Trunnion vs Floating Ball Valve: How the Ball Is Supported Changes Everything

Trunnion vs Floating Ball Valve: How the Ball Is Supported Changes Everything

The Short Answer: In a floating ball valve, the ball is held between two seats and pushed against the downstream seat by line pressure, which makes the design simple and economical for low-to-moderate pressure service. In a trunnion ball valve, the ball is held in place by upper and lower trunnions, so the seats are spring-loaded toward the ball instead—which allows high pressures, large sizes, lower operating torque, and dependable double sealing.

What Each Design Is

Floating Ball Valve

A floating ball valve has a ball that floats freely between the upstream and downstream seats. When the valve is closed, line pressure pushes the ball against the downstream seat to create the seal. The design is simple, with fewer parts, and is the standard choice for most general-purpose ball valves up to moderate sizes and pressures. The operating torque tends to rise with pressure because the ball is pressed harder into the seat.

Trunnion Ball Valve

A trunnion ball valve fixes the ball on upper and lower support shafts, so line pressure does not push the ball into the seats. Instead, spring-loaded seats press against the ball to maintain the seal. This keeps operating torque low and stable under high pressure and makes the valve suitable for large sizes, high-pressure service, and applications requiring reliable sealing on both sides of the ball.

Key Differences

Feature	Floating Ball Valve	Trunnion Ball Valve
Ball support	Held between seats	Fixed on trunnions
Seal mechanism	Pressure pushes ball to seat	Spring-loaded seats to ball
Pressure range	Low to moderate typical	High pressure typical
Size range	Small to medium	Medium to very large
Operating torque	Rises with pressure	Low and stable
Double seal (upstream + downstream)	Not typical	Typical
Cost	Lower	Higher

Which to Choose

Choose a floating ball valve for general isolation in low-to-moderate pressure systems, where simplicity and cost matter—most water, gas, and process lines up to a few hundred psi. Choose a trunnion ball valve for high-pressure service, large diameters, or applications that need sealing on both sides of the ball, such as pipelines, HIPPS, and critical process isolation. If the operating torque at high pressure becomes a problem for the actuator, that is a strong signal to move to a trunnion design.

Common Pitfalls

• Using a floating ball valve on a high-pressure large line—the torque and seat loading climb quickly and the valve becomes hard to operate.
• Buying a trunnion valve for a small low-pressure line—the extra cost is wasted.
• Ignoring seat materials: high-temperature service may require metal seats, which change the leakage class of both designs.
• Forgetting cavity relief—trapped pressure between the ball and seats can damage the valve unless the design provides relief.

Practical Recommendation

Match the ball support to pressure and size. Low-to-moderate pressure and small-to-medium sizes point to a floating ball valve; high pressure, large size, or double-sealing duty points to a trunnion design. Confirm the operating torque at the actual line pressure, check the seat and seal materials against the media and temperature, and verify the fire-safe and leakage class requirements on the datasheet before ordering.

A Field Example

A pipeline operator moving gas at high pressure originally considered a floating ball valve for a 16-inch isolation point. The calculated torque at operating pressure was excessive for the actuator, and the single-seat design could not provide the required double seal. The project moved to a trunnion ball valve with spring-loaded seats, which handled the pressure with a smaller actuator and gave the leak-tightness the specification demanded.

Conclusion

Floating ball valves are simple and economical where pressure is modest; trunnion ball valves take over where pressure, size, and sealing reliability matter. Let the line pressure and the torque requirement choose the design for you.

Sources & Further Reading

API, ASME

Tags: Ball Valve