Trunnion ball valves are a type of quarter-turn valve designed for high-pressure and large-diameter applications, where reliable flow control is essential. Unlike floating ball valves, trunnion ball valves feature a ball that is anchored by trunnions at both the top and bottom, ensuring stability and reducing operational torque. To understand how do trunnion ball valves work, it is important to note that the ball remains fixed in position while the seats move toward it to create sealing, allowing the valve to handle high-pressure conditions more effectively. This design provides tight shut-off and enhanced durability over long service periods. Trunnion ball valves play a crucial role in various industrial applications, including oil and gas pipelines, chemical processing, power generation, and water treatment, where precise flow control, safety, and long-term performance are critical. Their ability to operate reliably under high pressure and harsh conditions makes them a preferred choice for industries demanding efficiency and operational security.

What Is a Trunnion Ball Valve?

A trunnion ball valve is a type of quarter-turn ball valve where the spherical ball element is mechanically supported and fixed in place by shafts (called trunnions) at both the top and bottom. This design contrasts with other ball valves where the ball can move freely. Because the ball stays anchored, the seats (normally spring-loaded) move to form a tight seal around it, making the valve especially suited to high-pressure and heavy-duty applications.

In simpler terms, imagine the valve’s ball as a door on a hinge that only turns, rather than one that sits loosely and shifts with pressure. That fixed position reduces the force needed to operate the valve and improves performance when fluids or gases are under intense pressure or flow conditions.

Key Components of Trunnion Ball Valves

Trunnion-Ball-Valve-diagram

The valve body forms the pressure-containing shell of the valve. It houses all internal components and is designed to withstand line pressure, temperature changes and external loads from the pipeline. Body design and material selection are critical to long-term reliability.

Inside the body sits the ball, a precisely machined sphere with a bore through the centre. When the bore is aligned with the pipeline, flow passes through. When the ball is rotated 90 degrees, the solid section blocks the flow completely.

What makes this a trunnion ball valve is how the ball is supported. The ball is anchored by upper and lower trunnions, which are shafts that hold it firmly in place. These supports prevent both axial and radial movement caused by line pressure. Instead of shifting under load, the ball stays perfectly aligned with the seats, allowing sealing forces to be controlled more effectively.

The stem connects to the upper trunnion and transmits torque to rotate the ball. It passes through the body and is sealed to prevent leakage. Torque is applied either manually or through an actuator, depending on valve size, duty and automation requirements.

Actuation and control

The actuator provides the force needed to open and close the valve. This may be a manual gear operator, or a pneumatic, electric or hydraulic actuator for automated systems. Because trunnion ball valves have stable and predictable torque requirements, actuators can be sized accurately, improving reliability and control.

Seats, springs and sealing elements

Sealing is achieved by the seats, which are normally installed on both sides of the ball. In a trunnion design, these seats are often spring-loaded. The springs push the seats gently towards the fixed ball to create initial sealing at low pressure. As line pressure increases, pressure-assisted forces enhance the seal without increasing operating torque.

Additional seals, O-rings and gaskets are used throughout the valve, particularly around the stem and body joints, to prevent external leakage and maintain pressure integrity.

Bearings are fitted around the stem and trunnions to reduce friction during operation. By supporting smooth rotation and limiting metal-to-metal contact, bearings help reduce wear and extend the service life of the valve.

Body cavity relief and bleed ports

Many trunnion ball valves are equipped with body cavity relief or bleed ports. These allow pressure trapped between the seats to be safely released or monitored when the valve is closed. This feature supports double block and bleed functionality and improves safety during maintenance and operation.

How Trunnion Ball Valves Work

​At a functional level, a trunnion ball valve operates on a quarter-turn principle, meaning the valve moves from fully open to fully closed with a 90-degree rotation. While this sounds simple, the internal force balance is what sets a trunnion design apart from other ball valves.

Quarter-turn operation: open and closed positions

When the valve is in the fully open position, the bore through the ball is aligned with the pipeline. This creates a straight, unobstructed flow path, allowing fluid or gas to pass through with minimal pressure drop and turbulence. In this state, forces acting on the ball are balanced, and the seats maintain light contact to preserve sealing without excessive wear.

To close the valve, torque is applied through the actuator or manual operator, rotating the stem and ball by 90 degrees. In the fully closed position, the solid side of the ball blocks the pipeline bore completely, stopping flow. Line pressure then acts on the upstream side of the valve, but the way this pressure is handled is where the trunnion design becomes important.

