What Is a Trunnion Mounted Ball Valve?
A trunnion mounted ball valve is a quarter-turn rotary valve in which the ball is mechanically anchored by upper and lower trunnion shafts to reduce operating torque and distribute pressure loads across the valve structure. It is designed for high-pressure and large-diameter applications where floating ball designs would generate excessive torque and seat loading. Trunnion mounted ball valves provide reliable pressure-assisted sealing and stable structural performance under the most demanding pipeline service conditions, and represent the standard high-pressure ball valve configuration within the industrial valve types overview.
Key Takeaways
- The ball is supported by upper and lower trunnions — mechanical anchoring of the ball prevents axial displacement under pressure, eliminating the high seat contact force that makes large floating ball valves impractical to operate above NPS 6 at Class 600 and above.
- Seat rings provide sealing by moving toward the fixed ball — spring-loaded and pressure-assisted seats maintain consistent sealing contact stress against the stationary ball surface throughout the full range of operating pressures, from low-pressure startup conditions to maximum rated pressure.
- It is suitable for high-pressure and large-diameter pipelines — trunnion mounted designs per API 6D are the standard ball valve type for NPS 6 and above at Class 300 and higher, and for all sizes at Class 900 through Class 2500 in oil and gas transmission, refinery, and offshore service.
- It offers lower operating torque compared to floating ball valves — because line pressure acts on the seats rather than pushing the ball against the downstream seat, the operating torque of a trunnion mounted valve is substantially lower than a floating ball valve at equivalent bore size and differential pressure.
How It Works
Anchored Ball Support Mechanism
A trunnion mounted ball valve controls flow through the same quarter-turn rotational principle as all ball valve types — rotating the spherical ball 90 degrees aligns or blocks its through-bore with the pipeline. The critical structural distinction from floating designs is that the ball is mechanically anchored within the valve body by two trunnion shafts — the upper trunnion is integral with or connected to the operating stem, and the lower trunnion seats in a bearing in the body bottom. These two coaxial trunnions constrain the ball against axial displacement in any direction regardless of the differential pressure applied across the valve. In a floating ball valve, line pressure pushes the ball downstream against the downstream seat to generate the sealing contact stress — a mechanism that works effectively in small bore sizes and moderate pressure classes but produces enormous seat contact forces and corresponding operating torque as nominal size and pressure class increase. In the trunnion mounted design, the ball cannot shift downstream because the trunnions prevent axial movement — the mechanical loads from differential pressure are transmitted directly to the trunnion bearings and thence to the body structure, bypassing the seats entirely. The complete ball valve design principles that provide context for understanding both floating and trunnion-mounted configurations are addressed in the what is a ball valve reference.
Pressure-Assisted Seating Principle
With the ball fixed in position by the trunnions, sealing in a trunnion mounted ball valve is achieved by moving the seats toward the ball — the inverse of the floating ball mechanism. Each seat ring is mounted in a carrier that is free to move axially within the body, loaded toward the ball by a combination of spring force and line pressure acting on the seat carrier’s back face. This pressure-assisted seating mechanism produces a self-energizing sealing effect — as line pressure increases, the force pushing the seat against the ball surface increases proportionally, increasing the sealing contact stress precisely when it is most needed. At very low pressures near zero, the spring force alone maintains initial sealing contact, ensuring the valve seals reliably from the first moments of pressurization. This dual-mechanism seating — spring-preloaded at low pressure, pressure-assisted at operating pressure — is the source of the trunnion mounted valve’s reliable sealing performance across the full pressure range from atmospheric to maximum rated pressure. The design contrast with the floating ball valve’s downstream-pressure seating mechanism is examined in detail in the floating ball valve reference.
Main Components
Ball and Trunnion Assembly
The ball is a precision-machined sphere with a cylindrical through-bore whose diameter — in full-port designs equal to the pipeline bore, in reduced-port designs smaller — defines the valve’s Cv and pressure drop. The ball surface finish in the sealing zone is held to 0.4 µm Ra or better per API 6D to ensure uniform seat contact across the full sealing circumference. The upper trunnion is integral with the stem in most designs — the stem passes through the bonnet with anti-blowout shoulder design that prevents ejection under line pressure, a mandatory safety feature per API 6D for all pressure classes. The lower trunnion seats in a thrust bearing in the body bottom that carries the combined axial load of differential pressure and ball weight. Trunnion bearing materials — typically filled PTFE, bronze, or stainless steel depending on load and temperature — are selected for the combination of bearing pressure and surface speed at the design operating torque. Body construction options include split-body (two-piece), three-piece (center section removable for in-place ball and seat inspection), and top-entry designs (body remains in line while all internals are accessed from above) — top-entry construction is the standard for subsea and buried pipeline service where in-place maintenance without line break is mandatory.
