Flowserve 3400IQ Digital Positioner - Manual (Page 35)

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Logix 3400IQ Digital Positioner FCD LGENIM3401-00 – 06/06
flowserve.com
7.14 Alerts
7.14.1 FINAL_VALUE_CUTOFF
The FINAL_VALUE_CUTOFF or tight shutoff feature of the Logix 3400IQ digital positioner allows the
user to control the level at which the command signal causes full actuator saturation in the closed or
open position.
This feature can be used to guarantee actuator saturation in the closed or open position or prevent
throttling around the seat at small command signal levels. To enable, use configuration to apply the
desired FINAL_VALUE_CUTOFF threshold.
NOTE: The positioner automatically adds a 1 percent hysteresis value to the FINAL_VALUE_CUTOFF_
LO setting to prevent jumping in and out of saturation when the command is close to the setting.
7.14.2 Effects of FINAL_VALUE_CUTOFF on Operation
With the FINAL_VALUE_CUTOFF_LO set at 5 percent the positioner will operate as follows: 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, full actuator saturation will occur. The actuator will maintain full saturation below 5
percent command signal. Now, as the command increases, the positioner will remain saturated until
the command reaches 6 percent (remember the 1 percent hysteresis value added by the positioner).
At this point, the stem position will follow the command signal.
If the FINAL_VALUE_CUTOFF_LO is set to 3 percent but the valve will not go below 10 percent,
SOFTSTOP_LOW may be enabled. The lower soft limit must be less than or equal to 0 percent in
order for the FINAL_VALUE_CUTOFF_LO to become active.
If soft stops are active (ie: SOFTSTOP_LOW = 0 or SOFTSTOP_HIGH = 100) FINAL_VALUE_CUTOFF
is disabled.
7.14.3 Soft Limits
Unlike position alerts, soft limits prevent the stem position from going below or above the configured
limits. If the command signal is trying to drive the position past one of the limits, the yellow LED will
blink but the stem position will remain at the set limit.
7.14.4 Travel Accumulator
The travel accumulator is equivalent to a car odometer and sums the total valve movement. Using the
user defined stroke length and travel dead-band, the Logix 3400IQ digital positioner keeps a running
total of valve movement. When the positioner first powers up, high and low dead-band limits are
calculated around the present position. When the stem position exceeds the travel dead-band, the
movement from the center of the deadband region to the new position is calculated and added to the
travel accumulator. From this new position, deadband high and low limits are again calculated.