What Is a Trunnion Mounted Ball Valve?

A trunnion mounted ball valve is a type of quarter‑turn valve in which the ball is supported

by trunnions (short shaft extensions) at the top and bottom. These trunnions are anchored

in bearing supports inside the valve body. Because the ball is restrained from moving

downstream under line pressure, the seating action is provided by spring‑loaded or pressure‑energized

seats that move toward the ball.

In a trunnion mounted ball valve, the ball only rotates around its axis; it does not float

or shift axially. This design drastically reduces the operating torque, improves seat

performance at high pressure, and allows reliable sealing in large diameters where floating

ball valves would be impractical or unsafe.

Trunnion mounted ball valves are commonly used as:

• Mainline pipeline isolation valves
• Block valves in compressor and pump stations
• High pressure process isolation valves
• Export terminal valves for oil and gas
• Emergency Shutdown Valves (ESDVs) in critical services

Basic Construction and Components

Although there are many design variations, most trunnion mounted ball valves share a

common set of core components. Understanding these elements helps explain the engineering

advantages of the trunnion design.

Main Components of a Trunnion Mounted Ball Valve

The trunnion arrangement allows designers to optimize each component for high pressure,

safety, and service life in demanding industrial environments.

Working Principle of a Trunnion Mounted Ball Valve

The working principle of a trunnion mounted ball valve is based on a quarter‑turn rotation

of the ball and a seat design that moves relative to the fixed ball. The interaction between

line pressure, seat springs, and trunnion supports defines its performance.

Open Position

• The ball bore is aligned with the pipeline, allowing full‑bore or reduced‑bore flow.
• Line fluid flows through the port with minimal pressure drop.
• Seats are retracted slightly by springs and pressure balance, but maintain gentle contact with the ball.
• The body cavity pressure often equalizes with line pressure through seat design clearances.

Closing Operation

1. An operator or actuator rotates the stem 90°.
2. The trunnions guide the ball rotation without axial displacement.
3. As the ball rotates, the solid side of the ball approaches the seats.
4. Seat springs push the seats toward the ball, creating initial mechanical contact.

5. With increasing differential pressure, line pressure energizes the upstream seat

   (and often the downstream seat) against the ball, enhancing sealing.

Closed Position

• The ball bore is perpendicular to the pipeline, blocking flow.
• Seats press against the ball to provide tight shut‑off.

• In double block and bleed (DBB) trunnion mounted ball valves, both upstream and downstream

  seats seal, isolating the body cavity. The cavity can be vented or drained for verification.

• The trunnions absorb the line force acting on the ball, preventing excessive seat loading

  and enabling stable torque values.

Pressure‑Energized Sealing

A critical engineering characteristic of the trunnion mounted ball valve is pressure‑energized

seating. Unlike floating ball valves where pressure pushes the ball into the downstream seat,

the trunnion design uses line pressure behind the seats:

• Line pressure enters cavities behind the seats via designed clearances or channels.

• This pressure generates a force that pushes the seats toward the ball, increasing

  sealing as the differential pressure increases.

• Seat springs ensure contact at low pressure, while fluid pressure takes over at

  medium and high pressures.

The combination of fixed ball and movable seats explains why trunnion mounted ball valves

are preferred for very high pressures and large sizes: sealing performance improves

rather than deteriorates under pressure, while torque remains manageable.

Trunnion Mounted vs Floating Ball Valve

Understanding the differences between trunnion mounted ball valves and floating ball valves

is essential for correct valve selection. Both are ball valves, but their internal support

and seating mechanisms are fundamentally different.

Side‑by‑Side Comparison

From an engineering perspective, the trunnion mounted ball valve is the preferred solution

whenever pipeline diameter, system pressure, or safety criticality dictate a robust,

low‑torque, high‑integrity isolation valve.

