Trunnion-Mounted vs Floating Ball Valves: Which Design for Which Application

Ball valves are manufactured in two fundamentally different designs based on how the ball is supported within the valve body: floating ball designs where the ball is free to shift under process pressure, and trunnion-mounted designs where the ball is held in a fixed axial position by upper and lower trunnion supports. Understanding the differences between these two designs is essential for specifying ball valves correctly, particularly for larger sizes and higher-pressure applications where performance, operating torque, and reliability are critical.

Floating Ball Valve Design

In a floating ball valve, the ball is not supported by a shaft or trunnion. Instead, it rests in the valve body supported only by the two seat rings, one upstream and one downstream. The stem connects to the top of the ball to transmit torque for rotation. When the valve is in the closed position, line pressure on the upstream side of the ball pushes it against the downstream seat, creating the sealing force. The higher the line pressure, the greater the sealing force, which is why floating ball valves provide excellent sealing across a wide pressure range.

• Ball held between two seats, pushed downstream by process pressure for sealing

• Simple, fewer parts than trunnion design, lower cost for smaller sizes

• Available in sizes up to approximately NPS 12, limited by torque at high pressure

• Operating torque increases significantly with pressure as ball is pressed hard against seat

• Both seats loaded simultaneously, making double isolation not achievable

Trunnion-Mounted Ball Valve Design

In a trunnion-mounted ball valve, the ball is supported by upper and lower trunnions (cylindrical pins) that are held in bearings in the valve body. This prevents the ball from shifting axially under process pressure. Sealing is achieved by spring-loaded seats that are pushed against the ball by springs and process pressure behind the seats. The trunnion support bears the hydraulic load, so the seat contact force is determined by the spring preload rather than the full line pressure. This dramatically reduces operating torque compared to floating ball designs at the same size and pressure rating.

• Ball fixed in position by upper and lower trunnion supports with bearings

• Spring-loaded seats provide consistent sealing force independent of line pressure

• Operating torque much lower than floating ball design at equivalent pressure

• Standard design for NPS 6 and larger, and for any size in Class 600 and above

• Double piston effect seat design allows double isolation and bleed (DIB) capability

Selection Guidelines

For small valve sizes (NPS 2 and below) in Class 150 to 300 applications, floating ball valves are typically specified due to their lower cost, simpler design, and adequate performance for these conditions. As valve size increases or pressure class increases, the operating torque of floating ball designs increases rapidly, making trunnion mounting increasingly preferable. For NPS 6 and above in any pressure class, or for any size in Class 600 and above, trunnion-mounted designs are the industry standard. Applications requiring pipeline pigging, emergency shutdown service, or double isolation capability also favor trunnion-mounted designs.