What Is a Trunnion Mounted Ball Valve?

Direct Answer

A trunnion mounted ball valve is a quarter-turn valve in which the ball is mechanically anchored by upper and lower trunnions, allowing it to rotate while being supported on fixed bearings. This design reduces operating torque and enables reliable sealing under high-pressure and large-diameter service conditions.

Key Takeaways

The ball is mechanically supported by upper and lower trunnion bearings, preventing axial displacement under differential pressure.
Sealing is pressure-energized — line pressure acts on spring-loaded seats that move toward the stationary ball rather than pushing the ball against fixed seats.
Trunnion support absorbs pressure loading on the ball, significantly reducing operating torque compared to floating ball designs.
Suitable for high-pressure Class 600 and above service and large-diameter pipeline applications where floating ball designs are impractical.
Commonly manufactured to API 6D, ASME B16.34, and ISO 14313 standards.

How It Works

Definition of Trunnion Mounted Ball Valve

In a trunnion mounted ball valve, the ball is fixed in position by mechanical supports called trunnions located at the top and bottom of the ball body. The ball does not shift axially when pressure is applied. Instead, it rotates about a fixed axis supported by bearings integrated into the valve body, with the stem connecting the ball to the actuation interface above.

As part of the broader framework of valve terminology, the trunnion mounted design is distinguished from the floating ball design by its structural support mechanism and seat loading principle. Engineers referencing the valve terminology guide use this classification when specifying ball valves for high-pressure or large-diameter service where floating ball geometry becomes mechanically impractical. Port configuration — whether full port or reduced port — is specified independently and applies to trunnion mounted designs across all nominal sizes and pressure classes.

Structural Design and Sealing Mechanism

When the valve stem is rotated 90 degrees by manual or automated means, the ball rotates about the trunnion axis. In the open position, the ball bore aligns with the pipeline flow path. In the closed position, the bore is perpendicular to flow and the ball body blocks the passage. Throughout this rotation, the ball remains fixed in its axial position, supported by trunnion bearings that absorb lateral and axial loads from system pressure.

Sealing is achieved through pressure-energized seats. Spring-loaded seat assemblies maintain initial contact with the ball surface. When line pressure is applied to the closed valve, the upstream pressure acts on the seat face and pushes the seat more firmly against the ball, increasing contact stress and enhancing shutoff performance. Some designs incorporate a double piston effect seat that uses both upstream and downstream pressure differentials to energize sealing from both sides.

Seat design determines achievable leakage performance. Soft-seated trunnion valves with PTFE or polymer inserts can achieve seat leakage class VI, equivalent to bubble tight performance and functionally equivalent to zero leakage under standard test conditions. Metal-seated trunnion valves achieve Class IV or V leakage and are specified for high-temperature, abrasive, or erosive service where soft seat materials would degrade. For hydrocarbon and hazardous fluid service, trunnion mounted ball valves are commonly supplied as fire safe valves, incorporating secondary metal-to-metal seats and graphite stem packing to maintain controlled leakage if primary soft seats are destroyed by fire.

Pressure and Torque Considerations

Trunnion mounted ball valves are the standard design for high-pressure service. Because the trunnion bearings absorb the pressure load acting on the ball, the force transmitted to the seat surfaces is significantly lower than in a floating ball design where the entire pressure load is transferred through the ball to the downstream seat. This structural load path reduction allows trunnion valves to operate reliably across the full range of valve pressure classes from Class 150 through Class 2500 without excessive seat wear or seat deformation.

Understanding the relationship between pressure rating vs design pressure is essential when specifying trunnion mounted valves for high-pressure applications. The valve pressure rating at design temperature must equal or exceed the system design pressure, and the system working pressure must remain within the rated envelope at all operating conditions. The reduced structural loading in trunnion designs directly reduces valve torque requirements compared to floating ball valves at equivalent pressure and size, enabling smaller actuator selection and lower actuator cost in automated applications.

Interaction with Flow and Performance

Port configuration and internal bore geometry determine the flow performance of a trunnion mounted ball valve. The pressure drop across valve in the fully open position is governed by bore diameter relative to pipe size. Full port trunnion valves introduce negligible additional pressure drop; reduced port designs generate measurable restriction that must be included in system hydraulic calculations.

The Cv value and flow coefficient for a trunnion mounted ball valve are determined by internal bore area and body geometry, and must be verified against system flow requirements during sizing. For control applications using a characterized ball or V-port trim, control valve rangeability must be assessed to confirm that the valve provides stable, controllable flow across the required operating range. The valve actuator must be selected to provide sufficient torque for reliable operation at maximum differential pressure, including break-out torque at the start of valve travel, with appropriate safety factors applied for the specified service conditions.

Main Components

Trunnions

Upper and lower trunnions are precision-machined extensions of the ball body or separate structural elements that engage with bearing housings in the valve body. They provide axial and lateral support, preventing ball displacement under differential pressure and maintaining consistent ball-to-seat geometry throughout the valve service life.