Key Engineering Advantages of Trunnion Mounted Ball Valves

Trunnion mounted ball valves offer a set of engineering advantages that explain their

widespread use in demanding industries. These advantages directly address concerns about

torque, sealing, safety, and life‑cycle costs.

1. Reduced Operating Torque

• The ball is supported by trunnions and bearings, so it does not press heavily against

  the seats under pressure.

• The seating force is primarily generated by line pressure acting on seats and by

  springs, not by ball displacement.

• As a result, trunnion mounted ball valves require significantly lower actuation torque,

  especially in high pressure or large‑diameter applications.

• Lower torque enables the use of smaller actuators, reduces energy consumption,

  and limits mechanical stress on stems and gearboxes.

2. Superior Sealing at High Pressure

• The seat design in trunnion ball valves is pressure‑energized: increasing line

  pressure enhances the seat load on the ball.

• Unlike some valve types that lose sealing reliability at very high pressures,

  the trunnion mounted ball valve maintains or improves tightness under pressure.

• Both soft‑seated and metal‑seated trunnion designs can achieve high integrity

  shut‑off, including API 6D and ISO 5208 leakage classes.

3. Double Block and Bleed Capability

• Many trunnion mounted ball valves are designed for true double block and bleed (DBB)

  or double isolation and bleed (DIB) functionality.

• Upstream and downstream seats are capable of sealing simultaneously, isolating

  the body cavity from both sides.

• A bleed valve on the body cavity allows verification of seat integrity, line isolation,

  and safe depressurization for maintenance.

• DBB/DIB trunnion valves replace the need for two separate block valves with an

  intermediate bleed, simplifying piping layouts.

4. Improved Safety and Fire‑Safe Design

• Fire‑safe trunnion mounted ball valves are designed to maintain pressure‑containing

  capability during and after a fire event, typically tested according to

  API 607 or ISO 10497.

• Soft seats and stem seals can be backed by metal‑to‑metal fire‑safe sealing features

  that remain functional after soft materials are damaged by elevated temperatures.

• Trunnion support reduces the risk of blowout or internal displacement of trim

  components under abnormal conditions.

5. Suitability for Large Diameter Pipelines

• In large‑diameter pipelines (e.g., 16" to 60" and above), forces on the ball and seats

  become very large.

• Floating ball designs would require very high actuation torque and could experience

  high seat wear or instability.

• Trunnion mounted ball valves distribute forces through the trunnions into the body,

  enabling stable operation even in very large diameters.

6. Enhanced Service Life and Reliability

• Balanced load distribution and reduced seat stress mitigate wear and deformation.

• The ball experiences less scratching and scuffing due to controlled contact pressures.

• Reliable sealing performance over many operating cycles is achievable, reducing

  downtime and maintenance frequency.

7. Versatile Configuration and Customization

• Trunnion mounted ball valves can be configured as two‑piece, three‑piece, or fully

  welded body designs.

• Side‑entry and top‑entry configurations allow adaptation to installation and maintenance requirements.

• Bore options include full bore (full port), reduced bore, and special port shapes

  for piggable pipelines or throttling service.

Common Design Features and Options

Trunnion mounted ball valves can incorporate multiple advanced design features tailored

to specific service conditions. The following subsections summarize widely used industry‑standard

options.

Body Design Types

Seat Types and Configurations

• Soft‑Seated Trunnion Mounted Ball Valves:

  • Use polymer inserts such as PTFE, R‑PTFE, PEEK, or other engineered plastics.
  • Offer bubble‑tight shut‑off and very low leakage rates.
  • Recommended for clean fluids, gases, and non‑abrasive services.

• Metal‑Seated Trunnion Mounted Ball Valves:

  • Seats are metallic, often with hardfacing (e.g., Stellite, tungsten carbide).
  • Designed for high temperature, abrasive, erosive, or dirty services.
  • Provide robust sealing with acceptable leakage rates defined by standards.

