Trunnion vs Floating Ball Valve: How to Choose by Size, Class and Service

Technical guide · Supreme Valves India · Updated 2026-10-24

Ball valve RFQs are the most common enquiries we receive, and one of the first technical questions on any ball valve line is whether it should be a floating or a trunnion mounted design. The answer depends mostly on size and pressure class, but seat arrangement, torque and pipeline requirements also play a part.

How each design seals

Floating ball

The ball is held only by the two seats and connected to the stem through a slot, so it is free to move slightly along the pipe axis. When the valve is closed, line pressure pushes the ball against the downstream seat, and that pressure is what creates the seal. The downstream seat carries the full pressure load on the ball.

Trunnion mounted ball

The ball is fixed on a lower trunnion and the stem (or an upper trunnion), so it can rotate but not move along the pipe axis. The seats move instead: they are spring-loaded and pushed onto the ball by springs and by line pressure. The trunnion bearings carry the pressure load, not the seats.

Size and pressure class

Because the seat in a floating valve takes the entire thrust from pressure acting on the ball, seat stress and operating torque rise sharply with size and pressure. Typical practice is:

These are common limits, not rules from a standard. Manufacturers publish their own ranges, and project specifications often set the changeover point explicitly. For example, a specification may require trunnion mounted design for NPS 6 and above in all classes.

Operating torque and actuation

A trunnion valve's torque comes mainly from seat friction under spring load plus bearing friction, so it is much lower than a floating valve of the same size and class. On an actuated valve this decides the actuator size, the cost and sometimes whether a gear operator is needed for manual valves. For a large floating valve at high differential pressure, the breakaway torque can make the actuator larger than the valve justifies.

Seat arrangements: SPE, DPE, DBB and DIB

Trunnion valves allow a choice of seat behaviour, which floating valves generally don't offer:

• Single piston effect (SPE), self-relieving seat: seals against upstream pressure, and relieves cavity overpressure into the line.
• Double piston effect (DPE) seat: seals against pressure from either the line side or the cavity side. It cannot self-relieve.
• Double block and bleed (DBB): with the valve closed, both seats seal against pressure from their own side and the cavity can be vented through a bleed valve to verify isolation.
• Double isolation and bleed (DIB): DIB-1 has two DPE seats (both bidirectional). DIB-2 has one DPE and one SPE seat.

API 6D defines these terms. If the project needs positive isolation for maintenance, specify the arrangement explicitly rather than just "DBB".

Cavity relief

Liquid trapped in the body cavity of a closed valve can expand with temperature and build very high pressure. Trunnion valves with SPE seats relieve this automatically. Valves with two DPE seats (DIB-1) need an external cavity relief valve. Floating valves normally relieve to the upstream side through the seat. For liquefied gases and other services where thermal expansion is likely, confirm the relief arrangement on the datasheet.

Standards

• API 608 / ISO 17292: metal ball valves for petroleum, petrochemical and allied industries. Most floating ball valves for plant piping are built to these.
• API 6D / ISO 14313: pipeline and piping valves. Most trunnion valves for pipeline, gas and critical isolation service are specified to API 6D, often with an API monogram requirement.
• API 607 / API 6FA / ISO 10497: fire type testing for soft-seated valves in hydrocarbon service.
• ASME B16.34: pressure-temperature ratings and wall thickness for both designs.

Other features usually found on trunnion valves

• Emergency sealant injection fittings on the seats and stem, for pipeline valves.
• Drain and vent connections on the body.
• Full bore construction for pigging.
• Stem extensions for buried service.
• Metal seats with hard coatings (tungsten carbide or chrome carbide) for abrasive or high-temperature service.

Selection checklist

1. Size and pressure class: within the floating range for the manufacturer and the project specification?
2. Differential pressure at operation and the resulting actuator size.
3. Isolation requirement: simple shut-off, DBB, DIB-1 or DIB-2.
4. Risk of trapped liquid and the cavity relief method.
5. Pigging (full bore), buried service, sealant injection.
6. Fire-safe certification and fugitive emission class.
7. Design standard: API 608 for plant piping, API 6D for pipeline and critical service.

Send the line list with size, class, service and isolation requirement, and we will recommend floating or trunnion construction per line before quoting.

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