Chapter 6 — Adjustment Procedures
Part No. 750-184 6-7
As the load requirement changes, the firing rate will change
accordingly. Thus it is referred to as modulated firing.
Point D represents the maximum firing rate of the burner, or
highfire. In the event pressure or temperature drops while the
burner is firing at highfire, it indicates that the load exceeds the
capacity of the boiler.
The firing graph (Figure 6-6) shows that point B and point C do not
coincide. Extreme load conditions could require the points be
closely matched.
When set as shown, with a time lag between B and C, the burner
will be in a low fire position upon a restart and will fire at that rate
for a short period of time before falling pressure or temperature
requires an increase in the firing rate.
Note: On-Off cycling in excess of 8 cycles per hour will
shorten the life of the combustion air motor and cause
excessive wear on switch gear and pilot electrodes.
If points B and C overlap when restart occurs, the burner would
drive to a higher firing position immediately after the main flame
was proven.
Note: It is not recommended that the boiler controls be set so
as to overlap the 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 at a temperature 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 ratios have been set, make the required
adjustments to the controls to bring the boiler pressure or
temperature up to meet the load requirements.
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