45
At lower firing rates, a small movement of the servo is required to cause a significant change in the
air/fuel ratio as compared to high fire.
As can be seen from the above the trim limit is a progressively increasing number from low fire to
high fire. The following illustrates the trim limits from the above tables:
FIGURE 7.
Effect of Trim Limit Ratio on Trim Limits
GENERAL RULES FOR O
2
ALARM NOTIFICATION
The values entered represent the deviation from the current O2 target value. LO alarm refers to O2
levels under the O2 curve. HI alarm values refer to values above the O2 curve. The acceptable zone
lies between the LO curve and the HI curve. The following chart illustrates the HI and LO settings
for above and below the O2 curve.
It is the responsibility of the installing and operating personnel to ensure the trim limits
selected do not allow a hazardous combustion condition to occur. In the event of an O2
probe failure the untrimmed commissioned servo setpoints will be used.
Trim limit ratio = 3
Trim Limit = 2 degrees
Trim limit ratio = 3
Trim Limit = 3 degrees
Trim limit ratio = 4
Trim Limit = 3 degrees
Firing
rate%
Trim limit
Degrees
Firing
rate%
Trim limit
Degrees
Firing
rate%
Trim limit
Degrees
0
2.0
0
3.0
0
3.0
10
2.4
10
3.6
10
3.9
20
2.8
20
4.2
20
4.8
30
3.2
30
4.8
30
5.7
40
3.6
40
5.4
40
6.6
50
4.0
50
6.0
50
7.5
60
4.4
60
6.6
60
8.4
70
4.8
70
7.2
70
9.3
80
5.2
80
7.8
80
10.2
90
5.6
90
8.4
90
11.1
100
6.0
100
9.0
100
12.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0
10
20
30
40
50
60
70
80
90 100
D
e
g
r
e
e
s
Firing
rate
3,2
3,3
4,3
