Optical Flame Detectors

Flame detectors alarm at the presence of light from flames usually in the ultraviolet or infared range. The detectors are set to detect the typical light flicker of a flame. They may be equipped with a time delay features to eliminate false alarms from transient flickering light sources. 

There are six types of optical detector commonly used in the oil and gas industry. 

Each of the five types of detectors listed has advantages and limitations, making each more or less suitable for an application or a specific risk. There is not a uniform performance standard for flame detectors such as their is with smoke detectors. Flame detection for a particular model has to be analyzed by evaluation of its technical specification to expected fire development. 

---

The flame detector is an optical device that responds to the radiant energy that is given off by a flame. When a flame or explosion occurs within the field of view of the detector, the resulting electromagnetic radiation travels toward the detector at... 

An optical flame sensor installed at the beginning of the pipeline is the most suitable device for such an isolation system, since the propagating flame from the explosion has to be detected and extinguished. Pressure detectors alone are, in principle, not suited to the case on hand because there is no distinct separation between the pressure and flame fronts for explosion in pipelines. Optical ir sensors that have a relatively low sensitivity to daylight are normally chosen and have proved themselves amply in industrial practice. Therefore, daylight into the pipe in the vicinity of the sensor must be avoided. It is necessary to flush the optical lens with gas (e.g., nitrogen, air) to keep it dust-free. 

Supercritical fluid chromatography is compatible with both HPLC and GC detectors. As a result, optical detectors, flame detectors and spectroscopic detectors can be used. The FID is the most common detector used. However, the mobile phase composition, column type and flow rate must be taken into account when the detector is selected. Some care must also be taken such that the detector components are capable of withstanding the high pressures of SFC. 

Laser detectors operate in a similar way to optical beam detectors. The laser beam is directed across the area to be protected and is deflected or obscured by either the heat, the flame or the smoke rising from a fire, thus changing the intensity of lighf at the receiver. This triggers the alarm. 

For incandescent fires, optical fire detectors should be used. These may be UV detectors, IR detectors, or closed-circuit TV (or a combination of these). The choice will depend partly on the type of flame that is anticipated. For example, methanol bums with a nearly invisible flame, so a fire of this material cannot be detected by CCTV. 

Figure 6.3 A fiber-optically coupled two-color flame detector assembly originally developed for military aircraft, but now used extensively to protect automotive paint robots where flames are a real danger. A two-color Si/Si sandwich photodetector and the electronics and logic are in the red brick. One fiber optic cable conducts radiation from a remote sensing site to the detector, and the other is used for thru-the-lens self-test. (Image courtesy of Detector Electronics Corporation.)...

Other Detectors Two additional detectors are similar in design to a flame ionization detector. In the flame photometric detector optical emission from phosphorus and sulfur provides a detector selective for compounds containing these elements. The thermionic detector responds to compounds containing nitrogen or phosphorus. 

Flame Photometric Detector3 With the flame photometric detector (FPD), as with the FID, the sample effluent is burned in a hydrogen/air flame. By using optical filters to select wavelengths specific to sulfur and phosphorus and a photomultiplier tube, sulfur or phosphorus compounds can be selectively detected. 

In Chapter 5.4, optical ultraviolet radiation sensors are described, including UV-enhanced silicon-based pn diodes, detectors made from other wide band gap materials in crystalline or polycrystalline form, the latter being a new, less costly alternative. Other domestic applications are personal UV exposure dosimetry, surveillance of sun beds, flame scanning in gas and oil burners, fire alarm monitors and water sterilization equipment surveillance. 

---