Page 269 - Boiler_Operators_Handbook,_Second_Edition
P. 269

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
   264   265   266   267   268   269   270   271   272   273   274