Trunnion-Mounted Ball Valve

What Is a Trunnion-Mounted Ball Valve

In a trunnion-mounted ball valve, the ball is anchored at the bottom by a trunnion — a short shaft that fits into bearings in the valve body. This contrasts with floating ball valves, where the ball is held only by the two seats and free to move slightly downstream under pressure. The trunnion absorbs the pipeline loads and hydraulic forces that would otherwise push the ball against the downstream seat, making the design suitable for large diameters and high pressures.

Trunnion-mounted ball valves are the standard isolation valve in oil and gas pipelines, LNG terminals, and high-pressure process units worldwide. Their ability to provide double-block-and-bleed isolation, handle high differential pressures with low operating torque, and survive fire exposure makes them the valve of choice for the most demanding hydrocarbon services.

How the Trunnion Design Works

The ball has a lower extension (trunnion) that rotates in a bearing attached to the body. An upper extension connects to the stem. The ball is thus constrained to rotate around a fixed axis and cannot move downstream under line pressure. Instead of the ball floating into the downstream seat, the seats float — spring-loaded and pressure-activated rings that move toward the ball to create the seal.

Because the seats, not the ball, move to create the seal, the operating torque is lower and more predictable than a floating ball design. In a floating ball valve, the torque increases roughly with the square of the diameter and pressure. In a trunnion design, torque grows more slowly because the seats are mechanically loaded by relatively small springs, not by full-line-pressure acting on the ball area.

The trunnion also allows the valve body to absorb pipeline loads — axial forces from thermal expansion, weight, and occasional seismic loads — without distorting the ball-seat interface. This structural advantage becomes significant in large-diameter, above-ground installations.

Double Block and Bleed (DBB)

A DBB trunnion ball valve provides two independent sealing barriers — the upstream seat and the downstream seat — with a cavity between them that can be vented through a small bleed valve. If either seat leaks, the escaping fluid is diverted through the bleed port rather than passing through the valve. Operators monitor the bleed port for leakage; if fluid appears, they know one of the seats needs attention.

DBB isolation is required by many pipeline codes for critical block valve stations, launcher/receiver isolation, and connections to live process equipment. The advantage over two separate valves in series is that a single DBB valve occupies less space, has fewer potential leak paths, and costs less to install.

DIB — Double Isolation and Bleed

DIB is a more stringent version of DBB defined in API 6D. A DIB valve has two seats that both seal in the same direction (toward the cavity), so that each seat independently blocks flow from its respective side. This means the valve can provide double isolation regardless of which side has pressure — unlike a standard DBB, where each seat blocks flow from the opposite side only.

DIB is required when both sides of the valve may be pressurized independently, such as at pipeline tie-in points and cross-country station boundaries. API 6D classifies DIB valves as Type 1 (both seats single piston effect) or Type 2 (one seat double piston effect, one single piston effect).

Seat Designs

SPE seats are the most common. They use line pressure from one side to assist the spring in sealing the ball. DPE seats are designed so that pressure from either side pushes the seat ring toward the ball, providing double isolation. The trade-off with DPE seats is that cavity pressure can build up if both seats seal simultaneously — a cavity relief valve is mandatory.

Common Materials

Relevant Standards

• API 6D — Pipeline and piping valves (the primary standard for trunnion ball valves)
• API 608 — Metal ball valves
• API 607 / 6FA — Fire testing
• ASME B16.34 — Pressure-temperature ratings
• ISO 14313 — Pipeline valves (international adoption of API 6D)
• ISO 15848 — Fugitive emissions
• NACE MR0175 / ISO 15156 — Sour service material requirements

Typical Applications

• Pipeline Block Valves: DBB trunnion ball valves at block valve stations every 20–40 km on transmission pipelines.
• Pig Launcher/Receiver: DBB isolation on the barrel side and the pipeline side of the trap.
• LNG: Cryogenic trunnion ball valves with extended bonnets for LNG receiving terminals and liquefaction trains.
• Offshore: Subsea and topside trunnion ball valves for riser isolation, Christmas trees, and manifold connections.
• Refinery: High-pressure hydrogen, hot oil, and corrosive service isolation.
• Gas Distribution: City gate and regulator station isolation valves.

Advantages and Limitations

Advantages

• Low and predictable operating torque — easy to automate
• DBB/DIB capability in a single valve body
• Bidirectional sealing with appropriate seat design
• Fire-safe with metal-seat backup
• Handles high pressure and large diameter

Limitations

• More expensive than floating ball or gate valves
• More complex — trunnion bearings, springs, and seat rings are additional wear components
• Cavity relief required — trapped pressure between seats can overpressurize the body cavity
• Maintenance requires removing the valve or dismantling the body — more complex than a floating ball valve

Selection Guidelines

1. Specify trunnion for DN 200+ or Class 300+: Below that, floating ball valves are often adequate and more economical.
2. Choose DBB or DIB based on code requirement: Most pipeline codes require DBB at a minimum. DIB is needed where both sides may be independently pressurized.
3. Select seat material for the service: PTFE/PEEK for clean, moderate-temperature fluids. Metal seats for abrasive, high-temperature, or fire-safe service.
4. Require fire testing: API 607/6FA for all hydrocarbon service. Specify secondary metal-to-metal sealing.
5. Specify actuator mounting per ISO 5211: Direct-mount pad eliminates linkage, simplifies installation, and improves reliability.
6. For sour service: Ensure all pressure-containing and pressure-controlling parts comply with NACE MR0175 / ISO 15156.

Practical tip: When specifying a trunnion ball valve for pipeline service, always include a cavity relief valve (typically a small needle or relief valve piped from the body cavity to the upstream side). Without it, thermal expansion of trapped liquid in the cavity can generate pressures exceeding the body rating, especially in above-ground installations exposed to solar heating.