Frequently Asked Questions - Flowserve Logix 3400MD User Instructions

Logix 3400MD Digital Positioner FCD LGENIM3404-00 – 9/09

13 Frequently Asked Questions

Q: I set the Final Value Cutoff Low at 5 percent. How will the
positioner operate?
A: Assume that the present command signal is at 50 percent. If the
command signal is decreased, the positioner will follow the command
until it reaches 5 percent. At 5 percent, the spool will be driven fully
open or fully closed, depending on the air action of the valve, in order
to provide full actuator saturation and tight shutoff. The positioner
will maintain full saturation below 5 percent command signal. As the
command increases, the positioner will remain saturated until the
command reaches 6 percent (there is a 1 percent hysteresis value
added by the positioner). At this point, the stem position will follow the
command signal. While in Final Value Cutoff, the Logix 3400MD LEDs
will blink GGGY.
Q: I have Final Value Cutoff set to 3 percent but the valve will not go
below 10 percent.
A: Is a lower soft stop enabled? The lower soft stop must be less than
or equal to zero percent in order for the Final Value Cutoff to become
active. If a positive lower soft stop is written, this stop will take priority
over the Final Value Cutoff feature. When the lower soft stop is reached,
the positioner will blink a GYGYs code.
Q: Will soft stops prevent the valve from going to its fail position?
A: No.
Q: What is the difference between a model with Standard diagnostics
and a model with Advanced diagnostics?
A: The model with Advanced diagnostics adds top and bottom actuator
pressure sensors. This allows for more diagnostic calculations such as
loss of pressure, friction, advanced signatures, and troubleshooting.
The pressure sensors, if present, are also used in the positioner control
algorithm to enhance valve stability.
Q: Can I upgrade from a Standard to an Advanced?
A: Yes. Referencing the IOM, an advanced pressure sensor board
assembly can be purchased. Simply replace the pressure sensor plug
plate with the advanced pressure sensor board. Perform an actuator
pressure calibration.
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