Page 114 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 99

AUXILIARY TURBINE OPERATION through the turbine.
When compared to the typical electrical utility
Contrary to popular belief auxiliary turbines are plant where 60% of the heat from fuel ends up lost, your
not there just in case you lose electric power. I frequently auxiliary turbines are super efficient. Despite their econ-
hear an operator complain that the turbine driven aux- omies of scale, burning cheap coal, etc., the utility can’t
iliaries are a waste of time because they would lose make power as inexpensively as you can with auxiliary
everything on a power outage anyway. While it’s true turbines. That’s why you can typically power a piece of
that an auxiliary turbine will operate without electricity auxiliary equipment for one fourth of the cost of doing it
their more important function is reducing operating cost with an electric motor.
while contributing to the heat balance of the plant. If, on the other hand, you run too many auxiliary
The auxiliary turbines are an optional source of turbines so you’re dumping steam out the multiport
power and the wise operator will make best use of them (relief valve) to atmosphere you’re wasting all the en-
because, operated properly under the right conditions ergy that should have gone to the deaerator and it costs
they can reduce the cost of powering the auxiliary equip- more than ten times as much as electricity. The trick is to
ment by about 75%. I should also note that, if you run an operate the turbines so you’re putting as much as pos-
auxiliary turbine under the wrong conditions you can sible through the turbine without pushing any out the
increase the cost of powering the equipment by 1000%. multiport.
There’s no easier way I know of to get rid of a new The best auxiliary turbines to use are boiler feed
boss that doesn’t know anything about boiler plants and pump turbines. They require power proportional to
proves to be intolerable. I’m not suggesting you operate feedwater requirements and deaerator steam is propor-
auxiliary turbines improperly to bump up operating costs tional to feedwater requirements. Forced and induced
and get rid of a boss, but it is one trick I’ve seen used. draft fans are second best. Regrettably turbines don’t
There’s that term again, exactly what is a heat bal- use steam proportional to their power output, they
ance? In it’s truest sense a heat balance is the result of need a certain amount of steam to overcome friction
calculations that determine exactly where heat goes in and windage (like fighting the wind, it’s losses associ-
a boiler plant with the balance meaning heat out equals ated with the rotor of a turbine whirling in the steam)
heat in. The more common reference is the balance of so the steam consumption of an auxiliary turbine isn’t
heat into and out of a deaerator which could leave a lot perfectly proportional to its power output.
of you out when you don’t have a deaerator. There is a reasonable degree of proportionality
If you have a sparge line in a boiler feed tank and that is evident when you look at the Willians line for a
heat the boiler feedwater by injecting steam into that particular turbine. The Willians line is a line on a piece
line you’re operating with something similar but seldom of graph paper that shows the relationship of steam con-
use enough steam in that feed tank to effectively run a sumption to turbine power output and it looks some-
turbine. thing like that shown in Figure 2-8. Since there is a fixed
Maintaining a heat balance is operating a deaerator amount of energy needed just to keep it spinning there’s
and auxiliary turbines to get the most efficient use out some point where the turbine’s steam requirement per
of the steam going to the deaerator. When steam flows gallon of boiler feedwater pumped exceeds the require-
through an auxiliary turbine some energy is extracted ment for heating steam at the deaerator. When operating
from it to drive the pump, fan, or other auxiliary device. a feed pump turbine below that point some of the steam
The exhaust steam then flows to the deaerator where is wasted, when operating above that point the deaera-
it is used to preheat and deaerate the boiler feedwater. tor needs more steam than the pump does.
That steam condenses as it mixes with the feedwater Your basic task is to determine the boiler load
delivering virtually all the heat left in it to the feedwater closest to that point then operate one or more auxiliary
which is then fed to the boiler. turbines accordingly; run the turbine whenever you can
For all practical purposes (by ignoring the little without wasting steam. If you have more than one tur-
bit of heat lost from the piping and equipment through bine driven feed pump you have to determine the boiler
the insulation) all the energy in deaerator steam is load above which you can run two turbines. If the turbine
recovered and returned to the boiler. If it happens to drives are of different sizes and there are some for other
flow through a turbine on its way to the deaerator and services (like condensate pumping or driving fans) you
produce a little power, the cost of generating the power have to learn how to juggle them for making the most use
is only the little bit of heat lost by the steam as it passes of the auxiliary steam going to the deaerator.