1. What Is a Trunnion Mounted Ball Valve?

A trunnion mounted ball valve is a quarter-turn shut-off valve where the ball is supported by trunnions (upper and lower shafts or journals) that hold the ball firmly in position. Unlike a Floating ball valve, the ball in a trunnion mounted design does not move downstream under pressure. Instead, the seats are spring-loaded or pressure-energized to provide tight sealing against the fixed ball.

In high-pressure and large-size pipelines, trunnion mounted ball valves are preferred because they reduce operating torque, minimize seat wear, and offer reliable bi-directional sealing. These valves are widely used in transmission pipelines, block valves, terminal stations, compressor stations, and process plants.

Key characteristics of trunnion mounted ball valves include:

• Quarter-turn operation (typically 0°–90° rotation)
• Supported ball with upper and lower trunnions
• Spring-loaded or pressure-assisted seats
• Suitable for large diameters and high pressures
• Option for Double Block and Bleed (DBB) and Double Isolation and Bleed (DIB)
• Available in soft-seated and metal-seated designs

2. Key Components of a Trunnion Mounted Ball Valve

Understanding the main components helps explain the working principle of a trunnion mounted ball valve.

3. Working Principle of Trunnion Mounted Ball Valve

The working principle of a trunnion mounted ball valve is based on a fixed, supported ball and movable seats that create a tight seal under pressure. The core mechanism involves quarter-turn rotation and pressure-assisted seat sealing.

3.1 Flow Control by Quarter-Turn Rotation

Trunnion mounted ball valves use a 90-degree rotation of the ball to switch between open and closed positions:

• Open Position (0° or 90°, depending on design): The bore of the ball is aligned with the pipeline axis. Fluid flows through the full or reduced bore with minimal pressure drop.
• Closed Position (90° or 0°): The solid part of the ball blocks the flow path. The seats seal against the ball, preventing flow in one or both directions.

3.2 Role of Trunnions in the Working Principle

In a trunnion mounted ball valve, the ball is supported at two points:

• The upper trunnion connects to the stem or is integral with it.
• The lower trunnion supports the bottom of the ball.

This support arrangement ensures that:

• The ball does not move downstream under high differential pressure.
• The operating torque is reduced because the line pressure load is taken by the trunnions, not by the seats.
• The contact pressure between seat and ball can be controlled more effectively.

3.3 Seat Movement and Pressure-Energized Sealing

Unlike floating ball valves where the ball moves toward the downstream seat, in trunnion ball valves the seats move toward the ball. This movement occurs through spring force and differential pressure.

The sealing mechanism operates in two phases:

1. Low-Pressure Sealing: At low differential pressures, metallic or elastomeric springs behind the seats push them into contact with the ball, ensuring initial sealing.
2. High-Pressure Sealing: As line pressure increases, pressure acts on the back side of the upstream seat, forcing it more tightly against the ball. This pressure-energized effect enhances the seal.

3.4 Flow Direction and Bi-Directional Sealing

Trunnion mounted ball valves can be designed for:

• Uni-directional sealing: Effective sealing mainly in one flow direction.
• Bi-directional sealing: Effective sealing in both directions, often required in pipeline block valves.

Many modern trunnion ball valves are bi-directional, meaning they can provide tight shut-off whether flow comes from upstream or downstream side. Depending on design, they can be classified as DBB or DIB valves (discussed in a later section).

3.5 Cavity Relief and Body Cavity Pressure

The cavity between the seats and around the ball (body cavity) may become pressurized due to thermal expansion or trapped media. To prevent overpressure, trunnion mounted ball valves are often designed with:

• Self-relieving seats: The seat will unseal slightly at a defined pressure differential, allowing cavity fluid to relieve to the downstream side.
• External relief connections: Optional relief valves or bleed lines connected to the body cavity.

This cavity relief remains part of the working principle because it influences seat design and sealing direction.

