Floating vs Trunnion Ball Valve

Floating and trunnion are the two main ball support designs in ball valves. The design determines how sealing force is generated, the maximum practical operating pressure and size, and whether double block and bleed capability is available.

Definition

A floating ball valve has a ball that is not mechanically fixed to the body. Line pressure pushes the ball against the downstream seat, creating the seal. The sealing force increases with pressure, which works well at low-to-moderate pressures but becomes problematic at high pressures and large sizes.

A trunnion ball valve has a ball anchored by upper and lower trunnion bearings. The ball cannot move laterally. Instead, the seat rings are spring-loaded and press inward against the fixed ball. Sealing force comes from the seat springs and is largely independent of line pressure.

When Used

Floating ball: small-bore process piping (1/2” to 8”), Class 150-300, utility services, instrument root valves, and general isolation where cost is the primary driver. Standard per API 608.

Trunnion ball: pipeline isolation per API 6D, sizes 4” to 60”+, Class 150-2500, subsea, high-pressure gas, and any service requiring double block and bleed. Standard per API 6D and API 608.

Specs Table

Parameter	Floating Ball	Trunnion Ball
Ball support	Free-floating (no bearings)	Fixed on trunnion bearings
Sealing principle	Ball presses against downstream seat	Spring-loaded seats press against ball
Operating torque	Increases with pressure	Relatively constant (spring-driven)
Practical size limit	8”-10” (above this, torque is excessive)	60”+
Pressure classes	Class 150-600	Class 150-2500
Double block and bleed	No (single downstream seal)	Yes (both seats seal independently)
Cavity pressure relief	Not required	Required (self-relieving seat or external valve)
Entry type	Side entry (2-piece, 3-piece)	Side entry or top entry
Fire-safe design	API 607	API 607
Cost (6” Class 150)	Baseline	2-4x floating
Weight (6” Class 150)	~40 kg	~100 kg
Actuator size	Smaller (low-pressure)	Smaller relative to floating at high pressure
Standards	API 608	API 6D, API 608

Comparison

The critical pressure and size boundary between floating and trunnion designs is approximately 8” at Class 300. Above this, the force required to push the floating ball against the seat becomes so large that the actuator torque, seat stress, and operating effort become impractical. Trunnion design solves this by decoupling the sealing force from the line pressure.

Trunnion ball valves provide true double block and bleed (DBB): both the upstream and downstream seats seal independently, and a body cavity bleed valve confirms isolation integrity. Floating ball valves cannot provide DBB because only the downstream seat is loaded.

Cavity overpressure is a hazard unique to trunnion valves. When the valve is closed, fluid trapped in the body cavity can expand due to temperature increase, overpressuring the cavity. Trunnion valves include a self-relieving upstream seat or an external relief valve to manage this risk.

Common Mistake

Specifying a floating ball valve for pipeline service in sizes above 8” and Class 600 or higher. The operating torque becomes excessive, requiring an oversized actuator that costs more than the trunnion valve it was trying to avoid. Trunnion design is the correct and more economical choice at these ratings.

Both types reference ASME B16.34 for pressure-temperature ratings and common valve body materials.

Read the full guide to valve types