Seat Design and Sealing System
The seat ring assembly is the engineering heart of the trunnion mounted ball valve’s sealing performance. Each seat ring is a annular element with a sealing face machined to match the ball surface geometry — flat or contoured depending on the design — that is loaded toward the ball by a coil spring behind the seat carrier. In soft-seated designs, the seat face is PTFE or reinforced PTFE (glass-filled, carbon-filled, or metal-reinforced) — providing Class VI bubble-tight shutoff in clean service up to approximately 200°C. In metal-seated designs, the seat face carries a hard-face overlay — Stellite, tungsten carbide, or electroless nickel — that provides sealing integrity at temperatures above PTFE’s limit, in abrasive or erosive service, and in fire-safe applications requiring metal-to-metal seating at Class IV or V after a fire event per API 607. Advanced trunnion mounted ball valves for critical service include double block and bleed (DBB) capability — both upstream and downstream seats simultaneously provide independent pressure isolation, with a bleed port in the body cavity allowing the trapped cavity pressure to be vented, monitored, or tested for seat integrity verification without breaking the pipeline connection. Emergency sealant injection ports allow viscous sealant to be injected under pressure into the seat area through external fittings if seat sealing is compromised in service. The bore configuration selection criteria between full-port and reduced-port designs — which determines the valve’s Cv and fully-open pressure drop at the design flow rate — are addressed in the full port vs reduced port valve reference, which returns to the industrial valve types overview.
Advantages
High-Pressure and Large-Diameter Performance
The trunnion mounted design’s structural and torque advantages over floating ball designs become decisive as nominal size and pressure class increase. At NPS 8 Class 600, the differential pressure force on a full-bore ball approaches 200 kN — a load that would generate enormous seat contact stress and operating torque in a floating design but is carried entirely by the trunnion bearings in a trunnion mounted design, leaving seat contact stress determined solely by the spring preload and a fraction of the line pressure. This torque reduction enables direct actuation by standard pneumatic or electric actuators without requiring oversized actuation packages that would be necessary for equivalent floating ball designs. The double block and bleed capability of trunnion mounted designs provides a safety and maintenance advantage unavailable in floating ball valves and most other valve types — the ability to verify seat integrity in-service and safely isolate cavity pressure for maintenance without removing the valve from the line is a mandatory requirement in many oil and gas pipeline codes and safety management systems. For the comparison between trunnion ball valves and gate valves in high-pressure large-diameter isolation service, refer to ball vs gate valve design differences. For the comparison between ball valves and butterfly valves across the quarter-turn isolation valve application space, refer to butterfly vs ball valve. Both are classified within the industrial valve types overview.
Typical Applications
Transmission and Severe Service Systems
Trunnion mounted ball valves per API 6D are the standard isolation valve for high-pressure oil and gas transmission pipelines — crude oil, natural gas, and refined product lines at Class 600 through Class 2500, NPS 4 through NPS 60, where reliable shutoff at design pressure and direct actuation compatibility are mandatory requirements. In offshore and subsea service, trunnion mounted ball valves with subsea-rated body designs, corrosion-resistant alloy trim, and ROV (remotely operated vehicle) or hydraulic actuator interfaces provide reliable isolation at water depths to 3,000 meters where maintenance access is severely constrained and valve reliability is a safety-critical requirement. In refinery and petrochemical process units, trunnion mounted ball valves in Class 300 through Class 1500 with fire-safe metal seats per API 607 and API 6FA provide reliable process unit block valve service — their compact face-to-face dimensions per API 6D and direct actuator compatibility are advantages over gate valves in automated emergency isolation service. In power generation fuel gas and high-pressure auxiliary service, trunnion mounted ball valves provide fast automated isolation with lower torque requirements than equivalent gate valve gear-operated designs.
