Page 269 - Boiler_Operators_Handbook,_Second_Edition
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254 Boiler Operator’s Handbook
Imagine what happens someone shuts off the
steam to a tray type deaerator. Colder makeup and con-
densate still enter through the sprays but now there’s
no steam to heat it; what little steam is left condenses
almost immediately and a vacuum forms, right? Nope.
Below the deaeration section is a storage tank full of wa-
ter at the original steam temperature; it’s going to start
flashing off steam as the pressure falls so there is some
steam provided for deaeration. Assuming the sudden
flashing of all the feedwater doesn’t produce so much
cavitation in the feed pumps that they trip (turbine
driven ones normally do) the feedwater in storage will
boil as the colder makeup continues to enter the deaera-
tor. Before they started bolting down the trays the only
sign an operator had that something was wrong was
some clanging as the flashing steam and water swelled
up out of the storage section and lifted all the trays. Fre-
quently the insulation on the deaerator prevented the
noise reaching the operator’s ear so the next thing he
Figure 8-5. Tray type deaerator got to notice was all the sulfite in the boilers just disap-
peared. Of course, by the time an operator gets around
type deaerator costs a lot more but when compared to to discovering the sulfite was wiped out because the
the added cost of sulfite and blowdown over the operat- deaerator’s trays were all laying in the bottom of the
ing life of the boiler plant the additional cost is justified. storage tank, and not deaerating, a lot of oxygen had
I should mention that there’s a scrubber type reached the boiler to corrode it.
deaerator on the market that looks something like a You have to lower the operating pressure gradu-
combination of a vacuum and tray type, using packing ally until you get down to atmospheric conditions or
instead of trays. I also have a concern for those pieces of you’ll rattle a deaerator. A deaerator should also have a
equipment and will not recommend them because they vacuum breaker, normally a check valve installed back-
tend to channel at reduced loads, where all the water wards connected to the steam space to admit air should
goes down one path while the steam goes up another you lose steam pressure.
so it doesn’t do its job. Vacuum type deaerators are de- I should also say that you can shut down the steam
signed to operate continuously at one load so they don’t supplying a deaerator at full boiler load the odds are
normally experience that problem. that check valve used for a vacuum breaker will not al-
Occasionally I’ll see a deaerator that isn’t operating low enough air in once a vacuum starts forming and the
properly because the pressure control for the steam is at storage tank could be crushed by atmospheric pressure.
the control valve or senses the pressure in the steam line I’ve looked into the dearation section of many a
going to the deaerator. A proper installation, regardless tray type deaerator to see the trays all jumbled up. Other
of type, senses and controls the pressure in the top of the times they were stacked at different heights, indicating
storage tank after the scrubber or trays of the deaerator they shifted. One plant told me they had been that way
to eliminate the pressure drop through the scrubber, or for several years! Another problem that affects any unit
trays, and connecting piping. If you have a deaerator is a water spray valve coming apart. When that happens
problem, check where you’re sensing pressure. you have the equivalent of a fire hose hitting the trays
Why does a boiler operator have to know all this and no breakdown of the water initially so it isn’t heat-
stuff about deaerators? So he won’t screw them up! ed. When you have a feedwater temperature lower than
Modern tray type deaerators are normally furnished the saturation temperature matching the steam pressure
with tie-bolts to hold down the trays because some that’s a good sign that you have a defective water spray
people managed to dislodge all the trays. They work valve, regardless of the deaerator design.
properly only when the trays are all stacked properly Except for vacuum deaerators the feedwater tem-
and leveled so the water flows uniformly over the entire perature has to be above 212°F (unless you’re in Denver
bank of trays to interact with the steam. where it has to be above 203°F) or the deaerator isn’t