Ball

The precision-machined ball contains a through-bore sized for full port or reduced port configuration. Ball surface finish and sphericity directly affect sealing contact quality and leakage class achievability. Ball materials are selected based on process fluid, temperature, pressure, and corrosion resistance requirements.

Seats

Seats are spring-loaded assemblies installed in pockets in the valve body on each side of the ball. Soft-seated designs use PTFE or polymer inserts for tight shutoff. Metal-seated designs use hardened alloy or stellite-faced seats for high-temperature and erosive service. Seat design determines leakage class and fire-safe compliance.

Bearings

Bearings installed between the trunnions and valve body reduce rotational friction during ball operation, contributing to lower break-out and running torque. Bearing material selection must account for operating temperature, load conditions, and lubrication requirements.

Stem and Actuation Interface

The stem connects the ball to the manual operator or automated actuator mounting interface. Anti-blowout stem retention features prevent stem ejection under internal pressure. Stem packing material must be compatible with process temperature and fluid, and fire-resistant packing is required for fire safe certified assemblies.

Body Construction

Trunnion mounted ball valve bodies are available in two-piece, three-piece, and top-entry configurations. Top-entry designs allow in-line seat and ball maintenance without removing the valve from the pipeline, which is particularly valuable in large-diameter pipeline installations where valve removal is costly.

Advantages

Reduced Operating Torque: Trunnion support transfers pressure loading away from the seats, substantially reducing break-out and running torque compared to floating ball designs at equivalent pressure and size.
High-Pressure Capability: Structural support allows reliable sealing and operation in Class 600 and above systems where floating ball designs would generate excessive seat contact stress.
Improved Sealing Stability: Pressure-assisted seat design maintains and enhances shutoff contact force as differential pressure increases, supporting consistent leakage class performance.
Suitable for Large Diameters: Trunnion geometry is the standard design approach for ball valves above NPS 6 or NPS 8, where floating ball mass and pressure load make alternative designs impractical.
In-Line Maintainability: Top-entry body designs allow seat replacement and ball inspection without pipeline removal, reducing maintenance downtime in critical service applications.

Typical Applications

Oil and Gas Pipelines: Widely specified as mainline block valves and station isolation valves in long-distance hydrocarbon transmission pipelines under API 6D requirements.
Offshore Platforms: Used in high-pressure topside and subsea piping systems where reliable isolation and low operating torque are critical for safety and operational reliability.
Petrochemical Plants: Applied in high-pressure hydrocarbon process service including reactor feed, product transfer, and high-pressure separation systems.
LNG and Gas Processing: Specified for cryogenic and high-pressure gas service where consistent sealing performance and low torque actuation are required across extended service intervals.
Power Generation: Installed in high-pressure fuel gas, steam, and feedwater systems where pressure class and torque requirements exceed the capability of floating ball designs.

Frequently Asked Questions

What is the difference between trunnion mounted and floating ball valves?

In a floating ball valve, the ball is not mechanically supported and shifts slightly in the downstream direction under pressure, sealing against the downstream seat through pressure-induced contact force. In a trunnion mounted valve, the ball is fixed by trunnion bearings and does not move axially. Instead, spring-loaded seats move toward the stationary ball, and line pressure energizes the seat contact force from the upstream side.

Are trunnion mounted ball valves always full port?

No. Trunnion mounted ball valves are available in both full port and reduced port configurations. Port selection is specified based on system hydraulic requirements, pigging capability needs, and pressure drop allowance. Full port trunnion valves are required for pipeline pigging service; reduced port designs are acceptable for general isolation applications where the additional pressure drop is within system tolerance.

Do trunnion valves require less actuator torque?

Yes. Because differential pressure load is absorbed by the trunnion bearings rather than transmitted through the ball to the seats, the torque required to initiate and sustain ball rotation is significantly lower than for a floating ball valve at equivalent pressure and diameter. This enables smaller, lighter, and less costly actuators in automated applications.

Are trunnion mounted valves suitable for high-pressure service?

Yes. Trunnion mounted ball valves are the standard design for high-pressure and large-diameter service and are specified across pressure classes from Class 150 through Class 2500 under ASME B16.34 and API 6D. The structural support provided by the trunnions allows the valve to maintain sealing performance and manageable operating torque at pressures where floating ball designs would be impractical.

Conclusion

A trunnion mounted ball valve features a mechanically supported ball that rotates on fixed trunnion bearings, with pressure-energized seats providing reliable shutoff performance under high differential pressure. The trunnion design reduces operating torque, supports high-pressure class ratings, and enables large-diameter applications not practical with floating ball geometry. Compliance with API 6D, ASME B16.34, and ISO 14313 confirms pressure-temperature capability and dimensional compatibility within industrial piping systems. Trunnion mounted ball valve design is a fundamental element of valve terminology governing mechanical configuration and performance classification in pipeline and process valve engineering.