How the trunnions support the ball

Trunnion ball valve diagram (source: plastomatic)

In a trunnion ball valve, the ball is connected to two trunnions, one at the top and one at the bottom. These trunnions fit into bearings in the valve body, creating a stable axis around which the ball can rotate. The upper trunnion is connected to the stem, while the lower trunnion provides additional support and load sharing.

Together, these supports prevent both axial movement (side-to-side) and radial movement (off-centre rotation) of the ball. As a result, the ball remains perfectly aligned with the seats throughout opening and closing, regardless of pressure fluctuations in the pipeline. This fixed alignment is critical for consistent sealing and predictable operation.

Keeping forces under control

In high-pressure service, a large amount of force acts on the ball. In a floating design, that force pushes the ball downstream, increasing friction against the seat. In a trunnion design, the pressure load is transferred directly into the valve body through the trunnions, not through the seats or stem.

This load path reduces stress on sealing surfaces and internal components. It also allows the seats to do their job independently, moving slightly to maintain sealing without having to support the full pressure load.

Impact on torque and wear

Because the ball does not shift under pressure, the operating torque remains low and stable, even as differential pressure increases. This makes trunnion ball valves easier to operate, particularly in larger sizes where torque can otherwise become a limiting factor.

Lower torque also means less wear on seats, bearings and the stem, which helps extend service life. For automated valves, it allows for smaller actuators, reducing both initial cost and ongoing energy consumption.

Reliability in large and high-pressure valves

As valve size and pressure rating increase, the benefits of trunnion support become more pronounced. The fixed ball design improves reliability by maintaining alignment, reducing internal wear and ensuring consistent sealing over repeated cycles.

This video provides a snapshot of Tanggong Valve’s WCB trunnion ball valves, highlighting design details.

Trunnion vs Floating Ball Valves: Working Difference

When choosing between a trunnion ball valve and a floating ball valve, the key differences come down to how the ball is supported and how sealing force is created. These design choices directly affect pressure capability, valve size, operating torque and long-term performance.

Quick comparison at a glance

How ball movement changes seat loading

In a floating ball valve, the ball is held in place by the stem and seats. When the valve closes, line pressure pushes the ball downstream, forcing it against the seat to create a seal. As pressure rises, this force increases. While effective for smaller valves, it also leads to higher friction and greater stress on the seat.

In a trunnion ball valve, the ball is mechanically anchored and cannot move axially. Instead of the ball creating the seal by moving, the seats move towards the fixed ball, assisted by springs and line pressure. This means seat loading is controlled and more evenly distributed, regardless of pressure level.

Impact on pressure rating and size

Because the ball in a trunnion design does not shift under load, the valve can handle much higher pressures and larger pipe sizes without compromising performance. Floating ball valves, by contrast, become less practical as size and pressure increase due to rising torque and seat stress.

This is why trunnion ball valves are commonly specified for large-diameter pipelines and high-pressure duties, while floating ball valves are often used in smaller, lower-pressure systems.

Torque requirements and actuation

Ball movement has a direct impact on torque. In floating designs, higher pressure means higher torque, which can limit valve size or require larger actuators. Trunnion ball valves maintain lower, more predictable torque, making them easier to operate manually and more efficient to automate.

Leakage and long-term performance

With controlled seat loading and stable alignment, trunnion ball valves typically deliver better sealing reliability over time, especially under high differential pressure. Reduced wear on seats and sealing surfaces also contributes to longer service life.

Key Design Variants That Affect Operation

While the core working principle of a trunnion ball valve remains the same, design variations can significantly influence how the valve operates, how it is maintained, and where it is best used. Understanding these differences helps engineers and buyers match the valve design to real operating and lifecycle requirements.

One-piece, two-piece and three-piece body designs

A one-piece body trunnion ball valve is manufactured from a single solid body. This design offers excellent structural integrity and is well suited to high-pressure service. However, because the body cannot be disassembled, internal maintenance usually requires removing the valve from the pipeline.

A two-piece body design typically consists of a main body and an end connection. It balances strength with some level of serviceability and is commonly used in pipeline applications where robustness is a priority and maintenance intervals are long.

A three-piece body trunnion ball valve is designed with maintenance in mind. The central body section can be removed while the end connections remain welded or bolted to the pipeline. This makes it easier to inspect or replace seats and seals without cutting the valve out of the line. As a result, three-piece designs are often selected for process plants where downtime needs to be minimised.

