Trunnion vs Floating Ball Valves: When to Use Each Design

Ball valves are manufactured in two fundamental configurations distinguished by how the ball is supported within the valve body: floating ball and trunnion-mounted ball. In a floating ball design, the ball is not fixed to a shaft but is held between the two seat rings and is free to move slightly in the direction of flow under pressure. In a trunnion design, the ball is fixed to a shaft at top and bottom (the trunnions), which are supported by bearings in the body, and the seats are spring-loaded to contact the ball. These two configurations have different operating characteristics, pressure ratings, torque requirements, and size ranges that make each appropriate for different applications.

Floating Ball Valve Operation

In a floating ball valve, the ball is seated against upstream and downstream seat rings that contact both sides of the ball when the valve is closed. When differential pressure is applied, the ball is pushed downstream by the upstream pressure acting on the upstream face, pressing the ball firmly into the downstream seat ring and creating the primary sealing force. The seat sealing force is therefore proportional to the differential pressure, which means floating ball valves provide excellent shutoff performance proportional to the pressure across the valve. In the fully open position, the flow pushes the ball slightly away from the downstream seat. The operating torque required to rotate the floating ball is relatively low in smaller sizes but increases with valve size and pressure because the full line pressure acts over the cross-sectional area of the bore to create friction between ball and seats.

• Floating ball: ball moves under pressure, downstream seat provides primary seal

• Trunnion: ball fixed to shaft bearings, spring-loaded seats contact ball

• Size range: floating typically NPS 0.5 to NPS 6, trunnion from NPS 2 upward

• Pressure rating: floating ball limited by seat contact force, trunnion suitable for Class 150-2500

• Operating torque: trunnion lower torque at high pressure due to pressure-balanced design

Trunnion Ball Valve Design Advantages

Trunnion-mounted ball valves fix the ball on a vertical shaft (the trunnion) supported by upper and lower bearings in the body, preventing the ball from moving under line pressure. Instead, spring-loaded seats are pushed against the stationary ball to create the sealing contact. This design has two critical advantages at high pressure and large size. First, the trunnion shaft absorbs the line pressure load on the ball, preventing the ball from pressing into the downstream seat with force proportional to the full bore area times the line pressure, which at high pressure and large size would create impractically high seating friction and stem torque. Second, the spring-loaded seats provide consistent seating force independent of line pressure, allowing reliable shutoff in both directions and under low-pressure conditions where a floating ball might not seat as firmly.

Selection Criteria and Double Block and Bleed

Floating ball valves are the standard choice for general service in sizes NPS 2 and below and pressure classes up to approximately Class 600, where their simplicity, compactness, and low cost are clear advantages. Trunnion designs are preferred for large diameters (NPS 4 and above), high-pressure service (Class 600 and above), valves requiring low operating torque at full pressure, and applications requiring double block and bleed (DBB) capability. DBB ball valves provide positive sealing at both the upstream and downstream seats simultaneously, with a body bleed port between the seats that allows the body cavity to be vented to confirm both seats are sealing. Pipeline ball valves, subsea valves, and critical isolation valves in safety systems typically specify trunnion-mounted DBB designs.