IRIS P522 manual Orificing, Signal Sources

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.

machined-groove inside the flange opening from the housing side. The orifice disc is then fixed in place tightly between the two retaining rings. Install the retaining ring (or, “snap” ring) in the machined groove inside the flange, from the housing side.

Usually, an orifice disc with a larger-diameter hole is tried first. There will be a reduction in signal, so readjustments will be required following the steps outlined in the section INITIAL SETUP. If the dis- crimination is still not satisfactory, then try an orifice disc with a smaller-diameter hole.

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
1"	2	1
3/8"	5.3	1/8
1/4"	8	1/16
3/16"	10.7	1/32
1/8"	16	1/64

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 UV-type viewing heads as well as the	than flame can result in erroneous readings. Flame
solid-state flicker types. The UV tube in the S506 is	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
ter-hole diameters of 3/8, 1/4, 3/16 and 1/8 inch.
	and unwanted background.
These discs are installed with retaining rings in the
flange at the edge of the 1/2-inch NPT female pipe
thread for the purge air. An internal type retaining
ring is first installed by positioning the ring in the
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