Page 402 - Boiler_Operators_Handbook,_Second_Edition

Controls 387

ity. That requires a system that can stop and start each
boiler as needed and may include modulating controls
that fire the boilers at different rates. Several methods
using complex arrangements of linkage, modulating
motors that operate off another pressure control switch
connected to the common steam header and powered by
a shaft which in turn controlled switches and multiple
electric coils like the one in a pressuretrol were provided
and you may run into one of them.
Okay, I can’t explain it in one sentence and that
was part of their problem. The principles described for
a single burner control apply to them but they required
a lot of maintenance and are, for the most part, replaced
Figure 11-21. Settings for automatic three boiler control
by modern digital controllers that simulate their func-
tions. If you have one of those, read that instruction
manual several times and ensure yourself that you have boiler you just shut down will not generate so much
an understanding of what it’s supposed to do before you steam that the pressure rise associated with that steam
start making adjustments. Then watch what happens generation trips another boiler.
when you make adjustments because they may not do The difference in start (cycle on) settings has to
what you understood they should do. As of the writ- be sufficient to allow for the pressure drop that will oc-
ing of this book there is no national standard applied to cur while the boiler just started is purging and lighting
those devices so their descriptions, labels, and settings off. Figure 11-21 also shows how you can change the
vary considerably. Another problem is the people that modulating range. You’ll notice that the setup requires a
write their instructions and label the panels may have considerable swing in pressure to satisfy all the criteria.
completely different or erroneous perceptions of what If you want to change the order in which the boilers op-
gain, reset, and other control terms are. They will do a erate you have to change all the switch and pressuretrol
good job of controlling your boilers if you buy the right settings, a lot of work. So it’s much better to have one of
one and apply it properly. those digital controllers to do it all.
If you have multiple boilers, need more than one Many plants have lead-lag controls as part of the
in operation to serve all the loads, can’t be there to package for controlling their boilers. The adjustment
make a decision regarding when to start or stop another of settings in Figure 11-21 provides a form of lead-lag
boiler, and your boss won’t put up the cash to buy one control because it varies the number of operating boil-
of those modern controllers you’ll have to make do with ers, allowing Boiler 1 to carry the load until it can’t
the equipment you have. There’s no reason to change then bringing on Boiler 2 and finally Boiler 3 to handle
modulating control settings on the boilers unless you the maximum loads. All the boilers modulate together.
have limited turndown (two to one or less) or you have Lead-lag controllers were designed to accomplish it in a
adjusted turndown to the degree that you’re very inef- slightly different manner. They would run the first boiler
ficient at low fire. up to high fire and leave it there after starting the second
That, by the way, is a normal thing to do. Boiler op- boiler which would modulate to carry the load until the
erators don’t normally like to see a boiler shutting down load exceed the capacity of two boilers when the third
regularly. Creating load and other unwise operations are would start. Those controllers resolved a problem with
not the proper way to deal with it though. the scheme of Figure 11-21 which provided different re-
You achieve multiple boiler control by setting your sponses to load changes depending on how many boil-
operating pressure switches within the range you would ers were on line. The lead-lag controller always had only
use for one boiler. It’s easier to talk in terms of start and one boiler responding to load changes, the others were
stop pressures where the stop pressure is the setting of either on at high fire, or off. One controller was actually
the boiler pressure control switch and the start pressure capable of controlling as many as ten boilers.
is the switch setting less the differential. Figure 11-21 High pressure boiler plants can operate with the
shows the start and stop settings for three boilers to same simple modulating control we just reviewed as
achieve automatic control. The difference between stop long as there is no problem with the swinging pressure.
settings has to be enough that the residual energy in a Once a plant is large enough that someone installs a