A butterfly valve is a disc on a shaft that pivots inside a round body. Quarter-turn the handle and the disc swings to lie flat across the pipe, while a rubber liner seals its rim.
A butterfly valve is a quarter-turn valve with a disc mounted on a shaft, sitting inside a short round body clamped between two pipe flanges. The disc swings like a butterfly's wing to open or close the passage.
The outside is only a slim round ring with a handle. The sealing happens at the disc edge and the rubber liner around the bore, which you can only see with the body opened up.
Fluid enters through the inlet flange and meets the disc. The disc pivots on the shaft that runs across the body.
When the disc is turned so its edge faces the flow, fluid slips past on both sides. The disc is thin, so an open butterfly valve adds only a small obstruction.
Turning the handle 90 degrees swings the disc flat across the bore. Its rim now blocks the whole passage.
A rubber seat liner around the inside of the body grabs the disc's edge as it closes, compressing slightly to make a tight seal against leakage.
The stem carries the handle's turn into the disc, and the body — clamped between two flanges — holds everything rigid in the pipeline.
A soft rubber (or PTFE) liner around the bore grips the disc's rim as it closes. The slight compression of the liner against the edge is what stops fluid leaking past.
The disc pivoting on a central shaft looks like a pair of butterfly wings opening and closing, which gave the valve its name.
Part way, yes — unlike a ball valve it is often used for control, because the disc can be held at intermediate angles. But it should not be left near-closed for long, as the disc can chatter and wear the liner.
It is much shorter and lighter, and needs only a quarter-turn to operate. That makes it cheaper and quicker to fit in large pipes.
A little. Even fully open, the thin disc sits in the flow, so it causes a small pressure drop — more than a ball valve but far less than a globe valve.