Walchem W600 Series Instruction Manual page 71

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
PosiFlow 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 maintenance 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. Be-
cause 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
Interlock
Maintain
Wear Leveling Modes
After the Lead Lag group is defined, additional parameters can be configured within the settings list of the last
output 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 ener-
gized in the same order.
Duty Based
The order in which outputs are activated changes every time the Lead output is activated. How long each indi-
vidual 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 activat-
ed 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. When the Lead control mode (R1) determines the output should be activated, R2 turns on because it
has the lowest accumulated on-time. After 2 hours, if the output remains activated, the on-times are re eval-
uated and R2 turns off and R1 turns on because it now has the least accumulated total on time. The cycle
continues until the Lead control mode determines the feed is complete.
The Flow Verify alarm is never activated and the Lead Lag group is not affected
by the status of the PosiFlow input.
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.
When a Flow Verify alarm is activated, other outputs in the Lead Lag group are
activated instead if they are available; if no other outputs are available, or if ad-
ditional outputs are required due to Output Activation Mode settings, output(s)
reporting a Flow Verify alarm may still be activated as a last resort.
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