CleaverBrooks FLX Operation, Service And Parts Manual page 76

Chapter 5
100%
Firing Rate
Minimum Input
(Low Fire)
Burner Off
0%
modulating control range and operating
control range.
When firing a cold boiler, it is recommended that the burner
be kept at low fire, under manual flame control, until normal
operating pressure or temperature is reached. If the burner is
not under manual control on a cold start, it will immediately
move toward high fire as soon as the program control releases
the circuit that holds the burner in low fire. The modulating
control will be calling for high fire and the burner will move
to that position as rapidly as the damper motor can complete
its travel.
Note: Rapid heat input can subject the
pressure vessel metal and refractory to
undesirable conditions.
Do not operate the boiler in excess of 90% of the safety valve
relief setting. The closer the operating pressure is to the safety
valve relief pressure, the greater the possibility of valve
leakage. Continued leakage, however slight, will cause
erosion and necessitate early safety valve replacement. The
control settings on a hot water boiler must be within the
temperature limits of the boiler.
Ideally, the boiler operating controls should be set under
actual load conditions. Especially under new construction
conditions, the boiler is initially started and set to operate
under less than full load requirements. As soon as possible
thereafter, the controls should be reset to provide maximum
utilization of the modulating firing system. To accomplish
maximum utilization, and assuming that air/fuel combustion
5-4
6-4
D
Modulation Control Response
C
Modulated Firing
Range
(Burner ON)
Boiler Temp.or Press
Figure 5-3: Firing Graph
Operating Limit Control
Response
A
B
Falling Temp.
or Pressure
"ON - OFF"
Differential
Rising Temp.
or Pressure
(Burner OFF)
ratios have been set, make the required adjustments to the
controls to bring the boiler pressure or temperature up to meet
the load requirements.
To properly set the modulating control, carefully adjust it
under load conditions, until the load is maintained with the
burner firing at a steady rate. The firing rate at that point may
be full high fire or slightly less, depending upon the
relationship of the boiler size to the load.
When the modulating control is set and the burner is in full
high fire, the scale setting of the modulating pressure control
on a steam boiler will indicate the low point of the modulating
range. The scale setting of the modulating temperature
control on a hot water boiler will have a reading that indicates
the midpoint of the modulating range.
The operating limit control should now be adjusted and the
differential established. In an installation that does not require
a very close control of steam pressure or water temperature
the adjustable differential (Figure 5-3 A to B) should be set as
wide as conditions permit, since a wide setting will provide
less frequent burner cycling.
The high limit control provides a safety factor to shut the
burner off in the event the operating limit control should fail.
The setting of the control should be sufficiently above the
operating limit control to avoid nuisance shutdowns. The
setting, however, must be within the limits of the safety valve
Adjustment Procedures
High Limit Control
Safety Shutdown
(Burner OFF)
Increasing
750-177