4. Floating Ball Valve vs Trunnion Mounted Ball Valve

Understanding the difference between floating and trunnion mounted ball valves is crucial for proper valve selection. Both are ball valves, but they employ different working principles for sealing and load distribution.

5. Sealing Mechanism and Seat Design

The sealing mechanism of a trunnion mounted ball valve depends greatly on the seat and ball interface. Two main categories are soft-seated and metal-seated designs.

5.1 Soft-Seated Trunnion Ball Valves

Soft-seated trunnion ball valves use polymeric or elastomeric materials such as PTFE, reinforced PTFE, PEEK, or other plastics and rubbers as the primary sealing element. Typical features include:

• Very low leakage: Capable of achieving tight shut-off (often leakage rate A or ISO 5208 Rate A).
• Lower operating torque: Due to softer seat materials and smooth contact with the ball.
• Temperature limitations: Generally limited by the soft material’s thermal stability.
• Chemical compatibility: Seat composition chosen according to media (e.g., hydrocarbons, corrosive fluids).

5.2 Metal-Seated Trunnion Ball Valves

Metal-seated trunnion mounted ball valves use metallic seat rings and often coated balls (e.g., hardfacing, tungsten carbide). They are selected when:

• Operating temperatures exceed the limit of soft seats.
• Media are abrasive, erosive, or contain solids.
• Fire-safe performance is critical.

Characteristics include:

• Higher allowable temperature range.
• Increased wear resistance in harsh conditions.
• Slightly higher leakage rates compared to soft-seated designs, depending on class.

5.3 Single Piston Effect (SPE) and Double Piston Effect (DPE) Seats

Trunnion mounted ball valves often apply specific seat geometries known as Single Piston Effect (SPE) and Double Piston Effect (DPE) to manage sealing and cavity relief.

5.4 Fire-Safe Sealing Concept

Fire-safe trunnion mounted ball valves are designed such that if soft seals are destroyed by fire, secondary metal-to-metal seals maintain a certain level of tightness to prevent extensive leakage. Fire-safe certification typically follows internationally recognized standards.

6. Main Design Features of Trunnion Ball Valves

Modern trunnion mounted ball valves incorporate multiple design features to meet industrial requirements for safety, performance, and maintainability.

6.1 Body Construction Types

• Two-Piece Split Body: Body is split along a vertical or horizontal plane, with bolted joint. Common in medium and large sizes.
• Three-Piece Body: Central body section containing ball and seats with two end closures. Facilitates maintenance and seat replacement.
• Top Entry: Bonnet or cover on top enables access to internals without removing the valve from the line, suitable for welded-in Pipeline Valves.
• Fully Welded Body: No external body bolts; body joints are welded. Recommended for buried or subsea pipelines where leakage from body joints must be minimized.

6.2 Bore Types

• Full Bore (Full Port): The ball bore matches the nominal pipe diameter. Minimizes pressure drop and allows pigging in pipelines.
• Reduced Bore (Reduced Port): Ball bore is smaller than pipe ID. Reduces weight and cost; suitable when pressure drop is acceptable.

6.3 Stem Design

• Anti-Blowout Stem: Stem is inserted from inside the body and retained so it cannot be expelled under pressure.
• ISO Mounting Pad: Standard interface for actuators according to ISO 5211 or similar patterns.
• Low Emission Packing: Use of advanced packing systems to meet fugitive emission requirements.

6.4 Sealant Injection and Emergency Sealing

Many trunnion mounted ball valves in critical services provide:

• Stem sealant injection fittings: Allow temporary sealing enhancement if packing is damaged.
• Seat sealant injection: Provides emergency secondary sealing between seat and ball in case of primary seal damage.

6.5 Anti-Static and Grounding Devices

To prevent static buildup due to fluid flow and friction between ball and seats, anti-static devices are added between:

• Stem and body.
• Ball and stem.

This ensures electrical continuity and reduces risk of sparks in flammable environments.

