- Ball valves come in two fundamentally different configurations — floating ball and trunnion-mounted — and choosing the wrong type for your application results in either a valve that cannot be operated without damage, or one that fails to seal at the pressures and temperatures you need.
- The physics of how the ball is supported and how the seats are loaded determines not just the sealing performance but the operational torque, the seat wear characteristics, the suitability for different pressure classes, and the failure mode when the valve is damaged or worn.
- This guide explains the engineering differences between floating and trunnion ball valves, when to use each, and what the design implications mean for sizing, specification, and maintenance.
The Floating Ball Design: How It Works
In a floating ball valve, the ball is not fixed at the trunnions — it floats freely in the body, held in position only by the two seat rings on either side. The stem connects to the ball at the top but provides no radial support — it can transmit torque to rotate the ball but does not constrain its radial position. The seats, typically spring-loaded rings of PTFE, PEEK, or other seat materials, press against the ball from both sides and provide both the primary seal and the radial support for the ball.
The sealing mechanism in a floating ball valve is elegant in its simplicity. When the valve is closed and pressure is applied on the upstream side, the ball is pushed slightly downstream by the fluid pressure force. This movement increases the contact load between the ball and the downstream seat, creating a pressure-energised seal. The higher the process pressure, the greater the seating force, and the tighter the seal. This self-energising characteristic means that floating ball valves seal better at higher pressures — but it also means that the downstream seat carries the full pressure-induced load.
The downstream seat in a floating ball valve must withstand the full fluid pressure force acting on the projected area of the ball. For a given pressure and ball size, this force can be very large. For a 6″ (150mm) ball valve at Class 600 (maximum pressure ~100 bar), the force on the downstream seat is approximately: pressure × area = 100 bar × π/4 × (150mm)² ≈ 176 kN — roughly 18 tonnes. The seat material must be able to withstand this contact load without excessive deformation or extrusion. For small to medium valve sizes at moderate pressures, PTFE and PEEK seats can handle these loads adequately. For larger sizes and higher pressures, the contact stress on soft seats becomes impractical, which is why floating ball valves are generally limited to smaller sizes and lower pressure classes.
The Trunnion-Mounted Ball Design: How It Works
In a trunnion-mounted ball valve, the ball is supported on fixed trunnion bearings at both the top (stem) and bottom (bottom trunnion), constraining the ball’s radial position regardless of the pressure forces acting on it. The ball cannot move axially — it is held firmly in place by the trunnion bearings. Instead of the ball being pushed into the seats by process pressure, the seats are spring-loaded against the ball from both sides by independent seat springs. The initial seat contact force and hence the initial sealing performance come from the seat springs, not from the fluid pressure.
This has profound implications for both sealing performance and operating torque. Because the ball is constrained by trunnion bearings rather than being held between the seats, the seats do not carry the full pressure-induced force from the fluid — the trunnion bearings carry the radial load. This means the seat contact stress is controlled by the spring preload and is largely independent of process pressure. The result is: lower and more predictable operating torque, lower seat contact stress (allowing softer seat materials or lower contact stresses that extend seat life), and better suitability for large sizes and high pressures where floating ball designs would require impractically large actuators or would destroy soft seats.
The spring-loaded seats in a trunnion ball valve also serve a safety function: if the cavity between the two seats and the ball becomes over-pressurised (due to thermal expansion of trapped fluid), the downstream seat can deflect slightly under the cavity pressure to provide a pressure-relief path — the “double piston effect” that is an important consideration in the design and testing of double-block-and-bleed (DBB) valves.
Operating Torque: Why It Matters for Actuator Sizing
The operating torque of a ball valve is one of the most important parameters for actuator selection, and the difference between floating and trunnion designs in this regard is significant at larger sizes and higher pressures.
For a floating ball valve, the operating torque increases strongly with both valve size and pressure because the friction torque at the downstream seat is proportional to the seating force — which, as described above, increases with both ball diameter and pressure. In large floating ball valves at high pressure, the required operating torque can be very high, requiring large and expensive actuators. This is one of the primary reasons why floating ball valve designs are typically limited to NPS 4 or smaller for Class 600 and above, and NPS 6–8 for Class 300 and below.
For a trunnion-mounted ball valve, the operating torque is determined by the seat spring preload (not process pressure) and by the trunnion bearing friction. Because the trunnion bearings carry the radial load that would otherwise be borne by the downstream seat, the friction forces at the seats are dramatically lower. Trunnion ball valves can be operated at much lower torques than comparably sized floating ball valves, allowing smaller actuators, lower power consumption, and reduced wear on the seats over the valve’s operating life. This is why all large ball valves (NPS 6 and above in Class 600, NPS 10 and above in Class 300) are almost universally trunnion-mounted.
Seat Materials and Sealing Classes
Both floating and trunnion ball valves are available with soft seats (polymer inserts in the seat ring) and metal seats (hardened metal-to-metal contact between seat ring and ball). The choice between soft and metal seats has important implications for sealing class and temperature rating.
Soft-seated ball valves with PTFE or PEEK seats readily achieve ANSI/FCI 70-2 Class VI (bubble-tight) shutoff, which is the tightest standard leakage class. They are limited in temperature — PTFE seats are typically rated to +200°C, PEEK to +260°C — and they cannot be used in high-temperature service or in fluids that degrade the seat material. Soft seats are the correct choice for clean, moderate-temperature fluids where tight shutoff is required.
Metal-seated ball valves use hardened seat rings (typically with Stellite or chromium carbide hard-facing on both seat and ball) and achieve Class IV or Class V leakage in normal service. They can operate at much higher temperatures (500°C and beyond for all-metal designs) and in abrasive, erosive, or cryogenic service where soft seats would be destroyed or inadequate. The trade-off is higher torque requirements, more difficult seating if particles are present, and typically higher initial cost. Metal-seated ball valves are specified for high-temperature process duties, cryogenic service, steam service, and any application where soft seat material degradation is a risk.
The Size and Pressure Crossover: When to Use Which
The practical selection rule of thumb that most experienced valve engineers use is: floating ball for small sizes and lower pressures; trunnion-mounted for large sizes and higher pressures. The specific crossover point depends on the pressure class and the application, but a common industry guideline is to specify trunnion-mounted ball valves for NPS 4 and larger in Class 300 and above, and NPS 2 and larger in Class 600 and above. Below these thresholds, floating ball valves are generally more economical without performance penalty.
For pipeline isolation duties, many pipeline operators specify trunnion-mounted for all mainline block valves regardless of size, because the combination of full-bore requirement, high operating pressure, large actuators, and demanding seat leakage criteria all favour the trunnion design. The additional cost of a trunnion valve over a floating ball in NPS 2-4 sizes is typically modest relative to the installed cost of the valve assembly (actuator, positioner, controls), and the improved performance and lower torque justify it.
Understanding the engineering differences between floating ball and trunnion ball valves — not just the rule of thumb, but the underlying physics of why those rules exist — allows the engineer to make better decisions in the many real-world situations that fall outside the standard guidance. It is the kind of fundamental knowledge that separates the engineer who knows how to fill in a specification form from the one who understands why each box on that form matters.