1. Definition and Structural Difference
Both valves are quarter-turn ball valves; the difference is how the ball is supported. In a floating ball valve, the ball is held between two seats with no fixed lower trunnion - the ball can move slightly along the pipe axis, and when the valve is closed, line pressure pushes the ball against the downstream seat, energizing the seal. In a trunnion-mounted ball valve, the ball is rigidly fixed by an upper and a lower stem, supported in bearings, so it cannot move; sealing is achieved by spring-loaded seats (plus media pressure) pressing against the stationary ball. Because the ball cannot be pushed by pressure, trunnion designs seal reliably at both high and low pressure - a key advantage in pipeline service where valves may see low differential pressure during commissioning or drainage. Design references are API 6D (pipeline valves), API 608 (metal ball valves, flanged and butt-welding ends) and ISO 17292, with fire-safety testing to API 607 / ISO 10497.
2. Sealing Principles Compared
| Aspect | Floating Ball Valve | Trunnion-Mounted Ball Valve |
|---|---|---|
| Ball movement | Free to move along the axis; pressure pushes ball onto downstream seat | Fixed by upper and lower stems in bearings; no axial movement |
| Seat design | Two seats; seal energized mainly by media pressure (single-side forced seal) | Spring-preloaded seats, often with rotating or self-relieving seat options |
| Sealing at high pressure | Good - sealing force rises with pressure | Good - springs plus media pressure |
| Sealing at low pressure | Weaker - depends on seat preload and manufacturing precision | Excellent - spring preload maintains contact at zero/low differential |
| Operating torque | Higher at high pressure (ball presses harder on downstream seat) | Lower and more stable (ball is fixed; seats move) |
| Valve seat pressure | High seat loading at high class → seat wear concern | Controlled seat load → longer seat life in demanding service |
3. Typical Specifications and Boundaries
Floating ball valves are the economical choice in smaller sizes - commonly up to DN200 (8"), occasionally DN300 (12") - and moderate pressure classes up to about Class 600/PN100, in chemical plants, HVAC, water systems and general process lines. They are available with reduced or full bore, threaded, socket-weld or flanged ends per API 608 / GB/T 12237.
Trunnion-mounted ball valves are standard for large diameters (DN50 and above up to DN1500+) and high pressure classes (Class 300–2500 / PN40–PN420), in oil & gas transmission, storage, refining and power plants. Full-bore designs support pigging; double-piston-effect (DPE) seats or single-piston-effect (SPE) self-relieving seats are selected according to thermal expansion and cavity-pressure philosophy.
Double block and bleed (DBB): trunnion valves with two independent seats can be specified per API 6D to isolate both upstream and downstream pressure while bleeding the cavity - a capability floating valves do not offer in the same way.
4. Applications
| Application | Floating Ball Valve | Trunnion-Mounted Ball Valve |
|---|---|---|
| General chemical and process piping | ✓ First choice up to DN200/Class 600 | For large sizes or high class |
| Water and HVAC systems | ✓ Economical standard | When specified by project |
| Oil & gas pipelines and transmission | Small branch lines | ✓ Mainline isolation, piggable full bore |
| High-pressure gas storage and transmission stations | Limited by torque and class | ✓ Standard choice |
| Power and steam systems | Small-bore instrument lines | ✓ Large-bore, high-temperature alloy versions |
5. Selection and Operation Points
Size and pressure crossover: below ~DN200 and Class 600, floating valves win on cost; above that, or where low-pressure sealing must be guaranteed, move to trunnion design.
Torque and actuator sizing: trunnion valves generally show lower breakaway torque at rated pressure; size ISO 5211 actuators with ≥1.5× margin and verify with the manufacturer's torque chart.
Cavity pressure relief: for liquid service with thermal expansion, specify self-relieving (SPE) seats or cavity-relief holes to prevent body overpressure per API 6D philosophy.
Safety features: for hydrocarbon service require anti-static design, fire-safe construction (API 607 / ISO 10497) and blowout-proof stem as standard.
Testing: specify shell, seat and optional DBB tests per API 598 / ISO 5208 with the required leakage class; high-integrity pipeline valves may require additional low-pressure pneumatic seat tests per API 6D.
6. Common Misconceptions
"Trunnion valves are always better than floating." For small sizes and moderate service, a floating valve is lighter, cheaper and perfectly reliable; trunnion design earns its cost where size, pressure or low-pressure sealing demands it.
"Floating ball valves cannot handle high pressure." They can, within their class (typically up to Class 600/PN100); the trade-off is rising seat load and torque as pressure increases, not an absolute limit.
"Springs alone seal a trunnion valve." Springs provide low-pressure preload; at operating pressure the media force on the seat ring adds to the contact load. Both mechanisms are part of the design.
"All trunnion valves are double block and bleed." DBB depends on seat design (two independent seats with cavity bleed) and must be explicitly specified and tested per API 6D; not every trunnion valve is DBB-certified.
"Ball valves can be used for throttling." No - ball valves are isolation valves; partial opening erodes seats and the ball. Use control valves for regulation.
Frequently Asked Questions
At what size should I switch from floating to trunnion-mounted ball valve? s a rule of thumb, floating valves are economical up to about DN200 (8") and Class 600; larger sizes or higher classes favor trunnion design because the ball's pressure load grows with diameter and class, raising torque and seat stress beyond what a floating ball can manage efficiently. Confirm the crossover with the manufacturer for your specific duty.
Why does a trunnion ball valve seal better at low pressure? Because the seats are spring-preloaded against the fixed ball, contact pressure exists even at zero differential. In a floating valve, the sealing force comes mainly from media pressure pushing the ball onto the downstream seat - at very low pressure the seal depends on manufacturing precision and seat preload alone.
What is double-piston-effect (DPE) and why does it matter? In a DPE seat, pressure from either the line side or the cavity side pushes the seat harder against the ball, so the valve seals on both seats regardless of which side is pressurized - enabling double block and bleed. Single-piston-effect (SPE) seats seal from the line side only and automatically relieve cavity overpressure; the choice is a safety-and-operations decision for liquid-filled pipelines.
Do trunnion-mounted ball valves require special maintenance? They need the same periodic checks as any critical valve - seat and seal inspection, stem packing verification, spring condition, and torque/actuator function tests - plus cavity-pressure management in liquid service. Grease fittings on seats and stem (for lubricated-seat designs) need scheduled re-lubrication per the manufacturer's plan.
Which standard should I specify for pipeline ball valves? For pipeline transmission service, API 6D is the industry baseline (design, materials, DBB testing, fire-safe and anti-static requirements), with ISO 17292 or API 608 for plant and process ball valves. Pressure testing follows API 598 or ISO 5208; fire-safety type testing follows API 607 / ISO 10497.
Why is the operating torque of a trunnion valve lower at high pressure? In a floating valve, high pressure pushes the ball hard against the downstream seat, raising friction at the ball-seat interface and therefore torque. In a trunnion valve the ball is fixed by bearings and only the seat rings move, so seat load is controlled by springs plus a defined pressure area - keeping breakaway torque lower and more predictable for actuator sizing.