7. Relevant Standards and Design Codes

Trunnion mounted ball valves are engineered and tested in accordance with internationally recognized standards. Common standards include:

• API 6D: Specification for pipeline and piping valves; widely referenced for trunnion ball valves.
• API 608: Metal ball valves for general refinery and chemical service.
• ASME B16.34: Valves – Flanged, threaded, and welding end; defines pressure–temperature ratings.
• ISO 14313: Petroleum and natural gas industries – Pipeline transportation systems – Pipeline valves.
• ISO 5208 / API 598: Valve inspection and pressure testing.
• API 607 / ISO 10497: Fire test for soft-seated valves.
• ISO 15848 / EPA / TA-Luft: Fugitive emission standards (where applicable).

8. Typical Pressure–Temperature Ratings

The pressure–temperature rating of a trunnion mounted ball valve depends on material, design standard, and end connection. The following table provides generic examples based on common carbon steel materials:

Actual ratings must always be confirmed from the applicable design standard and material-specific charts. Soft seat materials may further limit maximum temperature.

9. Common Materials of Construction

Trunnion mounted ball valves are manufactured in a wide range of materials to handle different fluids, pressures, and temperatures.

10. End Connections and Face-to-Face Dimensions

Trunnion mounted ball valves can be supplied with various end connections to suit pipeline requirements.

10.1 Common End Connections

• Flanged Ends: Conform to ASME B16.5 or ISO standards; easy installation and removal.
• Butt Weld Ends: Conform to ASME B16.25; preferred in high-pressure pipelines and buried service.
• Hub or Clamp Connections: Used in offshore and specialized systems.

10.2 Face-to-Face and End-to-End Dimensions

Face-to-face and end-to-end dimensions for flanged and butt weld trunnion ball valves often follow:

• API 6D
• ASME B16.10
• ISO 5752

Standardized dimensions simplify replacement and interchangeability in pipeline systems.

11. Operation: Manual, Gear, and Actuated Trunnion Ball Valves

Because trunnion mounted ball valves often handle high pressure and large diameters, their operating torques can be substantial. Different operation methods are implemented:

11.1 Manual Lever Operation

Small-size trunnion ball valves at lower pressure ratings can be operated with a simple lever handle, allowing quick quarter-turn operation.

11.2 Gearbox Operation

For larger sizes or higher pressures, a manual gearbox is used:

• Reduces required hand force by providing mechanical advantage.
• Enables controlled, slow operation to prevent water hammer.

11.3 Actuated Operation

Automated trunnion mounted ball valves are common in modern process control and pipeline systems. Common actuators include:

• Pneumatic actuators: Suitable for fast operation and remote control.
• Hydraulic actuators: Used for high-torque, high-pressure applications, including subsea and Emergency Shutdown Valves.
• Electric actuators: Ideal where pneumatic or hydraulic supplies are limited and precise control is required.

Actuated trunnion ball valves may be configured as on/off, fail-safe (fail-open or fail-close), or modulating control valves when combined with positioners.

12. Typical Applications of Trunnion Mounted Ball Valves

Trunnion mounted ball valves are used in a wide range of industrial sectors due to their robust design and reliable sealing performance.

• Oil and Gas Transmission Pipelines: Mainline block valves, station isolation, scraper traps.
• Refineries and Petrochemical Plants: Process unit isolation, tank farm manifolds.
• Offshore Platforms: Wellhead manifolds, riser systems, topside and subsea isolation.
• Power Generation: Fuel gas lines, cooling water systems, auxiliary services.
• Chemical and Fertilizer Plants: Corrosive and toxic service isolation requiring reliable shut-off.
• Liquefied Natural Gas (LNG) and Cryogenic Service: Special cryogenic trunnion ball valves with extended bonnets.

13. Advantages of Trunnion Mounted Ball Valves

Using trunnion mounted ball valves in demanding applications offers multiple technical and economic benefits.

14. Selection Considerations

Selecting the correct trunnion mounted ball valve for a given service requires evaluation of several factors.

