Emerson Vilter VilTech Operation And Service Manual page 122

Lead list Processing
The sequencer processes through the lead list starting and stopping compressors as required. Lead list changes,
unavailable compressors, or a failure on one or more machines will cause special processing to be performed. This
special processing is described in the following sections:
Lead List Changes
When the operator changes the sequencer lead list, no compressors will be stopped or started immediately.
Instead, the compressors will be re-ordered and the last running compressor in the lead list will be given a
Trim command. The trim machine in the previous lead will be given a Full Load command. The standby
monitor timer is then started. When the standby monitor timer expires, the first compressor in Standby
which is above the new trim machine will be started with the Full Load command. The standby monitor timer
will then be restarted. The sequencer should now find a new operating condition which may result in the
current trim machine stopping. This process continues until all Standby machines above the trim machine
are started.
Unavailable Compressors
If a compressor in the lead list is not available when required it will be skipped and the next available machine
will be started. If the skipped compressor then becomes available it will be started immediately using the Full
Load command. The standby monitor timer will then be started, allowing the new configuration to find its
proper operating point. This process of starting a skipped compressor automatically maintains all running
compressors in the lead list order. It should be noted that a compressor becomes unavailable any time it is
stopped locally, not in sequenced mode or failed.
Trim Machine Failures
If the trim machine fails making it unavailable, two different scenarios can occur depending on the number of
compressors running. If the trim machine was the only compressor running the next available compressor in
the lead list is started immediately. If fully loaded compressors exist the previous compressor in the lead list
becomes the trim machine. Additional compressors may then be started using the normal staging process.
Fully Loaded Machine Failures
If a fully loaded machine fails making it unavailable, no immediate action is taken. Additional compressors
will be added through the normal staging process.
Sequencer Actions After Power Failures
After a power failure to the master, the sequencer will send a sequence stop command to all compressors in its lead
list. The compressors will be restarted using the normal staging process. Compressors that have failed because
the power was off for more than their power fail reset time will be off-line and unavailable for sequencing.
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