Page 102 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 87

refractory so some minor refractory damage may incur Like the old rule of thirds (page 111) I recommend
if a standby has to be brought on line immediately but you operate your plant so one boiler has half the total
the pressure parts will be uniformly heated and the operating hours and another has one third of the total
boiler will come on line quickly without danger of stress operating hours. The boiler with the most operating
cracking. hours will experience problems giving you a good in-
Now, quit heating up a boiler to maintain a stand- dication when to maintain, rebuild, or replace parts to
by. It wastes fuel, it increases environmental pollution, ensure the problems aren’t repeated on the other two.
it’s bad for the equipment, and it’s a waste of your time. You’ll also have two boilers with less wear than one and
I’ve discovered that plants which seem enamored one with less wear than the other two. If you only have
with the concept of standby boilers also like to rotate two boilers one should have twice as many hours as the
them frequently. They’re kept on standby so it’s easier to other.
rotate them. There’s also a bit of confusion regarding the Another perpetuated bit of foolishness is alternat-
status of a boiler on standby that should be cleared up; ing systems that are constantly switching boilers. Either
it seems to happen frequently in plants with multiple each time a boiler cycles or every day. Heating up a
heating boilers. Just because the pressure gauge shows boiler takes energy and switching to another results in
the same pressure as operating boilers doesn’t mean the all that energy being lost. Why waste it every day? If one
boiler is hot. Steam from the operating boilers will flow boiler is too big for the load (it is cycling) why would
to an idle boiler. A power boiler with a leaking non- you operate two to double radiation losses? Rotate the
return valve can hold a head of steam. boilers on at least a quarterly schedule so they get at
The problem is that pressure and temperature is least three month’s rest before you start them up again.
only above the water line; everything below can be dead Start-ups always put a strain on a boiler, why strain
cold, and in one case was actually freezing. For the same them any more frequently than necessary?
reasons that water circulates in a boiler when it’s firing Oh, that’s right, you would have to lay up the boil-
it will stagnate when it isn’t. I commonly come across er properly if you didn’t use it regularly. Collect some
boilers that show pressure where I can reach down and data, do a little math and you’ll discover that it’s costing
touch the bottom drum or a portion of the shell and find the owner a considerable amount of money to keep two
it cold. A boiler in that situation is not a hot standby, it’s boilers running when one is adequate. Lay one up for a
a bunch of thermally distorted steel. Any rapid changes summer, or a year. The little bit of work it takes to do the
in the water level can result in stress cracking of the job right will pay off in lower fuel bills that you can take
drum or shell and tube sheets. credit for.
Systems that simply drain the condensate off at the
surface of these boilers maintains an artificial state that
is dangerous. Those boilers should either be allowed to BOTTOM BLOWOFF
flood, so they’re all cold, with the condensate removed
in a section of piping above the boiler, or isolated and Some of you will argue this point because you’ve
put in lay up properly. It’s not too expensive to replace a used it for everything, everything but the only pur-
piece of piping compared to replacing a boiler. pose for bottom blowoff. Its only purpose is to remove
sludge, scale, and sediment that collects in the bottom
drum of the boiler. There is a prescribed procedure for it
ROTATING BOILERS with some variations depending on the type of bottom
blowoff valves that are on the boiler. Some of my cus-
The act of rotating boilers, sometimes called alter- tomers don’t perform a bottom blowoff… ever. That’s
nating although I prefer that label be used to refer to au- because their water pretreatment and chemistry meth-
tomatic rotation, is the operation of boilers in a manner ods don’t create any accumulation in that bottom drum
that assures that all the boilers have the same amount of and the little bit that does collect is removed with each
operating time. It has been common practice and many cleaning for annual inspection.
facilities have alternating controls that ensure every Yes, you may open the bottom blowoff valves to
boiler take its turn at operating. Why is it so important drain the boiler for its annual internal inspection (bian-
to make certain that all the boilers have an equal amount nual for some of you) but draining the boiler is not the
of use to improve the certainty that they all start having same as performing a bottom blow. Other reasons for
break downs at the same time? opening those valves are simply not acceptable. The bot-