I.R.I.S. FLAME MONITORING SYSTEM P522 - Manual (Page 23)

IRIS MODEL P522 APPLICATION MANUAL
Page 23
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.
Orificing will help improve the discrimination char-
acteristics for UV-type viewing heads as well as the
solid-state flicker types. The UV tube in the S506 is
not likely to saturate; however, if the count is over
1500, orificing will definitely improve the discrimi-
nation ratio.
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 cen-
ter-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 the ring in the
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.
SIGNAL SOURCES
Testing the viewing head with signal sources other
than flame can result in erroneous readings. Flame
is composed of a basic “brightness” radiation with
superimposed flicker of usually not more than five
per cent. These flicker components vary, in a ran-
dom fashion, from zero frequency (DC) to relatively
high frequencies (1K Hz and up). As explained pre-
viously, the viewing head has been designed to re-
spond to actual flame signals so that discrimination
can be achieved between the flame being monitored
and unwanted background.
e r u t r e p A
) e l o H (
n F
) p o t S f (
t h g il e v i t a l e R
r e w o p g n i s s a p
" 1 2 1
" 8 / 3 3 . 5 8 / 1
" 4 / 1 8 6 1 / 1
" 6 1 / 3 7 . 0 1 2 3 / 1
" 8 / 1 6 1 4 6 / 1
FIGURE 9 - ORIFICING: SIGNAL-
REDUCING CHARACTERISTICS OF DISCS