Trunnion Ball Valve - How They Work

Figure 1: Free floating ball valve (left) and a trunnion mounted ball valve (right)

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Trunnion ball valves are quarter-turn valves used to control the flow of media in a pipeline. Unlike the more common "free floating" ball valve design, trunnion ball valves have a unique configuration. In a trunnion ball valve, the ball is secured by connecting it to the stem on one side and a shaft called the trunnion on the other, ensuring the ball remains firmly in place. In contrast, a floating ball valve connects the ball only to the stem, allowing it to "float." This distinction is illustrated in Figure 1, highlighted by a red box.

Trunnion ball valves have many different advantages over free floating valves; they are suitable for both low and high-pressure applications and offer lower operational costs compared to free floating ball valves.

Trunnion Ball Valve Advantages

High Pressure Applications

For high pressure applications, a trunnion ball valve is the ideal ball valve design type. In floating ball valves, the ball has some freedom of movement. During use, the ball floats slightly downstream against the seat ring and creates a seal only on one side of the valve. The torque on the stem is also increased.

This is where the trunnion design offers a solution. Since the ball is not floating but is attached to the trunnion, it maintains a proper seal on both sides and does not contribute to added torque on the stem. Therefore, for high-pressure applications, a trunnion ball valve provides better sealing and requires less torque to operate.

Figure 2: A cross section of a trunnion ball valve, clearly showing the trunnion (A) and spring mounted seats (B)

Spring-loaded Seats

Trunnion mounted ball valves come with spring loaded seats. In floating ball valves, pressure forces the ball against the seats, creating a seal. Unfortunately, the ball valve requires high pressure to achieve an effective seal, which renders them ineffective in low pressure systems.

Trunnion mounted ball valves work effectively in both low and high pressure situations. In low or no-pressure circumstances, the spring-loaded seats create a seal, which also functions efficiently under high pressure.

Lower Operational Costs

A trunnion mounted ball valve results in lower operational costs because any additional pressure on the valve is absorbed by the trunnion and stem. This leads to lower operating torque, subsequently eliminating the need for large actuators.

To understand more about selecting the right ball valve for your application, check out our ball valve selection technical article!

Trunnion Mounted Ball Valve Weight

A trunnion ball valve is designed as per API608 or API 6D and complies with multiple industry standards. The API 6D standard specifies basic design, dimensions, and testing standards for the trunnion valve, including standard weights corresponding to various bore sizes and body dimensions.

Trunnion Double Block and Bleed (DBB) Ball Valves

Double block and bleed (DBB) involves shutting a pipe section on both sides of the valve, enabling a trunnion ball valve to block both upstream and downstream sides of the working area and then vent any remaining pressure in the piping and valve. A DBB setup is often utilized for stricter isolation of process fluids from other systems. It typically consists of two trunnion ball valves functioning as block valves, alongside a bleed valve (either ball or needle) directed to a safe location, adhering to local environmental regulations.

The DBB valve is a side-entry ball valve designed for usage in downstream, midstream, and upstream gas and oil applications, featuring either a welded or bolted body. It has two seats, with each providing a separate seal against pressure. When in a closed position and fluid flows from upstream, it pushes the seat against the ball to release sealing.

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When under pressure, the trunnion ball valve remains closed. The fluid traps within the ball results in pressure buildup. This pressure increases with temperature inside the ball, necessitating a vent or bleed when it becomes excessive. In trunnion ball valves that feature a bleed function, pressure inside the ball forces the seat back while compressing the spring behind it, leading to venting or bleeding between sealing surfaces. According to API 6D specifications, this venting is required at pressure levels below 1.33 times the design pressure rating for the ball valves; however, certain critical applications may require venting at below 1.1 times the originally designed pressure.

Discover more about ball valves in our extensive ball valve guide.

Figure 3: Double block and bleed valve

The Floating Ball Valve vs. The Trunnion Mounted Ball Valve

Floating ball and trunnion ball are general concepts, but understanding the exact differences between these designs is crucial for their application.

The primary distinction lies in the ball's construction and its assembly within the valve body. A trunnion ball is fixed and centered inside the valve through both a top shaft (the valve stem) and a bottom shaft (the trunnion). In contrast, a floating ball connects only to the valve body through the valve stem, allowing it to freely "float" against the seats.

In a floating ball design, in-line pressure pushes the ball against the downstream seat, accomplishing tightness. When transitioning from a closed to an open position, the ball must rotate against both in-line pressure and the seat's friction. This means that the torque to operate the valve results from both in-line pressure and seat characteristics. Torque requirements increase significantly with rising operating pressure, valve size, or if a more robust seat design, such as metal-seated, is introduced.

In a trunnion design, the ball is centralized via a trunnion, fixed between the stem and trunnion. Thus, it doesn't float but remains centered. The in-line pressure presses the seats against the ball to maintain tightness. Consequently, during operation, the ball only rotates against the seat pressure rather than both inline pressure and seat friction.

As a result, the torque required to operate a trunnion mounted ball valve typically is lower than that needed for a comparable floating ball valve. For instance, a DN200 metal-seated floating ball valve would necessitate a significantly larger actuator than a DN200 comparable trunnion valve, yielding lower overall package costs. Additionally, the trunnion design usually offers greater stability, making it suitable for extreme conditions and varying pressure levels.

Therefore, the trunnion-mounted ball valve is preferred for high-pressure applications with larger dimensions versus the floating ball valve. Another advantage of the trunnion design over the floating one is that a trunnion often integrates a drain or bleed connection, helping it serve as a dual-safe device. Furthermore, it operates as a relief valve when the central cavity pressure exceeds the spring force of the seats, relieving excess pressure back into the main line.

Despite being beneficial, a significant disadvantage of the trunnion design compared to the floating design is elevated costs. Hence, trunnions are typically utilized only when necessary.

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