Ball Valve Design Variations: Trunnion, Floating, and Top-Entry

Ball valves are manufactured in several design configurations that affect their pressure capability, seat loading, maintenance accessibility, and suitability for different applications. The three primary ball valve designs—floating ball, trunnion-mounted ball, and top-entry—each offer distinct advantages and limitations. Understanding these design variations enables engineers to select the most appropriate configuration for their specific application requirements.

Floating Ball Valve Design

In floating ball valve designs, the ball is not fixed axially and is free to float slightly within the valve body. Line pressure acting on the upstream face of the ball pushes it against the downstream seat, creating the sealing force. Floating ball valves are simple, economical, and suitable for moderate pressure applications (typically up to 600 Class). The ball-to-seat contact force increases proportionally with line pressure, creating potential seat deformation under very high pressure. Floating ball valves are available in 2-piece and 3-piece body configurations, with 3-piece designs allowing maintenance without removing the valve from the line.

• Floating ball: ball pushed against downstream seat by line pressure

• Sealing force: proportional to line pressure, increases with higher pressure

• Suitable pressure: typically Class 150 to 600, depending on size

• 2-piece body: economical, requires removal from line for maintenance

• 3-piece body: center section removable for seat replacement in-line

Trunnion-Mounted Ball Valve Design

Trunnion-mounted ball valves have the ball fixed in position by trunnion pins (stub shafts) at the top and bottom, preventing axial movement. Spring-loaded seats float against the ball, with springs (typically Belleville washers) maintaining initial seating contact at low pressure. At higher pressures, upstream pressure additionally loads the floating seat against the ball. The trunnion mounting prevents the ball from being pushed into and deforming the downstream seat at high pressure, making trunnion designs suitable for high-pressure (Class 600-2500) and large-diameter (4 inches and above) applications. Trunnion valves also feature a cavity vent or bleed valve to relieve trapped pressure in the body cavity.

Top-Entry Ball Valve Design

Top-entry ball valves allow removal of the ball and seats through the top of the valve body without removing the valve from the pipeline. The valve body is a one-piece design with a removable top cover that provides access to the internals. This design significantly reduces maintenance time and cost for in-line service. Top-entry designs are particularly valuable in buried service, where removing the valve from the line would be extremely costly. The top-entry configuration is available in floating and trunnion designs, in sizes from 2 inches through large diameter pipeline applications.