Floating vs Trunnion Ball Valves: Selection Guide
A floating ball valve seals by letting line pressure push the ball against the downstream seat, while a trunnion ball valve holds the ball on fixed trunnion bearings and uses spring-loaded seats, so it keeps torque low and sealing reliable at high pressure and large bore. In practice, floating designs are the economical choice for small to medium sizes and moderate pressure classes, while trunnion designs are specified for high-pressure, large-diameter, double-block-and-bleed, and pipeline applications. Choose by pressure class, bore size, torque, seat leakage requirement, and the design standard your specification invokes — not by price alone.
How a Floating Ball Valve Works
In a floating ball valve the ball is not fixed to the stem. The stem passes through the bonnet and engages a slot in the top of the ball, and the ball simply "floats" between two seats. When the valve closes, upstream pressure pushes the ball downstream, compressing it against the downstream seat. Sealing force therefore grows with differential pressure, which is why a floating ball valve often seals very well at moderate pressures.
The same mechanism sets the design's limits. Because the entire pressure load is transferred through the ball into one seat, seat contact stress and operating torque both rise quickly as pressure and bore increase. The stem carries bending loads as the ball moves, and the downstream seat must absorb the full seating load. This is why floating ball valves dominate in small and medium bores — commonly from DN15 (1/2 in.) up to around DN200 (8 in.), depending on pressure class and seat material — and why they are rare in high-pressure classes and large diameters.
Floating valves are simple: fewer parts, a compact body, and lower cost. Seals and seats are available as standard repair kits, and a two-piece or three-piece body design allows inline maintenance. For water, HVAC, general process, and low-pressure utility lines, a floating ball valve is usually the most economical shutoff solution.
How a Trunnion-Mounted Ball Valve Works
A trunnion ball valve fixes the ball on both ends. Trunnions — short shafts supported by bearings in the body or caps — hold the ball in a fixed axial and radial position, and the seats are pushed toward the ball by springs and pressure. Instead of the ball pressing into the seat, the seat presses onto the ball. This reverses the load path: seating force comes from engineered springs plus line pressure acting on the seat area, not from the ball floating downstream.
The consequences are significant for heavier duty:
Lower operating torque at high pressure, because the torque peaks typical of floating designs are avoided and bearing friction is controlled.
Suitability for large bores and high pressure classes, from large flanged valves to fully welded pipeline balls in gas transmission service.
Double block and bleed (DBB) capability: with both seats independently sealing and a body vent or drain between them, the cavity can be verified depressurized. Many pipeline specifications require DBB verifiable to API 6D.
Controlled cavity pressure: trunnion seats are commonly designed as self-relieving, so trapped cavity pressure vents to the pipeline instead of overpressurizing the body cavity — an important consideration for liquid service and thermal expansion.
The trade-offs are complexity and cost. A trunnion valve has more parts, tighter machining tolerances, and heavier bodies. Fully welded trunnion valves for buried service trade repairability for zero-emission performance and weight savings.
Key Differences at a Glance
Factor | Floating Ball Valve | Trunnion Ball Valve |
Ball support | Free to move with pressure | Fixed on trunnion bearings |
Seat loading | Ball presses into downstream seat | Spring- and pressure-energized seats press onto ball |
Typical bore | Small to medium (commonly up to ~DN200) | Medium to very large, including pipeline sizes |
Typical classes | Lower and medium pressure classes | Medium to high classes, including high-pressure gas |
Operating torque | Rises with pressure and size | Lower at high pressure |
DBB capability | Generally not certified | Commonly available and tested per API 6D |
Cost and weight | Lower | Higher |
Typical service | Water, HVAC, process utilities, general industry | Pipelines, compressor and metering stations, high-pressure process |
When to Specify a Floating Ball Valve
Specify a floating design when the line is small to medium bore, pressure class is moderate, and the medium is not abrasive. Typical applications include water treatment, HVAC, compressed air, and general process shutoff where tight shutoff with soft seats is expected. A floating valve is also attractive where inline maintenance matters: three-piece bodies allow seat and seal replacement without cutting the valve out of the line.
Check two practical limits before you standardize on floating designs. First, confirm the pressure-temperature rating for the body and seat combination against ASME B16.34 for the class you are buying. Second, ask the manufacturer for operating torque at your differential pressure, because actuator sizing for floating valves is sensitive to pressure — an undersized actuator on a floating ball valve is one of the most common commissioning problems.
When to Specify a Trunnion Ball Valve
Specify a trunnion design when bore size, pressure, or functional requirements exceed what a floating seat package can handle. Classic applications include gas transmission and distribution pipelines, compressor stations, tank farms, metering and regulator stations, high-pressure process headers, and any line where double block and bleed or cavity relief is required. If the specification calls for DBB, self-relieving seats, sub-sea or buried fully welded bodies, or large-bore actuated valves, the answer is almost always trunnion.
