which Design Should You Select?

Use the following as a practical guide:

• Choose a bolted bonnet Globe Valve for general industrial services at Class 150–600, where the medium is water, steam, oil, or non-aggressive chemicals, and where ease of maintenance is a priority.
• Choose a pressure-seal bonnet Globe Valve for Class 900 and above, especially in power generation main steam, boiler feed, and high-pressure gas injection services where leakage is unacceptable and temperatures exceed 450°C.
• If budget is a constraint and the service is Class 600, a bolted bonnet design with a ring-joint (RTJ) gasket and proper stud material is a cost-effective alternative — but verify that bolt relaxation under thermal cycling is acceptable for your specific application.
• Always match the body material to the service: F91 forged steel for ultra-supercritical steam; F22 for standard high-pressure steam; CF8M stainless casting for corrosive chemical service in bolted bonnet designs.

Understanding the interplay between bonnet design, material grade, and pressure class is essential for specifying a Globe Valve that delivers long-term reliability. The pressure-seal design is not simply a "stronger" version of the bolted bonnet — it is a fundamentally different engineering approach, purpose-built for the most demanding services in modern industrial infrastructure.