IRIS MODEL P522 APPLICATION MANUAL
To reiterate: The nonlinearity of the Si channel caused by the AGC action has been a major con- tributor to discrimination problems. The background will always be less intense, because it is further away (less by the square of the distance). Being less in- tense means it receives more amplification (AGC action) causing a worse discrimination ratio (the ra- tio of flame ON to flame OFF).
The PbS detector (S512) is linear up to the point satu- ration begins. Saturation will occur on bright flame signals. A reading of 1800 or more is an indication that the PbS cell is saturating. Only orificing can
correct this.
Remember: The S512 will read all fuels, propane and natural gas, pulverized coal, light and heavy oil, and all waste fuels. The S509 performs in a similar manner to the S512 by turning the Si channel off.
ORIFICING
Orificing the viewing head sight path will greatly improve performance in many applications, particu- larly with regard to discrimination between flames. The flame signal or radiation intensity can be suffi- ciently great that the IR type photodetectors (PbS) will be completely saturated. This will be true with large power utility boilers, where the burners are very large and relatively close to each other. If you can- not look at the flame with the naked eye without it hurting, then orificing should be used. This test works for incandescent type flames, such as oil and coal, but will not be valid for natural gas flames.
Usually, an orifice disc with a
Generally speaking, orificing should always be con- sidered when the burner(s) are large (50K lbs of steam and up), particularly when the S509 and S512 viewing heads are used. The table in Figure 9 shows the signal reducing characteristics of the different orifice discs.
Aperture | Fn | Relative light |
(Hole) | (f Stop) | passing power |
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1" | 2 | 1 |
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3/8" | 5.3 | 1/8 |
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1/4" | 8 | 1/16 |
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3/16" | 10.7 | 1/32 |
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1/8" | 16 | 1/64 |
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FIGURE 9 - ORIFICING: SIGNAL-
REDUCING CHARACTERISTICS OF DISCS
SIGNAL SOURCES
Orificing will help improve the discrimination char- | Testing the viewing head with signal sources other | |
acteristics for | than flame can result in erroneous readings. Flame | |
is composed of a basic “brightness” radiation with | ||
not likely to saturate; however, if the count is over | superimposed flicker of usually not more than five | |
1500, orificing will definitely improve the discrimi- | per cent. These flicker components vary, in a ran- | |
nation ratio. | dom fashion, from zero frequency (DC) to relatively | |
An orifice disc is simply a metal disc with a circular | high frequencies (1K Hz and up). As explained pre- | |
viously, the viewing head has been designed to re- | ||
hole in the center which is placed directly in front of | ||
spond to actual flame signals so that discrimination | ||
the lens. Four orifice discs are available with cen- | ||
can be achieved between the flame being monitored | ||
and unwanted background. | ||
These discs are installed with retaining rings in the | ||
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flange at the edge of the |
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thread for the purge air. An internal type retaining |
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ring is first installed by positioning the ring in the |
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