A ball valve seals by squeezing a soft seat ring against a polished ball. Turn the handle a quarter-turn, and the ball's bore swings away from the flow while the seats press shut.
A ball valve is a rotary valve built around a sphere with a hole drilled through it. Turning the handle a quarter-turn rotates the ball so the hole either faces the flow (open) or faces the wall (closed). It is one of the most common valves for on/off control of water, gas and oil.
From outside it is just a metal body and a lever. The part that decides whether it seals is hidden: the seat rings that hug the ball. Opening it up shows how the ball and seats work together â and why the seat material matters so much.
Fluid arrives through the inlet pipe and reaches the ball. If the valve is open, the drilled bore is lined up with the pipe, so the fluid flows straight through the sphere with very little pressure loss.
Because the passage through the ball is a smooth open tube, an open ball valve is almost like a piece of straight pipe. That is why ball valves are chosen for on/off duty rather than fine throttling.
Turning the handle 90 degrees rotates the ball so the bore now points sideways. The solid side of the ball swings into the flow path and blocks it. The movement is quick, which is useful for shutting off fast.
A soft seat ring sits on each side, pressed inward by a spring. As the ball turns shut, the seats keep hugging its polished surface, closing the tiny gap between the ball and the body so nothing leaks past.
The seat takes all the sealing duty. A soft elastomer seat (such as PTFE or rubber) conforms to the ball and seals at low pressure, but it limits the temperature and the abrasive fluids the valve can handle. Metal seats last longer but need more force to seal.
A quarter-turn â 90 degrees â does the whole job. Fully open to fully closed is a single quarter-turn, which is why they are favoured where a quick, definite shut-off is needed.
Usually a soft material such as PTFE or a rubber elastomer, sometimes metal. Soft seats conform to the ball and seal easily at low pressure; metal seats survive higher temperatures and abrasive fluids but need more force.
The spring keeps the seat pressed against the ball so it stays sealed even at low pressure, and it takes up wear as the seat and ball rub together over thousands of cycles.
Very little. With the bore lined up, the passage is a smooth straight tube, so an open ball valve causes only a small pressure drop compared with a globe valve.
Partially open, the ball presents a rough, awkward edge to the flow, which causes vibration, noise and rapid seat wear. Throttling is better done with a globe or needle valve.