types and designs:
* Valve actuator - which moves the valve's modulating element, such as ball or butterfly.
* Valve positioner - which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear.
* Valve body - in which the modulating element, a plug, globe, ball or butterfly, is contained.

Control action

thumbTaking the example of an air-operated valve, there are two control actions possible:

* "Air

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