• Single Piston Effect Seats:

  • Pressure on one side of the seat generates a force that pushes the seat toward the ball.
  • Excess cavity pressure can self‑relieve toward the line from that seat.

• Double Piston Effect Seats:

  • Pressure from either side of the seat (line or cavity) pushes the seat

    toward the ball, increasing sealing.

  • Requires a dedicated cavity relief system to avoid overpressure in the

    body cavity.

End Connections

Actuation Options

• Manual lever operation for small sizes and lower pressures.
• Manual gear operators for larger diameters or higher torque requirements.
• Pneumatic actuators (spring‑return or double‑acting) for fast, automated operation.
• Hydraulic actuators in subsea or high‑torque applications.
• Electric actuators for precise control and integration into plant automation.

Trunnion mounted ball valves are often supplied as complete automated packages,

especially when used as emergency shutdown valves or remote pipeline block valves.

Typical Materials of Construction

Material selection for a trunnion mounted ball valve is a critical engineering step,

influenced by internal pressure, temperature, fluid composition, and external environment.

Common Body and Bonnet Materials

Ball, Seat, and Stem Materials

• Ball:

  • Generally same grade as body or higher alloyed for corrosion resistance.
  • Can be overlayed or coated with hard materials for erosion/corrosion resistance.

• Seats:

  • Soft seats: PTFE, R‑PTFE, PEEK, Nylon, and other high‑performance polymers.
  • Metal seats: stainless steel or alloy steel with hardfacing (Stellite, tungsten carbide).

• Stem and Trunnions:

  • Typically stainless steel or alloy steel for strength and corrosion resistance.
  • Surface hardening or plating to prevent galling and wear in bearings and seals.

Selection must also consider standards such as NACE MR0175/ISO 15156 for sour service

and various end‑user specifications.

Applicable Standards and Design Codes

Trunnion mounted ball valves for industrial service are engineered and tested according

to recognized international standards. The following are commonly referenced in specifications:

Typical Technical Specifications for Trunnion Mounted Ball Valves

While actual specifications depend on project requirements, the following table summarizes

common ranges and options for engineered trunnion mounted ball valves.

Typical Industrial Applications

Due to their engineering advantages, trunnion mounted ball valves are used across a wide

range of industries that demand reliable isolation under challenging conditions.

Oil and Gas Production

• Wellhead and manifold isolation (onshore and offshore).
• Flowline and gathering systems.
• Gas lift and injection systems.

Transmission Pipelines

• Long‑distance crude oil and natural gas pipelines.
• Mainline block valves and sectionalizing valves.
• Pig launcher and receiver station isolation.

Refining and Petrochemicals

• Feedstock and product transfer lines.
• High‑pressure hydrogen and hydroprocessing units.
• Critical isolation in flammable and toxic service.

Power Generation

• Fuel gas and liquid fuel systems.
• High‑pressure steam systems (with appropriate materials and seats).
• Balance of plant process isolation.

LNG and Cryogenic Service

• LNG plants and receiving terminals.
• Storage tank inlets/outlets and transfer lines.
• Cryogenic isolation for liquefied gases.

Other Industries

• Chemical and specialty chemical plants.
• Mining and slurry handling (with abrasion‑resistant metal seats).
• Water injection and industrial water systems.

Engineering Selection Guidelines for Trunnion Mounted Ball Valves

Selecting the right trunnion mounted ball valve for an application involves evaluating

multiple engineering parameters simultaneously. The following guidelines outline

typical decision criteria.

Key Selection Criteria

• Pressure and Temperature:

  • Determine the maximum and minimum operating pressure and temperature.
  • Select pressure class and materials to handle design conditions with an adequate margin.

• Fluid Properties:

  • Assess corrosiveness, toxicity, flammability, and abrasiveness.
  • Decide between soft‑seated or metal‑seated trunnion ball valves.
  • Consider hydrogen, sour gas, or oxygen compatibility if applicable.

