The floating principle
In a floating ball valve the ball is not fixed to the stem or body; it is held between two annular seats and is free to move slightly along the pipe axis. A slot in the top of the ball engages the stem, transmitting rotation while allowing that float. When the valve closes, upstream pressure pushes the ball downstream so its polished surface presses into the downstream seat, and this pressure-assisted contact is the primary seal. The harder the line pushes, the tighter the valve seals, which is why floating designs hold bubble-tight at high differential pressure without springs or complicated seat hardware.
Sealing at low pressure
With little line pressure the seal depends on the mechanical preload built in at assembly, where the seats are slightly compressed between ball and body. This is why seat resilience matters: PTFE recovers elastically and keeps contact, but at elevated temperature it creeps and loses preload, and low-pressure sealing degrades first.
Why size and class are limited
The load on the downstream seat is line pressure acting over roughly the projected ball area, so it grows with the square of the bore. Past about DN200, or in class 600 and above, the soft seat is overstressed, cold flow accelerates, and the torque needed to break the ball free of the loaded seat becomes impractical for a lever. That combination, not any single failure, defines the handover point to trunnion mounted designs.
The body cavity
When the valve is closed, fluid is trapped in the cavity between the two seats. If a liquid-full valve is heated, the trapped liquid expands and cavity pressure can climb far above the line rating. Standard floating seats are designed to relieve: cavity pressure pushes the upstream seat away from the ball and vents inward to the line. Confirm this self-relieving behaviour for liquid service and heat-traced lines.
Stem and packing
The stem is inserted from inside the body with a shoulder so pressure cannot eject it — the anti-blow-out feature — and is sealed by adjustable packing or O-rings. An anti-static device maintains electrical continuity between ball, stem and body so charge cannot accumulate in dry gas or hydrocarbon service.
Fire-safe behaviour
In a fire the soft seats and polymer packing are destroyed. A fire-safe design to API 607 or ISO 10497 provides a secondary metal seat lip that the ball contacts once the soft seat has burned away, plus graphite stem packing and body gaskets, limiting leakage to defined rates during and after the fire.
Failure modes in service
The common ones are seat wear from partial-opening throttling, torque growth after long closed periods as the seat cold-flows around the ball, seat extrusion at temperature, and packing leakage from frequent cycling. These are specification issues more than manufacturing ones: matching seat material to the real temperature and duty cycle, and stating that duty in the enquiry, prevents most of them.
