12
Single acting positioner,
Direct function
Actuator with closing spring
When the control signal increases, the pres-
sure C+ to the actuator is increased. The
valve stem moves upward and rotates the
positioner spindle counter-clockwise. When
the control signal drops to zero, C+ is vented
and the valve closes.
Reverse function
Actuator with opening spring
When the control signal increases the press-
ure C+ to the actuator is increased. The valve
stem moves downward and the positioner
spindle rotates clockwise. When the control
signal drops to zero, C+ is vented and the
valve opens.
Double acting positioner,
Direct function
Double action actuator
When the control signal increases, the pressure
C+ to the actuator is increased. The valve stem
is pressed upward and rotates the positioner
spindle counter-clockwise. When the control
signal is reduced, the pressure C- to the
actuator increases and the valve spindle
is pressed downward. If the control signal
disappears, the pressure goes to C-, C+ vents,
and the valve closes.
Fail in place (Fail Freeze)
When the input signal drops below 3,75 mA,
the Fail Freeze function holds the actuator/
valve at current position.
Drift rate in midrange <0,1% after 30 seconds
and < 2% after 30 minutes.
C–
S
C+
C–
S
C+
C–
S
C+
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