Answer:
Good question
Usually motor operated valve used in big pipe lines sizes which it is need strong torque and for ON/OFF condition not to control the process, We can use rather than M.O.V pneumatic ON/OFF valve with piston actuator (Double Acting) but in this case the accessories it will cost you more because you need to provide pneumatic amplifier and big actuator depend on the pipe size.
That is why better to use M.O.V the motor will rotate the gears and the gears will rotate the valve with low cost.
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