Page 82 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 67

boiler and protecting the metal from corrosion. It also a factor on fixed fire units. Making the transition with
removed that material so it couldn’t burn on to produce full metering controls is another matter. Normally there
a permanent scale on the boiler heating surfaces. From is an interface between the combustion controls and the
completion of boil-out on those surfaces have to be pro- Burner Management systems which allows the Burner
tected by proper water treatment. Management system to control damper and valve posi-
After boil-out is complete, the safety valves and tions to satisfy requirements for purge and light-off (low
other materials removed for the boil-out should be fire) positions. At some point after a successful ignition
replaced. This can also produce an interruption in the of the main fuel the interface lets the automatic controls
dry-out of the refractory and require a gentle reheating take over. A stable, safe, and smooth transition between
before continuing. light-off and automatic operation requires more than a
Refractory dry-out is complete when the tempera- simple switching from one to the other.
ture of the refractory at any point has gradually raised To begin with, a cold boiler with modulation
to something higher than atmospheric boiling tem- shouldn’t be released to automatic control immediately.
perature. That’s usually 212°F but can be lower (203°F in There’s enough thermal shock for a boiler to experience
Denver, Colorado). Some people will accept termination going from relatively cold (even in what we would call
of water flowing out of casing drains, others are more a hot boiler room) to firing at low fire where the steel is
elaborate. The minor expense of some thermocouples lo- less than a millimeter from hot flue gases over 1,000°F. If
cated at certain points in the refractory and monitoring the controls simply shift to automatic that temperature
them is the best way to determine a dry-out is complete. difference will readily double. Limiting thermal shock
The controlling temperature is the temperature of as much as possible is important to extending boiler life
the refractory closest to the outer wall of the boiler not so provisions to prevent the controls running to high fire
the surface of the refractory in the furnace. right after ignition is important. The simplest approach
is you set the controls in manual before the boiler starts
Boiler Control Adjustments and make sure that the manual signal is adjusted to low
Now that I have spent some time training boiler fire. Other approaches include low fire hold systems and
operators in tuning boilers I’ve come to the realization ramping controls.
that I was somewhat cavalier in explaining tuning of a
new boiler in the original edition of this book. Tuning a Low Fire Hold
boiler is a complicated process and potential for an un- A low fire hold consists of provisions to keep the
trained individual to blow up the boiler is much higher burner at low fire until the boiler is near operating tem-
than I thought back then. While it’s far more dangerous perature. The normal arrangement is a pressure switch
with an older boiler the potential still exists with a new or temperature switch similar to the operating and high
one. Therefore, I must insist that, as a boiler operator, limit controls but with an electrical contact that’s nor-
you should not attempt burner adjustments without a mally open. The pressure or temperature has to reach
significant amount of training and hands-on experience. the switch setting before the contact closes to allow au-
The contractor, or the boiler manufacturer’s ser- tomatic operation. The switch has to be set lower than
vice technician should tune up a new boiler on startup. the normal pressure or temperature modulating controls
That’s because the boiler is the contractor’s property so the burner isn’t affected by the low fire hold system
until such time as the new owner accepts it. If you make after the boiler is up to operating conditions. Sometimes
any adjustments and the boiler blows up you bought it. during emergencies you’ll have to bypass the low fire
So, leave them at it. I have had the opportunity to inves- hold controls or the boiler will not get hot until spring.
tigate boiler explosions that occurred during startup un- Be certain you can operate in manual to over-ride low
der the supervision of a contractor or a manufacturer’s fire hold controls.
representative with sufficient regularity to justify that With the typical jackshaft control the switch pre-
instruction. vents an increase in firing rate above light-off position
until the pressure or temperature is reached. An auto-
Transition to Automatic or Manual Control matic low fire hold is very important for modulating
Another requirement for a new boiler is estab- boilers that are controlled by a thermostat. A few warm
lishing a smooth transition from light-off to automatic days could prevent the boiler operating until it was
operation. This is normally accomplished without any dead cold; the low fire hold will prevent the rapid heat-
trouble on boilers with jackshaft type controls and isn’t ing of that boiler on high fire with severe thermal shock.