Page 86 - Boiler plant and distribution system optimization manual

Combustion Analysis 71

point in a duct. The situation is further complicated by the
When a single-point probe is to be used, possibility of some CO monitoring instruments
compare several readings in the duct first, to find becoming insensitive and going off scale requir-
the most representative probe location. When ing a waiting period for them to come back to
existing ports are not satisfactory, drill or cut out operating range. During this period you may be
new ports and run traverse measurements. Re- blind to what is actually happening to the CO
member, unless you get truly representative data, level. One precaution that may be taken is to use a
your testing program will be of little value. combustibles analyzer along with the CO instru-
Flue-gas temperatures are subject to strat- ment, the combustibles analyzer is less sensitive
ification in ducts and a representative location and will indicate the actual situation over a wider
of thermometers or other temperature sensors range.
should be verified. Carbon monoxide measurements on oil and
Position them close to the boiler outlet, coal fired equipment is less often used because
because thermal losses can occur in the flue gas smoking or excessive carbon carryover usually
duct, especially in uninsulated sections. precedes the formation of large quantities of CO.
This is not always the case, however.
High CO levels have been measured on units
USING CARBON MONOxIDE where burner equipment had deteriorated or
TO MEASURE PERFORMANCE malfunctioned, impellers had burned off, oil tips
plugged, over fire air was too low, etc. Also, CO
On gas fired boilers carbon monoxide is the can be caused by chilling the combustion process
primary indicator of incomplete combustion and before the fuel is completely burned, two ways
usually determines the lowest practical level of this can happen is chilling of the flame with ex-
oxygen. The concentration of CO in the flue-gas cessive (cold) concentrations of combustion air in
should not exceed 400 ppm or .04% (the limit part of the flame and through flame impingement
established by many state and city ordinances, on the (cooler) boiler tubes.
industry codes and insurance companies). Knowing the CO level is very valuable, The
Once the final adjustments are made. It is CO analyzer should be capable of measuring
wise to observe the operation of a boiler for an from less than 100 to more than 2,000 ppm. While
extended period to insure that your adjustments Orsat analyzers have traditionally been used to
are final and there is no condition present that can determine CO, difficulties in accurate reading
increase the CO level above the acceptable limit. of concentrations less than 1000 ppm have pre-
When performing tests, occasional CO levels sented problems in the modern environment.
of up to 1000-2000 ppm may be encountered. Ad- Portable or permanently installed electronic type
equate boiler monitoring and flame observation CO analyzers have the ability to measure CO
are very important to assure stable conditions. Use continuously, having the advantage of indicating
caution at these levels because even a slight lower- excursions in CO that may not be detected with
ing of excess air can cause the CO level to skyrock- occasional spot readings.
et, which can lead to smoking, flame instability,
furnace pulsation and possibly an explosion.