I.R.I.S. FLAME MONITORING SYSTEM P522 - Manual (Page 20)

IRIS MODEL P522 APPLICATION MANUAL
Page 20
tion is present). Its output is randomly-spaced pulses,
with an average pulse rate proportional to the UV
radiation present. The digital count when reading
natural gas will be about the same as the S512, and
probably will be less when reading a heavy oil flame.
The very narrow spectral response of the UV tube
makes this type of detector ideal for discriminating
between flame, glowing refractory, and other burn-
ers. However, because of its response to only the
very short wavelengths, problems can occur because
this radiation is easily absorbed and masked by un-
burned fuel or other impurities in the coal (i.e., with
pulverized coal). Sour gas (H
2
S) will readily absorb
these short 200 nm wavelengths, as well as other
waste fuels resulting in a dropout of the flame sig-
nal. Even standard glass will absorb this wavelength;
therefore it is necessary to use a quartz lens with this
viewing head.
In general, the S506 viewing head will work well on
natural gas flames. It will respond to oil flames, but
with a lower signal level. The sighting should be
parallel to the axis of the burner and aimed at the
foot of the flame. Photographs of gas and oil flames
using visual- and UV-sensitive film clearly show that
the UV zone is much smaller than the visible zone,
and the highest UV intensity occurs near the flame
root. In addition, the zone of higher UV intensity
does not overlap the same zones of adjacent or op-
posing burners; so, with proper sighting, discrimi-
nation is predictable.
S506 SELF-CHECKING
VIEWING HEAD
The self-checking characteristics of the S506 are
implemented by using a shutter that blocks the flame
light path every second for 200 milliseconds. The
reason for doing this, presumably, is to detect a run-
away UV tube. The McGraw Edison-type tube tra-
ditionally used for UV flame detection is known to
have an avalanche mode (runaway condition) that
creates pulses when there is no flame present.
The UV tube used in the S506 is a different type of
tube, having an anode and cathode instead of the
dual-anode type used by others. This tube also uses
a relatively low voltage for operation, and we have
experienced a runaway condition with this tube only
once, with a tube that had been subjected to such a
significant shock that the electrodes inside were
clearly bent.
Why use self-checking if the tube does
not runaway? Because any runaway
failure, for any reason, is unacceptable,
and self checking monitors all electronic
components, from the UV tube to the
flame relay.
Each time the shutter closes, the flame signal is blocked,
resulting in a cessation of pulses. This is interpreted by
the P522 as a normal condition (no pulses during the
dark period) and the flame relay is kept energized. If
this does not occur, the P522 will go into a lockout
state, causing the flame relay to de-energize (see sec-
tion SELF CHECKING FUNCTION).
S509 VIEWING HEAD
As explained in the S509 product brochure, this view-
ing head employs a two-colour photodetector using
a “see-through” silicon (Si) front element and a lead
sulfide (PbS) back element. It is important to un-
derstand how this viewing head operates with respect
to different fuels and flame conditions.
The electromagnetic spectrum, or spectral range, for
the Si element is 350 nm to 1100 nm; for the PbS
element it is 1100 nm to 3000 nm. The Si element will
detect all incandescent type flames such as oil, pulver-
ized coal, and most waste fuels. It will not see natural
gas or propane fuels at all. The PbS element will detect
all flames, including natural gas and propane.
For this reason, the S509 viewing head will exhibit
stronger signals for oil and pulverized coal than for
natural gas, providing both the Si and PbS 25-turn
POTS are full clockwise (full on). Typical “bright”
signals for oil can read as high as 2900 on the digital
display, while natural gas may read as high as 2100.
For instance, a propane torch two inches away from
the viewing head will read 1900. (The above, typi-
cal readings are made with the filter switch in the
LL position.)
An important characteristic that must be explained
is the nonlinearity of the Si channel on “bright” sig-