Page 85 - Boiler plant and distribution system optimization manual

70 Boiler Plant and Distribution System Optimization Manual
Unlike off-stack analyzers, which sample from a
single point, they are unaffected by stratification
or stagnation of flue gases in various areas of the
stack.
The measurement of carbon monoxide or
combustibles is an important part of achieving
maximum combustion efficiency. The result will
be less fuel wasted and more money saved.
OPACITY
Smoking with oil and coal fuels indicates
the presence of flue-gas combustibles or unac-
ceptable flame conditions, and always should be
avoided. Some boilers, especially larger ones, are
equipped with smoke detectors, which can indi-
cate poor stack conditions. Ultimately, stack con-
ditions should be checked by visual observation.
Accurate spot check type smoke measure-
ments can also be made with the inexpensive,
portable hand pump with filter paper testers de-
scribed above.
These devices use the smoke spot number or
ASTM (American Society for Testing & Materials)
smoke scale(standard D-2156), and can be very
helpful in establishing optimum boiler condi-
tions.
STACK TEMPERATURE
Deposits and fouling of external tube surfac- pling site can be as important as the selection of
es with soot, ash and other products inhibit the
absorption of heat in the unit and lead to lower
efficiencies. Deposits are indicated by flue-gas
temperatures that are high compared to clean
conditions. The efficiency loss resulting from
dirty tubes can be estimated with the RULE OF
THUMB:
——————————————————————
EVERY InCREASE OF 40 DEG F In
STACK TEMPERATURE REDUCES
EFFICIEnCY BY ABOUT 1% .
——————————————————————
Waterside deposits caused by improper wa-
ter treatment also can lead to high stack tempera-
tures, but tube failures due to overheating gener-
ally occur before any substantial efficiency losses
are evident from these internal tube deposits.
Rising Stack Temperatures
Indicate a Problem
Stack-temperature measurements are an
easy and effective means for monitoring boil-
er-tube fouling. This is done by comparing the
present temperature to the start up temperature
or a temperature recorded when the boiler was in
a clean condition.
Since stack temperatures usually increase
with firing rate and excess air, make your com-
parisons at similar boiler operating conditions.
In the absence of previous data, flue gas tem-
peratures normally are about 150 to 200 F above
steam temperature for a boiler producing saturat-
ed steam at high firing rates.
Boilers equipped with economizers and air
preheaters should be judged by observing the
flue gas temperature immediately after leaving
the last of these heat recovery units.
ACCURACY OF SAMPLING TECHNIQUES
For oxygen, CO and smoke analyzers the
portion of the gas analyzed must be representa-
tive of the total gas stream. Location of the sam-
the proper measurement device.
To illustrate: on negative draft boilers, the
gas sampling point should be upstream of the air
preheater, if one is installed, or upstream of any
known air leaks. Reason is that air leakage into
the gas ducts can dilute flue gas and resultant
measurements won’t give a true indication of fur-
nace conditions. Air leakage in preheaters poses
the same problem.
Sample conditions immediately down-
stream of bends, dampers, or induced fans
should be avoided. Gases in such areas can strat-
ify or form pockets leading to errors, especially
when samples are withdrawn from a single