Trunnion-Mounted Ball Valves: Engineering for Large-Diameter and High-Pressure Pipeline Service

Trunnion-Mounted Ball Valves: Engineering for Large-Diameter and High-Pressure Pipeline Service

Trunnion-mounted ball valves are the dominant valve type for large-diameter, high-pressure pipeline and industrial applications where floating ball valve designs reach their practical limits. In a trunnion-mounted design, the ball is supported by mechanical trunnions (bearings) at both the top and bottom, fixing the ball's position in the valve body. This fixed support eliminates the load-dependent seat forces of floating ball designs, allowing trunnion valves to operate at much higher pressures and larger sizes with lower operating torque and longer seat life. Understanding the engineering principles behind trunnion-mounted ball valves is essential for pipeline engineers, project managers, and procurement specialists specifying valves for demanding applications.

Wofer Valve manufactures trunnion-mounted ball valves in sizes from 2 inch to 56 inch, pressure classes ASME 150 through 2500, in carbon steel, stainless steel, duplex stainless steel, and alloy steel materials. Our trunnion ball valves meet API 6D for pipeline service and API 608 for general industrial service.

Floating vs Trunnion: The Key Engineering Difference

In a floating ball valve, the ball is not mechanically constrained; it is free to move slightly in the flow direction under pressure. When the valve is closed, the upstream pressure pushes the ball against the downstream seat, creating a contact force proportional to the differential pressure times the ball's seating area. This self-sealing mechanism works well in smaller sizes and lower pressures, but at larger sizes or higher pressures the seating force becomes extremely high, causing severe seat wear and making the valve nearly impossible to operate manually. In a trunnion-mounted design, the ball is held in position by trunnion bearings, and spring-loaded seats are pushed against the ball by Belleville springs. The seat contact force is controlled by the spring design, not the line pressure, resulting in consistent and manageable operating torque regardless of pressure.

Spring-Loaded Seat Design

The spring-loaded seat is the key sealing mechanism of a trunnion-mounted ball valve. Each seat ring is spring-loaded from behind by Belleville disc springs that maintain constant contact pressure between the seat and the ball throughout the full range of operating pressures and temperatures. When the valve is closed, the differential pressure acting on the upstream seat's back face provides additional seating force (pressure-assisted sealing), while the downstream seat is loaded only by the spring force. This means trunnion ball valves can achieve bidirectional sealing (sealing in both flow directions) when both upstream and downstream seats are properly designed, making them suitable for double block and bleed (DBB) applications. The springs also provide self-compensation for seat wear over the valve's service life, maintaining sealing effectiveness longer than fixed seat designs.

Body Cavity Relief

An important safety feature of trunnion-mounted pipeline ball valves per API 6D is body cavity pressure relief. When the valve is closed and the body cavity is isolated between the two seats, thermal expansion of trapped fluid or vaporization of condensate can cause the cavity pressure to rise substantially. If the cavity pressure exceeds the seat design pressure, the seats can be damaged or the valve body can be overpressured. API 6D requires that the valve provide automatic cavity relief to the upstream or downstream piping when the cavity pressure exceeds a safe limit. This is typically achieved through a pressure relief hole in the upstream seat ring, or by a check valve in the body wall that opens to relieve excess cavity pressure to the pipeline.

Trunnion Valves for Sour Service

Trunnion-mounted ball valves used in sour gas and oil service (containing H2S) must comply with NACE MR0175/ISO 15156 for all wetted and pressure-containing materials. Key requirements include maximum hardness limits for carbon and low-alloy steel components (typically HRC 22 maximum for body, ball, seats, and stem), the use of austenitic stainless steel or other H2S-resistant alloys for springs and seats, and post-weld heat treatment (PWHT) of all welded pressure-containing components. Elastomeric seal materials must also be resistant to H2S and hydrocarbon exposure; HNBR and FKM are commonly specified. For particularly aggressive sour conditions (high H2S partial pressure combined with low pH), more exotic materials such as duplex stainless steel, Inconel 625, or Hastelloy C276 may be required for ball and seat components.

Subsea and Offshore Applications

Trunnion-mounted ball valves are extensively used in offshore and subsea oil and gas applications, where their reliability and long service life are critical. Subsea ball valves face extreme challenges: seawater depths of up to 3000 meters (with external hydrostatic pressures exceeding 4000 psi), cathodic protection requirements, remote operation by ROV (remotely operated vehicle) or subsea actuators, and service intervals measured in years without maintenance access. Subsea trunnion ball valves are typically constructed from super duplex stainless steel for corrosion resistance, with tungsten carbide-coated balls and seats for wear resistance. The actuator and hydraulic systems must be designed for the ambient temperature and pressure at the installation depth, and all seals must resist seawater ingress for decades without replacement.