CleaverBrooks 4WI, Руководство по эксплуатации, обслуживанию и запасным частям

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750-211 (revised 2009)
Promethean Boilers, Model 4WI Manual 6-19
6.17 — Gas Fuel Combustion Adjustment
Most flue gas analyzers in use today measure the content, by percentage of oxygen (O
2
) and carbon monoxide
(CO) either by percent or parts per million (ppm). Carbon dioxide (CO
2
) is not normally measured with today’s flue
gas analyzers, but may be displayed via a calculation.
The O
2
levels through the entire firing range of the burner, low-fire to high-fire, should be tested. The burner man-
ufacturer’s recommendations on turndown should also be followed and the turndown range of the burner should
not be exceeded.
It’s important to understand what the readings shown on an instrument refer to when setting combustion in a
boiler. To assist with this understanding, Figure 6-14 shows the relationship between O
2
levels (excess air) and the
products of combustion for a typical flue gas analysis (natural gas).
One of the products of combustion is CO
2
(carbon dioxide). This is shown in percentage.
Another product of combustion is CO (carbon monoxide) and is shown in both percentage and parts per million
(ppm). The maximum CO level standardly allowed is less than 400 ppm. However, this may change subject to local
regulations.
The percent O
2
recorded on an instrument equates to percent excess air, i.e. 3% O
2
is approximately 15% excess air
and 4% O
2
is approximately 20% excess air. The exact percentage of excess air is a mathematical calculation based
on an ultimate fuel analysis of the fuel being fired.
It is generally recommended that O
2
readings of between 3% to 4% be attained with less than 400 ppm CO at high-
fire.
Using information from Section 6.16, determine the standard conditions of gas pressure and flow for the size
boiler and the gas train on it. Calculate the actual pressure and flow through the use of correction factors that com-
pensate for incoming gas pressure and altitude.
Basically, gas adjustments are made with a gas pressure regulator, which con-
trols the pressure and with the butterfly gas valve which directly controls the
rate of flow.
In initially setting the linkage, back off the low-fire stopscrew on the butter-
fly valve so that the valve is closed. Then run the screw out to touch the
arm, and give it two complete turns. Adjust the connecting rod so that the
override tension is released and so that the arm is now just touching the
stopscrew. Tighten the locknuts on all ball joints.
The low-fire setting should be regarded as tentative until the proper gas
pressure for high-fire operation is established.
FIGURE 6-15.
Butterfly Gas Valve