Page 90 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 75

Frequently the reason for a boiler shutdown can be time spent in doing that will determine the extent of
determined, and corrected, quickly so it can be returned damage by thermal overstress.
to service. Many times it’s resolved quickly and the boil- Rather than heating all the water in the boiler you
er is returned to service even before anyone else notices can bring warm or hot water in to help accelerate the
you have a problem. When that can’t happen, then it’s warm-up. That’s especially true when the boiler is the
time for an emergency boiler start-up. only one you’re firing. Temporarily shutting off the
Any emergency that results in the shutdown of a makeup water and operating the boiler blowoff valves
boiler should be responded to with an instant evaluation to drop level so the heated water from the boiler feed
of the condition of that boiler. If you’re confident that it tank or deaerator displaces much of the cold water
cannot be returned to operation or are not sure why it in the boiler will both add to the heating of the boiler
went down the first step would be to start another boil- and provide some movement of water to help transfer
er, if you have one, so it’s warming up. Starting the other that heat to the thick parts of the boiler metal. Once
boiler takes time from finding the problem with the unit you’ve about drained a boiler feed tank you can restore
in operation but it also allows for a more gradual warm- the makeup. Let a deaerator sit until you’re producing
up of that boiler in the event the one that was running steam then bring the makeup on real slowly.
can’t be restarted. In an emergency you want to push the envelope as
Of course, if you know it went down because of a much as possible without damaging the boiler. Your di-
short power interruption, or other cause you know will saster plan should have been developed after some test-
not prevent a restart, there’s no reason to start that other ing that determines what firing rate provides the fastest
unit. If there’s water and steam pouring out of the boiler warm-up of the boiler within the limits recommended
that shut down or large gaping holes in what used to be by the boiler manufacturer so you can immediately set
square casing you know there’s no hope for the boiler that firing rate to get the fastest possible warm-up.
that went down and all you can do is secure it. If your normal procedure for warm-up includes
I define an emergency boiler start-up as one that shutting the burner down, don’t do it. I’ve never been a
requires operation of a boiler from a dead cold condition proponent of that activity other than for refractory dry
in as little time as possible. There are things you can do to out. If you think about it, the operation of the burner
limit the damage to the boiler in that process and actually followed by a purge produces dramatic swings in the
accelerate the start-up time. Which ones are available to metal’s exposure to temperatures on the fire sides. I
you will determine what you do. You can use these sug- think you will do less damage to the boiler by firing con-
gestions as guidelines to prepare your own disaster plan tinuously, although at low fire, than cycling the burner
that describes an emergency boiler start-up. on and off. In multiple burner boilers, where you’re only
Frequently heating boilers are allowed to sit idle operating one or a portion of the burners during warm-
with their steam valves open. This frequently gives the up, operation should consist of firing another burner
operator an impression that the boiler is ready to go before shutting one down as explained in the first dis-
because there’s pressure on it. Nothing could be further cussion on start-up.
from the truth and I discourage that practice because it Then, of course, there’s the matter of how serious
injects a considerable temperature swing in the shell of the need for steam is. Loss of steam for blanketing chem-
the boiler right at the water line. Steam at the surface ical reactions may be more critical than damage to the
is at saturation and hot, the original boiler water and boiler. In a hospital during a disaster where every oper-
condensate below the surface can be much cooler. Even ating room is handling emergency surgery maintaining
systems that drain the condensate from the bottom of steam for sterilization is a must. In such situations your
the boiler do not correct for the fact that the majority of disaster plan can call for ignoring the manufacturer’s
the water in the boiler is relatively cold. recommendations so you bring a boiler up to operation
Power boilers will always be considerably colder as fast as possible.
than normal steam condition. The principle concern in At some point you’ve established how much time
an emergency start of a boiler is the development of it will take to recover and documented it in your disaster
stresses in the boiler metal associated with rapid heat- planning. That information should be supplied to the
ing of the boiler. Whether it’s a low pressure firetube or facility served by the boiler plant so they’re aware of it
a large watertube doesn’t matter much, both have thick when preparing their disaster plans. Some facilities may
steel parts in contact with the boiler water that have reply with the question “is that the absolutely quickest
to be heated to normal saturation temperature and the you can do it?” In most cases you can answer “No, but