1. What Is a Fixed Ball Valve?

A fixed ball valve is a quarter-turn shut-off valve in which the

ball is supported by trunnions (upper and lower journals).

Unlike a Floating ball valve where the ball moves slightly downstream under

pressure, the ball in a fixed ball valve is mechanically held in place

and does not translate with line pressure. The sealing load is taken mainly by

the spring-loaded or pressure-energized seats, not by the ball

shifting against the seats.

This design allows fixed ball valves to handle higher pressures,

larger sizes, and more demanding service conditions

than standard floating ball valves. Fixed ball valves are widely used in:

• Oil and gas transmission and distribution pipelines
• Petrochemical and chemical processing plants
• Refining, LNG, and gas processing facilities
• HVAC and district heating systems (for large diameters)
• Power generation and industrial utility services

In many standards and technical documents, the term

trunnion ball valve is used interchangeably with

fixed ball valve.

2. Key Features of a Fixed Ball Valve

The fixed ball valve structure introduces several features that

distinguish it from other ball valve types:

• Trunnion-mounted ball eliminates axial movement of the ball.
• Seats are movable and often spring-loaded to ensure tight sealing.
• Lower operating torque compared with floating ball valves in the same size and pressure class.
• Suitable for large nominal diameters and high-pressure ratings.
• Can be designed for double block and bleed (DBB) and double isolation and bleed (DIB) functions.
• Available in full bore and reduced bore configurations.

3. Fixed Ball Valve Structure Overview

The structure of a fixed ball valve is built around a rigidly

supported ball and movable seats. A simplified description of the internal

layout is:

1. Central ball with upper and lower trunnions.
2. Seat rings on both upstream and downstream sides of the ball.
3. Body forming the pressure boundary and containing the ball and seats.
4. Bonnet or body covers for assembly, sealing, and maintenance access.
5. Stem connected to the ball to transmit actuation torque.
6. Sealing elements (soft or metal seats, body gaskets, stem packing).
7. Trunnion bearings or bushings supporting the ball and stem.
8. Actuator interface (manual lever, gear operator, or power actuator).

Because the ball is fixed, the process pressure pushes the seats

toward the ball instead of moving the ball toward the seats. This reduces seat wear

and increases seal reliability in heavy-duty applications.

4. Main Components of a Fixed Ball Valve

A typical fixed ball valve assembly contains the following main

components:

5. Detailed Description of Fixed Ball Valve Components

5.1 Valve Body

The valve body is the pressure-retaining outer shell of the

fixed ball valve. It is designed to withstand internal pressure,

external loads, and piping forces. Body design options include:

• One-piece body (usually smaller sizes; compact but limited maintenance access).
• Two-piece body (common in medium sizes and pressures; offers balance between cost and maintenance).
• Three-piece body (widely used for trunnion ball valves; superior maintainability and modular construction).
• Top-entry body (allows access by removing bonnet without removing valve from pipeline).

Typical body materials depend on service condition:

• Carbon steel for general service and non-corrosive applications.
• Stainless steel for corrosive environments and clean services.
• Duplex or super duplex for chloride-rich and offshore conditions.
• Alloy steels for high temperature and critical applications.

5.2 Ball and Trunnions

The ball is the core closure element. In a fixed ball valve, the

ball is supported by upper and lower trunnions. Important design

points:

• Ball may be solid or hollow depending on size and weight considerations.
• Port type: full bore (same diameter as pipeline) or reduced bore.
• Surface: precision machined and often hard-coated to improve wear resistance and sealing performance.

5.3 Seats and Sealing Rings

The seats are responsible for achieving tight shut-off

between the ball and the valve body. In a fixed ball valve, seats are usually

independent movable components that can slide slightly along

the valve axis.

Main design types:

• Soft seated fixed ball valves with PTFE, RPTFE, PEEK, or other polymers.
• Metal seated fixed ball valves with hard facing on seat and ball.
• Composite seats combining metal carrier ring with soft sealing insert.

5.4 Stem Assembly

The stem connects the ball to the external actuation mechanism.

In a fixed ball valve, the stem is typically non-rising and

top-mounted. Key functions:

• Transmits torque from handle, gearbox, or actuator to the ball.
• Maintains alignment with bearings and packing.
• Incorporates anti-blowout design to prevent stem ejection.

The stem is sealed by a combination of packing rings,

O-rings, and sometimes live-loaded springs to

compensate for wear and temperature changes.

5.5 Bearings, Bushings, and Support Elements

The fixed ball valve structure relies on low-friction support

components to ensure smooth operation:

• Stem bearings to center the stem and reduce wear.
• Trunnion bearings or bushings to support ball journals.
• Materials such as reinforced PTFE, composite bearings, or metal-backed PTFE.

Proper bearing design reduces operating torque and

extends the service life of the valve.

5.6 Springs and Seat Loading Mechanisms

Most fixed ball valves use spring-loaded seats.

The springs push the seats toward the ball to:

• Maintain sealing contact at low differential pressure.
• Compensate for wear, thermal expansion, or contraction.
• Allow bidirectional sealing for double block and bleed designs.

Springs are often made from corrosion-resistant alloys and

designed based on the valve size and pressure class.

5.7 Body Gaskets and Seals

Body and bonnet joints are sealed with gaskets, which can be:

• Spiral wound gaskets with graphite or PTFE filler.
• Ring joint gaskets for high pressure classes.
• Metal-to-metal contact with soft sealing inlay.

The choice of gasket type depends on pressure, temperature, and fluid

compatibility requirements.

5.8 Drain, Vent, and Injection Fittings

Industrial fixed ball valves frequently include:

• Body cavity drain to remove trapped medium.
• Vent port for depressurizing and testing.
• Sealant injection fittings for emergency seat or stem sealing in critical applications.

