Guide to Trunnion-Mounted Ball Valve in Pipeline Systems

In industrial production and energy transmission systems, fluid control is a key factor in ensuring safety and efficiency. As core components in piping systems, valves are responsible for essential functions such as shut-off, regulation, and flow diversion. Among various valve types, the trunnion-mounted ball valve is widely used in industries such as oil and gas, petrochemicals, power generation, and water treatment due to its stable structure, reliable sealing performance, and low operating torque.

The core design of a trunnion-mounted ball valve lies in the fact that the ball is fixed inside the valve body by upper and lower support bearings (trunnions). The ball only rotates without any translational movement. This structural feature allows the valve to maintain stable sealing performance even under high-pressure and large-diameter conditions, while significantly reducing the operating force required.

Whether in long-distance oil and gas transmission pipelines or high-temperature and high-pressure systems in chemical plants, trunnion-mounted ball valves provide reliable shut-off control. This article systematically introduces the definition, structure, working principle, main types, application areas, selection considerations, and maintenance practices of trunnion-mounted ball valves.

A trunnion-mounted ball valve (TMBV) is a high-performance industrial valve widely used in fluid control systems. Its defining structural feature is that the ball is supported and fixed by two trunnion bearings located at the top and bottom of the ball. The ball rotates inside the valve body without significant displacement.

This design gives the valve several advantages, including stable structure, reliable sealing, and low operating torque. It is particularly suitable for high-pressure, high-temperature, and large-diameter applications.

Unlike valves that rely on media pressure to push the ball into the seat for sealing, the ball position in a trunnion-mounted design is mechanically fixed by bearings. Even under high differential pressure, the operator can still open or close the valve with relatively low torque. This makes it highly advantageous in long-distance pipelines, oil and gas transmission systems, and applications requiring frequent operation.

Trunnion-mounted ball valves and floating ball valves are two common types of ball valves, but they differ fundamentally in structure and force distribution.

In a floating ball valve, the ball is not supported by a fixed structure. It moves slightly under medium pressure and is pushed against the downstream seat to achieve sealing. This design is simpler and less expensive, but under high pressure, the seat and ball experience greater stress and wear, limiting its pressure rating and service life.

In contrast, a trunnion-mounted ball valve has the ball fixed by upper and lower bearings. The pressure load is mainly absorbed by the trunnions rather than the seat. This reduces seat stress, improves sealing stability under high pressure, and enables bidirectional sealing capability. Although more complex and expensive, it offers significantly better reliability and service life in demanding conditions.

A trunnion-mounted ball valve consists of several key components working together, including the ball and trunnion system, valve body and stem, and seat and sealing system.

The ball is the core component of the valve and is typically made of metal materials compatible with the valve body to ensure strength and sealing performance. A flow-through hole is machined in the ball. When aligned with the pipeline, the valve is fully open; when perpendicular, it is fully closed.

The trunnion bearings are the key feature distinguishing this valve type from others. The ball has upper and lower trunnion shafts that are installed into bearing seats within the valve body. These trunnions support and position the ball.

The trunnions not only bear axial and radial loads generated by fluid pressure but also significantly reduce friction during rotation, ensuring low-torque operation even under high-pressure conditions.

The valve body serves as the external shell and is typically made of carbon steel or stainless steel to meet strength and corrosion resistance requirements. Based on maintenance requirements, valve bodies can be classified into side-entry and top-entry structures.

Top-entry designs allow internal components such as the ball and seats to be serviced from the top without removing the valve from the pipeline, making them suitable for in-line maintenance systems. Side-entry valves require removal from the pipeline for maintenance.

The stem connects the ball to the actuator and transmits torque. When the handwheel or actuator rotates the stem, it drives the ball through a 90-degree rotation. A sealing system is installed between the stem and valve body to prevent leakage along the stem.

The seat is a critical sealing element. Trunnion-mounted ball valves typically use either soft seats or metal seats.

• Soft seats are commonly made of PTFE and other polymer materials. They offer excellent sealing performance and low operating torque, suitable for medium and low-temperature applications.
• Metal seats are designed for high-temperature and high-pressure conditions. They require higher machining precision and provide durability in severe service environments.

When the valve is closed, the seat is pressed tightly against the ball by spring preload or medium pressure, forming a reliable sealing interface. This design enables low-leakage or zero-leakage performance, meeting strict environmental and safety requirements.

The trunnion-mounted ball valve is a typical quarter-turn valve. Its operation involves three key aspects: opening/closing movement, sealing mechanism, and the role of trunnion bearings.

When the valve is closed, the flow hole inside the ball is perpendicular to the pipeline, blocking fluid flow. To open the valve, the actuator rotates the stem by 90 degrees, which turns the ball so that the flow hole aligns with the pipeline, allowing fluid to pass.

Because the ball is fixed by trunnions, it does not move axially during rotation. This ensures smooth operation and relatively stable torque throughout the process. This feature is particularly important for large-diameter pipelines where operating torque stability directly affects safety and usability.

During closing, the ball rotates until its bore is perpendicular to the flow path, bringing the ball surface into contact with the seat.

