1. First, Clear Up the Terminology
A "float valve" is a level-control device in which a mechanical float opens or closes a port as the liquid level changes - it is not a ball valve at all. A "floating ball valve" is a quarter-turn isolation valve whose ball is not fixed to the trunnion: it floats slightly in the medium and is pressed against the downstream seat by line pressure to form the seal. This article compares the two common ball valve mounting styles: trunnion-mounted (fixed) ball valves and floating ball valves.
2. Structural Differences
Floating ball valve: the ball is held between two seats and moves a little under pressure. Simple, compact and economical, it delivers excellent sealing in medium and low pressure systems. The ball must absorb the full thrust from line pressure, so size and pressure class are limited.
Trunnion-mounted (fixed) ball valve: the ball is supported by upper and lower trunnions, so line pressure is transmitted to the bearings instead of the seats. This allows large diameters, high pressure classes and lower operating torque, and is the design of choice for large pipeline valves.
Floating ball valves are normally built to ASME B16.34 and ISO 7121 (soft-seated steel ball valves) or API 608/ISO 17292 (metal-seated), while large pipeline ball valves are supplied to API 6D with trunnion mounting and double block and bleed (DBB) capability.
3. Pressure, Temperature and Sealing Behavior
Sealing in a floating ball valve depends on line pressure pushing the ball onto the downstream seat; at very low pressure, spring-energized seats are used to maintain a bubble-tight seal. Trunnion ball valves seal through seat springs and line pressure acting on the upstream seat, giving reliable low-pressure sealing even on large bores.
Practical boundaries :
Floating ball valves: commonly Class 150–800 (PN16–PN130), sizes DN15–DN200, soft seats PTFE/RPTFE (−29 °C to about 200 °C) or metal seats for higher temperatures.
Trunnion ball valves: commonly Class 150–1500 and higher, sizes DN50–DN1200 and above, full-bore or reduced-bore, soft or metal seats, frequently fire-safe and anti-static per API 6D.
4. Flow and Control Characteristics
Both types can be full-bore, giving a straight-through passage with very low pressure drop. Because a ball valve is a quarter-turn device, it is mainly used for fast shut-off; for precise throttling, an actuated ball valve with a positioner, or a globe/control valve, is more appropriate. Trunnion valves with metal seats and low operating torque are easier to automate with pneumatic or electric actuators on large sizes.
5. Installation and Operating Considerations
Floating ball valves suit branch lines, skid piping and general industrial water, air and gas service where pressure is moderate.
Trunnion ball valves suit mainline isolation, pigging-friendly full-bore designs in transmission pipelines, high-pressure gas and oil services, and applications requiring DBB isolation.
Check the face-to-face dimension (ASME B16.10 for flanged ends), end connections (flanged RF, BW, SW, NPT/BSP), and mounting interface for actuators (ISO 5211).
Verify fire-safe (API 607 / ISO 10497) and anti-static (ISO 7121 / API 608, resistance not exceeding 10 Ω) options where hydrocarbons are handled.
6. Selection Guide
Select the mounting style by pressure class and diameter: below about DN200 and Class 600, floating ball valves are the economical standard choice; above that, or where low operating torque, DBB or metal seats are required, choose a trunnion-mounted design to API 6D. Always define media, temperature, required leakage class and automation needs in the inquiry so the supplier can confirm the seat material and fire-safe/anti-static options.
Frequently Asked Questions
Which is better, a trunnion or a floating ball valve? Neither is universally better. Floating ball valves are economical and seal well in moderate pressure and small-to-medium sizes; trunnion ball valves are required for large diameters, high pressure, low torque and DBB isolation. Choose by service conditions, not preference.
Why does a floating ball valve need line pressure to seal? In a floating design the ball is pushed against the downstream seat by the differential pressure across the valve. At very low pressure, spring-loaded seats keep the seal tight.
When is a trunnion ball valve specified by standards? API 6D pipeline valves for transmission service are almost always trunnion-mounted, especially above DN200/Class 600, and where full-bore pigging and DBB functionality are required.
Can a floating ball valve be used for natural gas? Yes, for distribution and moderate pressure gas service with appropriate seat material (e.g., RPTFE) and fire-safe/anti-static options per API 607 and ISO 7121. For high-pressure transmission mains, use an API 6D trunnion valve.
What does DBB (double block and bleed) mean? valve with DBB capability isolates the line upstream and downstream simultaneously and provides a bleed port between the two seals to verify isolation - a standard feature of trunnion ball valves used in pipelines.
How do I specify a ball valve correctly? Define size, pressure class (Class/PN), end connections, body and trim material, seat material, temperature, leakage class, fire-safe/anti-static requirements and actuator type. Reference ASME B16.34 and, where applicable, API 6D or ISO 7121.