Trunnion valves also make economic sense at large sizes even when pressure is moderate, because floating-ball torque at large bores would require very large actuators. When a project compares quotes for DN400 and larger valves, the trunnion design frequently wins on total installed cost despite a higher valve price.
Seat Materials and Leakage Standards
Seat material selection drives both sealing performance and torque. Soft seats — PTFE, reinforced PTFE, and PEEK are common — provide tight shutoff and low torque but are limited by temperature, pressure, and fluid compatibility. Metal seats, typically hardfaced, handle high temperature, abrasive media, and severe-service conditions, at the cost of higher required seating loads and achievable leakage class.
State the leakage acceptance class in the purchase specification. Common references include ISO 5208 for industrial valve leakage rates and API 598 for valve inspection and testing; pipeline valves built to API 6D carry their own seat and shell test requirements. For fire survival, soft-seated quarter-turn valves can be qualified to API 607 or ISO 10497, which verify that the valve still provides limited sealing after a simulated fire. Do not leave leakage class unspecified — "tight shutoff" without a test standard is not verifiable at receiving inspection.
Standards and Testing to Reference in Your Specification
ASME B16.34 — pressure-temperature ratings, minimum wall thickness, and materials for steel valves.
API 608 — metal ball valves with flanged, threaded, and welding ends for process service.
API 6D (with ISO 14313) — pipeline and piping valves, including DBB, cavity relief, and test requirements.
ISO 5208 and API 598 — leakage rates and pressure testing procedures.
API 607 / ISO 10497 — fire-test qualification for soft-seated quarter-turn valves.
NACE MR0175 / ISO 15156 — material selection for sour (H2S) service.
MSS SP-72 — ball valves with flanged or butt-welding ends for general service.
Cite the standard edition explicitly in the purchase order, and require material certificates (EN 10204 3.1), pressure test records, and nameplate marking per the invoked standard. For sour or low-temperature service, add the applicable material and test requirements at the specification stage — retrofitting them after manufacture is rarely possible.
Common Selection Mistakes and Risks
The most frequent mistakes with ball valve selection are predictable:
Choosing a floating valve by price where differential pressure or bore makes torque unmanageable, leading to actuator failures.
Ignoring body cavity overpressure in liquid service, where trapped thermal expansion can exceed body rating unless self-relieving seats or external relief are provided.
Ordering "fire-safe" without stating the fire-test standard and certificate requirement.
Assuming DBB without requiring it to be designed and tested per API 6D.
Omitting sour-service material requirements in H2S-containing streams.
Each of these is cheap to prevent at specification stage and expensive to discover during commissioning.
FAQ
Can a floating ball valve be used in high-pressure service?
Yes, within limits. Small-bore floating valves are available in higher pressure classes, and the pressure-temperature rating of ASME B16.34 governs the body. The practical constraints are seat stress and operating torque at the working differential, so confirm torque curves with the manufacturer before committing an actuator.
What is the practical size limit for a floating ball valve?
It depends on pressure class and seat design, but floating designs are most common in small and medium bores and become less economical as bore and class increase. Large-diameter valves are normally trunnion mounted because seat loads and torque grow with the square of the bore.
Do all trunnion ball valves provide double block and bleed?
No. DBB is a designed and tested capability. If your application requires it, state that the valve shall meet the DBB requirements of API 6D and verify the body vent or drain feature on the nameplate documentation.
Why does trapped cavity pressure matter?
Liquid trapped in the body cavity expands with temperature rise and can overpressure the cavity. Valves built to pipeline standards include self-relieving seats or other means of cavity relief; confirm the feature for liquid service.
Which standard covers fire-safe ball valves?
API 607 covers fire-test qualification for soft-seated quarter-turn valves, and ISO 10497 is the corresponding international fire-test standard. Require the certificate, not just the label.
Buyer / Engineer Checklist
Line conditions: medium, temperature range, differential pressure, and flow direction documented.
Pressure class verified against ASME B16.34 ratings.
Bore type (full or reduced) and size selected.
Floating vs trunnion decision justified by bore, pressure, torque, and DBB needs.
Seat material and leakage class specified with test standard (ISO 5208 / API 598 / API 6D).
Fire-test requirement and standard (API 607 / ISO 10497) stated if applicable.
Cavity relief requirement stated for liquid service.
Sour-service (NACE MR0175 / ISO 15156) and low-temperature requirements stated if applicable.
Anti-static and stem retention requirements confirmed with the manufacturer.
Actuator torque demand requested at actual differential pressure, with safety factor.
Documentation package defined: EN 10204 3.1 certificates, test records, nameplate per standard.
Conclusion
A floating ball valve is the economical shutoff solution when bore and pressure keep seat loads manageable, and its simplicity makes it easy to maintain and actuate. A trunnion ball valve is the correct specification when pressure, bore, or functional requirements — DBB, cavity relief, pipeline service — exceed the floating design's load path, and its spring-loaded seats keep torque and sealing stable under heavy duty. Specify the design standard, leakage class, and test requirements in writing, and the valve you receive will match the valve you ordered.
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