Page 101 - Boiler_Operators_Handbook,_Second_Edition

86 Boiler Operator’s Handbook

tripped in unison when at low fire; a sudden increase in operator’s (or an operator’s boss’s) lack of confidence
excess air will not produce an abnormal furnace envi- in the equipment and/or the operator’s skill. If you
ronment with a good control system. still disagree after reading this section you should also
Many times I hear the argument that switching at review your logs to see what has happened. You should
load is dangerous. As I said earlier, I don’t like low fire find that boiler operation is highly reliable, more reliable
switching and I consider shutting a boiler down, then than the electrical service, and should be treated that
starting on the alternate fuel, a little more dangerous. way.
There’s a reason most boiler explosions occur on light- Boilers do shut down unexpectedly and loss of
off. You’re creating an explosive mixture then trying to pressure or temperature will happen. You should find
get it to burn instantly. When a boiler is operating you your logs document that the shutdowns were primarily
have a fire so low fire changeovers or dual fuel firing due to loss of electrical service and an unexpected boiler
don’t involve that opportunity for an explosion. You’re failure is rare to nonexistent. So, I ask you, “why do you
also producing an inert gas while you’re firing so any continue firing another boiler to keep it hot just in case
injection of fuel that isn’t burned is surrounded by inert the operating unit fails?”
gas instead of air and it can’t burn. (There’s reasons to be Ever notice that you can’t break a wire by bending
cautious about this when you have boilers with a com- it once but you always can by bending it repeatedly?
mon breeching) As far as I’m concerned you are probably doing more
The low fire changeover method requires signifi- damage to your standby boiler by running the pressure
cant quantities of excess air so there is air there for any up regularly than you would if you poured the fire to it
introduced fuel to burn if it isn’t ignited immediately to get it up to pressure from a dead cold start the one or
by the existing fire. That’s a bit of a problem because two times in its life that was necessary.
the existing fire isn’t very stable and all that excess air A well maintained plant where equipment is tested
makes it even more unstable. Bringing on a second fuel regularly and maintained properly will not have boiler
when dual fuel firing with full metering controls results failures and has no need of keeping a boiler on standby.
in the combustion air increasing as the fuel starts flow- The damage to the boiler and the fuel and electricity
ing to the furnace. The fire of the existing fuel is above costs for keeping it hot normally outweigh any advan-
minimum to produce more heat and is more stable than tage of keeping it hot by regularly warming it up. On the
it would be at low fire. (Low fire position is normally other hand, the maintenance of pressure or temperature
determined to be when the fire is stable; anything lower may be so critical that loss of a boiler is unacceptable. In
being unstable) the 1980’s I had one customer with a simple formula: if
The method available to you for switching fuels the pressure dropped from 240 psig (normal operation)
should be documented by a detailed SOP for that opera- to 230 psig it cost the plant a quarter of a million dol-
tion because it is always possible for something to go lars. A standby boiler isn’t the solution in those cases,
wrong to produce an explosive condition. it’s having a sufficient number of boilers on line so loss
Finally, practice it. Before an operator is compelled of any one will not prevent maintenance of pressure or
to switch—it happens when the gas company called and temperature.
he or she can’t reach the chief or anyone else for help— There is simply no way I can justify the concept of
that operator should have done it under supervision at keeping a boiler on hot standby by firing it regularly. The
least twice each way. It’s also advisable to practice it in only means of maintaining a hot standby that I will agree
the early fall, before cold weather sets in, so everyone with are (1) installation of convection heaters and (2)
has the memory of it refreshed. blowdown transfer. By installing a heating coil in the bot-
tom drum of a boiler or installing a heat exchanger, cir-
culator and piping connecting the blowoff and feedwater
STANDBY OPERATION to heat the boiler water using steam from operating units
you can keep a boiler hot enough that it can be brought on
Whenever I bring up my opinion of standby op- line as fast as one that’s fired to keep it warm.
eration it provokes conversation. Before you sit down Blowdown transfer uses the continuous blow-
to write me a note or call to tell me I’m full of it, please down from operating boilers to keep an idle boiler hot.
read this whole section. You may just agree with me that Depending on the amount of blowdown it’s possible to
firing a boiler to keep one on standby is inefficient, bad keep more than one boiler in hot standby without firing
for the boiler, and nothing more than an indicator of an them. Either of these methods doesn’t apply heat to the