Tips To Select The Right Flame Detector

Consider a number of factors to determine the most appropriate option

By Edward Naranjo, Emerson Automation Solutions

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Fires and explosions are common hazards in the chemical and refining industries. Unexpected releases of flammable liquids and gases can ignite, sometimes violently, when mixed with air, leading to injury and property damage. Energy-sensing flame detectors can enhance the safety of processes involving flammable materials by triggering an alarm when a fire erupts, thus providing early warning and helping ensure people’s safety.

However, operating staff often debate about which type of device to select and where to place the sensors for best coverage. The confusion stems in part from the wide range of flammable materials that process plants may handle; a single process area may contain several different types. In addition, the development of new technologies like visual flame imaging and pattern recognition, while improving identification effectiveness, has created uncertainty around the choice of flame detectors. Fortunately, the particular application rather than detection technology determines the proper pick. In this article, we’ll examine the criteria for selection and review the detector types commonly used to protect against industrial fires.

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Key Considerations

Flame detectors respond to the radiant emissions from a flame — so, the first and perhaps most important consideration when selecting detectors is that they accurately and reliably identify flames. Detection equipment must suit the particular hazard; the devices’ spectral responses must match the spectral emissions of the flames to be detected [1]. Few plants fail to choose devices based on the process fluids present in the coverage area but many sites often don’t consider other fire hazards. For example, flame detectors at a gas compressor building in Brazil failed to detect a small fire produced by ethylene glycol. The fluid had leaked from a heat exchanger onto a manifold in a compressor and ignited upon contact with the hot surface. Because ethylene glycol flames produce little infrared (IR) light, the IR flame detectors installed to detect natural gas flames didn’t offer sufficient coverage to protect against alcohol fires. Luckily, personnel spotted the fire and quickly extinguished it.

Flame detectors vary in capabilities; each type presents advantages and limitations. You should consider several different types of performance variables when selecting the proper flame detector for a particular application:

• Detector range and response time. It’s important to understand the maximum distance within which the device recognizes flames, based on fuel type, and the amount of time the instrument will take to collect, process and report feedback for the radiated energy detected. The response must be quick — within 30 seconds according to the EN 54-10 fire detection safety standard [2] — and accurate. To achieve best economics, look for wide area coverage per device, coupled with the shortest time to detect a flame. Place flame detectors high up and in the edges or corners of a room.

• Immunity to unwanted alarms. A flame detector must mitigate the possibility of false alarms from non-fire sources. A false alarm incident may cause system shutdowns and evacuations, as well as result in investigations by the company or local authority having jurisdiction. Restarting a process may take hours or months, particularly when considering quality, environmental and process safety regulatory requirements. Moreover, false alarms lead to wasted effort by emergency responders and worker downtime as operations sit idle. In addition, they incur costs for replacement of extinguishing agents from fire suppression systems and materials lost within the process. Placing the detectors in more appropriate locations, decreasing their sensitivity setting and increasing their delay setting may mitigate false alarms. You also may tilt the flame detector so it has a better orientation for false alarm rejection.

• Field of view. Most flame detectors have a 90° to 120° horizontal field of view. Wide fields of view usually are desirable to protect closed modules where obstructions may limit area coverage over long distances. Place the detectors high off the ground so they have good line of sight to the area of concern; you don’t have to take ceiling height specifically into account.

• Environmental factors. Flame detectors at process plants must operate over a wide temperature range. You must consider high ambient temperature limits for devices installed inside compressor or turbine enclosures. In addition, flares and flare reflections may affect certain flame detector types like multispectral infrared and ultraviolet/IR (UV/IR) — particularly during blowdown to safely dispose of excess combustible gases or liquids in a flare.

• Communication capabilities. The role of flame detectors is to initiate suitable responses such as automatic control actions and alarms. This depends upon effective communication of outputs like 4–20-mA analog signals or relay contacts for remote alarm or fault indication. Bidirectional communication protocols like HART can make information pertaining to parameters, device configuration and device diagnostics available to central control or monitoring systems [3].

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