14.1 Process Conditions

• Fluid type (liquid, gas, multiphase, corrosive, dirty service).
• Operating pressure and temperature range.
• Presence of solids, sand, or particulates.
• Required shut-off class and leakage requirements.

14.2 Design Requirements

• Needed bore type (full or reduced bore) and piggability.
• Pressure class and connection type (flanged, welded, hub).
• Compliance with specific standards (API 6D, ISO, regional regulations).
• Need for DBB or DIB functionality and cavity relief method.

14.3 Operation and Control

• Manual versus automatic operation.
• Available utilities (air, hydraulic fluid, electricity).
• Fail-safe requirements and emergency shutdown logic.

14.4 Environmental and Safety Considerations

• Fire-safe and fugitive emission requirements.
• Hazardous area classification for actuators and accessories.
• Sour service compliance (e.g., sour gas with H₂S).

15. Maintenance, Inspection, and Safety Considerations

Proper maintenance ensures long-term reliability of trunnion mounted ball valves.

15.1 Routine Inspection

• Check for external leakage at body joints, stem packing, and end connections.
• Verify actuation performance and stroke time.
• Inspect for corrosion, coating damage, and mechanical damage.

15.2 Seat and Seal Maintenance

• Regularly exercise the valve to prevent sticking.
• Use sealant injection systems only as a temporary emergency measure, not as a permanent solution.
• In top-entry or three-piece designs, seats and seals can often be replaced without removing the valve from the line.

15.3 Safety During Operation

• Ensure system is depressurized before maintenance or disassembly.
• Confirm correct valve position before performing any pipeline work.
• Follow lockout-tagout procedures in automated systems.

16. Example Specification Tables for Trunnion Mounted Ball Valves

The following example tables illustrate typical specification data that engineers may reference when selecting or comparing trunnion mounted ball valves. Values are indicative and should not be used as design data.

16.1 Example Dimensional and Operational Data

16.2 Example Material and Design Summary

17. Frequently Asked Questions About Trunnion Mounted Ball Valve Working Principle

17.1 Why choose a trunnion mounted ball valve instead of a floating ball valve?

Trunnion mounted ball valves are preferred for high-pressure and large-diameter applications because they offer lower operating torque, reduced seat wear, and more reliable sealing. The fixed ball supported by trunnions ensures that pressure loads are transmitted to the valve body rather than to the seats, improving durability and safety.

17.2 How does the working principle affect leakage performance?

The working principle relies on spring-loaded and pressure-energized seats that move toward the stationary ball. This ensures a consistent sealing force across a wide pressure range. As a result, trunnion mounted ball valves can achieve very low leakage rates, making them suitable for critical isolation duties.

17.3 Can trunnion mounted ball valves be used for throttling?

Although a trunnion mounted ball valve can momentarily control flow, it is primarily designed as an on/off isolation valve. Prolonged operation in partially open positions may cause uneven seat wear, erosion, or cavitation damage. Where precise flow control is required, dedicated control valves are recommended.

17.4 What is Double Block and Bleed (DBB) in trunnion ball valves?

Double Block and Bleed refers to the ability of a single valve to provide two seating barriers (upstream and downstream) and an intermediate bleed connection to the body cavity. When the valve is closed, both seats isolate the cavity, and the bleed can be used to verify tight shut-off or to depressurize the cavity for maintenance.

17.5 What is Double Isolation and Bleed (DIB)?

Double Isolation and Bleed typically implies that both seats provide tight sealing against pressure from either side, often achieved with Double Piston Effect seats. In a DIB design, the body cavity is not self-relieving, and a separate relief system is used to protect against overpressure.

17.6 Are trunnion mounted ball valves piggable?

Full bore trunnion mounted ball valves are often designed to be piggable, meaning pipeline cleaning and inspection pigs can pass through without obstruction. This is particularly important in oil and gas pipelines where pigging is routine.