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:
LLmaster does not prevent it, and more important;
it will not confuse the LL master because it is implemented as a state machine that is in only one state at a time; for example:
• 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 Sola that is receiving varying firing rate requests to track the load.
First stage This is the Sola that was turned on first, when no slave Solas were firing.
Previous stage The Sola that was added to those stages that are firing. Just prior to the adding of the Sola that is under discussion.
Next stage The Sola that will or might be added as the next Sola to fire.
Last stage The Sola 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.
Lead boiler The Lead boiler is the Sola 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 Sola may be assigned to any of
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Date: 8-4-2010
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three groups: "Use First", "Equalize Runtime", or "Use Last". If one or more Solas 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 Sola 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.
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.