These features assist in maintenance,

pressure relief, and integrity testing.

6. Internal Flow Path and Sealing Principle

The flow path in a fixed ball valve is controlled by the ball

port alignment:

• Open position: ball port aligns with pipeline, allowing flow.
• Closed position: solid portion of ball blocks the flow path.

In a trunnion-mounted design, when the valve is closed and

pressure is applied:

1. Process pressure acts on the upstream seat.
2. The seat, being movable, is pushed against the ball.
3. Sealing contact pressure is created between seat and ball face.

Because the ball does not move axially, seat wear is reduced

compared to floating ball valves. Valve design may incorporate:

• Self-relieving seats to relieve overpressure in the cavity.
• Automatic body cavity relief to downstream side for safety.
• Bidirectional sealing seats for double block and bleed applications.

7. Common Materials for Fixed Ball Valve Components

Material selection for each fixed ball valve component depends

on process conditions such as pressure, temperature, and fluid composition.

8. Types of Fixed Ball Valve Body Designs

The structure of fixed ball valves can be divided into several

common body configurations:

8.1 Side-Entry (Split Body) Fixed Ball Valve

• Body made from two or three sections, assembled with bolts.
• Seats, ball, and trunnions inserted from the side during assembly.
• Most widely used design in pipeline and industrial applications.

8.2 Top-Entry Fixed Ball Valve

• Body has a top cover or bonnet allowing access from above.
• Ideal where online maintenance is required without removing valve from line.
• Often used in critical and high-value service locations.

8.3 Fully Welded Fixed Ball Valve

• Body is welded instead of bolted to minimize external leakage paths.
• Common in buried pipelines and gas distribution networks.
• Offers superior integrity but limited internal access for maintenance.

9. Operation and Actuation of Fixed Ball Valves

Fixed ball valves are quarter-turn valves. They can be operated

by:

• Manual lever for small sizes and low torque.
• Manual gearbox (worm gear) for larger diameters and higher class ratings.
• Pneumatic actuators for fast and frequent operation.
• Hydraulic actuators in high-torque, subsea, or remote locations.
• Electric actuators where electrical control is preferred.

Due to the trunnion-mounted design and low friction bearings,

the operating torque is significantly lower than that of

floating ball valves in comparable sizes. This reduces:

• Actuator size and cost.
• Manual operator effort.
• Potential for mechanical damage to seats and stem.

10. Advantages of Fixed Ball Valve Structure

The fixed ball valve structure offers several advantages for

demanding industrial applications:

11. Selection Criteria for Fixed Ball Valves

When selecting a fixed ball valve, engineers typically evaluate:

• Nominal size and bore (full bore vs. reduced bore).
• Pressure rating (ANSI / ASME Class, PN rating).
• Temperature range and corresponding material limits.
• Fluid type (clean gas, oil, slurry, corrosive chemicals, etc.).
• Seat type (soft or metal seating depending on leakage class and service).
• End connections (flanged, butt-weld, socket-weld, threaded).
• Actuation method and required accessories (limit switches, positioners, solenoids).
• Design standards and certifications applicable to the installation.

12. Typical Specification Table for Fixed Ball Valves

The following is an example of typical technical specifications

for industrial fixed ball valves. Actual values depend on detailed design and

applicable standards.

13. Typical Applications of Fixed Ball Valves

Because of their robust structure and reliable

main components, fixed ball valves are used in:

• Pipeline isolation in transmission systems for crude oil, natural gas, and refined products.
• Block valves in compressor stations and pumping stations.
• Process isolation in refineries and petrochemical units.
• Onshore and offshore wellhead and manifold systems.
• Tank farm inlet and outlet lines.
• Power plants and industrial utility services.

14. Maintenance Considerations for Fixed Ball Valves

The fixed ball valve structure is designed for long service

life, but periodic inspection is important:

• Check stem packing for signs of external leakage and adjust or replace if needed.
• Operate the valve periodically to ensure full movement and prevent sticking.
• Inspect body seals and gaskets during scheduled shutdowns.
• For valves with injection fittings, maintain adequate sealant according to procedures.
• Verify proper function of drain and vent ports.

Top-entry and split-body fixed ball valves allow internal components such as

seats and ball to be serviced or replaced without fully

removing the valve from the pipeline, depending on installation and design.

15. Fixed Ball Valve vs. Floating Ball Valve

Understanding the differences between fixed (trunnion) ball valves

and floating ball valves helps in choosing the appropriate type.

16. Safety and Design Options in Fixed Ball Valves

Modern fixed ball valve structures incorporate several safety

features:

• Anti-blowout stem design to prevent stem ejection.
• Antistatic devices to avoid static electricity build-up between ball and stem/body.
• Fire-safe construction with metal back-up sealing in case of seat damage by fire.
• Fugitive emission control with low-emission packing systems.
• Locking devices to prevent unauthorized operation.
• Position indicators and limit switches to confirm open/closed status.

These options depend on project specifications and regulatory requirements.

17. Conclusion

A fixed ball valve, or trunnion mounted ball valve,

is a robust and versatile solution for high-pressure,

large-diameter, and critical isolation

applications. Its structure, based on a trunnion-supported ball

and movable seats, provides low operating torque, long service

life, and reliable sealing.

Understanding the main components of a fixed ball valve—body,

ball, trunnions, seats, stem, seals, bearings, and actuation interface—helps

engineers and users to make informed decisions regarding valve selection,

specification, and maintenance. By focusing on

proper material selection, structural design, and quality components, fixed ball

valves can deliver safe and efficient service across a wide range of industrial

applications.