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Chapter 5
Operating Limit Control: Senses pressure or temperature and
automatically turns the burner on to initiate the startup
sequence when required and turns the burner off to initiate the
shutdown sequence when the demand is satisfied. The control
must be set to initiate startup only at the low-fire position.
High Limit Control: Senses the steam pressure or hot water
temperature. It is used as a safety limit to turn the burner off
in the event the operating limit control fails. The high limit
control should be set sufficiently above the operating limit
control to avoid nuisance shutdowns, because it has a manual
reset feature
When adjusting or setting controls, first be sure all control
devices are securely mounted and level. With the temperature
sensing control, make sure the sensing bulb is properly
bottomed in its well and is secured against movement. Be
sure the connecting tubing is not kinked.
The dial settings are generally accurate, although it is not
unusual to have a slight variation between a scale setting and
an actual pressure gauge or thermometer reading. Always
adjust control settings to agree with pressure gauge or
thermometer readings. Accurate instrument readings are
required. When necessary use auxiliary test equipment to set
controls.
Burner controls correctly set to match load demands will
provide operational advantages and achieve the following
desirable objectives:
•
The burner will be operating in low-fire position prior to
shutdown.
•
The burner will operate at low-fire for a brief period on
each start during normal operation.
•
Eliminates frequent burner on-off cycling.
The Hawk control system modulates the boiler firing rate
according to stored combustion curves. The Firing Graph (see
figure below) depicts a typical setting relationship of the
operating limit control, modulating control, and the high limit
control.
The burner will be "on" whenever the pressure or temperature
is less than point B and "off" whenever pressure or
temperature is greater than point A. The distance between
points A and B represents the "on-off" differential of the
operating limit control.
In normal operation, the burner will shut down whenever the
pressure or temperature rises above setting A. At that point
the switch in the operating limit control will open. As the
pressure or temperature drops back to B, the operating limit
control closes and the burner will restart. The boiler control
system will signal the actuators to be in a low-fire position. If
the load demands exceed the low-fire input potential, the
controls will increase the firing rate proportionately as
pressure or temperature falls toward point D. The controls
5-2
will stop at any intermediate point between C and D
whenever the fuel input balances the load requirement.
As the load requirement changes, the firing rate will change
accordingly. This it is referred to as modulated firing.
Point D represents the maximum firing rate of the burner, or
high-fire. In the event pressure or temperature drops while the
burner is firing at high-fire, it indicates that the load exceeds
the capacity of the boiler.
The Firing Graph 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.
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. It is therefore prudent to set the
modulating control a few pounds or degrees below the
operating control allowing the Low Fire to "catch the load"
before releasing to modulation.
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
! DANGER
NOTE: On-off cycling in excess of 8 cycles
per hour will shorten the life of the combus-
tion air motor and cause excessive wear on
switch gear and pilot electrodes. It also sub-
stantially reduces fuel efficiency.
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.
Note: It is not recommended that the boiler
c o n t r o l s b e s e t s o a s t o o v e r l a p t h e
modulation range and operating control
range.
Adjustment Procedures
CAUTION
750-364
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