Soft-seated vs metal-seated trunnion valves

Soft-seated trunnion ball valves, using materials such as PTFE or reinforced polymers, provide excellent tight shut-off at lower temperatures and cleaner services. They are widely used in oil and gas, chemical processing and general industrial applications where zero leakage is required.

Metal-seated trunnion ball valves are designed for harsher conditions. They can tolerate high temperatures, abrasive media and high flow velocities. While leakage rates are typically higher than soft-seated designs, metal seats offer superior durability and stability in severe service.

Fire-safe design considerations

In applications where fire risk is a concern, trunnion ball valves are often built to a fire-safe design. These valves incorporate secondary metal-to-metal sealing features that maintain shut-off capability if soft seals are damaged by fire.

In practice, this means that even after exposure to extreme heat, the valve can still isolate the pipeline, helping to limit escalation and protect personnel and equipment. Fire-safe design is a common requirement in oil and gas and petrochemical installations.

Special designs for extreme temperatures

For low-temperature services, such as LNG, trunnion ball valves are designed with extended bonnets, special materials and seals that remain flexible at cryogenic temperatures. These features ensure smooth operation and reliable sealing despite thermal contraction.

At the other end of the scale, high-temperature trunnion valves use heat-resistant materials, metal seats and carefully selected bearings to maintain performance without seizing or excessive wear.

Typical Applications and Industries

Trunnion ball valves are most commonly used in industries where pressure is high, valve sizes are large, and isolation reliability is critical. Their design is driven by practical operating demands rather than convenience, which is why they appear repeatedly in certain types of systems.

Oil and gas transmission, pipelines and block valve stations

One of the primary application areas for trunnion ball valves is oil and gas transmission. Long-distance pipelines operate under high pressure and often require large-diameter valves to isolate sections of the line. In these conditions, the fixed-ball design helps control internal forces and maintain tight sealing without excessive operating torque.

At block valve stations, trunnion ball valves are used as mainline isolation valves. These stations are designed for safety and maintenance access, so features such as double block and bleed, fire-safe construction and predictable shut-off behaviour are especially important. The trunnion design supports all of these requirements, making it a common specification choice for transmission projects.

Petrochemical, refining and power generation

In petrochemical and refining facilities, trunnion ball valves are used on high-pressure process lines, feed lines and critical isolation points around reactors, columns and heat exchangers. These systems often experience pressure fluctuations, temperature variation and strict safety controls. Stable torque and reliable sealing are key reasons the trunnion design is preferred.

Power generation plants, particularly those with high-pressure cooling water, steam-related services or auxiliary process systems, also rely on trunnion ball valves for dependable isolation. Here, consistent performance and long service life are often more important than compact size or lowest initial cost.

Industrial cooling and utility systems

In large industrial cooling systems, trunnion ball valves are used where pipe sizes increase and operating pressure remains significant. Their ability to handle frequent operation without excessive seat wear makes them suitable for critical cooling loops and isolation duties in heavy industry.

Advantages and Limitations of Trunnion Ball Valves

Advantages of trunnion ball valves

One of the most recognised advantages is lower operating torque. Because the ball is fixed in place and does not move under line pressure, the torque required to open and close the valve remains relatively stable. This is especially important in high-pressure systems and large valve sizes, where torque can otherwise become difficult to manage.

Trunnion ball valves also provide reliable sealing under high pressure. Spring-loaded, pressure-assisted seats move towards the fixed ball, maintaining tight shut-off across a wide pressure range. This controlled sealing mechanism reduces seat overload and helps preserve sealing performance over time.

Another key benefit is their suitability for large diameters. As pipeline size increases, floating ball designs become less practical due to rising torque and seat stress. Trunnion support allows the valve to scale up in size while maintaining smooth operation and dependable sealing.

Most trunnion ball valves are designed for bidirectional sealing, meaning they can seal effectively regardless of flow direction. This adds flexibility in system design and improves safety during isolation and maintenance, particularly in critical services.

Limitations to consider

Despite these advantages, trunnion ball valves are not without limitations. The most obvious is higher initial cost. The additional components, machining and testing involved in trunnion designs typically make them more expensive than floating ball valves of the same size and pressure class.

