Flowserve 400MD Logix - Manual (Page 64)

Pressure Alarms
Pressure alarms are only available on models with advanced diagnostics (Logix 3400MD digital positioner). Advanced diagnostic models
add top and bottom pressure sensors. These sensor readings and alarms are only accessible from the communicator when the configuration
has been set to Advanced.
Loss of Pressure: The loss of pressure alarm becomes active when the supply pressure is near the minimum positioner operating pressure of 30 psig. The
LEDs will blink Red, Yellow, Yellow, Green. This alarm is meant to alert the user to low supply pressure as well as complete loss of pressure.
Top Sensor, Bottom Sensor: Each sensor is checked during actuator calibration. If a calibration reading appears to be out of range, the appropriate alarm
will become active. The pressure sensors are located on the collector board assembly.
Internal Positioner Alerts
MPC: (FINAL_VALUE_CUTOFF_HI, FINAL_VALUE_CUTOFF_LO) The MPC or tight shutoff feature of the Logix 3400MD 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 satura-
tion 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
MPC threshold. Note: The positioner automatically adds a 1 percent hysteresis value to the MPC setting to prevent jumping in and out of saturation when
the command is close to the MPC setting.
Question: I set the FINAL_VALUE_CUTOFF_LO at 5 percent. How will the positioner operate?
Answer: 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 in order to provide full actuator saturation. The positioner will maintain full satura tion 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.
Question: I have FINAL_VALUE_CUTOFF_LO set to 3 percent but the valve will not go below 10 percent?
Answer: Is a lower soft limit enabled? The lower soft limit must be less than or equal to 0 percent in order for the MPC to become active. If soft stops are
active(SOFTSTOP_LOW>30, SOFTSTOP_ HIGH<100) FINAL_VALUE_HI or _LO is disabled.
Position Alerts
Position alerts notify the user that the valve has traveled past a configured limit. The default settings are –10 percent and 110 percent which
are outside normal travel and, therefore, disabled. Position alerts only notify the user that a limit has been exceeded and do not limit stem
movement. Position alerts will cause a green LED code to blink.
Soft Limits
Unlike position alerts, soft limits (SOFTSTOP_LOW, SOFTSTOP_HIGH) prevent the stem position from going below or above the configured limits. If the com-
mand signal is trying to drive the position past one of the limits, the green LED code will blink, but the stem position will remain at the set limit.
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 dead band region to the new position is calculated and added to the travel accumulator. From this new position, dead band high
and low limits are again calculated.
Example: The Logix 3400MD digital positioner has a default dead band configuration of 20 percent and 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, which is
outside the present dead band. The Logix 3400MD digital positioner calculates the stem movement and adds this number to the travel accumulator.
80percent(presentposition)–50percent(previous)=30percentmovementx4-inchstroke=1.2 inches
Logix 3400MD Digital Positioner LGENIM3405-02 11/13
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