page_title: Control valve
section_title: Control action
hierarchical_section_title: Control valve / Control action
caption_attribution_description: English: Example of current loops used for sending and control. Specific example of a smart valve positioner used.
context_page_description: A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
  In automatic control terminology, a control valve is termed a "final control element".
context_section_description: Taking the example of an air-operated valve, there are two control actions possible:
  "Air or current to open" - The flow restriction decreases with increased control signal value.
  "Air or current to close" - The flow restriction increases with increased control signal value.
  There can also be failure to safety modes:
  Air or control signal failure to close" - On failure of compressed air to the actuator, the valve closes under spring pressure or by backup power.
  Air or control signal failure to open" - On failure of compressed air to actuator, the valve opens under spring pressure or by backup power.
  The modes of failure operation are requirements of the failure to safety process control specification of the plant. In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed.
caption: Example of current loops used for sensing and control transmission. Specific example of a smart valve positioner used.
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