In This Article
- 1.Floating Ball Valve — How It Works
- 2.Trunnion-Mounted Ball Valve — How It Works
- 3.Key Differences: Floating vs Trunnion
- 4.The Engineering Selection Rule
- 5.When Does Pipeline Service Require Trunnion?
Both floating and trunnion-mounted ball valves look similar externally — a spherical ball with a through-bore, a hand-lever or gear operator, and flanged end connections. But their internal seating mechanisms are fundamentally different, and the correct choice between them depends on line size, operating pressure, fluid type, and whether double block and bleed (DBB) is required.
Floating Ball Valve — How It Works
In a floating ball valve, the ball is not mechanically supported at top and bottom (no trunnions). It is held in place only by the two seat rings and the process pressure. When the valve is closed, upstream pressure pushes the ball against the downstream seat, creating the sealing force. The seat sealing force therefore depends on line pressure — at low pressures, the sealing is adequate; at high pressures (above Class 300 in larger sizes), the seat load becomes excessive, causing seat deformation and premature failure.
Trunnion-Mounted Ball Valve — How It Works
In a trunnion-mounted ball valve, the ball has integral trunnions (shaft extensions) at top and bottom, supported by bearings in the valve body. This fixed support means the ball does not move under pressure. The seats are spring-loaded and move towards the ball — the seat-to-ball contact is maintained by the spring load, not by line pressure. This makes the operating torque essentially independent of line pressure and allows the valve to seal at zero differential pressure (unlike a floating ball valve where line pressure is needed to push the ball onto the downstream seat).
Key Differences: Floating vs Trunnion
| Criterion | Floating Ball Valve | Trunnion Ball Valve |
|---|---|---|
| Sealing mechanism | Line pressure pushes ball onto downstream seat | Spring-loaded seats move to contact fixed ball |
| Sealing at zero ΔP | Poor — no upstream pressure means no sealing force | Excellent — spring-loaded seats seal independently of ΔP |
| Operating torque vs pressure | Torque increases with line pressure (seat friction) | Torque essentially constant (spring pre-load, not line pressure) |
| Typical size range | ½" to 6" (DN15 to DN150) | 2" to 60" (DN50 to DN1500) |
| Typical pressure class | Class 150 to Class 600 | Class 150 to Class 2500 |
| DBB capability | Cannot achieve true DBB — single-piston effect only | Can achieve DBB with dual-piston effect seats or dedicated DBB design |
| Body cavity pressure relief | Required — ball trapping can cause body pressure build-up | Less critical — trunnions provide structural support |
| Weight (same size) | Lighter | Heavier (trunnion bearings, stronger body) |
| Cost | Lower | Higher (typically 2–3× for same size/class) |
| API 6D compliance | Yes — for smaller sizes and lower pressures | Yes — standard for pipeline service above Class 300 / DN150 |
The Engineering Selection Rule
The industry rule of thumb is:
- Use floating ball valves: DN15 to DN100 (½" to 4"), Class 150 to Class 300, for non-critical general service where operating torque is manageable and DBB is not required
- Use trunnion ball valves: DN150 and above (6"+) in any pressure class, OR any size in Class 600 and above, OR when DBB / DIB isolation is required, OR when the valve must seal at zero differential pressure (e.g., first isolation after a pump start)
- Exception: some manufacturers offer floating ball valves to DN150 / Class 600 for cost-sensitive applications where operating torque is acceptable — but verify torque requirements carefully
When Does Pipeline Service Require Trunnion?
- API 6D pipeline applications DN150 (6") and above — API 6D effectively mandates trunnion mounting for large-diameter pipeline valves
- Metering and custody transfer: DBB ball valves at meter inlets/outlets are always trunnion-mounted with dual-piston seats
- ESD (Emergency Shut-Down) valves: must seal at zero ΔP (after emergency); floating valves cannot guarantee this
- High-pressure gas service Class 900 and above: operating torque for a floating ball valve becomes unmanageable manually; actuator sizing becomes impractical
- Subsea valves: all subsea ball valves are trunnion-mounted (ROV-operable, high torque environment, zero-ΔP sealing requirement)
- LNG cryogenic service: extended-body trunnion ball valves with PCTFE seats are standard for LNG at -161°C
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