They also have a more complex internal structure. Trunnions, bearings, springs and sealing systems increase the number of parts inside the valve. While this complexity supports performance in demanding applications, it can be unnecessary for low-pressure or non-critical services.

Finally, maintenance may require higher skill levels. Servicing trunnion ball valves, particularly in large sizes or critical applications, often calls for trained personnel who understand the valve’s internal load paths and sealing mechanisms. Improper maintenance can negate the benefits of the design.

How to Choose the Right Trunnion Ball Valve

The first and most critical factor is pressure class. Trunnion ball valves are often selected for high-pressure duties, but the pressure rating must match both normal operating pressure and potential upset conditions. Always consider maximum allowable pressure, not just average values.

Valve size also matters. Larger diameters benefit most from trunnion support due to torque and seat-loading considerations. For smaller sizes, it is worth confirming whether a trunnion design is truly necessary.

The process medium plays a major role in material and seat selection. Clean gas, crude oil, corrosive chemicals or abrasive fluids all place different demands on the valve. This ties directly into temperature range, which influences body materials, sealing elements and whether soft or metal seats are appropriate.

End connections should be chosen based on pipeline design and installation practice. Flanged connections allow easier removal, while welded ends reduce potential leakage points and are common in high-pressure pipeline service.

Actuation is another key decision. Manual, pneumatic, electric or hydraulic actuation should be selected based on valve size, operating frequency, available utilities and safety requirements. One advantage of trunnion ball valves is their predictable torque, which simplifies actuator sizing when done correctly.

Finally, always confirm compliance with relevant standards and certifications, such as API, ASME, ISO and fire-safe requirements. These standards are not just paperwork; they reflect design and testing practices that directly affect safety and performance in service.

Installation, Operation, and Maintenance Best Practices

Installation and commissioning

Proper installation starts with pipeline alignment. The valve should not be used to correct misaligned pipework. Excessive stress from poor alignment can distort the body, affect seat contact and lead to leakage over time. Adequate pipe supports on both sides of the valve are equally important, especially for large and heavy trunnion valves.

Before commissioning, the pipeline should be thoroughly flushed to remove debris such as welding slag, sand or scale. These contaminants can damage seats and sealing surfaces during the first operation cycle, when surfaces are still new and most vulnerable.

During installation, it is also important to confirm that the valve is installed in the correct orientation, particularly when features such as drain, vent or sealant injection ports are present. Initial operation during commissioning should be slow and controlled, allowing the seats and seals to settle properly.

Operating practices that protect sealing performance

Trunnion ball valves are designed primarily for on-off service, not throttling. Keeping the valve either fully open or fully closed avoids unnecessary erosion and seat wear caused by high-velocity flow across partially exposed sealing surfaces.

Where the valve is actuated, correct actuator sizing and stroke adjustment are essential. Over-torquing can damage seats and stem seals, while under-torquing may prevent full closure. Operating the valve within its design limits, even though it can handle high pressure, remains good practice.

Routine inspection and preventative maintenance

Regular inspection helps identify issues before they become serious. Typical checks include monitoring operating torque, external leakage, stem seal condition and body flange integrity. Any unusual increase in torque or signs of leakage should be investigated promptly.

For valves in critical service, planned maintenance may include seat and seal inspection or replacement during shutdowns. Trunnion designs, especially those with maintenance-friendly body configurations, allow this work to be done efficiently when planned in advance.

How Tanggong’s Trunnion Ball Valves Are Engineered to Work Better

Tanggong Valve’s trunnion ball valves are engineered with a clear understanding of internal load paths and sealing behaviour. Material selection is tailored to service conditions, taking into account pressure, temperature and the nature of the medium. Body and ball materials are chosen for strength and stability, while stem, trunnion and bearing materials are selected to minimise friction and wear.

Seat design receives particular attention. Spring-loaded seats are engineered to deliver consistent contact with the fixed ball at low pressure, while allowing pressure-assisted sealing to take over at higher pressures. Depending on the application, both soft and metal-seated options are available, each designed to balance tight shut-off with durability.

Before leaving the factory, every valve is subjected to comprehensive testing procedures. These typically include pressure testing, seat leakage testing and functional operation checks to confirm torque and smooth movement. Fire-safe testing and additional inspections are carried out where required by the project or applicable standards.

All designs are aligned with international standards, such as API, ASME and ISO, ensuring that performance, safety and interchangeability meet global expectations. Compliance is treated as a baseline, not a selling point.​