IRIS S55XX SERIES VIEWING HEADS APPLICATION MANUAL
PAGE 9
the UV radiation present is from a very small region.
This small region emitting UV is not the case how-
ever, for low NOx gas burners. The UV radiation is
usually much less in intensity and spread out with this
type of burner, and relatively high readings can be ob-
tained from all over the furnace when many burners
are on. This is particularly true when flue gas recir-
culation is used. There will however, be a relatively
stronger signal near the “root” of the flame and this
more intense spot should be located during the aiming
or sighting process. This “root” or intense spot may be
further out than with the standard gas burner so it is
imperative that a swivel mount be used when making
these sighting adjustments.
Another factor that needs to be considered when
aiming the viewing head, is the load condition of the
boiler. The flames from a burner can be radically dif-
ferent at different loads. This is one of the reasons for
trying to choose a sighting initially that will minimize
this changing flame at different loads. If the flame
extends further out at higher loads then the parallel
to the burner axis sighting will be the most effective.
The use of a large diameter sight pipe with a swivel
mount is recommended for all installations on large
multi burner boilers.
ORIFICING
In some applications, the extreme brightness of the
burner flames can cause the sensors in the viewing
head to saturate. Under these conditions, orificing of
the viewing head sight path will greatly improve the
performance, particularly with regard to discrimina-
tion between flames. Saturation is more probable with
large power utility boilers where the burners are very
large and relatively close to one another. If the flame
is bright enough to cause the eye to hurt when looking
at the flame, then orificing should be used. This eye
test applies to incandescent type flames such as oil
and coal but will not be valid for natural gas flames.
Orificing may help improve the discrimination char-
acteristics for the UV detector as well as the IR. The
UV detector is not likely to saturate; however, orific-
ing will usually improve the discrimination ratio for
large size burners.
An orifice disc is simply a metal disc with a circular
hole in the center which is placed directly in front of
the lens. Four orifice discs are available with center
hole diameters of 3/8, 1/4, 3/16 and 1/8 inch. 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 a ring in the machined groove
inside flange opening from the housing side. The ori-
fice disc is then inserted. Use a second retaining ring
to hold it in place so that it is sandwiched tightly be-
tween the two retaining rings.
Usually an orifice disc with a larger diameter hole is
tried first. There will be a reduction in the signal level
so if the associated viewing head and signal processor
has been previously calibrated, it will be necessary to
calibrate the processor again so that the gain settings
will be correct. If the discrimination is still not satis-
factory then an orifice disc with a smaller diameter
hole should be tried.
Generally speaking, orificing should always be con-
sidered when the burner(s) are large (50K lbs of steam
and up), particularly when the IR detector is used
which would be the case for oil or pulverized coal.
The following table shows the signal reducing charac-
teristics of the different orifice disks.
Aperture Hole
Fn (f Stop)
Relative light
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
DEFAULT SETTINGS FOR THE S550B
A new signal processor will set the attached viewing
head default values as follow:
UV Gain = 32
(out of 0-99 range)
Filter = Filter 3
(high pass above 33Hz)
IR Gain = 451
(out of 0- 699 range)
These are nominal settings that should allow for ini-
tial sightings and adjustments. Both UV and IR sen-
sors are active in the default mode. If the settings for
the viewing head has been changed from the default
values, they can be reset to the factory default from