Excitation redundancy configuration S-073N

CUSHION LOADING
As a general rule of thumb, the cushion loading pressure is set at 50% of the supply pressure.
If the valve or controlled mechanism places a considerable force on the actuator, the cushion loading pressure
should be altered to compensate for it. Basically, what you are striving for is an equal split of forces to drive the
actuator in either direction. The cushion loading pressure is properly set when a positioner output, equal to 50% of
the supply pressure, just starts to move the actuator.
AIR FAILURE CIRCUIT
Figure 4 shows the components and connections required for an “air failure” circuit. This circuit applies only to
Model 73N-B. During an air failure, this circuit will cause the actuator to move downward. The regulator used in
this circuit must have soft supply and exhaust seats that provide a tight shut-off.
ZERO ADJUSTMENT
The only adjustment that can be made to the positioner is a zero adjustment. The zero adjusting screw is located
under the positioner top cover.
To adjust the zero, set the instrument air pressure to the midpoint of its span, and turn the zero adjustment until the
valve is at the mid-point of its stroke.
In some cases, valve shut-off or opening may be required at a specific instrument pressure. To zero the positioner at
this point, set the instrument signal at the specific pressure and turn the zero adjustment screw until the valve
reaches the required position.
A slight change of the instrument pressure should start to move the valve.
The valve stroke for a given span may also be suppressed or shifted to the desired range by means of the zero
adjusting screw.