Cryogenic and High-Pressure Extreme Service
Trunnion mounted ball valves are engineered for both extreme pressure and extreme temperature service conditions that represent the boundaries of standard industrial valve design. At Class 1500 and 2500, forged body trunnion mounted ball valves with pressure-seal end connections, metal-to-metal hard-faced seats, and anti-static stem design per API 6D provide reliable isolation for the highest-pressure hydrocarbon service — full design requirements are addressed in the what is a high-pressure valve reference. For LNG liquefaction, LNG shipping terminal, and industrial gas service at temperatures to −196°C, cryogenic trunnion mounted ball valves with extended bonnets that keep the stem packing above the cold zone, low-temperature-qualified body materials per ASME B31.3 chapter VIII, and PTFE or metal seats qualified for cryogenic dimensional stability provide reliable quarter-turn isolation at temperatures that would cause standard soft-seat materials to become brittle and non-sealing — full cryogenic service requirements are addressed in the what is a cryogenic valve reference. Both extreme-service categories are classified within the industrial valve types overview.
Frequently Asked Questions
What is the difference between a trunnion mounted and a floating ball valve?
A trunnion mounted ball valve anchors the ball with upper and lower trunnion shafts, preventing axial displacement under pressure — the seats move toward the fixed ball for sealing. A floating ball valve allows the ball to shift slightly downstream under line pressure, pressing against the downstream seat to generate sealing contact stress. Trunnion designs are required above NPS 6 at Class 600 and above, where floating designs would generate torques too high for practical actuation. Floating designs are appropriate and cost-effective for NPS 4 and below at Class 600 and lower.
Why are trunnion mounted ball valves used in high-pressure systems?
At high pressure classes and large nominal sizes, the differential pressure force on the ball in a floating design would generate seat contact stresses and operating torques that exceed the capability of standard actuation systems. The trunnion mounted design transfers these pressure loads to the trunnion bearings and body structure rather than to the seats, keeping seat contact stress at the design value determined by the spring preload and a controlled fraction of line pressure. This maintains manageable operating torque at all pressure and size combinations covered by API 6D, enabling direct actuation without oversized actuator packages.
What is double block and bleed in a trunnion ball valve?
Double block and bleed (DBB) refers to the trunnion mounted ball valve’s ability to simultaneously provide independent pressure isolation at both the upstream and downstream seats — both seats seal against their respective line pressures at the same time — while a bleed port in the body cavity allows the trapped pressure between the two seals to be vented, monitored, or tested independently. This configuration provides positive verification that both seats are sealing before maintenance work is performed on downstream equipment, and satisfies the pipeline isolation requirements of safety management systems that mandate independent verification of isolation integrity before breaking containment.
Are trunnion mounted ball valves suitable for automation?
Yes — the trunnion mounted design’s inherently lower operating torque compared to equivalent floating ball designs makes it directly compatible with standard pneumatic rack-and-pinion, scotch yoke, and electric quarter-turn actuators without requiring oversized actuation packages. Anti-static stem design, position feedback transmitters, and solenoid valve accessories for fail-safe venting integrate the trunnion mounted ball valve into DCS and safety instrumented system architectures for automated emergency shutdown and remote-operated block valve service.
Conclusion
A trunnion mounted ball valve is the standard high-pressure, large-diameter ball valve design for oil and gas transmission, refinery, offshore, and severe service applications — its anchored ball structure, pressure-assisted seating mechanism, and double block and bleed capability address the specific performance requirements of high-pressure pipeline service that floating ball designs cannot meet at large bore sizes and elevated pressure classes. Correct specification requires confirming the nominal size and pressure class combination against API 6D design requirements, selecting the seat material (soft or metal-seated) for the operating temperature, fluid chemistry, and fire-safe requirements, specifying the body configuration (split, three-piece, or top-entry) for the installation and maintenance access requirements, and sizing the actuator to the maximum operating torque at design differential pressure with the required safety factor. Engineers requiring a comprehensive framework that integrates trunnion ball valve selection with floating ball valve comparison, bore configuration, and the full industrial valve classification should consult the industrial valve types overview as the governing reference for all ball valve engineering decisions.