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Another type of pneumatic-assisted control valve is a butterfly valve. From the diagram in Ill. 1 notice that the valve mechanism for this type of valve has a valve plate with a rod mounted through its axis that becomes the valve stem.
The valve's pneumatic control mechanism is linked to the valve stem so that it can be rotated to cause the valve plate to move to a position that's horizontal with the flow or vertical to the flow, which would cause the flow to stop.
The pneumatic control mechanism for this type of valve is similar to the one shown in the previous figure. When air is sent to the pneumatic chamber of the valve, it will press on a diaphragm and cause the valve linkage to move up and down. In this type of valve, the up-and-down movement causes the valve stem for the butterfly valve to rotate to the open or closed position. Springs provide the counterbalance to the diaphragm, which allows the valve to be proportional.
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