For soft-seated valves, the seat material deforms slightly to fill microscopic gaps, achieving tight sealing. For metal-seated valves, higher precision machining and appropriate contact pressure are required to ensure sealing performance.

Since most pressure load is absorbed by the trunnion bearings, the seat experiences reduced stress, which minimizes wear and extends service life while maintaining stable sealing under high pressure.

Trunnion bearings play a critical role in valve operation. First, they absorb the load generated by fluid pressure, preventing excessive force on the seat. Second, they reduce friction during rotation, enabling easier operation. Third, they improve alignment accuracy between the ball bore and valve body flow passage, reducing flow resistance.

Trunnion-mounted ball valves can be classified into several types based on body structure and application conditions.

• Two-piece body valves consist of two connected parts joined by bolts. They are simple to manufacture and widely used in medium and low-pressure applications.
• Three-piece body valves consist of two end caps and a central body section. They allow internal maintenance without removing the entire valve from the pipeline.
• Top-entry trunnion-mounted ball valves allow full maintenance from the top, making them ideal for critical systems requiring regular inspection.
• Side-entry trunnion-mounted ball valves require removal from the pipeline for maintenance but are suitable for compact installations.
• Fully welded trunnion-mounted ball valves eliminate potential leakage paths by using a welded structure. They offer extremely high integrity and are widely used in oil, gas, and high-pressure pipelines.
• Split-body trunnion-mounted ball valves offer flexible installation and maintenance and are commonly used in water treatment, petrochemical, oil and gas, and power industries.

• Low-temperature trunnion-mounted ball valves are designed for cryogenic environments such as LNG processing and storage. They use specialized materials and structural designs.
• Subsea trunnion-mounted ball valves are used in offshore drilling and underwater production systems. They are designed to withstand seawater corrosion and high pressure, often with reinforced sealing and protective coatings.

Trunnion-mounted ball valves are widely used across multiple industrial sectors due to their excellent performance characteristics.

In upstream operations, they are used in wellhead systems and gathering pipelines for flow control and shut-off. In midstream transportation, they are widely used in long-distance pipelines requiring high-pressure and reliable shut-off performance. In downstream refining, they provide isolation for process lines, ensuring operational safety.

In power plants, they are used in boiler feedwater systems, cooling systems, and steam pipelines, where fast and reliable shut-off is required.

In chemical and petrochemical industries, they handle corrosive media, acids, and volatile chemicals. Material selection is adjusted based on medium characteristics to ensure corrosion resistance and sealing reliability.

In water and wastewater systems, they control raw water, treated water, and chemical dosing. In mining applications, they handle slurry and abrasive media, requiring wear-resistant materials. In pharmaceutical industries, they are used in sterile processes and batch production, requiring high cleanliness and sealing performance.

Proper selection of trunnion-mounted ball valves is essential for ensuring long-term reliable operation.

Key parameters include pressure rating, temperature range, and medium characteristics. Pressure must be within the valve’s rated capacity. Temperature affects material selection, especially for high or cryogenic conditions. Media properties such as corrosiveness, viscosity, and solid particles must also be considered.

Common body materials include carbon steel, stainless steel, alloy steel, and duplex stainless steel. Carbon steel is economical and suitable for general applications. Stainless steel offers good corrosion resistance. Alloy and duplex steels are used in severe service conditions.

Seat materials are equally important. PTFE provides excellent sealing but limited temperature resistance. Metal seats offer high-temperature capability but require higher torque and cost.

Selection should comply with international standards such as API and ASME, which define design, manufacturing, testing, and inspection requirements.

Manufacturer capability is also critical. Production equipment, testing methods, quality systems, and after-sales service should be evaluated to ensure long-term reliability.

Proper maintenance extends service life and ensures safe operation.

Regular inspection should focus on wear, corrosion, and deformation of the body, ball, and seats. The stem and actuator should be checked for smooth operation and abnormal noise or sticking.

Trunnion bearing lubrication is essential. Grease should be applied according to manufacturer recommendations. Insufficient lubrication increases wear and operating torque.

Seat wear should be monitored regularly. If leakage or increased torque is observed, sealing components should be inspected or replaced.

In high-temperature or corrosive environments, inspection frequency should be increased. High temperatures may cause seal aging, while corrosive media may reduce wall thickness and structural integrity.

For automated systems, actuator air, electric, or hydraulic systems should be regularly checked to ensure proper response to control signals.

Trunnion-mounted ball valves play a vital role in industries such as oil and gas, power generation, chemicals, and water treatment due to their stable structure, reliable sealing, and low operating torque.

Their design, where the ball is supported by trunnion bearings, makes them especially suitable for high-pressure, large-diameter, and long-cycle applications. Compared with floating ball valves, they offer broader applicability and longer service life.

A clear understanding of their structure, working principle, types, selection criteria, and maintenance requirements enables engineers to make informed decisions. With proper selection, installation, and maintenance, trunnion-mounted ball valves can ensure safe, efficient, and long-term stable operation in various industrial systems, providing reliable fluid control for modern industry.