Understanding What Is a Trunnion Ball Valve

Trunnion ball valves show up in various applications wherever engineers need shutoff performance that stays dependable at high pressure and in large pipe sizes. They look like ordinary ball valves from the outside, but inside they solve problems that floating ball valves struggle with at scale. Two bearings, called trunnions, anchor the ball so it only rotates. That one choice changes how forces travel through the valve, how the seats seal, and how much torque you need to turn it.

Let’s unpack what that means in practice, how a trunnion design is built, when to specify one, and how to buy and maintain them with confidence.

What a Trunnion Ball Valve Is

A trunnion ball valve is a quarter-turn valve with a spherical closure element that has a hole drilled through it. When the port lines up with the pipeline, flow passes; when rotated 90 degrees, it blocks the line.

The difference lives in the mounting. Instead of allowing the ball to float downstream under pressure, the ball is held by shafts at its top and bottom, effectively redirecting forces that would typically act upstream. Those supports carry the pressure loads that would otherwise press the ball into the downstream seat. The result is a stable ball, lower operating torque at pressure, and more predictable sealing at large diameters and high differential pressure.

Inside the Mechanism: Core Components

A modern trunnion ball valve is more than a body, ball, and handle. It is a compact machine designed for high loads and safe operation.

Figure 2: Ball Valve Component Assembly – showing all major parts and their relationships

Body: One piece, two piece, or three piece, often forged for high pressure. Common pressure classes range from ASME 150 to 2500 and beyond.

Ball: Precision machined with a full-bore or reduced-bore port. Surface treatments like hard chrome, tungsten carbide, or HVOF coatings improve wear and corrosion resistance.

Trunnions: Fixed shafts that locate the ball and carry radial loads. Designs may include upper and lower trunnions or a single bottom trunnion with the stem acting as the upper support.

Seats: Soft materials like PTFE, reinforced PTFE, PEEK, or elastomers for typical service, and metal seats for high-temperature or abrasive duty. Seats are often spring-loaded for consistent contact.

Stem: Blowout-proof geometry, anti-static paths, and robust packing stacks with live-loading where fugitive emissions limits apply.

Bearings and bushings: Low-friction supports on the stem and trunnions that cut torque and resist galling.

Cavity management: Pressure equalizing ports, cavity relief features, and optional double block and bleed arrangements.

Each piece is chosen for service conditions. For example, sour hydrocarbon service will drive material choices to meet NACE MR0175/ISO 15156.

Figure 3: Detailed Trunnion Ball Valve Assembly Drawing – complete technical specification

How Trunnion Mounting Changes Performance

Anchoring the ball changes the load path. Line pressure that once pushed a floating ball into one seat now routes into the trunnions and body. Seats do not carry the full axial thrust of the ball, which lowers friction and actuation torque at pressure.

Seat energization is carefully engineered:

Single piston effect (SPE): Upstream pressure energizes the seat toward the ball. If cavity pressure rises, it can push the seat away and self-relieve.

Double piston effect (DPE): Pressure on either side pushes both seats toward the ball. This blocks in both directions, which is useful when paired with a bleed between seats for double block and bleed testing.

Many pipeline valves combine one SPE seat and one DPE seat to balance cavity relief and bi-directional sealing priorities. The key benefit is consistent sealing without needing the ball to wedge into a single seat.

Operation and Actuation

Operation is a simple quarter turn. The stem rotates the ball without lateral shift, which keeps friction consistent.

Manual actuation: Lever or gear operator sized for breakaway torque at maximum differential pressure.

Automated actuation: Pneumatic, electric, or hydraulic actuators with positioners, limit switches, and partial stroke testing for safety instrumented functions.

Fire-safe behavior: If soft seats degrade in a fire, springs and geometry bring metal-to-metal sealing surfaces into contact to limit leakage.

Tight shutoff: Full-bore trunnion valves offer low pressure drop and can be piggable in pipeline service.

Because trunnion ball valves support the ball, the required torque growth with pressure is modest compared to floating designs. That often reduces actuator size and cost, especially on large valves.

Where They Shine: Typical Applications

Trunnion ball valves handle duty that pushes lighter designs past their comfort zone.

Figure 4: Trunnion Ball Valves in Oil & Gas Industry Applications

Oil and gas

• Pipeline block valves and station isolation
• Refineries and petrochemical units with high pressure and hazardous media
• Storage terminals that require double isolation and bleed for maintenance

Chemical processing

• Corrosive and high-pressure service with polymer or metal seats
• Reactor feed and high-temperature loops

Water and wastewater

• Large diameter mains, pump stations, and desalination trains
• Full-bore designs that support pigging in long lines

Power generation

• Steam and feedwater isolation
• Gas turbine fuel gas lines

Marine, mining, and pulp and paper

• Abrasive slurries with metal seats and hard coatings
• Compact layouts with automated isolation and safety features

Large pipe sizes, high differential pressure, and strict safety expectations are the recurring themes.

Figure 5: Large-Scale Pipeline Valve Applications in Refinery Operations

Advantages that Justify Selection

Engineers pick trunnion ball valves for a short list of strong reasons:

High pressure capability: Built and qualified for high classes where single-seated floating designs struggle.

Wide size range: Commonly from 3 inches up to 48 inches and larger by project.

Lower operating torque: Supported ball and spring-preloaded seats reduce friction under load.

Safety features: Blowout-proof stems, anti-static paths, fire-safe construction, and double block and bleed options.

Serviceability: Split-body or top-entry designs that allow in-line seat and seal maintenance on many models.

Durability: Reduced seat wear at pressure and better stability across thermal cycles.

