The working mechanism is true to description. In the ball valve, it is an actual ball-shaped hunk of material that moves up and down in the valve seat to close and open the flow. In the butterfly valve, you have a disc-shaped piece of metal, that can turn on a central axis doing same.A ball valve is a sphere with a hole through it attached to the handle of the valve. When the hole lines up with the inlet/outlet the fluid flows; when the handle turns the hole 90 degrees to the inlet/outlet, the flow stops. The butterfly, or gated, valve is a round disk attached to the handle. When the disk is turned narrow to the inlet/outlet, flow occurs and stops when the disk is turned across the opening.A ball valve has a ball with a hole drilled through it when you turn the handle you line the hole up inside the valve to allow fluid to flow turn the handle the other way the ball will block the fluid passage . A butterfly valve has a plate attached to the handle when you turn the handle you spin the plate on its axes and it blocks fluid flow!

Ball valves have a ball shaped rotating part with a hole in it which by turning that part the liquid flow rate could be set and it is available in severalconfigurations
Butterfly valve has a big flat circular plate as rotating part and just uses for on and off flows and can not be used to adjust liquid flow rate
Another difference is that ball valves could be mounted with controllers and actuators to form controll valves ,and they are using a lot in process industeries.
Butterfly valves could not be on high pressure streams which is no limitation for ball valves。

The general description of the ball and butterfly valve in the answers so far are pretty accurate.

There are some errors or omissions.

Butterfly valves can be found for high pressures. I don’t no the current limit but I am sure they can be found for pressures of up to 1000 psi.

Unless specifically designed, neither is intended to be used for throttling or control service. They are primarily on/off valves.

They can be found in a multitude of materials with many different types of seats and seat materials.

Either can be equipped with an actuator for automated service.

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