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Switch change (for example, energize a second pump to maintain a tank level when the low-low level switch opens
Control Operation
Backup Pump Control
The default control operation for the Lead Lag group is that if a condition exists that prevents one relay from being
activated, it is skipped and the next output in the group is turned on instead. This situation may occur if the output
is experiencing an active Flow Verify alarm or the output is not in Auto mode. Backup control using a Lag output
does not require any additional settings and could be used to create an output for a backup pump to be activated
only if the main pump loses prime and/or is taken out of service for maintenance.
Example: A Lead Lag group consisting of R1, R2 & R3 is configured (R1←R2←R3). All three pumps have Posi
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Flow monitors wired to inputs D1, D2 & D3, respectively. R1 uses On/Off mode to control caustic feed to maintain
a pH set point above 7.0. R1 and R3 pumps are in Auto mode, R2 pump has been taken out of service for mainte-
nance and is currently in HOA Off mode. The process pH falls below 7.0 and R1 is energized. Before the pH rises
to satisfy the dead band, the D1 PosiFlow input monitors an error condition and activates a Flow Verify alarm for
the R1 pump. The Lead Lag system de-energizes R1 and checks the status of R2. Because R2 is not is service, R3
is energized to maintain caustic feed.
Each digital input channel set up as a Feed Monitor type has a Flow Alarm Mode setting used to specify how the
pump output is handled when Flow Verify alarms are identified. Based on this setting, the Lead Lag group responds
as follows:
Disabled
The Flow Verify alarm is never activated and the Lead Lag group is not affected by
the status of the PosiFlow input.
Interlock
When a Flow Verify alarm is activated, the related output is immediately turned
off; if available, other outputs in the Lead Lag group are activated instead.
Maintain
When a Flow Verify alarm is activated, other outputs in the Lead Lag group are ac-
tivated instead if they are available; if no other outputs are available, or if addition-
al outputs are required due to Output Activation Mode settings, output(s) reporting
a Flow Verify alarm may still be activated as a last resort.
Wear Leveling Modes
After the Lead Lag group is defined, additional parameters can be configured within the settings list of the last out
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put in the group. These options optimize the behavior of the Lead Lag functionality. Several different wear leveling
options can be selected to control the order in which outputs are activated.
Disabled
The order in which the Lead and Lag outputs turn on does not change automatically. They are always energized in
the same order.
Duty Based
The order in which outputs are activated changes every time the Lead output is activated. How long each individu-
al pump has been running is not considered.
Example: When the Lead output, set for On/Off control, drops below the setpoint, R1 is activated. R1 turns off
after its deadband is satisfied. The next time the measurement goes below the setpoint, R2 is activated and R1
remains off. After all outputs in the group have been exercised for one feed cycle, the process begins again with the
first output (R1).
Time Balanced
Time balanced mode alternates outputs in a manner that equalizes the runtime of all connected pumps. This mode
takes into account how long each output in the Lead Lag group has been running (since a manual reset) and selects
the output that has the lowest on-time during each cycle. If the output remains energized longer than the specified
cycle time, the time-on for each output is recalculated and a different output may be activated to balance the usage
of each.
Example: In a two-pump Lead Lag group, time balanced wear leveling is selected with a cycle time of 2 hours.