IRIS P522 manual S506 SELF-CHECKING Viewing Head, S509 Viewing Head

IRIS MODEL P522 APPLICATION MANUAL

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 (H2S) 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-

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