The combination pays off in lower life-cycle cost where downtime or leakage risk has a penalty.

Trunnion vs Floating: Side-by-Side View

Sizing, Materials, and Seat Selection

Correct specification is the difference between a good asset and a source of unplanned downtime. Work through these items when sizing:

Pressure class and temperature

Choose ASME class to cover maximum operating pressure with margin.

Validate temperature ratings for body, seats, and seals. Metal-seated designs address very high temperatures.

Bore selection

Full bore for pigging, minimal pressure drop, and lower velocities.

Reduced bore to control cost and weight where full bore is not needed.

End connections

Flanged for ease of maintenance and alignment.

Butt-weld for pipelines and reduced leak paths, with attention to weld-end metallurgy and PWHT requirements.

Materials

Carbon steel and stainless steel cover most services.

Duplex, super duplex, or nickel alloys for chloride stress cracking, sour service, or aggressive media.

Seats and seals

PTFE and reinforced PTFE for general chemical compatibility.

PEEK for higher temperature and pressure.

Metal seats with hard coatings for high temperature or solids.

Elastomer O-rings and packing selected for media and temperature.

Sealing architecture

Decide on DPE or SPE seats per isolation philosophy and cavity relief needs.

Consider double isolation and bleed designs where code or risk analysis calls for it.

Document fire-safe, anti-static, and fugitive emissions requirements early so the right test standards appear in the datasheets.

Installation, Operation, and Maintenance Practices

Trunnion ball valves reward good practices with long service.

Handling and storage

Keep ends capped. Store clean and dry. Avoid side loading the stem.

Pre-commissioning

Verify pressure class, materials, and direction indicators.

Stroke test with the actual actuator and confirm limit switch settings.

Hydrotest per project spec, then dry and preserve as needed.

Commissioning

Bring up pressure slowly to avoid slamming seats.

Check packing gland tightness and live-load settings if applicable.

Operation

Avoid throttling at small openings that can erode seats. Ball valves are primarily on/off devices.

For fast-acting actuators, evaluate surge and water hammer.

Maintenance

Inspect periodically for external leakage and cycling torque drift.

Use in-line service capability where designed, noting top-entry access or split-body removal steps.

Where fitted, emergency sealant injection ports can be a temporary mitigation, not a substitute for repair.

A short preventive maintenance checklist reduces surprises:

• Verify actuator health and air supply quality.
• Trend torque requirements and cycle counts.
• Check gland packings for cold flow or relaxation.
• Confirm bleed ports and cavity relief paths are clear.

Reliability, Failure Modes, and Preventive Measures

Common issues and how to avoid them:

Seat damage from debris

Use strainers upstream during commissioning and consider filtration for dirty service.

Choose metal seats and hard coatings for solids-laden media.

Pressure locking of the body cavity

Specify SPE seat on at least one side or cavity relief devices that prevent trapped overpressure.

Provide a bleed port between seats for testing and relief.

Thermal expansion issues

Confirm temperature transients and specify materials that tolerate cycling.

Verify stem packing selection for thermal range and emissions targets.

Excessive actuation torque

Right-size actuators for worst-case differential pressure and temperature.

Pick low-friction bearings and coatings on the trunnions and stem.

Fugitive emissions from stem packing

Use live-loaded packing and qualified emission designs where required by regulation.

Follow OEM torque and adjustment values to avoid under or over tightening.

Reviewing the valve’s torque curve, seat design, and cavity pressure behavior as a set helps avoid most of these traps.

Standards and What They Mean to Your Specification

Key standards give buyers a common language and assure performance:

• API 6D or ISO 14313 for pipeline valves
• ASME B16.34 for pressure-temperature ratings and design
• API 607 or API 6FA for fire testing
• ISO 15848 or API 641 for fugitive emissions
• NACE MR0175/ISO 15156 for sour service materials
• MSS SP-25 for marking and identification
• End connection standards such as ASME B16.5 flanges and ASME B16.25 butt-weld ends

Procurement packages should call out testing, documentation, and upstream requirements:

• Hydrostatic shell and seat tests with pressures listed
• Low-pressure gas seat leakage tests where appropriate
• Fire-safe certification and emission test reports when specified
• Material test reports and traceability
• Coating systems and thickness for external corrosion

Quick Selection Pointers

Choose trunnion mounting when any of the following apply:

• Pipe sizes above 2 to 3 inches or where large-bore isolation is needed
• High pressure classes where torque and seat load become limiting factors
• Safety features like double block and bleed, fire-safe, and anti-static
• Automated isolation with a desire to minimize actuator size

Choose a floating ball valve when:

• Lines are small and pressures moderate
• Cost and simplicity dominate and the service is non-critical
• Throttling is minimal and cycles are light

A Short Sizing Example

An operator needs a block valve on a 24 inch natural gas transmission line at 900 class with pigging capability and remote actuation. The specification calls for fire-safe construction, double isolation during maintenance, and low fugitive emissions.

Bore: Full bore to allow pig passage and minimize pressure drop.

Body: Forged steel to match pressure class and project materials.

Seats: DPE on both sides to permit double isolation and a bleed valve between seats for verification.

Cavity relief: Confirm a design that prevents trapped overpressure.

Actuation: Pneumatic actuator with sufficient torque at minimum temperature and maximum differential pressure, with partial stroke testing capability for various applications.

Compliance: API 6D, API 607, ISO 15848.

Extras: Anti-static device, blowout-proof stem, external coating for buried service.

The trunnion ball valves design keeps torque reasonable at pressure, supports the large diameter, and delivers the isolation philosophy the operator requires.