FLAME DETECTOR & FM3260:

The performance of flame detectors is defined by the test results utilizing the FM 3260 test.FM 3260 testing provides a set of results, that is often referred to as the “cone of vision” of the detector. A typical cone of vision for an optical fire detector is presented in this post.Gray area where design basis fire (1 ft. x 1 ft. n-Heptane pan fire) were to be located, it would be “seen” (i.e. detected) by the fire detector.But there is issue.The performance presented in the FM 3260 results is calculated in ideal laboratory conditions, whereas in a process plant there are conditions present that will act to attenuate, or reduce, the effective distance at which a design basis fire can be seen.Attenuating factors are divided into three categories.

1) due to reduction in sensitivity due to false stimuli (Attenuating Factor 1)=

E.g if a fire detector is capable of viewing a design basis fire at 100 meters in the FM 3260 test, but during field trials there are many false stimuli sources the detector can actually only see the fire from 75 meters, then the attenuation factor for false stimuli is 75/100 or AF1= 0.75.

2) due to dirty optics (Attenuating Factor 2)=

if dirty optics alarms set at 70% transmissivity, then the dirty-optics factor should be 100+70/100, or AF2=0.85.

3) due to off-center line view (Attenuating Factor 3)=

The cone-of-vision drawing above, you will note that fires that are directly in front of the fire detector can be seen at 210 feet, but fire that are seen at a 45 degree angle can only be seen at 150 feet. As such, if your FGS mapping technique cannot elegantly account for different detection distance at different angle, you must conservatively use the smallest distance in the cone of vision, as presented in the figure below, this distance would only be 150 ft, resulting in an off-center line factor of 150/210, or AF3=0.71.

AF3 is only required if the mapping software or mapping technique cannot elegantly model different viewing distances and different off-center line angles

These factors are used to decrease the effective distance at which a fire can be seen.

The cone of vision angle one can draw in 2D or 3D to understand the coverage of Flame detection.