CleaverBrooks CFC Operation, Service & Parts Manual page 382

CB FALCON LEAD/LAG
The LL master may be aware of slave falcons minimum firing
rate and use this information for some of its algorithms, but
when apportioning rate it may also assign rates that are less
than this. In fact the add-stage and drop-stage algorithms may
assume this and be defined in terms of theoretical rates that
are possibly lower than the actual minimum rate of the falcon
control. In any case a falcon that is firing and is being
commanded to fire at less than its minimum modulation rate
will operate at its minimum rate: this is a standard behavior for
a falcon control in stand-alone (non-slave) mode.
If any slave under LL Master control is in a Run-Limited
condition, then for some algorithms the LL master can
apportion to that stage the rate that it is actually firing at.
Additionally when a slave imposes its own Run-limited rate
this may trigger the LL
Master to add a stage, if it needs more capacity, or drop a
stage if the run-limiting is providing too much heat (for
example if a stage is running at a higher-than commanded
rate due to anti-condensation).
By adjusting the parameters in an extreme way it is possible
to define add-stage and drop-stage conditions that overlap or
even cross over each other. Certainly it is incorrect to do this,
and it would take a very deliberate and non-accidental act to
accomplish it. But there are two points in this:
1. LL master does not prevent it, and more important;
2. it will not confuse the LL master because it is imple-
mented as a state machine that is in only one state at a
time; for example:
Lead boiler The Lead boiler is the falcon that is the first stage to fire among those stages which are in the equalize runtime
(Lead/Lag) group. If a boiler is in the "Use first" group it may fire before the Lead boiler fires.
First boiler A falcon may be assigned to any of three groups:
"Use First", "Equalize Runtime", or "Use Last". If one or
more units are in the "Use First" category, then one of
these (the one with the lowest sequence number) will
always be the first boiler to fire. If there is no falcon in
the "Use First" category and one or more are in the
"Equalize Runtime" category, then the First boiler is also
the Lead boiler.
Add-stage method
Add-stage detection timing
Add-stage request An Add-stage method implements the
criteria for adding another stage. Criteria that may apply
are the firing rate of a stage or stages vs. a threshold,
the amount of operating point versus setpoint error seen
by the master, the rate at which setpoint error is
developing, and the rate at which a stage or stages are
approaching their maximum or baseload firing rate.
• if its add-stage action has been triggered, it will
remain in this condition until either a stage has been
added,
or
• the criteria for its being in an add-stage condition is
no longer met; only then will it take another look
around to see what state it should go to next.
Assumptions:
Modulating stage The modulating stage is the falcon that is
receiving varying firing rate requests to track the load.
First stage This is the falcon that was turned on first, when no
slaves were firing.
Previous stage The falcon that was added to those stages
that are firing. Just prior to the adding of the unit that is
under discussion.
Next stage The falcon that will or might be added as the next
unit to fire.
Last stage The falcon that is firing and that was added the
most recently to the group of slaves that are firing.
Typically this is also the modulating stage, however as
the load decreases then the last-added stage will be at
its minimum rate and the previous stage will be
modulating.
Typically these use Add-stage detection timing to
determine how long these things have persisted. When
all criteria have been met for a sufficient time, then an
Add-stage request is active.
Drop-stage method
Drop-stage detection timing
Drop-stage request A Drop-stage method implements the
criteria for dropping a stage. Criteria that may apply are
the firing rate of a stage (or stages) vs. a threshold, the
amount of operating point versus setpoint error seen by
the master, the rate at which setpoint error is
developing, and the rate at which a stage or stages are
approaching their minimum firing rate.
Typically these use Drop-stage detection timing to
determine how long these things have persisted. When
all criteria have been met for a sufficient time, then an
Drop-stage request is active.
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