Bryan Boilers 200 Lead DROP-STAGE on Error, LAG Selecton Method Sequence ORDER, Measured RUN Time

hysteresis is ignored if one or more lag boilers are firing.

• If LDSE is enabled:

The Burner Off threshold provided by LL off hysteresis is ignored also for the lead boiler when it is firing solo (i.e. when no lag boilers are firing).

• If LDSE is disabled:

When the lead is firing solo and the operating point reaches the Burner Off threshold specified by LL off hysteresis turns off LL master burner demand (and thus the lead boiler).

As usual, whenever LL master burner demand is turned off by its hysteresis block, it does not recur until the operating point falls below the Burner On threshold.

Summary of the burner-off thresholds that are used:

4.7.11LEAD DROP-STAGE ON ERROR

- LDSE:

If LDSE is enabled then either Drop-stage method1 must be enabled to provide staging based on "Error threshold"; otherwise a parameter error lockout occurs.

Normally, for a lag boiler, dropping a stage based on error involves meeting three criteria: 1) the operating point temperature must exceed an offset from setpoint, 2) this condition must persist for a period of time, and 3) the measured time starts only when the modulating boilers are firing at the minimum modulation rate. And normally when LDSE is not enabled, the lead boiler is special case that is not affected by a drop-stage event: it shuts down only when the operating point reaches the burner-off threshold (or ABO threshold, if that is enabled).

If LDSE is enabled:

•Enabling (or disabling) LDSE has no effect on the dropstage behavior for a lag boiler; however

•When only the lead boiler is firing then an error based drop-stage event does act to drop the lead boiler, and moreover, only one of the three criteria above are considered by the method in this case: the operating point temperature. Thus dropping the lead does not depend on exceeding this temperature for a period of time, nor does it require the lead to be at minimum

73

Date: 8-4-2010

Revision: 0

Form: 2396

modulation rate. When LDSE is enabled and the lead is firing solo, then simply reaching the drop- stage threshold causes a dropstage event that causes the lead to turn off and [rf3259] which thus ends LL master demand until the operating point again falls to the Burner On threshold.

Sequencer

The Sola sequencer determines which Sola will be the next one to turn on or turn off whenever an Add-stage event occurs. It maintains the following variables:

LeadBoilerSeqNum - sequence number of the current lead boiler in the Slave Status table.

Lead BoilerRunTime - the cumulative time that the current lead boiler has been running.

In all cases, if a boiler sequence number is needed and Slave sequence order is 0, then the boiler's ModBus address is used as its sequence number.

In all cases, if two boilers being compared have the same effective sequence number, then the one that is selected is undefined (either may prevail).

Sequencer Parameters

LEAD SELECTION METHOD: ROTATE IN SEQUENCE ORDER, MEASURED RUN TIME This determines the selection method for lead selection and sequencing, as described below.

LAG SELECTON METHOD: SEQUENCE ORDER, MEASURED RUN TIME

This determines the selection method for lag selection and sequencing, as described below.

LEAD ROTATION TIME: HH:MM OR NONE This determines the lead rotation time as defined below.

FORCE LEAD ROTATION TIME: HH:MM OR NONE

If this parameter is a non-zero time, then it is used to force the rotation of the lead boiler if it stays on longer than the time specified.

Sequencer Add Boiler Selection

The sequencer selects the next boiler to be added according to a sorted order. This description assumes this is implemented by