- Floating ball valves rely on the ball itself shifting slightly downstream under line pressure to seat against the downstream seal, which works well at smaller sizes and moderate pressures but places increasing load on the stem and bearing as size and pressure rise.
- Trunnion-mounted ball valves support the ball on bearings at both top and bottom, holding it fixed in place while spring-loaded seats move to seal against it, a design that scales to much larger sizes and higher pressures without the stem loading problems floating designs face.
- The crossover point between floating and trunnion designs isn’t a fixed rule of thumb; it depends on the specific combination of size, pressure class, and torque requirements, and treating a floating valve as a drop-in solution at large size and high pressure risks operating torque and seat life problems that don’t show up until the valve is already in service.
Two Different Ways to Seal a Ball
Both floating and trunnion-mounted ball valves use the same basic sealing concept: a spherical ball with a bore through it rotates against seat rings to open or block flow. Where they differ fundamentally is how the ball itself is supported and how sealing force against the seat is generated. A floating ball valve suspends the ball between two seats without a fixed lower support, allowing line pressure to push the ball slightly downstream so it seats firmly against the downstream seal. A trunnion-mounted design instead fixes the ball’s position with bearings at top and bottom (the trunnions), and instead relies on spring-loaded or pressure-assisted seats that move slightly to press against the fixed ball. This distinction drives nearly every other difference between the two designs.
Stem Loading and the Size Limitation of Floating Designs
In a floating ball valve, the stem has to both rotate the ball and, in many designs, tolerate the ball’s small downstream shift under pressure, which means the stem and its bearing see combined torsional and lateral loading. As valve size and pressure class increase, the total force pushing the ball against its downstream seat grows substantially, and the stem and upper bearing structure has to absorb a share of that reaction force. This is the core reason floating ball valve designs become impractical at larger sizes and higher pressure classes: the stem and bearing simply weren’t designed to carry that scale of load, and pushing a floating design beyond its practical range shows up as excessive operating torque, accelerated stem bearing wear, or in severe cases, stem bending.
How Trunnion Mounting Solves the Scaling Problem
By fixing the ball’s position with dedicated top and bottom bearings, a trunnion-mounted valve removes the stem from the primary sealing load path entirely; the stem’s job becomes purely rotating the ball, while the trunnion bearings carry the pressure-induced thrust load the floating design’s stem would otherwise absorb. This is what allows trunnion-mounted ball valves to be built in large sizes and high pressure classes, up to and including some of the highest pressure ratings used in oil and gas transmission service, without the stem and bearing problems that would appear in an equivalently sized floating design. The trade-off is a more complex, heavier, and more expensive valve body, since it needs to accommodate the additional bearing structure and spring-loaded seat mechanism.
Operating Torque Comparison
Because the stem in a trunnion-mounted valve only needs to rotate the ball rather than also managing the ball’s seating force, trunnion designs generally offer lower and more consistent operating torque across their pressure range compared to a floating design pushed toward the upper end of its practical size and pressure limits. This matters directly for actuator sizing: a floating valve operating near its practical limits may require a disproportionately larger, more expensive actuator to reliably overcome breakaway torque compared to a trunnion design of the same nominal size and pressure class, an economic factor that often tips the selection toward trunnion mounting even before the mechanical scaling limits become the deciding factor.
Sealing Performance and Seat Design Differences
Trunnion-mounted valves typically use spring-energized seats that maintain sealing contact even at low or zero differential pressure, since the seats themselves generate sealing force rather than relying entirely on line pressure pushing the ball against them. This gives trunnion designs an advantage in bidirectional sealing and in applications with variable or reversing flow direction. Floating ball valves, by contrast, seal most effectively in one direction (the direction that pushes the ball against its designed sealing seat) and may show reduced sealing performance in the reverse direction unless specifically designed with symmetric double-seating for bidirectional service, a distinction worth checking carefully for any application where flow direction isn’t strictly one-way.
Cost and Weight Trade-offs
Floating ball valves are mechanically simpler, lighter, and less expensive to manufacture than trunnion-mounted valves of the same nominal size, which is exactly why they remain the default choice for smaller sizes and moderate pressure classes where their mechanical limitations aren’t a practical concern. The added cost of a trunnion-mounted valve, driven by the bearing structure, spring-loaded seats, and generally more robust body construction, only becomes justified once size, pressure, or bidirectional sealing requirements exceed what a floating design can reliably deliver, which is why blanket rules like “always specify trunnion above a certain size” need to be checked against the actual pressure class and torque requirements of the specific application rather than applied mechanically.
Making the Selection
Choosing between floating and trunnion-mounted ball valves comes down to checking actual size and pressure class against the practical limits of floating designs for the specific valve family and manufacturer being considered, confirming whether bidirectional sealing is a genuine requirement, and comparing actuator sizing and operating torque implications between the two options rather than assuming the cheaper floating design will scale without penalty. For critical, large-diameter, or high-pressure isolation duty, trunnion mounting is very often the safer default, but for the large population of smaller, moderate-pressure applications, a floating ball valve remains the simpler and more economical choice.