Honeywell C7076A Operating Instructions Manual page 4

8.6 sighting angle
The first 30% of a flame closest to the burner nozzle (the flame root)
emits the most ultraviolet energy. Also, if the detector sights the flame
at an angle instead of perpendicularly, it views more flame depth.
Therefore, the best sighting angle is nearly parallel to the axis of the
flame, permitting the detector to view a large portion of the first 30%
of the flame closest to the burner nozzle.
Detector in good sighting position
(low angle sighting)
unburned fuel
Burner nozzle
Detector in poor
sighting position
Detector sighting angle
Low angle sighting permits the detector to view a greater depth of
flame, thus reducing the effects of instabilities in the flame pattern.
Also, the environment near the burner nozzle is usually cleaner than
at any other part of the combustion chamber. This provides a clearer
line of sight and can keep the viewing window cleaner, thus reducing
the maintenance required.
➔ When possible, it is desirable to tilt the detector and sight pipe
downward to prevent the buildup of soot in the pipe or on the
viewing window.
In most installations, the detector needs to respond to the pilot flame
alone, then the pilot and main burner flame together, and finally the
main burner flame alone.
The detector must meet all sighting requirements that apply:
– Pilot flame alone: The smallest pilot flame that can be detected
must be capable of reliably igniting the main burner.
– Pilot and main burner flame together: The detector must sight
the junction of both flames.
– Main burner flame alone: The detector must sight the most
stable part of the flame for all firing rates.
8.7 screening effects
Smoke, fuel mist, dirt and dust are masking agents that absorb
ultraviolet radiation from the flame. They create a screen that re-
duces the amount of ultraviolet radiation reaching the detector and
may cause flame signal deterioration resulting in a shutdown. The
adverse affects of screening may be minimized by proper burner
adjustment, increasing the detector viewing area (shorten sight pipe
and/or increase its diameter), and optimizing detector sensitivity.
8.8 Multiburner requirements (flame discrimination)
In addition to meeting the requirements for a single burner, a multi-
burner installation requires discrimination between flames. Flame dis-
crimination can be defined as locating all flame detectors so that each
detector responds only to the flame of the burner it is supervising.
In multiple burner systems, not every detector can be positioned so
its line-of-sight does not intercept flames from other burners. For
example, this situation occurs in front-fired boiler-furnaces having
more than one row of burners, or in multilevel opposed-fired furnaces
where the burners face each other.
When planning such an installation, locate each flame detector so
that it has the best possible view of the first 30% closest to the burner
nozzle (the flame root) it is supervising, and the worst possible view
of all other flames.
flame depth angle view
flame depth-
perpendicular view
Flame A
Detector A
Example of flame discrimination problem (opposed fired burners).
This figure above illustrates a critical detector application problem that
requires flame discrimination. Flame discrimination is accomplished
for Detector A by repositioning it until the flame relay (in the flame
safeguard control) does not respond to Flame B. Note that Detector
A is aimed at the first 30% of Flame A where the ultraviolet radiation
is most intense. It sights the tip of Flame B, but it is not aimed at the
first 30% of Flame B where UV is intense. Detector A is repositioned
to assure maximum response to Flame A while rejecting Flame B.
Similarly, Detector B is positioned to assure maximum response to
Flame B while rejecting Flame A.
If you reposition a detector and still cannot achieve flame discrim-
ination, try reducing the viewing area by increasing the length or
decreasing the diameter of the sight pipe, or adding an orifice plate.
8.9 Multifuel requirements
Detectors supervising burners that alternately fire more than one fuel
may require a different sensitivity level for each fuel. For example, a
higher sensitivity is required to reliably sense pulverized coal or No.
6 fuel oil in contrast to natural gas or to No. 2 fuel oil. Reliable flame
sensing and flame discrimination may not be maintained simulta-
neously (without changing the sensitivity setting) when alternating
between two fuels.
The C7076 has two integral sensitivity adjustments that can be re-
motely and automatically selected. The two sensitivity adjustments
can be chosen by means of the fuel selector switch (refer to Remote
Sensitivity Selection in the Installation section). The sensitivity adjust-
ments should be made for both fuels as described in the section on
multiburner requirements, using one adjustment pot for each fuel.
8.10 Parallel flame detectors
Two C7076 detectors can be connected in parallel to the same
flame signal amplifier and still provide independent sensitivity adjust-
ment. This capability is particularly useful for multiburner, multifuel
applications.
Shifting flame patterns, commonly encountered on burners with wide
turndown ratios, may require parallel detectors to prove the flame at
the highest and lowest firing rates. In this case, one detector super-
vises the pilot (interrupted) and both detectors supervise supervise
the main burner flame. During the main burner run period, either
detector is capable of maintaining system operation.
In addition to assuring more reliable flame detection, parallel detectors
facilitate maintenance during burner operation. Each detector can
be removed in turn without shutting down the supervised burner.
However, a flame simulating failure occurring in the flame signal
amplifier or in either detector will cause a shutdown.
8.11 Redundant Flame Detection system
Two C7076 detectors connected to two flame signal amplifiers wired
in parallel comprise a redundant flame detection system. In addition
to the features of parallel flame detectors, a redundant system in-
creases reliability and is therefore recommended for critical burner
applications. A flame failure, flame signal loss, or flame simulating
failure occurring in either detector subsystem will cause an alarm
(not a shutdown) allowing corrective action to avert a shutdown.
EN-4
Flame B
Detector B