Flowserve 3400MD Digital Positioner - Manual (Page 22)

Logix 3400MD Digital Positioner FCD LGENIM3404-08-AQ –5/15
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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, STOP_LO_POS 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: STOP_LO_POS = 0 or STOP_HI_POS =
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 3400MD 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 calcu-
lated and added to the travel accumulator. From this new position,
deadband high and low limits are again calculated.
Example: The Logix 3400MD digital positioner has a default dead-
band configuration of 20 percent. The valve has a 4 inch linear stroke.
When the valve first powers up, the command signal is 50 percent.
The unit will calculate a high travel threshold of 70 percent (50
Percent present position plus 20 percent dead-band) and a low
travel threshold of 30 percent (50 percent present position minus 20
percent dead-band). As long as the stem position remains greater
than 30 percent and less than 70 percent, no additions are made to
the travel accumulator. Now, assume the stem position moves to 80
percent that is outside the present dead-band. The Logix 3400MD
digital positioner calculates the stem movement and adds this
number to the travel accumulator.
80 percent (present position) - 50 percent (previous) =
30 percent movement x 4-inch stroke = 1.2 inches
So, 1.2 inches is added to the travel accumulator. New dead-band
thresholds of 100 percent (80 percent present position plus 20
percent dead-band) and 60 percent (80 percent present position
minus 20 percent dead-band) are calculated. This process continues
as the stem position moves throughout its stroke range.
7.14.5 Cycle Counter
The cycle counter is another means of monitoring valve travel. Unlike
the travel accumulator, the stem position must do two things to count
as a cycle: exceed the cycle counter dead-band and change direction.
A cycle counter limit can also be written into the positioner. If this
limit is exceeded, the yellow LED will blink.
7.14.6 Position Deviation
If the stem position differs from the control command by a certain
amount for a given length of time, the yellow LED will blink to signify
excess deviation. The trip point and settling times are set from the
transducer function block.
7.14.7 Advanced and Features
NOTE: These features can be activated for full diagnostic functionality
of the system. These are contained in the transducer blocks. Refer to
the Installation and Reference Manual for a more detailed explanation.
7.14.8 Standard vs. Advanced Diagnostics
Advanced diagnostics models add top, bottom, and supply pressure
sensors. This allows for diagnostic functionalities such as loss of
pressure, advanced signatures, etc. Pro diagnostics adds the full
compliment of diagnostic features offered by the Logix 3400MD.
7.14.9 Temperature and Pressure Units
The desired temperature and pressure units can be set during configu-
ration. Once set, all readings will be displayed in the desired units.
7.14.10 Stroke Length
Stroke length is used by the travel accumulator. When the stroke
length and units are set, the length is used to determine the total
travel accumulated. The travel accumulator will have the units
associated with stroke.