• Pipeline Diameter and Layout:

  • For DN ≥ 50–80 mm and high‑pressure service, trunnion mounted ball valves

    are usually preferred.

  • Check space availability for actuator and maintenance access.

• Leakage and Tight Shut‑off Requirements:

  • Define acceptable leakage class; many applications require bubble‑tight

    shut‑off from trunnion ball valves.

  • Confirm DBB or DIB requirements and specify seat design accordingly.

• Fire‑Safe and Safety Integrity:

  • Specify fire‑safe design and testing where hydrocarbons or flammable fluids are present.
  • For safety instrumented functions, integrate the trunnion mounted ball valve

    and actuator into the SIL assessment.

• Actuation and Control:

  • Decide on manual or automated operation, fail‑safe positions, and stroking times.
  • Ensure compatibility with control and safety systems.

Installation, Operation, and Maintenance Considerations

Proper installation and maintenance practices are vital for realizing the full engineering

advantages of trunnion mounted ball valves.

Installation Best Practices

• Verify valve orientation (flow direction) if the design is unidirectional or DIB type.
• Ensure cleanliness of piping; remove debris that could damage seats during first operation.
• Support heavy trunnion ball valves adequately to avoid piping stress.
• Align flanges or welding ends carefully to prevent body distortion.
• Perform field leak tests according to project and standard requirements.

Operation and Actuation

• Operate the valve only within its rated pressure and temperature limits.
• Avoid throttling with soft‑seated trunnion ball valves unless specifically designed for control service.
• Regularly test emergency shutdown and remote‑operated valves to confirm response times and functionality.

Maintenance and Inspection

• Inspect stem packing, body seals, and external surfaces for leakage or corrosion.
• Verify function of drain and vent connections and DBB/DIB arrangements.
• For valves with injection fittings, test and maintain sealant systems as per procedure.
• Follow manufacturer’s guidelines for lubrication of bearings and stem if required.

Frequently Asked Engineering Questions About Trunnion Mounted Ball Valves

Why choose a trunnion mounted ball valve over a floating ball valve?

A trunnion mounted ball valve is preferred when system pressure is high, pipeline diameter

is large, or tight shut‑off with low torque is required. The trunnion design provides

stable support for the ball, minimizes seat wear, and allows double block and bleed configurations.

Can trunnion mounted ball valves be used for throttling?

Standard trunnion ball valves are primarily designed for on/off isolation service. Limited

throttling may be acceptable in some designs and operating envelopes, but long‑term control

duty should use a dedicated control valve or specially engineered trunnion ball control valve.

What is the difference between double block and bleed (DBB) and double isolation and bleed (DIB)?

In many trunnion mounted ball valve designs, DBB means the valve uses seats that provide

isolation from both sides, with a bleed on the cavity to verify sealing. DIB typically

involves double piston effect seats that can seal from both directions simultaneously,

with external relief devices for cavity overpressure. Exact terminology depends on standards

and project specifications, so definitions should be confirmed.

Are trunnion mounted ball valves piggable?

Full bore trunnion mounted ball valves are typically piggable, meaning pipeline inspection

gauges or cleaning pigs can pass through. Full bore design is widely used in pipeline systems

to facilitate pigging operations.

Do trunnion mounted ball valves require special foundations or supports?

In large diameters and high‑pressure applications, trunnion ball valves can be heavy.

Proper pipe supports or valve supports are necessary to avoid excessive loads on piping,

actuators, and bodies. Pipe stress analysis should consider the valve weight and dimensions.

Summary

Trunnion mounted ball valves provide a powerful combination of engineering advantages:

reduced operating torque, high‑integrity sealing under extreme pressure, suitability

for large‑diameter pipelines, and versatility across a wide range of industrial applications.

By understanding the trunnion support mechanism, pressure‑energized seat design, applicable

standards, and available configuration options, engineers can confidently specify and

apply trunnion mounted ball valves to improve reliability, safety, and life‑cycle performance

in demanding systems.